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UBC Theses and Dissertations

A comparative study of DC and AC vortex stabilized arcs Gettel, Lorne Edward 1980

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A COMPARATIVE STUDY OF DC AND AC VORTEX STABILIZED ARCS 1 LORNE EDWARD GETTEL B . S c , U n i v e r s i t y o f B r i t i s h Columbia, 1974 M.Sc, U n i v e r s i t y o f B r i t i s h Columbia, 1975 A THESIS SUBMTTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF THE FACULTY OF GRADUATE STUDIES (Physics Department) We accept t h i s t h e s i s as conforming to the re q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA September 1980 DOCTOR OF PHILOSOPHY i n Lome Edward G e t t e l , 1980 In presenting th i s thesis in pa r t i a l fu l f i lment o f the requirements f o r an advanced degree at the Univers i ty of B r i t i s h Co lumb ia , I ag ree that the L ibrary shal l make it f ree ly ava i lab le for r e f e r e n c e and study. I further agree that permission for extensive copying of th is thesis for scho lar ly purposes may be granted by the Head of my Department o r by his representat ives. It is understood that c o p y i n g o r p u b l i c a t i o n o f th i s thes is for f inanc ia l gain sha l l not be allowed without my written permission. Department of The Univers i ty of B r i t i s h Columbia 2075 Wesbrook P l a c e Vancouver, Canada V6T 1W5 Date SEPT lOJ^O ABSTRACT A comparative study of high i n t e n s i t y DC and AC vortex s t a b i l i z e d a r c s o p e r a t i n g i n argon (at p r e s s u r e s of one to f i v e atmospheres) has been conducted. The energy balance f o r both the AC and DC a r c s has been determined c a l o r i m e t r i c a l l y . From these measurements the r a d i a t i v e e f f i c i e n c y ( r a d i a t i o n l o s s e s / i n p u t power) has been c a l c u l a t e d . I t was found over the c u r r e n t range examined (150-450 amperes) t h a t the r a d i a t i v e e f f i c i e n c y of the AC vortex s t a b i l i z e d a r c was comparable to the DC a r c . Since DC vortex s t a b i l i z e d a r c s have been used as a high i n t e n s i t y r a d i a t i o n source, these r e s u l t s i n d i c a t e t hat the AC v o r t e x s t a b i l i z e d a r c shows promise f o r use as a high i n t e n s i t y r a d i a t i o n source. From the energy balance r e s u l t s the heat t r a n s f e r to the w a l l was s u r p r i s i n g l y found to s c a l e l i n e a r l y with the r a d i a t i o n l o s s e s . The w a l l l o a d i n g i s not due to a b s o r b t i o n of r a d i a t i o n and i s much l a r g e r than t h a t expected from laminar r a d i a l heat t r a n s f e r . To i n v e s t i g a t e t h i s f u r t h e r a simple channel model was developed f o r the luminous DC a r c core. From t h i s model the r a d i u s and temperature of the luminous arc core was determined as a f u n c t i o n of c u r r e n t . The p r e d i c t e d r a d i i were i n good agreement with time i n t e g r a t e d photographs of the luminous core of the a r c . At h i g h c u r r e n t (I>350 amperes) the DC a r c r a d i u s was e s s e n t i a l l y c o n s t a n t . The w a l l heat t r a n s f e r c o n t i n u e d to i n c r e a s e when the a r c r a d i u s was e s s e n t i a l l y c o n s t a n t , so t h a t h i g h l y e f f i c i e n t heat t r a n s f e r p r o c e s s e s must be t a k i n g p l a c e o u t s i d e the c e n t r a l luminous a r c c o r e . I t i s b e l i e v e d t h a t t u r b u l e n t m i x i n g might be p r e s e n t i n t h i s r e g i o n and be r e s p o n s i b l e f o r the l a r g e w a l l heat t r a n s f e r . The heat t r a n s f e r p r o c e s s e s to the a r c e l e c t r o d e s have been measured c a l o r i m e t r i c a 1 l y and the e l e c t r o d e s u r f a c e t e m p e r a t u r e has been measured s p e c t r o s c o p i c a l l y . For b o t h AC and DC e l e c t r o d e s the heat t r a n s f e r s c a l e d l i n e a r l y w i t h the a r c c u r r e n t . The e l e c t r o d e v o l t a g e d r o p i s s t r o n g l y dependent on gas f low d i r e c t i o n w i t h the v o l t a g e d r o p always l a r g e r f o r f low towards the e l e c t r o d e than f o r f low away from the e l e c t r o d e . These r e s u l t s a r e not due to c o n v e c t i v e heat t r a n s f e r e f f e c t s . The geometry o f the e l e c t r o d e a r c a t t a c h m e n t r e g i o n changes when the f low d i r e c t i o n i s r e v e r s e d . I t i s b e l i e v e d t h a t both the anode and cathode f a l l p o t e n t i a l s a r e a l t e r e d when the f low d i r e c t i o n i s r e v e r s e d , and t h i s i s r e s p o n s i b l e f o r the d i f f e r e n c e i n e l e c t r o d e v o l t a g e d r o p when the flow d i r e c t i o n i s r e v e r s e d . From the e l e c t r o d e s u r f a c e t e m p e r a t u r e measurements the heat t r a n s f e r to the a r c e l e c t r o d e s was shown to be e s s e n t i a l l y o n e - d i m e n s i o n a l i n n a t u r e . A model o f the AC e l e c t r o d e heat t r a n s f e r was d e v e l o p e d u s i n g the DC heat t r a n s f e r r e s u l t s which p r e d i c t s r e s u l t s i v f o r the e l e c t r o d e v o l t a g e drop t h a t are i n good agreement w i t h the e x p e r i m e n t a l r e s u l t s . The AC e l e c t r o d e heat t r a n s f e r was found to be <50% of the anode heat t r a n s f e r i n a DC a r c a t the same c u r r e n t . In the DC a r c the anode heat t r a n s f e r i s much l a r g e r than the cathode heat t r a n s f e r . For a p r a c t i c a l DC v o r t e x s t a b i l i z e d arc r a d i a t i o n source anode f a i l u r e i s a s e r i o u s p r o b l e m , so t h a t the r e s u l t s f o r the AC e l e c t r o d e heat t r a n s f e r i s of c o n s i d e r a b l e p r a c t i c a l i m p o r t a n c e . TABLE OF CONTENTS A b s t r a c t i i T a b l e o f C o n t e n t s v L i s t o f T a b l e s v i i i L i s t o f F i g u r e s i x Acknowledgements x i i i 1 INTRODUCTION 1 2 RADIATION AND ELECTRODE ENERGY TRANSFER MECHANISMS IN VORTEX STABILIZED ARCS 11 2.1 I n t r o d u c t i o n 11 2.2 V o r t e x S t a b i l i z e d A r c s 11 2.2.1 B a s i c P r i n c i p l e s o f O p e r a t i o n 11 2.2.2 E n e r g y B a l a n c e and R a d i a t i o n L o s s e s o f a V o r t e x S t a b i l i z e d A r c 14 2.2.3 R a d i a t i o n L o s s e s from t h e V o r t e x S t a b i l i z e d A r c 17 2.3 E l e c t r o d e Heat T r a n s f e r Mechanism 19 2.3.1 Power B a l a n c e a t t h e Anode 20 2.3.2 Power B a l a n c e a t t h e C a t h o d e 25 3 APPARATUS AND EXPERIMENTAL PROCEDURE 31 3.1 The V o r t e x S t a b i l i z e d A r c 31 3.2 A r c Power S u p p l y 39 3.3 V o l t a g e - C u r r e n t C h a r a c t e r i s t i c s o f t h e V o r t e x S t a b i l i z e d A r c 40 3.4 A r c D i a g n o s t i c s .43 3.4.1 E n e r g y B a l a n c e 43 3.4.2 E l e c t r o d e S u r f a c e T e m p e r a t u r e Measurements 48 3.4.3 Time R e s o l v e d O b s e r v a t i o n s o f AC A r c E l e c t r o d e s 56 EXPERIMENTAL RESULTS 60 4.1 I n t r o d u c t i o n 60 4.2 Energy B a l a n c e 60 4.3 E l e c t r o d e Heat T r a n s f e r 71 4.4 E l e c t r o d e S u r f a c e Temperature Measurements ...87 4.4.1 A c c u r a c y of the Temperature Measuring System 87 4.4.2 AC E l e c t r o d e S u r f a c e Temperature R e s u l t s 95 4.4.3 DC E l e c t r o d e S u r f a c e Temperature R e s u l t s 103 4.5 Time R e s o l v e d O b s e r v a t i o n s o f AC E l e c t r o d e Arc Attachment Region 107 4.6 Time I n t e g r a t e d Photographs of the DC and AC Arc Column 110 DISCUSSION 117 5.1 I n t r o d u c t i o n 117 5.2 Arc Energy E a l a n c e : S i g n i f i c a n t Parameters ..117 5.3 Energy B a l a n c e 119 5.3.1 DC Energy B a l a n c e 119 5.3.2 AC Energy B a l a n c e 140 5.3.3 P r a c t i c a l C o n s i d e r a t i o n s : R a d i a t i v e E f f i c i e n c y 147 5.4 Heat T r a n s f e r to the Arc E l e c t r o d e s 149 5.4.1 E l e c t r o d e Heat T r a n s f e r S c a l i n g 149 5.4.2 Anode Heat T r a n s f e r 150 5.4.3 Cathode Heat T r a n s f e r 155 v i i 5.4.4 Model of AC E l e c t r o d e Heat T r a n s f e r 157 5.4.5 A P o s s i b l e E x p l a n a t i o n of the Flow Dependence of the E l e c t r o d e Heat T r a n s f e r 162 5.4.6 AC E l e c t r o d e Heat T r a n s f e r : P r a c t i c a l C o n s i d e r a t i o n s 171 5.5 E l e c t r o d e Surface Temperature Measurements 172 6 CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK 186 6.1 Co n c l u s i o n s 186 6.2 Suggestions f o r Future Work 193 B i b l i o g r a p h y 197 Appendix A Arc Power Supply 201 Appendix B S t a r t i n g C i r c u i t f o r the Vortex S t a b i l i z e d Arc 205 Appendix C Arc C a l o r i m e t r y 210 Appendix D Determination of the AC Arc Input Power 214 Appendix E AC Arc Time Constant 217 Appendix F S a f e t y C o n s i d e r a t i o n s 221 v i i i LIST OF TABLES I Energy Balance f o r the AC Arc (45° C o n i c a l E l e c t r o d e s ) 62 II Energy Balance f o r the AC Arc (Truncated 45 C o n i c a l E l e c t r o d e s ) 6'3 II I Energy Balance f o r the DC Arc (Gas Flow from Cathode to Anode) 67 IV Energy Balance f o r the DC Arc (Gas Flow from Anode to Cathode) 68 V The E f f e c t of Gas Flow C o n d i t i o n s on E l e c t r o d e Heat T r a n s f e r 90 VI Summary of DC E l e c t r o d e Heat T r a n s f e r Measurements 91 VII Summary of AC E l e c t r o d e Heat T r a n s f e r Measurements 92 VIII Values of the AC E l e c t r o d e Voltage Drop (V) and Current Independent Heat T r a n s f e r Term (C) 163 I-C T y p i c a l C o o l i n g Water Flow-Rates f o r the Arc Components 212 i x LIST OF FIGURES 1- 1 Vortex S t a b i l i z e d Arc 1 2- 1 DC Vortex S t a b i l i z e d Arc (Union Carbide) 12 2-2 Arc S t a b i l i z a t i o n Mechanism 13 2- 3 Total Radiation Loss from an Argon Plasma 18 3- 1 AC Vortex S t a b i l i z e d Arc 32 3-2 Tangential Gas Jet Assembly 33 3-3 Gas Exit Heat Exchanger Assembly 34 3-4 Geometry of Vortex Gas Column 36 3-5 Electrode Tip Geometry 38 3-6 AC Voltage vs Time and Current vs Time 41 3-7 AC Voltage Current Waveforms 42 3-8 DC Voltage vs Time and Current vs Time 44 3-9 DC Voltage Current Waveforms 45 3-10 Arc Radiation Absorber 47 3-11 Electrode Surface Temperature Measuring System 50 3-12 Decay of Plasma Radiation Following Current Interruption 54 3-13 Decay of Plasma Radiation and Electrode Thermal Radiation Following Current Interruption 55 3-14 System for Time Resolved Photographs of the AC Electrodes 57 3- 15 Synchronous Motor-Stroboscope Unit 58 4- 1 AC Radiative E f f i c i e n c y vs Current (Truncated 45° Conical Electrodes) 65 4-2 AC Radiative E f f i c i e n c y vs Current (45* Conical Electrodes) 66 X 4-3 DC Radiative E f f i c i e n c y vs Current (Flow from cathode to Anode) 69 4-4 DC Radiative E f f i c i e n c y vs Current (Flow from Anode to Cathode) 70 4-5 Typical AC Electrode Heat Transfer vs Current 72 4-6 Typical DC Electrode Heat Transfer vs Current 74 4-7 AC Electrode Heat Transfer vs Current (both flow directions) 75 4-8 DC Electrode Heat Transfer vs Current (Flow from Cathode to Anode) 76 4-9 DC Electrode Heat Transfer vs Current (Flow from Anode to Cathode) 77 4-10 AC Electrode Heat Transfer vs Current for Different Pressure (Flow Away) 79 4-11 AC Electrode Heat Transfer vs Current for Different Pressure (Flow Towards) 80 4-12 DC Electrode Heat Transfer vs Current for Different Pressure (Flow from Cathode to Anode) 81 4-13 DC Electrode Heat Transfer vs Current for Different Pressure (Flow from Anode to Cathode) 82 4-14 AC Electrode Heat Transfer vs Current for Different Flow Rate (Flow Away) 83 4-15 AC Electrode Heat Transfer vs Current for Different Flow Rates (Flow Towards) 84 4-16 Anode Heat Transfer vs Current for Different Flow Rates (Flow Towards) 85 4-17 Cathode Heat Transfer vs Current for Different Flow Rates (Flow Away) 86 4-18 Cathode Heat Transfer vs Current for Different Flow Rates (Flow Towards) 88 4-19 Anode Heat Transfer vs Current for Different Flow Rates (Flow Towards) 89 x i 4-20 Anode Thermal R a d i a t i o n Decay Following Current I n t e r r u p t i o n 94 4-21 AC E l e c t r o d e Thermal R a d i a t i o n Decay Following Current I n t e r r u p t i o n ( F l a t Tipped E l e c t r o d e ) 96 4-22 AC E l e c t r o d e Thermal R a d i a t i o n Decay Following Current I n t e r r u p t i o n (45 C o n i c a l E l e c t r o d e Tip) 97 4-23 AC E l e c t r o d e Surface Temperature vs Time ( F l a t Tip) 100 4-24 AC E l e c t r o d e Surface Temperature vs Time (45 C o n i c a l E l e c t r o d e Tip) 101 4-25 AC E l e c t r o d e Hot Spot Temperature vs Time ....102 4-26 AC E l e c t r o d e Surface Temperature vs Current ..104 4-27 AC E l e c t r o d e Surface Temperature vs Current ..105 4-28 Anode Thermal R a d i a t i o n Following Current I n t e r r u p t i o n f o r D i f f e r e n t Currents 106 4-29 Anode Temperature vs Current 108 4-30 Anode Temperature vs Pressure 109 4-31 Time Resolved Photographs of the AC E l e c t r o d e T i p (1=240 A) I l l 4-32 Time Resolved Photographs of the AC E l e c t r o d e T i p (1 = 240 A) 112 4-33 Time Resolved Photographs of the AC E l e c t r o d e T i p (1 = 380 A) 113 4-34 AC E l e c t r o d e During the Cathode Half Cycle (1 = 100 A) 114 4-35 DC Arc Column (1=450 A) 115 4- 36 AC Arc Column (1 = 380 A) 116 5- 1 DC R a d i a t i o n Losses vs (Current) 120 5-2 DC Input Power vs Current 121 5-3 DC Wall Heat T r a n s f e r vs R a d i a t i o n Losses ....123 x i i 5-4 Radiation Losses from an Argon Plasma vs Temperature 132 5-5 Current Normalized Input Power vs Radius 133 5-6 DC Arc Radius vs Current 136 5-7 DC Arc Temperature vs Current 137 5-8 Axial Arc Temperature vs Current Density 139 5-9 AC Radiation Losses vs Current 141 5-10 AC Input Power vs Current 142 5-11 Maximum AC Radiative E f f i c i e n c y vs Pressure ..143 5-12 AC Wall Heat Transfer vs Radiation Losses ....145 5-13 Predicted AC Electrode Heat Transfer vs I e /VT 160 5-14 Predicted AC Electrode Heat Transfer vs I0/\/T 161 5-15 Arc Column Anode Attachment Region 165 5-16 Arc Column Cathode Attachment Region 168 5-17 Electrode Hot Spot Geometry 174 5-18 Electrode Tip Geometry 177 5-19 Electrode Temperature vs Distance (Input Power = 5 kWatts) 179 A-1 AC Arc Power Supply 202 A-2 DC Arc Power Supply 204 B-l DC Arc Starting C i r c u i t 207 B-2 AC Arc Starting C i r c u i t 208 C-l Cooling System for the Arc Components 211 C-2 D i f f e r e n t i a l Temperature Measuring System ....213 D-l AC Arc Input Power Measuring C i r c u i t 215 x i i i ACKNOWLEDGEMENTS I wish to thank Dr. F.L. Curzon for his enthusiastic supervision of thi s investigation. I would also l i k e to thank Dr. R. Nodwell for suggesting this project. In addition I would l i k e to thank Dr. B. Ahlborn for his suggestions for improving the presentation of t h i s thesis. I would also l i k e to acknowledge the assistance of Dr. D. Camm, Dr. R. Kerr and Dr. S. Richards of Vortek Industries for many useful suggestions in the early stages of this work. Many thanks to Mr. A. Cheuck for his assistance with the electronics and general maintenance of the equipment. The assistance of Mr. C. Sedger in the student workshop, and of Mr. P. Haas and Mr. B. Meyer in the construction of parts of the apparatus i s g r a t e f u l l y acknowledged. I would also l i k e to thank Mr. K. Renneberg for generously allowing the use of equipment from the Optics Laboratory. Special thanks to a l l members of the UBC Plasma Physics group for their friendship and help during the course of this work. The asistance of summer research students Mr. R. Clark and Mr. G. Auchenleck i s also g r a t e f u l l y acknowledged. F i n a l l y I would l i k e to thank my parents for their support and encouragement. Financial assistance from the National Research x i i i i C o u n c i l , UBC F e l l o w s h i p s and Transport Canada has been g r a t e f u l l y r e c i e v e d . T h i s work was supported by a grant from the N a t i o n a l Science and Engineering Research C o u n c i l of Canada. 1 Chapter 1 INTRODUCTION An arc which i s burning with i t s column on the a x i s of a r o t a t i n g gas or water w a l l as shown i n F i g u r e 1-1 i s c a l l e d a vortex s t a b i l i z e d a r c . E L E C T R O D E GAS VORTEX A R C P L A S M A F i g u r e 1-1: Vortex S t a b i l i z e d Arc Recently DC vortex s t a b i l i z e d a r c s have shown c o n s i d e r a b l e promise f o r use as high i n t e n s i t y r a d i a t i o n sources (Anderson (1965), Denis (1965), S t r e s i n o (1966), Tuchman (1967), M a l l i a r i s (1970), Tam (1972) and Camm (1974)). At present such sources are used s u c c e s s f u l l y as s o l a r s i m u l a t o r s by the Avco C o r p o r a t i o n and Vortek I n d u s t r i e s . These vortex s t a b i l i z e d a r c s have much higher 2 e f f i c i e n c i e s and o u t p u t power t h a n c o n v e n t i o n a l l i g h t s o u r c e s and w i l l u n d o u b t e d l y be used i n the f u t u r e f o r wide a r e a i l l u m i n a t i o n , p a r t i c u l a r l y f o r t r a n s p o r t a t i o n f a c i l i t i e s . The a r c column of a c o n v e n t i o n a l v o r t e x s t a b i l i z e d l i g h t s o u r c e r u n s down the c e n t r e of a gas v o r t e x t h e r e b y making i t p o s s i b l e t o m a i n t a i n a l o n g w e l l c o n s t r i c t e d l u m i n o u s p l a s m a . F o r any p r a c t i c a l a r c l i g h t s o u r c e , c o n s t r i c t i o n o f the a r c column i s r e q u i r e d to maximize tne r a d i a t i o n l o s s e s . By c o n s t r i c t i n g t h e a r c column a t f i x e d c u r r e n t , the c u r r e n t d e n s i t y i s i n c r e a s e d r e s u l t i n g i n a h i g h e r plasma t e m p e r a t u r e . The t e m p e r a t u r e i n c r e a s e produc ed by c o n s t r i c t i n g the c o l u m n , i n t u r n , g r e a t l y i n c r e a s e s the r a d i a t i o n l o s s e s from the a r c . Even though DC v o r t e x s t a b i l i z e d a r c s (VSA) have shown promise f o r use as a h i g h i n t e n s i t y r a d i a t i o n s o u r c e t h e r e are s t i l l a number o f s i g n i f i c a n t problems with DC a r c s . The anode heat t r a n s f e r i n h i g h i n t e n s i t y DC v o r t e x s t a b i l i z e d a r c s i s t y p i c a l l y much l a r g e r t h a n the c a t h o d e h e a t t r a n s f e r { M a l l i a r i s (1970), Camm (1977)). C a t a s t r o p h i c t h e r m o - m e c h a n i c a l f a i l u r e of t h e anode u s u a l l y l i m i t s the l i f e t i m e of the a r c (Camm (1977)) . By o p e r a t i n g t h e v o r t e x s t a b i l i z e d a r c w i t h a l t e r n a t i n g c u r r e n t t h e heat t r a n s f e r t o each e l e c t r o d e s h o u l d be r o u g h l y t h e average of the anode and c a t h o d e heat t r a n s f e r i n a DC a r c . T h i s r e d u c t i o n of t h e e l e c t r o d e heat t r a n s f e r s h o u l d t h e r e f o r e i n c r e a s e the e l e c t r o d e l i f e t i m e . 3 The other minor problem with DC vortex s t a b i l i z e d arcs concerns the power supply for the arc. Most re a d i l y available high current power supplies are alternating current supplies. Hence DC vortex s t a b i l i z e d arcs reguire costly high power r e c t i f i e r c i r c u i t r y . Both these problems with high i n t e n s i t y DC vortex s t a b i l i z e d arcs may be overcome by operating the VSA with alternating current. The work reported i n t h i s thesis i s based on experiments performed on both alternating and direct current vortex s t a b i l i z e d arcs operating i n argon. Previous work on AC arcs i s very limited compared to the immense volume of work performed on DC arcs. The work performed on AC vortex s t a b i l i z e d arcs i s even more l i m i t e d . The only other work reported on AC vortex s t a b i l i z e d arcs was that done by Andrada and Erfurth (1962). Hence th i s thesis touches on a large number of aspects of the behaviour of the AC VSA, establishing o v e r a l l features rather than dealing with the precise d e t a i l s of operation of the arc. Throughout t h i s work the behaviour of the AC arc has always been compared to that of a DC VSA operating under the same conditions. For the AC vortex s t a b i l i z e d arc (VSA) to be useful as a high intensity radiation source i t must have reasonably high r a d i a t i v e e f f i c i e n c y (radiative efficiency=radiation losses/input power). If the 4 - r a d i a t i v e e f f i c i e n c y of the AC arc was only a small f r a c t i o n of that for the DC VSA then any advantages i n electrode l i f e t i m e offered by the AC arc would be meaningless. The e f f i c i e n c y of the AC VSA, however, does not necessarily have to be as high as that of the DC arc for i t to be useful as a p r a c t i c a l l i g h t source. If the electrode l i f e t i m e for the AC arc i s subs t a n t i a l l y longer than for the DC arc, then any reductions in e f f i c i e n c y caused by operating AC would not be s i g n i f i c a n t . Hence i t i s e s s e n t i a l to determine the radi a t i v e e f f i c i e n c y of the AC VSA and compare i t with the DC r e s u l t s . To accomplish t h i s the energy balance for both AC and DC operation of the vortex s t a b i l i z e d arc as a function of current and gas flow conditions has been determined. From these results the rad i a t i v e e f f i c i e n c y has been calculated. From the energy balance measurements new scaling r e l a t i o n s have been determined r e l a t i n g the various power loss mechanisms present i n the arc. It was found that the heat transfer to the wall of the arc i s proportional to the radiati o n losses and i s considerably larger than that predicted by laminar r a d i a l heat transfer. To investigate t h i s further a simple channel model was developed. In t h i s model the temperature of the luminous core of the arc i s assumed to be constant. The input power to the luminous arc core i s equated to the experimental heat losses. From t h i s model the radius of the luminous core has been determined as a function of temperature at given 5 c u r r e n t . T h i s model i n d i c a t e d t h a t the vortex s t a b i l i z e d a r c behaves l i k e a w a l l s t a b i l i z e d arc a t high c u r r e n t . At high c u r r e n t the r a d i a l heat t r a n s f e r to the wall continued to i n c r e a s e while the a r c r a d i u s was approximately c o n s t a n t . T h i s means t h a t h i g h l y e f f i c i e n t r a d i a l heat t r a n s f e r processes were o c c u r r i n g , p o s s i b l y the r e s u l t of t u r b u l e n t mixing processes i n the a r c . In order to determine i f o p e r a t i n g the VSA with a l t e r n a t i n g c u r r e n t would o f f e r s i g n i f i c a n t advantages i n e l e c t r o d e l i f e t i m e , the e l e c t r o d e heat t r a n s f e r processes have been i n v e s t i a g a t e d . The t o t a l heat t r a n s f e r to the a r c e l e c t r o d e s has been measured f o r both AC and DC o p e r a t i o n as a f u n c t i o n of the s i g n i f i c a n t arc parameters ( c u r r e n t , gas pressure, gas flow r a t e and gas flow d i r e c t i o n ) . The e l e c t r o d e s u r f a c e temperature has a l s o been measured f o r both AC and DC o p e r a t i o n of the arc s i n c e l o c a l melting of the e l e c t r o d e i s a c r u c i a l f a i l u r e mechanism. With the system used f o r these measurements s p a t i a l r e s o l u t i o n of the e l e c t r o d e s u r f a c e temperature was p o s s i b l e which i s important i f hot spots occur on the e l e c t r o d e s i n c e f a i l u r e over even s m a l l p a r t s of the water c o o l e d e l e c t r o d e can r e s u l t i n breakdown of the whole system. The temperature measurements p l u s the t o t a l heat t r a n s f e r measurements provided an idea of the nature of the e l e c t r o d e heat t r a n s f e r processes f o r vortex s t a b i l i z e d a r c s . Since we were i n t e r e s t e d i n whether 6 l o c a l heating occurred on the e l e c t r o d e time r e s o l v e d photographs of the AC e l e c t r o d e a r c attachment r e g i o n have been taken. The time e v o l u t i o n of the e l e c t r o d e arc attachment r e g i o n was s t u d i e d from these photographs as w e l l as the development of e l e c t r o d e hot s p o t s . The r e s u l t s of these measurements show t h a t the AC vortex s t a b i l i z e d a r c holds promise f o r use as a high i n t e n s i t y r a d i a t i o n source. The r a d i a t i v e e f f i c i e n c y of the AC arc was found t o be comparable to the DC VSA o p e r a t i n g under the same c o n d i t i o n s . The AC e l e c t r o d e heat t r a n s f e r was found to be <50% of the anode heat t r a n s f e r i n the DC a r c o p e r a t i n g at the same c o n d i t i o n s . Hence the major problem area with the DC vortex s t a b i l i z e d a r c s ( e l e c t r o d e f a i l u r e ) seems to be much improved by o p e r a t i n g the arc with a l t e r n a t i n g c u r r e n t . T h e r e f o r e we b e l i e v e i t i s worth pursuing f u r t h e r s t u d i e s of the AC vortex s t a b i l i z e d arc with a view t o developing high i n t e n s i t y r a d i a t i o n s o u r c e s . Although we are p r i m a r i l y i n t e r e s t e d i n the arc as a r a d i a t i o n source other i n v e s t i g a t o r s have used arcs f o r many purposes ( i n c l u d i n g l i g h t i n g a p p l i c a t i o n s ) . The i n t r o d u c t i o n i s concluded with a b r i e f survey of some of the e a r l i e r developments l e a d i n g up t o t h i s t h e s i s and an o u t l i n e of the t h e s i s . As mentioned p r e v i o u s l y the work re p o r t e d i n t h i s t h e s i s i s based on experiments performed on AC and DC vortex s t a b i l i z e d a r c s . DC e l e c t r i c a rcs have been 7 s t u d i e d e x t e n s i v e l y f o r n e a r l y n i n e t y y e a r s . The work done on AC a r c s i s not n e a r l y as e x t e n s i v e as that done on DC a r c s . E a r l y work on AC a r c s (Finkelnburg (1950), Ter Horst and Rutgers (1953)) was confined t o f r e e burning a r c s employing carbon e l e c t r o d e s or to c i r c u i t breaker a p p l i c a t i o n s (Casie (1939), Mayr (1943), F r a n c i s (1948)). P h i l l i p s (1967) examined the w a l l s t a b i l i z e d cascade arc i n the 1960's. The m o t i v a t i o n behind t h i s work was the production of a high temperature gas heater f o r use i n r e -entry s t u d i e s . As mentioned e a r l i e r Andrada and E r f u r t h (1962) s t u d i e d a long vortex s t a b i l i z e d n i t r o g e n a r c that was used as an a r c h e a t e r . The reason they used a l t e r n a t i n g c u r r e n t f o r t h e i r arc was the l a c k of a v a i l a b i l i t y of a high power DC supply. T h e i r a r c had a segmented copper c o n f i n i n g w a l l s i m i l a r t o t h a t of cascade a r c s . The vortex gas flow f i e l d allowed very long arc columns ( 750 mm) to be produced. Other i n v e s t i g a t o r s ( D e t l o f f and Uhlenbusch (1970), Fang (1973), G i l l e t t e (1973) and Lowke (1975)) have s t u d i e d the cascade AC arc o p e r a t i n g at low c u r r e n t s ( u s u a l l y l e s s than 100 amps). This work was mainly concerned with the behavior of the a r c near c u r r e n t zero and the d e t e r m i n a t i o n of some of the s i g n i f i c a n t a r c parameters (temperature, d e n s i t y , c u r r e n t , v o l t a g e , and e l e c t r i c f i e l d ) throughout the AC c y c l e . None of the AC a r c s mentioned above a r e u s e f u l as high i n t e n s i t y l i g h t sources. The vortex s t a b i l i z e d AC arc c o n s i d e r e d i n t h i s t h e s i s , on the ether hand, i s intended 8 to be used e n t i r e l y as a high intensity radiation source. Vortex s t a b i l i z e d arcs were f i r s t used by the chemical industry (Schonherr (1909)) to produce high temperature gases. Important work on vortex s t a b i l i z e d arcs was f i r s t done i n the years 1949-51 by Maecker and Burhorn at K i e l (Maecker (1951), Maecker and Burhorn (1951)). Their arc ran in a tube whose inner surface was cooled by flowing water. The water was injected tangentially into the tube and was made to sti c k to the inner surface by c e n t r i f u g a l force. The objective of t h e i r work was to obtain a high temperature arc plasma source by constricting the arc i n a very small diameter tube. The water layer net only acted to cool the confining tube but also to cool the outer regions of the arc which resulted in a much higher temperature on the arc column axis. The f i r s t extensive work on gas vortex s t a b i l i z e d arcs was done by the Plasmadyne Corporation in the l a t e 1950*s and early 1960*s. These arcs were to be used as solar simulators and arc heaters. The arc column ran down the centre of a gas vortex which was used to c o n s t r i c t i t . The gas vortex was confined by a quartz tube around i t , the confining tube and arc column being c o a x i a l l y arranged. The quartz tube was cooled by running water past i t s outer surface. The f i r s t detailed performance study of a vortex s t a b i l i z e d arc was performed by Anderson (1965) on a 9 twenty k i l o w a t t a r c . F u r t h e r work on the VSA was p r i m a r i l y concerned with i n c r e a s i n g the i n p u t power. A 180 kW VSA was developed by Stresino(1966) which produced r a d i a t i o n l o s s e s of 60 kW. The i n p u t power was i n c r e a s e d t o 500 kW by M a l l i a r i s (1969). The r a d i a t i v e e f f i c i e n c y of t h e i r arc was approximately 40%. In a l l of the above work, the VSA c o n s i d e r e d c o n s i s t e d of two c o - a x i a l quartz tubes with c o o l i n g water f l o w i n g between them. The gas was i n t r o d u c e d t a n g e n t i a l l y i n t o the arc chamber near the i n n e r guartz tube. Recently a new type of VSA has been developed by Vortek I n d u s t r i e s (1974). T h e i r a r c uses only a s i n g l e guartz tube u n l i k e other vortex s t a b i l i z e d a r c s which a l l use a double guartz w a l l . Water i s i n j e c t e d t a n g e n t i a l l y i n t o the a r c chamber near the inner s u r f a c e of the guartz w a l l and the flow r a t e i s arranged so t h a t a t h i n l a y e r of water adheres to the i n n e r s u r f a c e . The water l a y e r c o o l s the c o n f i n i n g wall as w e l l as keeping i t c l e a n . Other j e t s i n s i d e the l a y e r of water (water wall) i n j e c t gas t a n g e n t i a l l y i n t o the a r c chamber. The a r c i s c o n s t r i c t e d by the gas flow as well as by the l a y e r of water on the i n n e r w a l l . T h i s system operates at i n p u t powers up to 125 kW with t y p i c a l r a d i a t i o n l o s s e s as high as 50% of the i n p u t power. 10 O u t l i n e of the Thesis The t h e s i s i s d i v i d e d i n t o s i x c h a p t e r s . Chapter 2 c o n t a i n s a b r i e f d i s c u s s i o n of the vortex s t a b i l i z e d a r c and i t s r a d i a t i v e c h a r a c t e r i s t i c s . The dominant heat t r a n s f e r processes at the a r c e l e c t r o d e s are a l s o summarized. Chapter 3 c o n t a i n s d e t a i l s of the vortex s t a b i l i z e d a r c used i n t h i s work and presents i n f o r m a t i o n on how the s u r f a c e temperature, heat t r a n s f e r to the e l e c t r o d e s and arc r a d i a t i v e e f f i c i e n c y were measured. A simple system used to o b t a i n time r e s o l v e d photographs of the AC e l e c t r o d e r e g i o n i s a l s o d e s c r i b e d . In Chapter 4 the experimental r e s u l t s of the energy balance, e l e c t r o d e heat t r a n s f e r and s u r f a c e temperature are given f o r both AC and DC o p e r a t i o n of the a r c . Time r e s o l v e d photographs of the AC e l e c t r o d e r e g i o n are a l s o presented. Based on the heat t r a n s p o r t measurements a channel model of the DC a r c i s presented i n Chapter 5. T h i s model i n d i c a t e s t h a t the a r c behaves l i k e a w a l l s t a b i l i z e d arc at c u r r e n t s exceeding 350 (A). Chapter 5 a l s o c o n t a i n s a d i s c u s s i o n of the e l e c t r o d e heat t r a n s f e r and s u r f a c e temperature r e s u l t s presented i n Chapter 4. C o n c l u s i o n s , o r i g i n a l c o n t r i b u t i o n s of t h i s work and suggestions f o r f u t u r e work are given i n Chapter 6. 11 Chapter 2 RADIATION AND ELECTRODE ENERGY TRANSFER MECHANISMS IN VORTEX STABILIZED ARCS 2i_\ I n t r o d u c t i o n In t h i s chapter a b r i e f d i s c u s s i o n i s presented concerning the o p e r a t i o n of a vortex s t a b i l i z e d a r c . The r a d i a t i o n l o s s e s from such an a r c are a l s o d i s c u s s e d . In the second p a r t of the chapter the predominant energy t r a n s f e r mechanisms o c c u r r i n g at the a r c e l e c t r o d e s are c o n s i d e r e d . 2.2 Vortex S t a b i l i z e d Arcs 2.2.1 B a s i c P r i n c i p l e s of Operation A vortex s t a b i l i z e d a r c burns i n a c y l i n d r i c a l guartz tube i n which a s w i r l i n g gas flow i s c r e a t e d by t a n g e n t i a l i n j e c t i o n of the gas through j e t s l o c a t e d at one end of the tube. The water cooled metal e l e c t r o d e s are mounted along the a x i s of the guartz tube and are u s u a l l y f r e e t o move along i t so that the arc l e n g t h can be v a r i e d . A schematic diagram of a c o n v e n t i o n a l DC vortex s t a b i l i z e d arc developed by Union Carbide (Anderson e t a l (1965) ) i s shown i n F i g u r e 2-1. 12 CATHODE QUARTZ WALL -VORTEX GAS FLOW F i g u r e 2-1: DC Vortex S t a b i l i z e d Arc (Union Carbide) The gas flow i n the qua r t z tube i s e s s e n t i a l f o r the s t a b i l i t y as w e l l as c o n s t r i c t i o n of a vortex s t a b i l i z e d a r c . I t can be shown t h a t the azimuthal component (v. ) of the gas v e l o c i t y c o n f e r s s t a b i l i t y on D the a r c column. I f the d i s t a n c e between the a r c and part of the quartz tube decreases, then v Q i n c r e a s e s . A consequent i n c r e a s e i n the r a d i a l pressure g r a d i e n t r e s u l t s which pushes the a r c column back towards the a x i s . A second c h a r a c t e r i s t i c of the flow which enhances the confinement of the arc near the tube a x i s i s that the gas vortex reduces the d e n s i t y on a x i s . The a r c c u r r e n t tends t o flow where the gas d e n s i t y i s a minimum which' i s the a x i s of the a r c tube. The a r c s t a b i l i z a t i o n mechanism i s i l l u s t r a t e d i n Fig u r e 2-2. The arc column has been 13 EQUILIBRIUM POSITION QUARTZ WALL F i g u r e 2 - 2 : Arc S t a b i l i z a t i o n Mechanism 14 d i s p l a c e d from i t s e g u i l i b r i u m p o s i t i o n as shown i n 2-2 ( a ) . In 2-2 (b) and 2-2 (c) the r e s u l t i n g r a d i a l d e n s i t y and pressure p r o f i l e s are shown. I t i s c l e a r t h a t a net f o r c e a c t s on the a r c column which b r i n g s i t back to i t s e g u i l i b r i u m p o s i t i o n . In a d d i t i o n t o the flow s t a b i l i z a t i o n processes present i n the vortex s t a b i l i z e d a r c , the e l e c t r o d e s a l s o c o n t r i b u t e to the a r c s t a b i l i t y i n the e l e c t r o d e r e g i o n of the a r c . Any o f f - a x i s motion of the a r c column near the e l e c t r o d e region i s c o n s t r a i n e d by the e l e c t r i c f i e l d s present which d r i v e the c u r r e n t towards the e l e c t r o d e s . 22m2^m2 Energy Balance and R a d i a t i o n Losses of a Vortex S t a b i l i z e d Arc In t h i s s e c t i o n the energy l o s s mechanisms f o r a vortex s t a b i l i z e d arc are b r i e f l y d i s c u s s e d . In an arc plasma the heating mechanism i s predominantly due to the Joule e f f e c t . For a high pressure a r c (atmospheric pressure or above), the e l e c t r o n s which have been a c c e l e r a t e d i n the e l e c t r i c f i e l d of the a r c l o s e most of t h e i r energy i n c o l l i s i o n s with n e u t r a l atoms or i o n s . Hence g e n e r a l l y a conditon of thermal e q u i b l i b r i u m e x i s t s between the i o n s , e l e c t r o n s and n e u t r a l s i n the arc plasma. The energy i n p u t i n t o the arc column i s d i s s i p a t e d by r a d i a t i o n , thermal conduction and c o n v e c t i o n . The energy balance f o r the a r c can be 15 expressed using the Elenbaas-Heller eguation (Elenbaas (1935)) which can be written as a(T) E 2 = -V-k(T) VT + p (T )C p (T ) v g - v T + U R (T) _ where a (T)=electrical conductivity E=electric f i e l d k (T)=thermal conductivity p (T) =density C p (T) =specif i c heat Vg=gas ve l o c i t y UR (1) =radiation loss/unit volume The term on the l e f t hand side i s the input power per unit volume due to the e l e c t r i c f i e l d . For a l l regions of the arc except close to the electrodes the e l e c t r i c f i e l d i s a constant for fixed arc conditions. 1 The terms on the r i g h t hand side are successively the net energy loss per unit volume due to thermal conduction, convection and r a d i a t i o n . When the entire arc i s considered the heat transfer to the electrodes must be added to eguation 2-1. The t o t a l input power to the arc i s just given by IV T where I=arc current and VT= t o t a l arc voltage (V T includes the voltage drop i n the electrode lfiegions e x i s t around both the anode and cathode where the e l e c t r i c f i e l d i s higher than i t i s i n the arc column. These regions around the electrodes are referred to as electrode sheath regions. The length of the electrode sheath regions i s t y p i c a l l y the order of the mean free path of electrons. For an atmospheric pressure arc this i s the order of a few microns. 16 sheath regions) given by The power balance for the arc i s then VJ = L T -V-k(T) VT + pCpV g.vT + U R (T ) j Zvrdr + QA + Qc (2 -2 ) where L=arc length Q^=heat transfer to anode Q =heat transfer to cathode C E=arc radius For the AC vortex s t a b i l i z e d arc an additional term must be included i n the energy balance due to the temporal variation in arc temperature. This additional term i s given by p C p 3 T / 8 t where t=time. The power balance for the AC arc i s then given by < VTI > = < L j f -V • k(T) VT + p(T) Cp(T) Vg • VT + pC p — + U R J . 2T r rd r> + Q E L + Q £ L ( 2 . 3 ) where < > refers to the time average for one cycle °r ELI 2 = t ^ i n e a v e r a 9 e < i heat transfer to th'e electrodes For a vortex s t a b i l i z e d arc there i s a strong interaction between the flow f i e l d and the arc column. As a result the energy loss mechanisms for the arc expressed i n 17 eguations 2-2 and 2-3 can be g r e a t l y enhanced by t u r b u l e n t mixing processes (Andrada and E r f u r t h (1962)). 2.2.3 R a d i a t i o n Losses from the Vortex S t a b i l i z e d Arc For the measurements presented i n l a t e r c h a p t e r s both the AC and DC vortex s t a b i l i z e d arcs were run e x c l u s i v e l y i n argon. Hence the d i s c u s s i o n to f o l l o w w i l l be r e s t r i c t e d to the r a d i a t i o n p r o p e r t i e s of argon. The r a d i a t i o n emitted from an argon arc plasma c o n s i s t s of a spectrum with l i n e r a d i a t i o n superimposed on a continuum (Anderson (1965)). The continuum resembles a blackbody at a temperature lower than t h a t of the plasma . The l i n e r a d i a t i o n i s from the bound-bound e l e c t r o n t r a n s i t i o n s . The continuum r a d i a t i o n from an argon plasma c o n s i s t s of two p a r t s : free-bound or e l e c t r o n - i o n recombination r a d i a t i o n and f r e e - f r e e or bremsstrahlung r a d i a t i o n (Morris (1969), Ghosh Roy (1972), Goldbach (1972)). Free-bound r a d i a t i o n occurs when an e l e c t r o n i s captured i n t o a bound energy s t a t e of an i o n , with any s u r p l u s energy being c a r r i e d away by a photon. Bremsstrahlung r a d i a t i o n mainly r e s u l t s from the Coulomb c o l l i s i o n s between ions and f r e e e l e c t r o n s i n the plasma. At low temperature the r a d i a t i o n from argon i s dominated by l i n e r a d i a t i o n . At t y p i c a l temperatures f o r an argon a r c plasma (10,000-14,000 K, Olsen (1963)) the recombination and bremsstrahlung r a d i a t i o n become 18 s i g n i f i c a n t as was demonstrated by Yakubov (1964). The t o t a l r a d i a t i o n from an argon plasma i s s t r o n g l y dependent on plasma temperature and p r e s s u r e as was demonstrated by Evans and T o n k i n (1967). T h e i r r e s u l t s are shown i n F i g u r e 2 -3 . 10 20 TEMPERATURE (x10 K) F i g u r e 2-3 : T c t a l R a d i a t i o n Losses from an Argon Plasma I t can be seen t h a t the t o t a l r a d i a t i o n e m i s s i o n c o e f f i c i e n t r i s e s very r a p i d l y above 10000 K. The t o t a l r a d i a t i o n l o s s e s a l s o i n c r e a s e d r a m a t i c a l l y wi th p r e s s u r e which i s p r o b a b l y due to the l a r g e r number o f i n t e r a c t i o n s between s p e c i e s of the plasma per u n i t volume when the 19 pressure i s i n c r e a s e d . A g u a n t i t a t i v e understanding of the e f f e c t of pressure on t o t a l r a d i a t i o n l o s s e s i s complicated due to changes i n the nature of the i n t e r a c t i o n s when the pressure i s i n c r e a s e d . I t i s c l e a r from the r e s u l t s of Evans t h a t a VSA which i s t o be used as a r a d i a t i o n source should be operated at p r e s s u r e s as high as p o s s i b l e . In a vortex s t a b i l i z e d a r c the diameter of the a r c can be somewhat c o n t r o l l e d by the gas flow. For f i x e d c u r r e n t , the c u r r e n t d e n s i t y i n c r e a s e s when the a r c i s c o n s t r i c t e d by the gas flow. When the c u r r e n t d e n s i t y i s i n c r e a s e d the plasma temperature i n c r e a s e s which r e s u l t s i n l a r g e i n c r e a s e s i n U R . Hence f o r a p r a c t i c a l arc l i g h t source i t i s e s s e n t i a l t o c o n s t r i c t the a r c column so that i t operates at a high temperature i n order to i n c r e a s e the r a d i a t i o n l o s s e s . 2 i 3 E l e c t r o d e Heat T r a n s f e r Mechanisms The energy t r a n s f e r mechanisms at the e l e c t r o d e s of an e l e c t r i c a r c are extremely complex due to the strong c o u p l i n g between the e l e c t r o d e s and the arc column. In t h i s s e c t i o n f o l l o w i n g Ecker (1961), the major f a c t o r s c o n t r i b u t i n g to the energy balance at the anode and cathode f o r the vortex s t a b i l i z e d a r c s used i n t h i s work are given. 20 J i J i J Power Balance at the Anode At the surface of the anode the t o t a l current I i s e n t i r e l y composed of the electron current I_ , since n e g l i g i b l e ion emission occurs from the tungsten anodes used i n t h i s work. Away from the anode surface an ion current I + exists, with the t o t a l current given by I=I_ +I + . The anode power gain and loss terms given below contain terms that are e x p l i c i t l y dependent on current and those which are not. (A) Anode Power Gain Terms (1) Electron Condensation Energy _[C.ECL When an electron (charge e) st r i k e s the anode the energy e i s released, where <p i s the work function of the anode material. The t o t a l power supplied to the anode i s then Q_ ~ = I <p, where I i s the electron current at the anode surface. (2) Electron Kinetic Energy J P . K E 1 Electrons which s t r i k e the anode transfer t h e i r random thermal k i n e t i c energy to the electrode. Each electron c o l l e c t e d by the ancde transfers an energy of (5/2) KT g, where K i s Boltzmann's constant and T £ i s the electron temperature. The t o t a l power transferred i s then given by Q K £= (5/2) KT c I_. 21 (3) E l e c t r o n K i n e t i c Energy. Acguired i n Anode F a l l iQ^-i At the anode, approaching e l e c t r o n s are a c c e l e r a t e d by the anode f a l l p o t e n t i a l , (V ) . This p o t e n t i a l r e s u l t s from the r e d u c t i o n of the plasma e l e c t r i c a l c o n d u c t i v i t y near the anode and the charge imbalance t h a t e x i s t s near the anode which i s necessary to maintain c o n t i n u i t y of c u r r e n t . The anodes used i n t h i s experiment were water cooled and the anode i s thus an a d d i t i o n a l heat s i n k . The plasma temperature decreases i n the neighbourhood of the anode which r e s u l t s i n an i n c r e a s e i n the e l e c t r i c a l r e s i s t i v i t y . T h i s causes an i n c r e a s e i n the e l e c t r i c f i e l d which p a r t i a l l y o f f s e t s the decrease i n temperature. The anodes used i n these experiments had tungsten t i p s so that emission of p o s i t i v e i o n s from the anode i s n e g l i g i b l e . At the anode s u r f a c e I + i s zero and I _ = 1 . Hence i n the v i c i n i t y of the anode an a d d i t i o n a l e l e c t r i c f i e l d i s necessary to maintain c u r r e n t c o n t i n u i t y i n the r e s t of the a r c column by the g e n e r a t i o n of a d d i t i o n a l charge c a r r i e r s . The charge imbalance i s l a r g e at d i s t a n c e s up to roughly one e l e c t r o n mean f r e e path from the anode c r e a t i n g a p o t e n t i a l d i f f e r e n c e which a c c e l e r a t e s most of the e l e c t r o n s before they are c o l l e c t e d by the anode. A s m a l l f r a c t i o n of the e l e c t r o n s c o l l i d e with gas atoms to c r e a t e the ions which are r e g u i r e d t o maintain charge c o n t i n u i t y i n the area of the 22 anode. The power deposited at the anode due to the t o t a l anode f a l l potential V ^ i s given by Q A F = V A F I • (4) Thermal Conduction from the Arc Column JO J L The vortex s t a b i l i z e d arc considered i n t h i s work i s a high pressure arc so that thermal equilibrium i s assumed to hold i n the arc column (ie T =T.=T where e 1 o T ,I.,and T are respectively the temperature of the e I o electrons, ions and neutral gas atoms). Hence the temperature of the neutral gas atoms i s guite high and s i g n i f i c a n t power can be transferred to the anode from the arc column by conduction of heat from neutral and excited atoms. (5) Plasma Radiation JL2 p RL The vortex s t a b i l i z e d arcs used in t h i s work had radiat i o n losses as high as 35% of the input power to the arc. Hence the anode surface receives an appreciable radiation flux from the arc column. (6) Connective Heat Transfer JL2CV1 In a vortex s t a b i l i z e d arc there i s an axial flow of gas past the downstream electrode. The temperature of the gas passing by the electrode i s higher than that of the electrode, hence there i s a convective power transfer from the gas to the anode given by 23 Q c v=h(T o - T A ) where h=convective heat t r a n s f e r c o e f f i c i e n t , T =anode temperature and A, = e f f e c t i v e anode A A a r e a . In some of the experiments performed on the DC arc the gas flow d i r e c t i o n was r e v e r s e d so t h a t there was flow from the anode to the cathode. For t h i s flow d i r e c t i o n the c o n v e c t i o n term would be a power l o s s term as the c o l d gas passing by the anode would remove heat. JB) Anode Power Loss Terms ( 1) Thermal Conduction thro ugh the E l e c t r o d e M a t e r i a l J L ) -»- c 0- LThe heat t r a n s f e r r e d to the anode i s p r i m a r i l y c a r r i e d away by thermal conduction through the e l e c t r o d e m a t e r i a l . A l l the e l e c t r o d e s used i n t h i s work were water cooled so t h a t e s s e n t i a l l y a l l the heat t r a n s f e r r e d to the e l e c t r o d e s was removed by the c o o l i n g water. (2) E l e c t r o d e Thermal R a d i a t i o n JL j_ Pv In t h i s work the anode s u r f a c e temperature was found to be q u i t e h i g h , so t h a t thermal r a d i a t i o n l o s s e s from the anode must be i n c l u d e d i n the anode power balance. The thermal r a d i a t i o n l o s s e s are given by 24 4 !• = e (T ) a T A where e i s the e m i s s i v i t y of the R A o A H anode m a t e r i a l , a Q=Stefan-Boltzmann c o n s t a n t and A =area of the hot anode s u r f a c e . Summing the i n d i v i d u a l anode energy g a i n and l o s s terms given above the anode energy balance i s given by the f o l l o w i n g e x p r e s s i o n L c + L R = 4,1- + f K J e _ I - + V A F I- + Q p R + Q c v + Q c o e (2-4) In the heat t r a n s f e r measurements performed i n t h i s work the heat t r a n s f e r through the e l e c t r o d e t i p to the c o o l i n g water has been measured, so that t h i s measured heat t r a n s f e r i s given by QA * L c = I- (<)> + |- KTg + V A p ) + (Q p R - L R ) + Q c o + Q c v ( 2-5) 25 where I_=electron c u r r e n t <j> =anode work f u n c t i o n V A F=anode f a l l p o t e n t i a l T e = e l e c t r o n temperature Qp R=plasma r a d i a t i o n C c v = c o n v e c t i o n heat t r a n s f e r 0^, o=thermal conduction heat t r a n s f e r L c=thermal conduction through e l e c t r o d e L R = r a d i a t i o n l o s s from anode t i p 2.3.2 Power Balance at the Cathode The energy balance at the cathode i s i n g e n e r a l much more complicated than that at the anode. There are a number of power l o s s and g a i n terms which are s i m i l a r f o r both the anode and cathode. These have been d i s c u s s e d p r e v i o u s l y i n the s e c t i o n on the anode energy balance. The power t r a n s f e r terms common to both the anode and cathode are: Qp R, 0^,0, L c , and 1^. The remaining power t r a n s f e r terms to the cathode are summarized below. 26 JAL A d d i t i o n a l Cathode Power Loss Terms (1) E l e c t r o n Evaporation Energy, J L ]_ ~ ~ —EE When an e l e c t r o n i s f r e e d from the cathode t h e r e i s an energy l o s s of e$, i f thermionic emission i s assumed t o be the dominant emission process a t the cathode spot. The t o t a l power l o s s i s then L =1 <p where I i s the v EE e e e l e c t r o n c u r r e n t due to t h e r m i o n i c emission. In the experiments on vortex s t a b i l i z e d a r c s presented i n t h i s t h e s i s tungsten e l e c t r o d e s were used e x c l u s i v e l y . The emi s s i o n process from a tungsten cathode i s b e l i e v e d t o be dominated by t h e r m i o n i c emission (Compton (1923), Hoyaux (1963)). Hence as a crude approximation the e l e c t r o n c u r r e n t can be viewed as almost e n t i r e l y due t o t h e r m i o n i c e m i s s i o n so t h a t I £ I and the power l o s s i s then given by L =1 4 . EE -(2) Connective Heat T r a n s f e r (Q ) In the vortex s t a b i l i z e d arc i f the cathode i s the upstream e l e c t r o d e then there should be a c o n v e c t i v e heat t r a n s f e r from the cathode t o the c o l d gas e n t e r i n g the a r c chamber given by Q =h (T -T ) A where T =cathode CV C 0 C C temperature and A c = e f f e c t i v e area. For some experiments performed on the DC VSA the gas flow d i r e c t i o n was from the anode t o the cathode. In t h i s case the c o n v e c t i v e term would be a power gain term r a t h e r than a l o s s term. 27 J l l A d d i t i o n a l Cathode Power Gain Terms {1) K i n e t i c Energy of Ions J ^ g l Ions which s t r i k e the cathode t r a n s f e r some of t h e i r random thermal k i n e t i c energy t o the cathode. The t o t a l power t r a n s f e r t o the cathode i s given by a (5/2)KT.I + e 1 where a + i s the accomodation c o e f f i c i e n t (Compton (1932)) d e f i n e d as a+ = energy of incident ion - energy after r e f l e c t i o n energy of incident ion and I i s the i o n c u r r e n t . + (2) Ion K i n e t i c Energy Acguire i n the Cathode F a l l J ^ p l Ions e n t e r i n g the cathode r e g i o n are a c c e l e r a t e d by rhe cathode f a l l p o t e n t i a l . The i n c r e a s e i n i o n k i n e t i c energy i s given by e V , and the t o t a l power CF t r a n s f e r r e d to the anode i s Q^=V_,I. CF + (3) Ion n e u t r a l i s a t i o n Energy J £ N l Ions t h a t are n e u t r a l i z e d at the cathode t r a n s f e r some of the n e u t r a l i s a t i o n energy to the cathode. The t o t a l energy a v a i l a b l e per i o n i s e(U^- <f>) s i n c e an energy e <j> i s r e l e a s e d t c f r e e each e l e c t r o n f o r 28 n e u t r a l i s a t i o n . The t o t a l heat t r a n s f e r t o the cathode i s gi v e n by Q N =I + (U\- ^Ja where a N i s an accomodation c o e f f i c i e n t of which p r e c i s e d e t a i l s are not known and i s the i o n i z a t i o n p o t e n t i a l f o r the gas used i n the a r c . Summing the i n d i v i d u a l cathode power l o s s and gain terms given above, the cathode energy balance i s given by the f o l l o w i n g e x p r e s s i o n . L R + I - 4, = a + (|) K7JI+ + V C F l+ + a n (U^ - <(.)I + + Qcv + Q. CO QPR (2-6) Again the cathode heat t r a n s f e r measured i n these experiments was the heat l o s s c a r r i e d away by the c o o l i n g water, so t h a t the measured heat t r a n s f e r Q i s given by 29 Q C E L c = I + ( a + | KJj + V C F + a p ( U i - c p ) - I- <p (2-7) + (QRR - LR) + Q C V Q C O where I =ion c u r r e n t + a =ion accomodation c o e f f i c i e n t + V C F=cathode f a l l p o t e n t i a l U \ =ionization p o t e n t i a l a n e u t r a l i s a t i o n accomodation c o e f f i c i e n t N I = e l e c t r o n c u r r e n t For both the anode and cathode the measured heat t r a n s f e r c o n s i s t s of a term t h a t i s e x p l i c i t l y dependent on c u r r e n t and one that i s not. For the anode the heat t r a n s f e r i s c f the form Q =A*BI_=A+BI and f o r the cathode i s of the form Q =ODI -EI + -The terms A,B,C #E f and E c o n t a i n no e x p l i c i t dependence on I, however, these terms are i n a l l l i k l i h o o d a f f e c t e d by I i n some way. The power t r a n s f e r mechanisms c i t e d above f o r 30 the anode and cathode play a s i g n i f i c a n t r o l e i n determining when e l e c t r o d e s f a i l due t o e x c e s s i v e heat l o a d . The next chapter presents d e t a i l s of the VSA used f o r the heat t r a n s f e r measurements r e p o r t e d l a t e r i n t h i s t h e s i s . 31 Chapter 3 APPARATUS AND EXPERIMENTAL PROCEDURE 3. 1 The Vortex S t a b i l i z e d Arc The vortex s t a b i l i z e d arc used i n a l l the experiments i s shown s c h e m a t i c a l l y i n F i g u r e 3-1. The arc chamber c o n s i s t s of two c o - a x i a l q u a r t z tubes of i n n e r diameter 27 and 43 mm r e s p e c t i v e l y , through which c o o l i n g water flows. The a r c i s s t a b i l i z e d by v o r t e x i n g argon gas which e n t e r s the a r c chamber t a n g e n t i a l l y through two 1.0 mm diameter j e t s l o c a t e d at one end of the arc chamber. The t a n g e n t i a l j e t assembly i s shown i n F i g u r e 3-2. The argon flow r a t e used i n the experiments ranged from 0.48 t o 1.20 l i t r e s / s e c o n d with i n i t i a l azimuthal flow v e l o c i t i e s ranging from 25 to 65 (m/s) at the h i g h e s t arc pressure c o n s i d e r e d . The argon flow r a t e was measured with a Brooks flowmeter (Model 1305) which was a c c u r a t e to 10%. The argon emerged through s i x 3.2 mm diameter tubes mounted i n the e l e c t r o d e h o l d e r assembly which i s shown i n F i g u r e 3-3. The e l e c t r o d e h o l d e r assembly was water c o o l e d and the argon was c o o l e d t o room temperature a f t e r i t passed through t h i s u n i t . The argon was then passed through a F a i r c h i l d (Model 10) back pressure r e g u l a t o r . T h i s u n i t allowed the a r c chamber pressure t o be v a r i e d F i g u r e 3-1: AC Vortex S t a b i l i z e d A r c F i g u r e 3-2: T a n g e n t i a l Gas J e t Assembly F i g u r e 3-3: Gas E x i t Heat Exchanger Assembly 35 c o n t i n u o u s l y from one to f i v e atmospheres. In order t o maintain a constant flow r a t e when the back pressure was i n c r e a s e d r e q u i r e d the i n p u t pressure to the j e t s to be i n c r e a s e d . The a r c chamber pressure was measured by i n s e r t i n g a g u a r t z c a p i l l a r y tube i n t o the a r c chamber through one of the gas e x i t tubes. The e x t e r n a l end of the c a p i l l a r y was a t t a c h e d t o a standard d i f f e r e n t i a l pressure gauge. The c a p i l l a r y c o u l d a l s o be moved along the chamber a x i s to examine the pressure d i s t r i b u t i o n i n the a r c . No s i g n i f i c a n t a x i a l p ressure g r a d i e n t was observed. Since the c a p i l l a r y tube was 8mm from the chamber a x i s only the pressure at the outer edge of the a r c column co u l d be measured. When the a r c i s running a r a d i a l pressure g r a d i e n t must e x i s t to provide a r c s t a b i l i z a t i o n . A crude estimate of t h i s g r a d i e n t can be made us i n g the gas flow c o n d i t i o n s at constant temperature. The gas column i n the a r c chamber i s e s s e n t i a l l y c y l i n d r i c a l i n shape and c y l i n d r i c a l symmetry w i l l be assumed f o r t h i s simple c a l c u l a t i o n (Figure 3-4). 36 F i g u r e 3-4: Geometry of Vortex Gas Column The flow c o n d i t i o n s i n the arc chamber are deterimined from the Navier-Stokes eguation which i s given by r r 9 3 3 3 i v e z , - 9 P r (3-1) where p=gas d e n s i t y v r = r a d i a l v e l o c i t y v z = a x i a l v e l o c i t y v Q=azimuthal v e l o c i t y P=pressure The azimuthal v e l o c i t y v i s much l a r g e r than v and v 9 XT Z which can t h e r e f o r e be set equal t o zero . The r a d i a l pressure g r a d i e n t i s then given by 37 9P_ 3r (3-2) I f A. i s the area of the gas j e t s then the mass flow r a t e i s tt= p A_. V q and the r a d i a l pressure g r a d i e n t i s g i v e n b y 3P_ = _1 3r pr m A A (3-3) For the flow c o n d i t i o n s present i n the a r c the maximum value f o r the r a d i a l pressure g r a d i e n t i s approximately 0.5 atm/cm. The e l e c t r o d e s used i n the a r c were c o n s t r u c t e d of 12.7 mm and 6.3 mm diameter c o a x i a l brass tubes with tungsten (1% thorium) t i p s . The tungsten t i p s were a l l o y e d to a t h i n copper s u b s t r a t e t h a t was s i l v e r s o l d e r e d to the outer brass tube. The c o a x i a l c o n s t r u c t i o n allowed the e l e c t r o d e t i p s t o be water c o o l e d . Various e l e c t r o d e t i p geometries have been used: f l a t , t r u n c a t e d 45° c o n i c a l and 45° c o n i c a l t i p s (see F i g u r e 3-5). The diameter of the e l e c t r o d e t i p s was 12.7 mm and the l e n g t h was t y p i c a l l y 10 mm. Each e l e c t r o d e p o s i t i o n could be changed using a rack and p i n i o n assembly t o move the e l e c t r o d e so that the arc l e n g t h c o u l d be v a r i e d from a few m i l l i m e t r e s up to 120 mm. 38 45° CONICAL TIP \ W / TRUNCATED 45° CONICAL TIP W F L A T TIP F i g u r e 3 - 5 : E l e c t r o d e Tip Geometry 39 3.2 Arc Power Supply The vortex s t a b i l i z e d arc d e s c r i b e d i n S e c t i o n 3.1 could be operated with e i t h e r a l t e r n a t i n g or d i r e c t c u r r e n t . The AC arc power supply had an open c i r c u i t v o l t a g e of 208 v o l t s RMS (60 Hz) and was capable of s u p p l y i n g c u r r e n t s up to 400 (A) RMS f o r an arc length of 100 mm. The DC power supply had an open c i r c u i t v o l t a g e of 320 v o l t s at a maximum c u r r e n t of 480 (A) . (For complete d e t a i l s of the arc power s u p p l i e s r e f e r to Appendix A). The arc was s t a r t e d using a high v o l t a g e pulse transformer whose secondary was i n s e r i e s with the a r c . The secondary of t h i s transformer had a l a r g e c u r r e n t c a r r y i n g c a p a c i t y (750 A RMS) and was l e f t i n s e r i e s with the arc a f t e r the arc was s t a r t e d . The i n i t i a l i n t e r - e l e c t r o d e gap was t y p i c a l l y 20 mm and was broken down by a 60 kV, 4MHz pulse t r a i n produced by the high v o l t a g e transformer. T h i s pulse t r a i n maintained the i o n i z a t i o n i n the arc column for £ 0.5-1.0 seconds a f t e r which the arc power supply was able to s u s t a i n the a r c . (For complete d e t a i l s of the arc s t a r t i n g c i r c u i t r e f e r to Appendix B). T h i s method of s t a r t i n g the arc e l i m i n a t e d damage to the e l e c t r o d e s which would have occurred i f i t had been s t a r t e d by touching the e l e c t r o d e s t o g e t h e r . 40 .3.3 Voltage-Current C h a r a c t e r i s t i c s of the Vortex S t a b i l i z e d Arc T y p i c a l voltage vs time and c u r r e n t vs time t r a c e s f o r the AC a r c are shown i n F i g u r e 3-6 ( a ) . The v o l t a g e s i g n a l was obtained by means of a voltage d i v i d e r connected across the e l e c t r o d e s . The c u r r e n t s i g n a l (obtained with a c a l i b r a t e d low inductance shunt) i s not a p e r f e c t s i n u s o i d a l waveform f o r low c u r r e n t s as can be seen i n F i g u r e 3-6 (b). The v o l t a g e waveform e x h i b i t s both i g n i t i o n and e x t i n c t i o n s p i k e s f o r low c u r r e n t s and s m a l l a r c l e n g t h . These v o l t a g e s p i k e s disappear f o r s u f f i c i e n t l y l a r g e c u r r e n t s and a r c l e n g t h as shown i n Fi g u r e 3-6 (c) . Other i n v e s t i g a t o r s ( P h i l l i p s (1967), D e t l o f f (1 970) and Lowke(1975)) have observed both i g n i t i o n and e x t i n c t i o n s p i k e s f o r low c u r r e n t AC a r c s (I <100 A RMS). Since these v o l t a g e s p i k e s disappear f o r s u f f i c i e n t l y high c u r r e n t i t i s c l e a r that the arc c o n d i t o n s i n t h i s work d i f f e r s i g n i f i c a n t l y from those e x i s t i n g i n the low c u r r e n t a r c s of P h i l l i p s , D e t l o f f and Lowke. A s m a l l phase s h i f t e x i s t s between the voltage and c u r r e n t f o r the AC a r c . The vol t a g e l a g s the c u r r e n t by approximately 5 as shown i n Figure 3^6 (d). T y p i c a l V-I curves f o r the AC a r c are shown i n F i g u r e 3-7. These o s c i l l o g r a m s were obtained using a Type 41 VOLTAGE 76 V/div. CURRENT 2 0 0 A/div. TIME 2 ms/div. 76 V/div. 2 0 0 A/div. TIME 0 . 4 ms/div. F i g u r e 3 - 6 : AC Voltage vs. Time and Current vs. Time F i g u r e 3-7: AC V o l t a g e - C u r r e n t Waveforms 43 536 T e k t r o n i x o s c i l l o s c o p e using two Type D p l u g - i n u n i t s which were a d j u s t e d so t h a t no i n h e r e n t phase s h i f t e x i s t e d between the two channels. For low c u r r e n t s and pressure the arc e x h i b i t s a negative dynamic impedance. As the c u r r e n t and pressure are i n c r e a s e d the a r c has a p o s i t i v e dynamic impedance f o r most of the AC c y c l e . T y p i c a l v o l t a g e and c u r r e n t vs time t r a c e s f o r the DC a r c are shown i n F i g u r e 3-8. The v o l t a g e and c u r r e n t both e x h i b i t a l a r g e 120 Hz r i p p l e caused by poor f i l t e r i n g of the DC power supply. U n f o r t u n a t e l y a b e t t e r f i l t e r i n g network would have r e g u i r e d e x c e s s i v e l y l a r g e c a p a c i t o r s . T y p i c a l V-I curves f o r the DC arc are shown i n F i g u r e 3-9. At h i g h e r c u r r e n t s the arc has a p o s i t i v e impedance as shown i n F i g u r e 3-9. 3.4 Arc D i a g n o s t i c s 3.4.1 Energy Balance The energy balance f o r the AC and DC a r c s has been determined by continuous flow c a l o r i m e t r y . The heat t r a n s f e r to each e l e c t r o d e , to the guartz wall and to the exhaust gas was measured. The output r a d i a t i o n has a l s o been determined c a l o r i m e t r i c a l l y using a water cooled absorbing j a c k e t placed around the a r c . The brass absorbing j a c k e t was 15.2 cm long and had an i n t e r n a l 44 VOLTAGE 76 V/div CURRENT 1 6 0 A / d i v 2 ms/d iv . F i g u r e 3-8: DC Voltnqe vs. Time and C u r r e n t vs. Time 45 VOLTAGE 1 5 V/div. CURRENT 7 0 A/div. CURRENT 1 4 0 A/d iv V t F i g u r e 3-9} DC Volta g e Current Waveforms 46 diameter of 5.1 cm. The absorber (Figure 3-10) was coated with a commercial b l a c k e n i n g compound (Brassblack, Birchwood Casey). The a b s o r p t i o n f o r t h i s s u r f a c e was g r e a t e r than 90% i n the v i s i b l e . The r a d i a t i o n end l o s s e s were c a l c u l a t e d to be y 5% and t h i s c o r r e c t i o n was i n c l u d e d in the energy balance. The c o o l i n g water flow r a t e s to the va r i o u s arc components were measured using c a l i b r a t e d Brooks flowmeters (Model 1305). The water temperature i n c r e a s e i n the c o o l i n g l i n e s was monitored by N a t i o n a l Semiconductor LX5700A temperature t r a n s d u c e r s p l a c e d i n the c o o l i n g l i n e s . (For complete d e t a i l s of the c o o l i n g system f o r the v a r i o u s arc components and the d i f f e r e n t i a l temperature measuring system r e f e r to Appendix C). The water flow i n the a r c components was s t a r t e d s e v e r a l minutes before s w i t c h i n g on the a r c so as t o permit them to achieve a steady temperature. F o l l o w i n g any changes i n the arc parameters ( c u r r e n t , pressure and gas flow rate) the water temperature i n the c o o l i n g l i n e s was allowed to s t a b i l i z e before the temperature i n c r e a s e was measured. The t o t a l i n p u t power to the AC a r c was determined as f o l l o w s . S i g n a l s p r o p o r t i o n a l to the arc v o l t a g e (V^) and cu r r e n t (I) were fed to a c i r c u i t which m u l t i p l i e d the s i g n a l s and then i n t e g r a t e d the r e s u l t i n g waveform over one c y c l e . In t h i s way i t was p o s s i b l e to measure the power t a k i n g i n t o account the n o n - s i n u s o i d a l waveforms of both V m and I . (For complete d e t a i l s of the 47 C R O S S SECTION TOP HALF OF RADIATION ABSORBER F i g u r e 3 - 1 0 : A r c R a d i a t i o n Absorber 48 c i r c u i t used t o measure the a r c i n p u t power r e f e r to Appendix D). The i n p u t power co u l d a l s o be determined by adding the heat l o s s e s i n the v a r i o u s a r c components to the power r a d i a t e d by the a r c . The two methods y i e l d e d r e s u l t s t h at were equal w i t h i n an e r r o r of about 5%. The i n p u t power to the DC a r c was determined i n a s i m i l a r f a s h i o n . 3 . _4? 2 E l e c t r o d e Surface Temperature Measurements The e l e c t r o d e s u r f a c e temperature was determined from the i n t e n s i t y r a t i o of the e l e c t r o d e thermal r a d i a t i o n at two d i f f e r e n t wavelengths using the Planck, r a d i a t i o n law with the assumption of gray r a d i a t i o n from the e l e c t r o d e . The i n t e n s i t y r a t i o was determined using a two channel o p t i c a l d e t e c t i o n system, which w i l l be d i s c u s s e d i n d e t a i l l a t e r . The e l e c t r o d e s u r f a c e temperature f o r a r c s and plasmatrons have been determined by other i n v e s t i g a t o r s ( S t u c k e l b e r g (1928), Bauer (1954), Borodin (1968), Hugel (1969), Borodin (1969), Kimblin (1969), Pustogarov (1973), and Boxman (1975)) using a number of d i f f e r e n t t e c hniques. Boxman e t a l (1975) have measured the anode s u r f a c e temperature f c r a vacuum arc using a one channel o p t i c a l system to monitor the thermal r a d i a t i o n from the e l e c t r o d e . To determine the s u r f a c e temperature by t h i s 49 method r e q u i r e s a knowledge of the e l e c t r o d e s u r f a c e e m i s s i v i t y , which i s s t r o n g l y dependent on the s u r f a c e c o n d t i o n s . The v a r i o u s photographic techniques (Pustoqarov (1973)) used f o r measuring the s u r f a c e temperature a l s o r e l y on a knowledge of the l o c a l e m i s s i v i t y which l i m i t s the accuracy of these methods. The two channel o p t i c a l system used i n t h i s work does not r e l y on a knowledge of the e l e c t r o d e s u r f a c e e m i s s i v i t y when the assumption of gray r a d i a t i o n i s used. Hence t h i s method has the p o t e n t i a l f o r b e t t e r accuracy than those t h a t r e l y on a d e t a i l e d knowledge of the s u r f a c e e m i s s i v i t y at a s i n g l e wavelength. For the high i n t e n s i t y a r c s used i n t h i s work the i n t e n s i t y of l i g h t emitted by the e l e c t r o d e s i s s m a l l compared to the i n t e n s i t y of l i g h t emitted by the arc column. This makes i t d i f f i c u l t t o measure the thermal r a d i a t i o n from the e l e c t r o d e . When the a r c c u r r e n t i s i n t e r r u p t e d the plasma r a d i a t i o n decays very r a p i d l y while the thermal decay time of the e l e c t r o d e i s orders of magnitude g r e a t e r . In these experiments the thermal r a d i a t i o n was t h e r e f o r e monitored a f t e r the arc had been switched o f f and the plasma r a d i a t i o n had decayed. The system used to measure the e l e c t r o d e s u r f a c e temperature i s shown s c h e m a t i c a l l y i n F i g u r e 3 - 1 1 . Lens L c o l l i m a t e s the thermal r a d i a t i o n from the s u r f a c e of 1 the e l e c t r o d e . The s h u t t e r and aperture u n i t permit t h i s r a d i a t i o n to enter the monochromators through l e n s e s L_ 51 and at a s u i t a b l e d elay a f t e r the arc has been e x t i n g u i s h e d . L and L were chosen so t h a t t h e i r f 2 3 numbers matched the e f f e c t i v e f number of the monochromators. The monochromators used were i d e n t i c a l 0.5 metre J a r r e l - A s h u n i t s . Model 82-000, with a o d i s p e r s i o n of 16 A/mm and e f f e c t i v e aperture r a t i o of f/8.6. Both p h o t o m u l t i p l i e r s were BCA Model C31034 using a Type 128 photocathode which has high quantum e f f i c i e n c y i n the red and near i n f r a r e d r e g i o n s . 100 micron wide i n l e t s l i t s were used on each monochromator and the width of the o u t l e t s l i t s ranged from 200 um to 2 mm. Apertures 1 and 2 l i m i t e d the e l e c t r o d e r e g i o n examined to a 2 mm diameter c i r c u l a r r e g i o n . The e l e c t r o d e s u r f a c e temperature was determined from the i n t e n s i t y r a t i o of thermal r a d i a t i o n emitted at wavelengths of 600+30 nm and 700*30 nm using the Planck r a d i a t i o n f u n c t i o n with the assumption of gray r a d i a t i o n from the e l e c t r o d e . Over the wavelength range c o n s i d e r e d the e m i s s i v i t y of tungsten v a r i e s by about 3% (De Vos (1953)) so that the assumption cf gray r a d i a t i o n i s v a l i d . The s p e c t r a l response of the two monochromator-p h o t o m u l t i p l i e r systems was determined using a standard tungsten lamp and o p t i c a l pyrometer. The l i n e a r i t y of the p h o t o m u l t i p l i e r s was e s t a b l i s h e d using c a l i b r a t e d n e u t r a l d e n s i t y f i l t e r s . The i n t e n s i t i e s 1^ and I ^ o f the thermal r a d i a t i o n d e t e c t e d by channels 1 and 2 are given by 52 -5 I l (Xi, T s) = _ i i i 3= S__ g hc/XiKT s _ 1 _ 5 ( 3 " 4 ) j n T <> = A ? c l x? e O ^ L i l s ) hc/X 2KT s e -1 where C ^ h c / k c=speed of l i g h t h=Planck.'s constant K=Boltzmann's constant X, 9 =wavelength s e t t i n g f o r channel 1,2 T c = e l e c t r o d e s u r f a c e temperature e (^  # T) = s u r f ace e m i s s i v i t y The c o n s t a n t s A^ and i n c o r p o r a t e a geometric f a c t o r f o r each channel as well as the r e l a t i v e s e n s i t i v i t y f o r each c h a n n e l . With the assumption of gray r a d i a t i o n ( i e e ( x 1 #T)= £ (X ,T) the e l e c t r o d e s u r f a c e temperature i s give n by (3-5) The r a t i o A^  /A^ was determined e x p e r i m e n t a l l y by sampling the thermal r a d i a t i o n from the e l e c t r o d e with both channels set at the same wavelength. The temperature measuring system s t a r t e d m onitoring thermal r a d i a t i o n when the s h u t t e r - a p e r t u r e 53 u n i t opened 10 ms a f t e r c u r r e n t i n t e r r u p t i o n . The c u r r e n t was i n t e r r u p t e d by removing the v o l t a g e from the primary of the a r c power supply transformer (see Appendix A). T h i s was done using a r e l a y c o n t r o l l e d breaker t h a t was opened using a t r i g g e r p u l s e . Due t o the l a r g e inductance i n the secondary of the a r c power supply t r a n s f o r m e r (choke c o i l and s a t u r a b l e r e a c t o r ) the c u r r e n t d i d n ' t f a l l t o zero u n t i l % 3ms a f t e r the v o l t a g e was removed from the primary. The decay time f o r the arc column has been c a l c u l a t e d using a simple model where the energy of the a r c column i s l o s t by r a d i a l c o nduction and r a d i a t i o n (Refer to Appendix E) . The time c o n s t a n t f o r the arc decay was found t o be t y p i c a l l y 2 ms which agrees q u i t e well with the observed decay of the plasma column shown i n Figure 3-12. When the temperature measuring system s t a r t e d monitoring thermal r a d i a t i o n , the plasma r a d i a t i o n had decayed to much l e s s than 1% of i t s i n i t i a l v a l u e , as shown i n F i g u r e 3-13. Before every s u r f a c e temperature measurement, a c a l i b r a t i o n run was performed with both channels monitoring the same wavelength. T h i s procedure provided a check on the alignment of the o p t i c a l system t o ensure t h a t both channels were l o o k i n g at the same re g i o n of the e l e c t r o d e . I t a l s o provided a check on the r e p r o d u c i b i l i t y of the a r c condtions and e l e c t r o d e attachment r e g i o n from run t c run. 54 PT VOLTAGE 5V/div T IME 2 0 ms/div. i i i < TIME F i g u r e 3-12: Decay o f Plasma R a d i a t i o n F o l l o w i n g Current I n t e r r u p t i o n 55 PHOTOMULTIPL IER OUTPUT VOLTAGE 2 V/div. PHOTOTRANSISTOR OUTPUT VOLTAGE 5 V/div. « TIME 2 0 ms/div. F i g u r e 3-13: Decay o f Plasma R a d i a t i o n and E l e c t r o d e Thermal R a d i a t i o n F o l l o w i n g C u rrent I n t e r r u p t i o n 56 3.4.3 Time Resolved Observations of AC Arc E l e c t r o d e s Time r e s o l v e d photographs of the AC e l e c t r o d e -a r c attachment r e g i o n have been taken using a simple o p t i c a l system which i s d e s c r i b e d i n t h i s s e c t i o n and i s d e p i c t e d s c h e m a t i c a l l y i n F i g u r e 3-14. The image of the a r c e l e c t r o d e formed by l e n s L passes through a d i s k with a narrow r a d i a l s l i t t h a t i s r o t a t e d by a synchronous AC motor. T h i s y i e l d s an image of the a r c e l e c t r o d e at a f i x e d time during the AC c y c l e at IP (image p l a n e ) . Due to the f i n i t e width of the s l i t the image of the e l e c t r o d e appearing at IP i s i n t e g r a t e d over £ 1 ms. Hence the r e s o l u t i o n of t h i s system was of t h i s order. The synchronous motor was mounted on a p l a t f o r m which allowed the s t a t o r t o be r o t a t e d about the s h a f t of the r o t o r as shown i n F i g u r e 3-15. The angular displacement 6 of the s t a t o r determines the time during the c u r r e n t c y c l e at which the photographic f i l m i s exposed to l i g h t from the e l e c t r o d e s . Hence by changing 6 the time e v o l u t i o n of t h e e l e c t r o d e spots can be c o n t i n u o u s l y observed at IP with a t y p i c a l r e s o l u t i o n of 1 ms. The r e g i o n of the AC c y c l e examined f o r a given 6 was determined using a F a i r c h i l d (FPT 100) p h o t o t r a n s i s t o r l o c a t e d at the image of the e l e c t r o d e at IP. The output of the p h o t o t r a n s i s t o r was d i s p l a y e d together with the c u r r e n t waveform on a T e k t r o n i x 551 dual beam o s c i l l o s c o p e . Hence i t was p o s s i b l e to check d i r e c t l y what part of the AC c u r r e n t was 57 F i g u r e 3-14: System f o r Time Resolved Photographs o f the AC E l e c t r o d e s 59 being examined f o r any p o s i t i o n , e , of the synchronous motor. Photographs of the e l e c t r o d e region at d i f f e r e n t p a r t s of the AC c y c l e were taken using a standard 35 mm SLR camera and high speed f i l m . These photographs which are presented l a t e r were time i n t e g r a t e d with an i n t e g r a t i o n time of roughly 1 ms. 60 Chapter 4 EXPERIMENTAL RESULTS i i * . ! I n t r o d u c t i o n In t h i s chapter the r e s u l t s of the experiments performed on AC and DC vortex s t a b i l i z e d a r c s are given. The chapter i s d i v i d e d i n t o four s e c t i o n s . The f i r s t s e c t i o n , ( 4 . 2 ) , deals with the energy balance r e s u l t s f o r both the AC and DC vortex s t a b i l i z e d a r c . The e l e c t r o d e heat t r a n s f e r r e s u l t s are given i n s e c t i o n 4 . 3 , and the e l e c t r o d e s u r f a c e temperature r e s u l t s are given i n s e c t i o n 4 . 4 . In the l a s t s e c t i o n the time r e s o l v e d o b s e r v a t i o n s of the AC e l e c t r o d e - a r c attachment r e g i o n are presented. 4 ^ 2 Energy Balance The energy balance f o r both AC and DC vortex s t a b i l i z e d a r c s has been determined c a l o r i m e t r i c a l l y . As mentioned i n Chapter 3 the heat l o s s e s t o each e l e c t r o d e , to the gu a r t z w a l l , and to the exhaust gas have been determined. In a d d i t i o n the arc r a d i a t i o n l o s s e s have a l s o been measured c a l o r i m e t r i c a l l y . The sum of these energy l o s s r a t e s was t y p i c a l l y found to be w i t h i n +5% of the i n p u t power determined from the i n t e g r a t e d power waveforms (Refer to Appendix D) . 6 1 The primary concern of the energy balance r e s u l t s was to determine the r a d i a t i v e e f f i c i e n c y f o r the AC VSA and to compare these r e s u t s t o those f o r the DC VSA. Since the r a d i a t i o n l o s s e s from the arc plasma should be a f f e c t e d by the arc c u r r e n t and p r e s s u r e , the energy balance was determined as a f u n c t i o n of c u r r e n t and p r e s s u r e . For the AC a r c the energy balance was determined f o r two d i f f e r e n t e l e c t r o d e t i p geometries. F o r t y - f i v e degree c o n i c a l e l e c t r o d e t i p s and c y l i n d r i c a l f l a t t i p s were used i n these experiments. For the DC arc o the cathode t i p was a 45 cone and the anode t i p was a f l a t s u r f a c e d c y l i n d e r . T h i s cathode geometry was chosen to ensure that the p o s i t i o n of the cathode spo t s , which are e s s e n t i a l f o r the o p e r a t i o n of the a r c , were w e l l l o c a l i z e d a t the t i p of the cathode. With t h i s geometry the cathode spots were always l o c a t e d at the very t i p of the cathode due to the high temperautres i n t h i s r e g ion caused by a huge heat f l u x at the t i p of the e l e c t r o d e . S i n c e anodes spots are not r e q u i r e d to maintain the arc a c o n i c a l anode i s not r e q u i r e d . A f l a t anode was chosen so as to minimize the average heat f l u x at the s u r f a c e . Experiments were a l s o performed, however, using a c o n i c a l anode. Runs of long d u r a t i o n were rendered i m p o s s i b l e by melting at the e l e c t r o d e t i p . The r e s u l t s of the energy balance f o r the AC a r c are given i n Table I and I I . The r a d i a t i v e e f f i c i e n c y 62 TRMS P P GAS p R A D QEL1 QEL2 PW PIN A atm (kW) (kW) (kW) (kW) (kW) (kW) 140 1.4 0.7 0.95 1.2 0.7 1.6 5.2 200 1.4 1.1 1.8 1.5 1.1 2.4 7.9 250 1.4 1.5 2.6 1.8 1.5 3.2 10.7 300 1.4 2.0 3.5 2.1 1.9 4.0 13.5 390 1.4 2.7 5.4 2.5 2.5 5.8 18.9 180 2.1 1.0 2.0 1.5 1.0 2.8 8.3 240 2.1 1.3 3.2 1.8 1.4 3.6 11.3 290 2.1 1.6 4.1 2.1 1.8 4.6 14.2 340 2.1 2.0 5.3 2.3 2.2 5.5 17.3 380 2.1 2.2 6.1 2.5 2.6 6.4 19.8 400 2.1 2.4 6.5 2.6 2.7 6.7 20.9 180 3.7 1.0 3.1 1.7 1.2 3.6 10.6 250 3.7 1.3 4.5 2.0 1.8 4.9 14.5 310 3.7 1.4 5.9 2.3 2.2 6.0 17.8 350 3.7 1.7 7.0 2.5 2.6 6.9 20.6 380 3.7 1.7 7.7 2.7 3.0 7.7 22.8 400 3.7 1.9 8.3 2.7 3.2 8.3 24.4 180 5.1 0.6 3.6 1.9 1.2 4.0 11.3 240 5.1 0.8 5.3 2.3 1.8 5.6 15.8 300 5.1 1.1 6.5 2.5 2.3 6.9 19.3 340 5.1 1.3 7.9 2.8 2.7 8.0 22.7 380 5.1 1.4 9.1 3.0 3.0 9.0 25.5 400 5.1 1.6 9.5 3.0 3.2 9.6 26.9 QEL1 - (Flow Towards Electrode) Q _ - (Flow Away from the Electrode) Table I: Energy Balanae for the AC Arc (45 Conical Electrodes) 63 IRMS P p GAS p RAD Q E L 1 QEL2 P I N A atm (kW) (kV7) (kW) (kw) (kw) (kw) 180 1.4 0.8 1.7 1.7 1.1 2.1 7.4 250 1.4 1.2 2.6 2.0 1.5 3.3 10.6 300 1.4 1.4 3.5 2.2 1 . 9 4.0 13.0 340 1.4 1.7 4.2 2.4 2.2 4.7 15.2 380 1.4 2.2 5.0 2.6 2.5 5.5 17.8 400 1.4 2.2 5.3 2.7 2.6 5.7 18.5 200 2.1 0.7 2.4 2.2 1.1 3.4 9.8 240 2.1 0.9 3.3 2.6 1.5 4.3 12.6 280 2.1 1.2 4.0 2.8 1.8 4.9 14.7 320 2.1 1.4 4.7 3.0 2.1 5.7 16.9 370 2.1 1.6 5.8 3.3 2.5 6.7 19.9 390 2.1 1.8 6.3 3.4 2.7 7.2 21.4 210 3.7 0.9 3.3 1.7 1.2 3.5 10.6 260 3.7 1.5 4.4 2.0 1.5 4.5 13 . 9 310 3.7 1.8 5.7 2.3 1.7 5.7 17.2 340 3.7 1.8 6.6 2.3 2.0 6.5 19.2 380 3.7 2.1 7.7 2.6 2.2 7.4 22.0 400 3.7 2.3 8.2 2.6 2.4 7.8 23.0 200 5.1 1.0 4.0 1.8 1.2 4.3 12.3 260 5.1 1.1 5.1 2.1 1.4 5.4 15.1 310 5.1 1.4 6.3 2.4 1.7 6.6 18.7 350 5.1 1.7 7.9 2.6 2.0 7.6 21.8 390 5.1 2.0 9.1 2.8 2.2 8.7 24.8 Q E L^ - (Flow Towards E l e c t r o d e ) Q E L 2 - (Flow Away from the E l e c t r o d e ) T a b l e I I : Energy Balance f o r the AC A r c (Truncated 45 C o n i c a l E l e c t r o d e s ) 64 ( n = r a d i a t i o n l o s s / i n p u t power) as a f u n c t i o n of c u r r e n t with pressure as a parameter i s shown i n F i g u r e s 4-1 and 4-2 f o r both e l e c t r o d e t i p geometries. For both e l e c t r o d e s used n i n c r e a s e s l i n e a r l y with c u r r e n t over the c u r r e n t range examined. The r a d i a t i v e e f f i c i e n c y i n c r e a s e s with the arc chamber pressure f o r both e l e c t r o d e geometries. The e l e c t r o d e energy balance was found t o be s l i g h t l y dependent on the gas flow r a t e , while n was found t o be e s s e n t i a l l y independent of flow r a t e . Since n was independent of flow r a t e the energy balance as a f u n c t i o n of flow r a t e was not examined. The r e s u l t s of the energy balance f o r the DC arc are given i n Table I I I and IV. The energy balance was determined f o r both flow f r c a the cathode to the anode and from the anode to the cathode. The e l e c t r o d e energy balance was found to depend markedly on gas flow d i r e c t i o n . Hence the a r c energy balance was determined f o r both flow d i r e c t i o n s . The r a d i a t i v e e f f i c i e n c y as a f u n c t i o n of c u r r e n t with pressure as a parameter i s shown i n F i g u r e s 4-3 and 4-4 f o r both flow d i r e c t i o n s . n i s again l i n e a r with c u r r e n t . n a l s o i n c r e a s e s with pressure f o r both flow d i r e c t i o n s . From the r e s u l t s presented f o r n f o r both AC and DC o p e r a t i o n i t i s c l e a r t h a t t o maximize n the arc s h o u l d be operated at c u r r e n t s around 400 A and at pressures of at l e a s t 5 atm f i f not h i g h e r . 65 • 1 ' 1 if) O if) o ro ro c\j C\J (%) A0N3I0IJJ3 3A!lVIQVa F i g u r e 4-1: AC R a d i a t i v e E f f i c i e n c y v s . Current (Truncated 45 ° C o n i c a l E l e c t r o d e s ) 66 (%) A0N3IDIJJ3 3 A l l V i a V y F i g u r e 4-2: AC R a d i a t i v e E f f i c i e n c y v s. C u rrent (45* C o n i c a l E l e c t r o d e s 67 I P p GAS P RAD Q C QA PW P I N A atm (kW) (kw) (kW) (kW) (kW) (kW) 175 1.4 1.4 3.5 1.5 2.6 3.7 12.7 245 1.4 2.1 5.4 1.8 3.3 5.3 17.9 290 1.4 2.6 6.5 2.0 3.6 6.3 21.0 345 1.4 3.1 7.6 2.1 4.1 7.3 24.2 380 1.4 4.0 10.1 2.4 4.7 9.7 30.9 195 2.1 1.1 4.0 1.9 2.6 4.9 14.5 285 2.1 2.0 6.1 2.2 3.6 6.8 20.7 330 2.1 2.4 7.3 2.4 4.1 7.9 24.0 370 2.1 3.1 8.8 2.6 4.6 9.3 28.4 430 2.1 3.8 10.5 2.7 5.1 11.1 33.2 210 3.7 2.1 5.2 1.5 2.7 5.2 16.6 310 3.7 3.4 7.5 1.8 3.2 7.3 23.2 360 3.7 4.0 9.2 1.9 3.7 8.9 27.7 410 3.7 5.0 11.4 2.2 4.2 11.2 34.0 470 3.7 5.9 13.8 2.3 4.7 13.4 40.1 175 5.1 2.0 5.3 1.5 2.2 5.2 16.2 255 5.1 3.2 8.0 1.8 3.0 7.5 23.5 290 5.1 3.8 9.7 2.0 3.4 9.0 27.9 325 5.1 4.7 11.6 2.1 3.9 10.9 33.2 365 5.1 6.5 13.9 2.3 4.5 13.1 40.3 425 5.1 6.0 15.3 2.4 4.8 14.5 42.9 Table I I I : Cathode to Energy Balance for the DC Arc (Gas Flow from Anode) I P PGas p RAD A atm (kW) (kW) 230 1.4 2.3 3.9 290 1.4 3.2 5.7 335 1.4 3.8 7.1 365 1.4 4.4 8.9 440 1.4 5.1 10.6 490 1.4 5.4 11.5 240 2.1 2.5 4.3 320 2.1 3.3 6.3 370 2.1 4.0 7.9 410 2.1 4.5 9.6 480 2.1 5.3 11.4 220 3.7 2.1 4.6 290 3.7 3.0 6.8 310 3.7 3.9 8.8 390 3.7 4.4 10.7 425 3.7 4.7 12.4 460 3.7 5.4 13.6 195 5.1 2.0 4.8 290 5.1 3.1 7.2 345 5.1 4.0 9.3 400 5.1 4.5 11.0 450 5.1 5.0 13.4 470 5.1 5.4 14.9 Table IV: Energy Balance Anode to Cathode) Q, (C QA PW PIN kW) (kW) (kW) (kW) 1.4 3.2 4.2 15.0 1.8 3.7 6.0 20.4 2.1 4.1 7.5 24.6 2.4 4.5 9.5 30.3 2.7 4.8 11.8 35.0 2.9 5.1 13.1 38.2 1.3 3.2 4.6 15.9 1.8 3.8 6.7 21.9 2.0 4.1 8.4 26.4 2.3 4.5 10.1 31.0 2.6 4.8 12.3 36.4 1.1 3.1 5.0 15.9 1.4 3.6 7.2 22.0 1.8 4.0 9.1 27.6 2.1 4.4 11.0 32.6 2.4 4.7 12.7 37.1 2.5 5.0 14.3 40.8 1.1 2.9 5.1 15.9 1.4 3.6 7.6 22.9 1.8 4.0 9.9 29.2 2.0 4.4 11.9 33.7 2.3 4.8 14.5 40.4 2.5 5.0 16.4 44.2 for the DC Arc (Gas Flow from 69 F i g u r e 4 - 3 : DC R a d i a t i v e E f f i c i e n c y v s . Current (Flow from Cathode to Anode i-h H O 3 O 0) c H 0) I rt O a o n rt o fD 0) — rt p-< CD M M> H> H-o 3 o < CO o c M M CD 3 rt *1 t—1 O 3 5 U z 3 0 UJ y LL L L Ld ^ 2 5 P < < a: 2 0 j. PRESSURE o - 1.4 a tm • - 2.1 A - 3.7 ° - 5.1 J L 200 300 CURRENT 4 0 0 (A) o 71 3 E l e c t r o d e Heat T r a n s f e r The t o t a l heat t r a n s f e r to the a r c e l e c t r o d e s has been measured c a l o r i m e t r i c a l l y f o r both AC and DC o p e r a t i o n , as a f u n c t i o n of the s i g n i f i c a n t arc parameters. I t was found e x p e r i m e n t a l l y t h a t the arc e l e c t r o d e heat t r a n s f e r was s t r o n g l y dependent on the arc c u r r e n t and gas flow d i r e c t i o n . There was a much weaker dependence on the arc chamber pressure and argon flow r a t e . The heat t r a n s f e r to both AC and DC e l e c t r o d e s under a l l gas flew and pressure c o n d i t i o n s was found to s c a l e l i n e a r l y with the a r c c u r r e n t . T h i s l i n e a r r e l a t i o n s h i p was found to be v a l i d from approximately 150-400 (A) RMS f o r AC o p e r a t i o n and 150-450 (A) f o r DC o p e r a t i o n . I t was not p o s s i b l e to determine the heat t r a n s f e r f o r c u r r e n t s l e s s than 150 (A) because the arc became un s t a b l e . Long running times are r e q u i r e d to s t a b i l i z e the temperature i n the e l e c t r o d e c o o l i n g water a f t e r the c u r r e n t i s changed. Low c u r r e n t a r c s (K100 A ), which were g u i t e u n s t a b l e , tend to go out before the e l e c t r o d e c o o l i n g water temperature had s t a b i l i z e d . T y p i c a l AC r e s u l t s are given i n F i g u r e 4-5, which c l e a r l y show the l i n e a r r e l a t i o n between e l e c t r o d e heat t r a n s f e r and c u r r e n t under d i f f e r e n t arc conditons. Both e l e c t r o d e s used f o r these runs were i d e n t i c a l i n o shape and c o n s t r u c t i o n {45 c o n i c a l tungsten t i p s , t i p 72 1 1 1 1 1 1 ' 100 2 0 0 3 0 0 4 0 0 CURRENT (A) F i g u r e 4 - 5 : T y p i c a l AC E l e c t r o d e Heat T r a n s f e r v s . Current 73 l e n g t h £ 10 mm). T y p i c a l DC r e s u l t s are given i n F i g u r e 4-6, which again shows the l i n e a r r e l a t i o n between heat t r a n s f e r and c u r r e n t . The cathode used f o r these runs was i d e n t i c a l to the e l e c t r o d e s used i n the AC runs. The anode used had a c y l i n d r i c a l tungsten t i p and the s u r f a c e where the a r c attached was f l a t . The e l e c t r o d e heat t r a n s f e r f o r both AC and DC o p e r a t i o n was s t r o n g l y a f f e c t e d by the gas flow d i r e c t i o n . Normally f o r DC o p e r a t i o n the gas flow was d i r e c t e d from the cathode to the anode. The DC a r c co u l d a l s o be operated with gas flow from the anode t o the cathode. T h i s was accomplished by i n t e r c h a n g i n g the j e t and gas e x i t - h e a t exchanger u n i t s . For both AC and DC o p e r a t i o n i t was found t h a t the slope of the heat t r a n s f e r vs. I graphs was s i g n i f i c a n t l y l a r g e r when flow was d i r e c t e d towards the e l e c t r o d e . T h i s occurred f o r both the anode and cathode f o r DC o p e r a t i o n . T y p i c a l r e s u l t s f o r the AC arc are gi v e n i n F i g u r e 4-7. The slope of the graph f o r flew towards the e l e c t r o d e i s s i g n i f i c a n t l y i n c r e a s e d ( ^ 20%) and cannot be accounted f o r by experimental u n c e r t a i n t y . T y p i c a l r e s u l t s f o r the DC arc are given i n F i g u r e s 4-8 and 4-9. The i n c r e a s e i n the slope f o r flow towards the anode i s ^40% and t h i s d i f f e r e n c e i s a l s o not a t t r i b u t a b l e to experimental u n c e r t a i n t y . S i m i l a r r e s u l t s apply f o r the cathode where the i n c r e a s e i n the s l o p e f o r 7 4 / / i. 1 1 1 1 ' ' 1 ' 100 200 300 4 0 0 CURRENT (A) F i g u r e 4-6: T y p i c a l DC E l e c t r o d e Heat T r a n s f e r vs. Current 75 5 £ 4 . 0 cr LU L L CO 2 3.0 < LU I LU Q O c r h -u LU _ l UJ (J < 2.0 h 1.0 h A - FLOW TOWARDS o - FLOW AWAY 100 2 0 0 3 0 0 CURRENT (A) 4 0 0 F i g u r e 4-7: AC E l e c t r o d e Heat T r a n s f e r v s . C u r r e n t (both flow d i r e c t i o n s ) 76 / ' ' 1 100 200 3 00 4 0 0 CURRENT (A) F i g u r e 4-8: DC E l e c t r o d e Heat T r a n s f e r v s . Current (Flow from Cathode to Anode) 77 F i g u r e 4-9: DC E l e c t r o d e Heat T r a n s f e r v s . C u r r e n t (Flow from Anode to Cathode) 78 flow towards the cathode i s ^ 35%. The arc chamber pressure and the gas flow rate had a small e f f e c t on the electrode heat transfer. The pressure had no measurable e f f e c t on the slope of the heat transfer vs I results for both AC and DC operation. The current independent part of the AC electrode heat transfer (intercept of the AC graphs) was affected by the arc chamber pressure as shown i n Figures 4-10 and 4-11. For flow towards the electrode the curves are s l i g h t l y shifted downwards for increasing pressure. For flow away from the electrode the curves are shifted upwards for increasing pressure. The current independent part of the cathode and anode heat transfer was also mildly affected by the arc pressure as shown i n Figures 4-12 and 4-13. For AC operation the flow rate also affected the intercept of the heat transfer vs I graphs while leaving the slope unchanged (Refer to Figures 4-14 and 4-15). The intercept increased with increasing flow rate for both flow d i r e c t i o n s . The e f f e c t was, however, more pronounced for flow away from an electrode than for flow towards an electrode. The gas flow rate affected both the slope and intercept of the graphs cf heat transfer vs I for the cathode i n the DC runs as can be seen i n Figure 4-16 for flow away from the cathode. The gas flow rate had no e f f e c t on the anode heat loading for flow towards the anode as shown by Figure 4-17. 79 l i t 100 200 300 4 0 0 CURRENT (A) P i g u r e 4-10: AC E l e c t r o d e Heat T r a n s f e r v s . C u r r e n t f o r D i f f e r e n t Pressure (Flow Away) 80 4.0 3.0 2.0 1.0 P R E S S U R E ° - 1.3 a t m • - 2.4 A - 3.7 / ' s S '/ S * Ss » JL 100 200 300 CURRENT (A) 4 0 0 Figure 4-11: AC Electrode Heat Transfer vs. Current for Different Pressure (Flow Towards) 81 1 0 0 2 0 0 3 0 0 4 0 0 CURRENT (A) F i g u r e 4-12: DC E l e c t r o d e Heat T r a n s f e r v s . Current f o r D i f f e r e n t Pressure (Flow from Cathode to Anode) 82 100 ' 2 0 0 ' 366 ' 4&6 CURRENT (A) F i g u r e 4 - 1 3 : DC E l e c t r o d e Heat T r a n s f e r v s . C u r r e n t f o r D i f f e r e n t P r e s s u r e (Flow from Anode t o Cathode) 83 F i g u r e 4-14: AC E l e c t r o d e Heat T r a n s f e r vs. Current f o r D i f f e r e n t Flow Rate (Flow Away) 4.0 3.0 h 2 0 b 1.0 CURRENT F i g u r e 4-15: AC E l e c t r o d e Heat T r a n s f e r vs. Current f o r D i f f e r e n t Flow Rates (Flow Towards) 85 5.0 F LOW R A T E c - 0 . 4 7 A -0.71 ° - 1.06 i tres/s 4.0 c r m CO < cr h- 3.0 I— < LLJ X UJ Q 2 . 0 o 1.0 V x 100 200 CURRENT 300 (A) 400 F i g u r e 4-16: Anode Heat T r a n s f e r v s . Current f o r D i f f e r e n t Flow Rates (Flow Towards) F i g u r e 4-17: Cathode Heat T r a n s f e r vs. Current f o r D i f f e r e n t Flow Rates (Flow Away) 87 When the flow d i r e c t i o n was r e v e r s e d the r e s u l t s of flow r a t e on e l e c t r o d e heat t r a n s f e r were symmetric. The cathode l o a d i n g was found to be independent of flow r a t e as shown i n F i g u r e 4-18. The i n t e r c e p t of the anode l o a d i n g r e s u l t s i n c r e a s e d with flow r a t e i n c r e a s e s while the slope was e s s e n t i a l l y the same, as shown i n F i g u r e 4-19. As s t a t e d p r e v i o u s l y the graphs of e l e c t r o d e heat t r a n s f e r tQ^i) v s 1 a r e l i n e a r f o r both AC and DC e l e c t r o d e s under a l l flow c o n d i t i o n s . For both the AC and DC e l e c t r o d e s the s l o p e s of the Q_T vs I graphs are always l a r g e r f o r flow towards the e l e c t r o d e than f o r flow away from the e l e c t r o d e . In Table V the e f f e c t on the s l o p e s and i n t e r c e p t s of the Q vs I graphs due to i n c r e a s e s i n the gas flow r a t e and pressure have been summarized. In Table VI and VII the n u m e r i c a l r e s u l t s f o r the slope and i n t e r c e p t s of the Q vs I graphs are summarized. 4 ..4 E l e c t r o d e Surface Temperature Measurements 4.4.1 Accuracy of the Temperature Measuring System The s u r f a c e temperature measurements performed on a DC anode provided a convenient check on the accuracy 88 4.0 F L O W R A T E o - 0.47 l i t r e s / s A - 0.71 ° - 1.06 ^ 3 0 LU J U LL CO z < or i— h-2 O < LU LU Q O X u 1.0 100 200 CURRENT 3 0 0 (A) 4 0 0 F i g u r e 4-18: Cathode Heat T r a n s f e r v s . Current f o r D i f f e r e n t Flow Rates (Flow Towards) 89 5.0 5 4.0 cr -U JL CO z < LT f - 3.0 I-< LU I U J Q2.0 o 1.0 FLOW RATE o - 0.47 l i tres/s A - 0.71 ° - 1.06 100 200 CURRENT 300 (A) 400 F i g u r e 4-19: Anode Heat T r a n s f e r v s. Current f o r D i f f e r e n t Flow Rates (Flow Towards) DC RESULTS Cathode Slope I n t e r c e p t Pressure Increase N.C. S.D. Pressure Increase N.C. S.D. Gas Flow Increase I . I. Gas Flow Increase N..C. N.C. Anode Flow D i r e c t i o n Slope I n t e r c e p t N.C. S.D. C—>A N.C. S.D. A—>C N.C. N.C. C—>A N.C. I. A — » C AC RESULTS Flow Towards E l e c t r o d e Slope I n t e r c e p t Flow Away From E l e c t r o d e Slope I n t e r c e p t Pressure Increase N.C. S.D. N.C. S.I. Gas Flow Increase N.C. S.D. N.C. S.I. N.C. = No Change S.D. = S l i g h t Decrease S.I. = S l i g h t Increase I. «• Increase Table V: The E f f e c t o f Gas Flow C o n d i t i o n s on E l e c t r o d e Heat T r a n s f e r 91 DC RESULTS Anode Vft Current Independent Term (volts) (kWatts) +5% +5% Pressure atm Flow Direction 10.7 10.95 11.0 0.44 0.56 0.46 5.1 3.7 2.1 T T T 7.4 7.5 1.46 1.58 5.1 3.7 A A Cathode Current Independent Term Pressure Flow Direction 4.65 0.18 5.1 T 4.95 0.18 3.7 T 3.8 0.88 5.1 A 3.5 0.84 3.7 A 3.8 1.1 2.1 A T = flow towards electrode A = flow away from the electrode V c = slope of Q c vs. I curve V"A = slope of Q A vs. I curve Table vi: Summary of DC Electrode Heat Transfer Measurements AC RESULTS V Current Independent Pressure Flow D i r e c t i o n Term ( v o l t s ) (kWatts) atm + 5 % + 5 % 5.3 0.72 5.1 A 5.45 0.66 3.7 A 5.4 0.48 2.4 A 5.25 0.50 1.3 A 5.8 -0.05 5.1 T 6.7 0 3.7 T 6.9 -0.05 2.4 T 7.0 0.15 1.3 T V = sl o p e of Q E L v s . I curve T = flow towards the e l e c t r o d e A = flow away from the e l e c t r o d e T a b l e vii: Summary o f AC E l e c t r o d e Heat T r a n s f e r Measurements 93 of the two channel temperature measuring system. For these runs the anode had a t h i c k 45 ° c o n i c a l t i p ( i d e n t i c a l to the usual AC e l e c t r o d e ) . When the DC arc was operated a t high c u r r e n t t h i s anode t i p showed s i g n s of m e l ting. T y p i c a l anode thermal r a d i a t i o n waveforms from the two channel o p t i c a l system f o l l o w i n g c u r r e n t i n t e r r u p t i o n are shown i n F i g u r e 4-20. The observed p l a t e a u i n the i n t e n s i t y waveforms corresponds to the l i g u i d - s o l i d phase t r a n s i t o n f o r tungsten. Hence t h i s p r o v i d e s a d i r e c t check on the c a l i b r a t i o n of the temperature measuring system. The s u r f a c e temperature was c a l c u l a t e d i n the p l a t e a u r e g i o n using the i n t e n s i t y decay waveforms. The temperature i n t h i s r e g i o n was found to be 3850*150 K. The accepted value f o r the melting p o i n t of tunsten i s 3653 K, so t h a t the accuracy of the s u r f a c e temperature measuring systen was q u i t e good. The temperature measured f o r t h i s p l a t e a u r e g i o n was s y s t e m a t i c a l l y higher than the accepted value f o r the m e l t i n g po i n t of tungsten. T h i s i s probably due to the assumption of gray r a d i a t i o n used to c a l c u l a t e the temperature. No data i s a v a i l a b l e f o r the e m i s s i v i t y of tungsten at the melting p o i n t . However, up to 3000 K the e m i s s i v i t y decreases by t y p i c a l l y 2-3% (De Vos (1953)) from 600 to 700 nm. Hence the c a l c u l a t i o n overestimates the i n t e n s i t y of the 700 nm term, which would r e s u l t i n s y s t e m a t i c a l l y higher temperatures. 94 C H A N N E L 1 2 V/div A , =700 nm C H A N N E L 2 5 V/div A 2 = 6 0 0 nm A 1 = A 2 = 6 0 0 nm < TIME 20 ms/div. F i g u r e 4 -20 : Anode Thermal R a d i a t i o n Decay F o l l o w i n g C u r r e n t I n t e r r u p t i o n 95 4.4.2 AC E l e c t r o d e Surface Temperature R e s u l t s The e l e c t r o d e s u r f a c e temperature f o r AC o p e r a t i o n has been determined f o r two d i f f e r e n t e l e c t r o d e t i p geometries. Experiments on the AC a r c were performed using f l a t , c y l i n d r i c a l and f o r t y - f i v e degree c o n i c a l e l e c t r o d e t i p s . T y p i c a l i n t e n s i t y decay waveforms f o r the f l a t c y l i n d r i c a l e l e c t r o d e t i p s are shown i n F i g u r e 4-21. S i m i l a r decay waveforms f o r the 45 ° c o n i c a l t i p e l e c t r o d e s are shown i n F i g u r e 4-22. Comparing the waveforms f o r the two d i f f e r e n t e l e c t r o d e t i p geometries, i t i s c l e a r t h a t the thermal decay time constant f o r the c o n i c a l e l e c t r o d e i s much s h o r t e r than f o r the f l a t e l e c t r o d e . Hence the heat t r a n s f e r c o n d i t i o n s at the e l e c t r o d e should be q u i t e d i f f e r e n t f o r the two e l e c t r o d e geometries, and w i l l be d i s c u s s e d f u r t h e r i n Chapter 5. As o u t l i n e d i n Chapter 3, the AC c u r r e n t i n the a r c could not be i n t e r r u p t e d at a s p e c i f i e d time during the c u r r e n t c y c l e . T h i s proved not to be a handicap f o r the t h i c k e l e c t r o d e t i p s {tungsten t i p t h i c k n e s s £ 9-10 mm) used i n these experiments. For the c o n i c a l e l e c t r o d e s i t was found t h a t the e l e c t r o d e s u r f a c e temperature was con s t a n t throughout the AC c y c l e over the a r c attachment r e g i o n examined. The s i z e of the e l e c t r o d e r e g i o n examined was ^ 2 mm i n diameter which was approximately the s i z e of the arc attachment r e g i o n f o r the c o n i c a l e l e c t r o d e s . Within t h i s attachment r e g i o n time v a r y i n g 96 A1 =700 nm A2 = 6 0 0 nm 2 V/div 2 0 ms/div. T IME F i g u r e 4-21: AC E l e c t r o d e Thermal R a d i a t i o n Decay F o l l o w i n g Current I n t e r r u p t i o n ( F l a t Tipped E l e c t r o d e ) 97 A, =700 nm 0.5 V/div. A 2 =600 nm 0.2 V/div. TIME A 1=A 2=600 nm 10 ms/div. F i q u r e 4-22: AC E l e c t r o d e Thermal R a d i a t i o n D e c a y F o l l o w i n g Current I n t e r r u p t i o n (4 5 ° C o n i c a l E l e c t r o d e T i p ) 98 e l e c t r o d e hot s p o t s of e x t r e m e l y s m a l l s i z e were o b s e r v e d . (Refer t o S e c t i o n 4.5 f o r a d e t a i l e d d i s c u s s i o n of the t i m e r e s o l v e d o b s e r v a t i o n s o f the AC a r c e l e c t r o d e s ) . Both anode and c a t h o d e hot s p o t s ( F i n k e l n b u r g (1944), Froome (1946) , Schmidt (1949), E c k e r (1954), Wiencke (1958)) were o b s e r v e d on t h e AC e l e c t r o d e t i p . At the c a t h o d e hot s p o t s were always o b s e r v e d as t h e y a r e r e q u i r e d t o m a i n t a i n t h e a r c (Hoyaux ( 1 9 6 8 ) ) . Anode hot s p o t s however were not always o b s e r v e d as they are not n e c e s s a r y f o r m a i n t a i n e n c e of t h e a r c . The anode s p o t s were always o b s e r v e d at the former c a t h o d e hot spot l o c a t i o n . S i n c e t h e s e hot s p o t s were n o t o b s e r v e d by t h e t e m p e r a t u r e measuring system they had t o be e x t r e m e l y s m a l l . In o r d e r to examine t h e s e s m a l l h o t s p o t s i m p r o v e d s p a t i a l r e s o l u t i o n was r e q u i r e d which was c e r t a i n l y p o s s i b l e with the o p t i c a l d e t e c t i o n system u s e d . However, much i m p r o v e d s e n s i t i v i t y of the d e t e c t i o n system would a l s o have been r e q u i r e d . F o r the f l a t e l e c t r o d e t i p s the s u r f a c e t e m p e r a t u r e was a l s o c o n s t a n t t h r o u g h o u t t h e AC c y c l e . The s p a t i a l v a r i a t i o n s i n the s u r f a c e t e m p e r a t u r e were a l s o very s m a l l a c r o s s the s u r f a c e of the f l a t e l e c t r o d e . When measurements were c o n d u c t e d at t h e edge of the e l e c t r o d e o c c a s i o n a l l y h o t s p o t s were o b s e r v e d which had h i g h e r t e m p e r a t u r e s and much s h o r t e r decay t i m e s t h a n the r e s t of the e l e c t r o d e s u r f a c e . U n f o r t u n a t e l y t h e s e hot s p o t s moved a r c u n d the edge o f t h e e l e c t r o d e so t h a t i t 99 was d i f f i c u l t to examine them i n d e t a i l . T y p i c a l s u r f a c e temperature decay curves c a l c u l a t e d using the i n t e n s i t y decay waveforms (Figure 4 -21) are shown i n F i g u r e 4 - 2 3 . The s u r f a c e temperature decays e x p o n e n t i a l l y with time and the decay time constant i s approximately 6 0 0 ms . The s p a t i a l v a r i a t i o n i n s u r f a c e temperature as s t a t e d p r e v i o u s l y i s q u i t e small a c r o s s the e l e c t r o d e . T y p i c a l temperature decay curves f o r the 4 5 0 c o n i c a l AC e l e c t r o d e are shown i n F i g u r e 4 - 2 4 . These temperature measurements were made on a 2 mm diameter r e g i o n at the t i p of the e l e c t r o d e . Again the s u r f a c e temperature decays e x p o n e n t i a l l y with time. The t y p i c a l thermal decay time co n s t a n t f o r the c o n i c a l e l e c t r o d e t i p was 8 0 - 1 0 0 msec. The temperature decay curve f o r the hot spots o c c a s i o n a l l y observed on the edge of the f l a t e l e c t r o d e s i s shown i n F i g u r e 4 - 2 5 . The temperature decay was a l s o found to be e x p o n e n t i a l with time. The i n i t i a l temperature of the hot spot i s c o n s i d e r a b l y higher than the r e s t of the e l e c t r o d e s u r f a c e . The decay time c o n s t a n t which was ^ 100 ms was a l s o much s h o r t e r than t h a t f o r the r e s t of the e l e c t r o d e t i p . The e l e c t r o d e s u r f a c e temperature was determined as a f u n c t i o n of c u r r e n t and pressure f o r the c o n i c a l e l e c t r o d e t i p s . This e l e c t r o d e geometry was used e x c l u s i v e l y f c r these measurements s i n c e the arc 101 F i g u r e 4-24: AC E l e c t r o d e Surface Temperature vs Time (45 C o n i c a l E l e c t r o d e Tip) 1 0 2 F i g u r e 4-25: AC E l e c t r o d e Hot Spot Temperature vs Time 103 attachment region was l o c a l i z e d at the t i p of the electrode which provided much better s t a b i l i z a t i o n of the arc column near the electrode than that provided by the f l a t electrode. The electrode surface temperature as a function of I i s shown in Figure 4-26. The temperture r e s u l t s were determined from at least six separate temperature measuring runs. The uncertainty i n the temperature measurements i s the standard deviation of the mean. The surface temperature as a fuction of pressure i s shown in Figure 4-27 for a current near the maximun (1=380 A BMS) of the power supply. 4.4.3 DC Electrode Surface Temperature Results The anode surface temperature f o r a DC VSA has been determined. The reason why the anode was singled out fo r examination i s due to the much higher heat transfer rates occurring at the anode than at the cathode. In order that a direct comparison between the AC and DC surface temperatures could be made, the anode geometry was the same 45 0 conical shape as that used for the AC electrodes. Typical i n t e n s i t y decay waveforms for the anode are shown i n Figure 4-28 as a function of arc current. For the higher currents the anode i s i n i t i a l l y above or at the melting point of tungsten as indicated by the plateau 104 2.8 O X PRESSURE = 3.7 a t m LU cr 1-2.4 < cr bJ CL LU r -LU O 2 0 cr h-<J LLI _ l LU 200 300 CURRENT ( A ) 400 F i g u r e 4-26: AC E l e c t r o d e Surface Temperature v s . Current 2.0 3.0 4 .0 5 .0 ARC PRESSURE (atm) F i g u r e 4-27: AC E l e c t r o d e Surface Temperature vs. Pressure \~-> o cn 106 1 = 4 5 0 A (a) A1 =700 nm 2V/d iv A 2 = 6 0 0 nm 5V/djv A=A =600 nm 1 2 P i q u r e 4-28: Anode Thermal R a d i a t i o n F o l l o w i n g Current I n t e r r u p t i o n f o r D i f f e r e n t C u r r ents TO 7 r e g i o n s . The s u r f a c e t e m p e r a t u r e d e t e r m i n e d from t h e above waveforms i s shown as a f u n c t i o n of c u r r e n t i n F i g u r e 4 - 2 9 . The s u r f a c e t e m p e r a t u r e e s s e n t i a l l y s c a l e s l i n e a r l y with I u n t i l the m e l t i n g p o i n t o f t u n g s t e n i s r e a c h e d . The anode s u r f a c e t e m p e r a t u r e as a f u n c t i o n of p r e s s u r e i s shown i n F i g u r e 4 - 3 0 . The anode t e m p e r a t u r e i n c r e a s e s d r a m a t i c a l l y a f t e r the p r e s s u r e i s i n c r e a s e d above 3.5 a t m o s p h e r e s . 4 ,_5 Time R e s o l v e d O b s e r v a t i o n s o f AC E l e c t r o d e Arc Attachment Region The time e v o l u t i o n o f the AC e l e c t r o d e - a r c a t t a c h m e n t r e g i o n has been examined u s i n g t h e system d e s c r i b e d i n s e c t i o n 3 . 4 . 3 . F o r t h e s e e x p e r i m e n t s t h e o 45 c o n i c a l e l e c t r o d e t i p s were used e x c l u s i v e l y s i n c e the a t t a c h m e n t r e g i o n f o r t h e s e e l e c t r o d e s was always w e l l l o c a l i z e d a t t h e t i p o f t h e e l e c t r o d e . For the cathode h a l f - c y c l e an i n t e n s e plasma j e t was o b s e r v e d which s t a r t s a t t h e e l e c t r o d e t i p and e x t e n d s f o r a p p r o x i m a t e l y 10 mm i n t o the a r c c o l u m n . When t h e p o l a r i t y of the e l e c t r o d e changed the plasma j e t d i s a p p e a r e d and was not o b s e r v e d t h r o u g h o u t t h e anode h a l f c y c l e . A hot s p o t a p p r o x i m a t e l y 2 mm i n d i a m e t e r was found a t t h e t i p o f t h e e l e c t r o d e f o r both the anode and c a t h o d e h a l f - c y c l e s . W i t h i n t h i s main hot s p o t very s m a l l hot s p o t s ( t y p i c a l l y 0 . 2 - 0 . 4 mm d i a m e t e r ) were o b s e r v e d which had h i g h e r l u m i n o s i t i e s and, 108 4.0 h co O X Ul CL Z> I-< cr UJ Q_ Ul h-Ul Q o 3.0 h 2.0 h 200 300 4 0 0 CURRENT (A) 5 0 0 F i g u r e 4-29: Anode Temperature vs. Current 109 4 .0 I = 3 2 0 A O X K H LU cr 3.5 < cr in CL LU ill a o z < 3.0 K H K H K H i 2.0 3.0 4.0 5.0 ARC PRESSURE (atm) F i g u r e 4-30: Anode Temperature vs. Pressure 110 of course, temperatures than the main hot spot. The l u m i n o s i t i e s of these s m a l l hot s p c t s v a r i e d throughout the AC c y c l e . They were predominantly observed on the cathode h a l f c y c l e . The l u m i n o s i t y of the main hot spot was e s s e n t i a l l y independent c f time. Time r e s o l v e d photographs of the AC e l e c t r o d e running at 240 and 380 (A) RMS are shown i n F i g u r e s 4-31, 4-32, and 4-33. In these photographs the plasma j e t and main e l e c t r o d e hot spot are c l e a r l y seen. The time v a r y i n g cathode hot spots are much more d i f f i c u l t to see i n the photographs as they are obscured by the i n t e n s i t y of the plasma j e t . In F i g u r e 4-34 the cathode phase f o r low c u r r e n t (1=100 A ) i s shewn. The s i z e of the main e l e c t r o d e hot spot i s much s m a l l e r than was the case f o r higher c u r r e n t s . Only one s m a l l time v a r y i n g hot spot was observed f o r t h i s low c u r r e n t run while at 1=380 amps up to 5 cathode hot spots were observed w i t h i n the main hot spot. 4 ..6 Time I n t e g r a t e d Photographs of the DC and AC Arc Column Time i n t e g r a t e d photographs were taken using a standard 35 mm SLR camera. The r e s u l t s f o r the DC a r c are shown i n F i g u r e s 4-35 while the r e s u l t s f o r the AC arc column are shown i n F i g u r e 4-36. I l l Cathode: 2ms a f t e r c u r r e n t zero Cathode: 6 ms a f t e r c u r r e n t zero F i g u r e 4-31: Time Resolved Photographs of the AC E l e c t r o d e T i p (1=240 A) 112 Cathode: 8 ms a f t e r c u r r e n t zero Anode: 10 ms a f t e r current zero Figure 4-32: Time Resolved Photographs of the AC Electrode T i P < W = 2 4 0 A ) 113 Anode: 15 ms a f t e r c u r r e n t zero -Figure 4-33: Time Resolved Photographs of the AC E l e c t r o d e T i p ( 1 ^ = 3 8 0 A) 114 F i g u r e 4-34: AC E l e c t r o d e During the Cathode H a l f C y c l e (1^5=100 A) Pressure=1.3 atm Pressure=5.1 atm F i g u r e 4-35: DC A r c Column (1=450 A) Arc Length =100 mm Pressure=3.7 atm Pressure=5.1 atm Figure 4-36: AC Arc Column (1^2=380 A) Arc Length =100 mm 117 Chapter 5 DISCUSSION 5_.J I n t r o d u c t i o n In t h i s chapter a d i s c u s s i o n of the r e s u l t s p resented i n chapter 4 i s g i v e n . The format of t h i s chapter i s s i m i l a r to t h a t of Chapter 4. The arc parameters t h a t a f f e c t the arc energy balance are d i s c u s s e d i n the f i r s t s e c t i o n . The second s e c t i o n d e a l s with the energy balance, the t h i r d with the e l e c t r o d e heat t r a n s f e r and the l a s t s e c t i o n with the e l e c t r o d e s u r f a c e temperature measurements. 5.-2 Arc Energy Balance: S i g n i f i c a n t Parameters The r e s u l t s presented i n Chapter 4 show the arc energy balance as a f u n c t i o n of pressure and c u r r e n t f o r both AC and DC o p e r a t i o n . I t was found t h a t these two parameters most s t r o n g l y a f f e c t e d the energy balance, p a r t i c u l a r l y the r a d i a t i o n l o s s e s . As mentioned i n S e c t i o n 4.3 the e l e c t r o d e heat t r a n s f e r was a f f e c t e d by both the flow r a t e and d i r e c t i o n . The energy balance f o r DC o p e r a t i o n was performed f o r both flow d i r e c t i o n s to see i f the o v e r a l l energy balance was a f f e c t e d by flow 1 18 d i r e c t i o n as the e l e c t r o d e heat t r a n s f e r had been. The r e s u l t s show t h a t the energy balance i s not n o t i c e a b l y a f f e c t e d by flow d i r e c t i o n (Table I I I and IV) . The r a d i a t i v e e f f i c i e n c y f o r DC o p e r a t i o n was only s l i g h t l y a f f e c t e d by flow d i r e c t i o n as shown i n F i g u r e s 4-3 and 4-4. These r e s u l t s show that changes i n flow d i r e c t i o n only i n f l u e n c e the arc r e g i o n immediately around the e l e c t r o d e , and the energy l o s s mechanisms i n the arc column are e s s e n t i a l l y independent of flow d i r e c t i o n . n was a l s o examined as a f u n c t i o n of gas flow r a t e . When the flow r a t e was i n c r e a s e d the r a d i a t i o n l o s s e s i n c r e a s e d . However the input power a l s o i n c r e a s e d so t h a t n showed no measurable i n c r e a s e over the flow range c o n s i d e r e d . T h i s r e s u l t i s c o n s i s t e n t with B a t u r i n e t a l (1968) who detected only s l i g h t i n c r e a s e s i n n when the flow r a t e was i n c r e a s e d f o r an atmospheric pressure argon a r c . In t h i s work we were p r i m a r i l y concerned with the r a d i a t i o n p r o p e r t i e s of vortex s t a b i l i z e d a r c s . Since n was not a f f e c t e d by flow r a t e the arc energy balance was not examined as a f u n c t i o n of gas flow r a t e . 119 5».3 Energy Balance 5.3.1 DC Energy Balance (1) R a d i a t i o n Losses The r a d i a t i o n l o s s e s from the DC arc were found 2 to s c a l e l i n e a r l y with I ( f o r I above 200 A ) . T y p i c a l r e s u l t s are shown i n F i g u r e 5-1 ( r e s u l t s from S e c t i o n 4.2). T h i s strong dependence of the r a d i a t i o n l o s s e s ( P R A D ) on I has been observed by other i n v e s t i g a t o r s . Anderson (1965) found that the r a d i a t i o n f l u x f o r a 20 kW 1. 4 vortex s t a b i l i z e d DC a r c s c a l e d with I '. The i n p u t power t o the DC a r c was found to s c a l e l i n e a r l y with I as shown i n F i g u r e 5-2. The r a d i a t i v e e f f i c i e n c y ( n ^ ) i s of the form n = Si I 2 + ^ DC S 2 I + t 2 ^ { 5 _ 1 } = S I (1 + t i / Si i 2 ) ( l + -gl^) 2 The experimental values f o r 2.1 atm are given below. S = f l = 4.5 x 10 3 H = 6.3 x , 1 0 3 ^2 = -75 s 2 s 1 s 2 N D C D S 9^- v e n b y 5-1 i s e s s e n t i a l l y l i n e a r with I over the c u r r e n t range examined (20u_< I _<480) as shown i n F i g u r e s PRESSURE 0.5 1.0 CURRENT 1.5 2.0 (A 2 ) x10' F i g u r e 5-1: DC R a d i a t i o n Losses vs. (Current) 121 F i g u r e 5-2: DC Input Power v s . C u r r e n t 122 4-3 and 4-4. I t i s c l e a r t h a t n D C must s a t u r a t e at some c u r r e n t above 480 ( A ) s i n c e n D C can not i n c r e a s e i n d e f i n i t e l y with I . U n f o r t u n a t e l y the DC power supply does not have the c a p a b i l i t y of p r o v i d i n g c u r r e n t s above 480 ( A ) so t h a t i t wasn't p o s s i b l e to determine the s a t u r a t i o n value of n D C was a l s o found to i n c r e a s e with arc chamber pressure as shown i n F i g u r e 4-6. The reason f o r the i n c r e a s e i n n D C with P i s due to the d e n s i t y i n c r e a s e t h a t accompanies any pressure i n c r e a s e . T h i s r e s u l t s i n more i n t e r a c t i o n s per u n i t volume which i n c r e a s e the r a d i a t i o n l o s s per u n i t volume, as has been shown by Evans (1967). (2) B a d i a l Heat T r a n s f e r As mentioned p r e v i o u s l y the r a d i a l heat t r a n s f e r from the a r c to the c o n f i n i n g wall (P w) has been measured (Sec t i o n 4.2). P^ was found to be l i n e a r l y r e l a t e d to the r a d i a t i o n l o s s e s ( p R A D ) under a l l flow c o n d i t i o n s with t y p i c a l r e s u l t s shown i n F i g u r e 5-3. The heat t r a n s f e r to the exhaust gas { F G A S ^ w a s a l s o found to s c a l e l i n e a r l y with the r a d i a t i o n l o s s e s . T h i s r e s u l t would seem to i n d i c a t e t h a t the w a l l l o a d i n g i s due to a b s o r p t i o n of r a d i a t i o n by the quartz wall and i t s c o o l i n g water r a t h e r than by r a d i a l heat t r a n s f e r . I f t h i s were the case then £ 50% of the r a d i a t i o n energy would have to be c o n t a i n e d i n the wavelength band above 1 micron (where water has s i g n i f i c a n t a b s o r p t i o n bands) and below 0.2 microns where 123 2 0 1 6 1 2 8 P R E S S U R E A - 3.7 a t m • - 5.1 U W A L L » » • 1 ' < A6 8 P RAD 12 16 ( k W ) F i g u r e 5-3: DC W a l l Heat T r a n s f e r v s . R a d i a t i o n Losses 124 the t r a n s m i s s i o n of quartz i s n e g l i g i b l e . Other i n v e s t i g a t o r s who have examined the spectrum of high i n t e n s i t y argcn a r c s (Anderson (1965), M a l l i a r i s (1970) and Decker (1972)) have fcund t h a t l e s s than 10% cf the r a d i a t i o n energy l o s s e s from the a r c occur i n these wavelength bands. Hence the w a l l l o a d i n g can not be the r e s u l t of a b s o r p t i o n of r a d i a t i o n . The g u e s t i o n now a r i s e s as to what heat t r a n s f e r process i s r e s p o n s i b l e f o r P„. The r a d i a l heat t r a n s f e r to the wall has been estimated using the temperature and r a d i u s of the arc which have been c a l c u l a t e d using a simple channel model t h a t w i l l be d e s c r i b e d i n d e t a i l l a t e r i n t h i s s e c t i o n . The temperature f o r the c e n t r a l core of the a r c column obtained using t h i s simple channel model was t y p i c a l l y 11000 K, while the r a d i u s of the luminous core i s t y p i c a l l y 7-8 mm. The temperature of the i n n e r s u r f a c e of the c o n f i n i n g quartz tube has been estimated t o be 600-1000 K. Osing these r e s u l t s the r a d i a l heat t r a n s f e r (P T 7) has been estimated to be ^ 4-5 kW based on laminar r a d i a l heat t r a n s f e r . For the vortex s t a b i l i z e d a r c s used i n t h i s work there i s s i g n i f i c a n t a x i a l flow so t h a t the value given above f o r the r a d i a l heat t r a n s f e r would i n f a c t be s m a l l e r . The l a r g e s t observed value f o r P i s ^ 15kW, hence laminar r a d i a l heat t r a n s f e r cannot account f o r the wall l o a d i n g . We b e l i e v e the w a l l l o a d i n g i s due to t u r b u l e n t heat and mass t r a n s f e r processes. Other i n v e s t i g a t o r s (Andrada and E r f u r t h (1962)) have reported 125 t u r b u l e n t mixing processes i n a r c s with strong gas flow f i e l d s . We expect s i m i l a r t u r b u l e n t behavior f o r the arcs used i n t h i s work. As mentioned p r e v i o u s l y i s l i n e a r l y r e l a t e d to P^p • We would l i k e to be a b l e t o e x p l a i n t h i s r e s u l t i n the context of the t u r b u l e n t heat t r a n s f e r processes t h a t have been proposed to e x p l a i n the high v a l u e s of P . U n f o r t u n a t e l y t h i s r e g u i r e s a d e t a i l e d W knowledge of the c o n d i t i o n s i n the r e g i o n j u s t beyond the luminous area of the a r c , which we do not have at present. (3) Channel Model f o r the DC Arc Column In t h i s s e c t i o n a simple channel model f o r the DC a r c column w i l l be presented. T h i s model w i l l make use of the energy balance r e s u l t s presented i n S e c t i o n 1.2 and w i l l allow the r a d i u s and temperature of the arc column to be determined as a f u n c t i o n of c u r r e n t . Simple channel models have been used i n the past ( F i n k e l n b u r g and Maecker (1956)) to determine the temperature d i s t r i b u t i o n i n DC a r c s . In the s i m p l e s t channel model the e l e c t r i c a l c o n d u c t i v i t y i s assumed to be c o n s t a n t i n a symmetrical channel along the a r c a x i s . O u t s i d e t h i s i o n i z e d channel the e l e c t r i c a l c o n d u c t i v i t y i s assumed to be z e r o . The r a d i u s of the conducting channel i s then determined using Steenbeck's minimum p r i n c i p l e . According t o t h i s p r i n c i p l e the c o r r e c t value f o r the r a d i u s of the conducting channel i s the one which 126 l e a d s to a minimum value of the e l e c t r i c f i e l d . The r e s u l t i n g temperature p r o f i l e f o r the channel i s a maximun on the arc a x i s and decreases p a r a b o l i c a l l y . More r e f i n e d channel models have been presented by Maecker(1959). In t h i s model the e l e c t r i c a l c o n d u c t i v i t y i s assumed to be l i n e a r l y r e l a t e d t o the heat t r a n s f e r f u n c t i o n S (S= J"k(T)dT ). Outside the i o n i z e d channel the e l e c t r i c a l c o n d u c t i v i t y i s again assumed t o be ze r o . The r a d i u s of the i o n i z e d channel i s determined from the curves of a vs S which are presumably known, so t h a t Steenbeck's minimum p r i n c i p l e does not have to be used. The a r c temperature p r o f i l e s determined from t h i s model are given by B e s s e l f u n c t i o n s and l n ( r ) . In t h i s simple channel model of the a r c column, the e l e c t r i c a l c o n d u c t i v i t y as well as the temperature i s assumed to be independent of r a d i u s f o r the high temperature r a d i a t i n g core (T >10000 K) as shown below. hi cc z> I-< cc UJ 0_ LU 1-u / Cr: / < RADIUS 127 For r Q < r < r w i r w = t h e r a d i u s of the c o n f i n i n g guartz wall) the a r c i s assumed t o be n o n - r a d i a t i n g and r a d i a l temperature g r a d i e n t s e x i s t . In the channel model that f o l l o w s the arc r a d i u s r e f e r s t o the r a d i u s of the luminous core of the a r c . For conduction dominated a r c s (Maecker (1959), Hoyaux (1968)) the temperature p r o f i l e i s p a r a b o l i c or i s give n by B e s s e l f u n c t i o n s , while f o r r a d i a t i o n dominated a r c s the temperature i s independent of r a d i u s (Lowke (1970)). The a r c s used i n t h i s work t y p i c a l l y had r a d i a t i o n l o s s e s of 30-35% of the input power. Hence some f l a t t e n i n g of the temperature p r o f i l e would be expected f o r the ar c s used i n t h i s work and the crude approximation of a constant temperature channel should be approximately v a l i d . In t h i s model a l l of the r a d i a t i o n from the arc w i l l be assumed to be frcm the c e n t r a l core. Since the r a d i a t i o n l o s s e s f o r argon are n e g l i g i b l e below 10000 K (Evans (1967)) the maximum temperature o u t s i d e of the c e n t r a l core (ie r>r ) i s 10000 K. I t w i l l be f u r t h e r o assumed t h a t the m a j o r i t y of the c u r r e n t flows i n the c e n t r a l core and a l l the i n p u t power i s d e p o s i t e d t h e r e . The energy balance per u n i t volume f o r the a r c column i s given by O(T) E 2 = U R (T, P) + q w + q G ( 5 _ 2 j where g^=radial heat t r a n s f e r i n c l u d i n g laminar 128 and t u r b u l e n t heat t r a n s f e r g G=heat t r a n s f e r t o g a s / u n i t volume o (T) = e l e c t r i c a l c o n d u c t i v i t y E = e l e c t r i c f i e l d U. = r a d i a t i o n l o s s e s / u n i t volume R The t o t a l power f o r t h i s simple model i s then j u s t 2 r , 2 ( aE J i n c l u d i n g e l e c t r o d e l o s s e s i s then given by •nrQ oE L where L=arc l e n g t h . The t o t a l energy balance I 2 L 2 TTr o ^ ( f ) UR 7 T o + ^ (ro> rw> T) L + Q G ( r o S r w , T)L + (F+GI) (5-3) where (l,=heat t r a n s f e r to the w a l l Q G=heat t r a n s f e r to the exhaust gas F+GI=heat t r a n s f e r to the e l e c t r o d e s (F and G are constants U R i s s t r o n g l y dependent on temperature and pressure (Evans (1967)) over the temperature range (10000-13000 K) encountered i n arc plasmas. Over t h i s temperature range 0 can be approximated from Evans' R r e s u l t s by U_-(T, P) = B(P) T n (5-4) R where n i s t y p i c a l l y g r e a t e r than 10. As mentioned p r e v i o u s l y the heat t r a n s f e r to the wall and the exhaust gas were found e x p e r i m e n t a l l y to s c a l e l i n e a r l y with the r a d i a t i o n l o s s e s under a l l flow c o n d i t i o n s . These r e s u l t s are given below 129 B(P) T n T i r 0 L £ Q w ( r Q , r w , T) L (5-5) Q w ( r , r w , T) S 2.95 Q G ( r D , r w , T) L Using these experimental s c a l i n g r e l a t i o n s and Evans' r a d i a t i o n data f o r argon, eguation (5-3) can be w r i t t e n as I 2 L - (F + Gl) y 2.34 B ( P ) T n r 2 L 2 (5-6) v o = 2.34 U R Hence the e n t i r e enery l o s s from the a r c ( d i s r e g a r d i n g e l e c t r o d e l o s s e s ) can be r e p r e s e n t e d as a f u n c t i o n of the r a d i a t i o n l o s s e s . The l e f t hand s i d e of (5-6) can be w r i t t e n as , T 1 2 G, 2 1 2 2 4 G 2 2 F 2 o L 4 0 L o L 2 airr o T y p i c a l values f o r the arc parameters are given below I ^200-400 A 1 10000- 1300C K CT^30-50 mho/cm G ^ 13 v o l t s F ^1-2 kw Using these values i t i s r e a d i l y shown that the terms c o n t a i n i n g F and G are n e g l i g i b l e i n comparison to the remaining terms. Eguation ( 5 - 6 ) can then be w r i t t e n as 130 ( I - \ o*rQ2 £) 2 = 2.34 B(P) T n (N + MT) T T T ^ ^ where a (T) = N*MT 9000 <T <J20C0 K from Devoto (1973) S o l v i n g f o r T y i e l d s T = C(P) (I - h a T r r 02 G/L) 2 n+1 , _1_ , M ^ n+1 ; N where C(P) = , 1 [2.34 N B(P) l n + 1 (5-8) Now both I and r are unknowns i n eguation (5-8) so that another eguation i n T and r Q i s r e q u i r e d . The e x t r a r e l a t i o n i s provided by the r a d i a t i o n l o s s from the arc and i s given by PRAD " B ( P ) T ™ L (5-9) where P =measured r a d i a t i o n l o s s from the arc RAD The arc r a d i u s and temperature have been determined as a f u n c t i o n of c u r r e n t f o r an a r c pressure of 2.1 atm. The a x i s pressure i n these experiments i s u n f o r t u n a t e l y unknown but i t w i l l be assumed that the pressure i n the hot core of the a r c i s £ 2 atm. To determine r and^ T r e q u i r e s a knowledge of B(P) and n which are determined from Evans' graphs. Evans' r e s u l t s f o r 2 atm are shown i n 131 Figure 5-4. From t h i s graph n can be estimated quite e a s i l y . However, due to the extremely stronq temperature dependence of P small errors i n n can r e s u l t in r RAD s i g n i f i c a n t errors in B (P) . In what follows a graphical technigue to calculate r and T i s presented which i s o completely eguivalent to the a n a l y t i c a l method above. Once r and T have been determined for one current value o then B (P) can be calculated from equation (5-8) using the estimated value for n from Figure 5-4 and a value of r and 1 determined graphically. As stated previously the input power to the arc 2 2 f o r t h i s simple channel model i s given by L r / u r o (I). In o Figure 5-5 curves of input power/unit length normalized with respect to current have been plotted as a function of radius with temperature as a parameter. From the energy balance results presented i n Section 4.2 the input power per unit length i s known as a function of I. It i s obtained by subtracting electrode heat losses from the measured value of the e l e c t r i c a l input power. These experimental values of input power per unit length (again normalized with respect to current) have been plotted on 2 Figure 5-5 and appear as curves of constant P I N / L I • Clea r l y a l l the possible values of TQ and T for the arc column must be on the l i n e P T/LI =ccnstant. A value of r T N 2. I N ' " ~ ~ ~o 2 (say r =r_) and T (say T=Tn) on the l i n e P /LI =constant O 1 1 IN i s then used to calculate the radi a t i v e power loss 2 ^ P R A D = U R ^ T l 1 T r i where u R ( T 1 » p ) 1 S t n e radiation loss 133 ARC RADIUS (mm) F i g u r e 5-5: C u r r e n t Normalized Input Power v s . Radius 134 per u n i t volume at temperature T^ given by Evans* r e s u l t s . However P„,„ i s a l s o a measured q u a n t i t y . Hence by R A D 1 v a r y i n g (r ,T ) the computed and measured r a d i a t i o n l o s s e s o can be brought i n t o c o i n c i d e n c e , thereby y i e l d i n g the arc r a d i u s and temperature. For example the value of (r , T) f o r 1=480 A using t h i s g r a p h i c a l technique was r=7.65 mm and T=11,050 K. There are a number of p o i n t s that should be made about t h i s g r a p h i c a l technigue t o determine (r ,TJ. In o the g r a p h i c a l method the r a d i a t i o n l o s s e s U (T,P) are read d i r e c t l y from Evans' graphs (Figure 5-4), which can be determined with reasonable accuracy. A knowledge of n and 8 (P) t (U R= (B (P) T11) , i s not even r e g u i r e d to determine ( r Q , l ) using t h i s g r a p h i c a l method. T h i s i s i n marked c o n s t r a s t to the a n a l y t i c a l r e l a t i o n which r e q u i r e s a c c c u r a t e values of n and E (P). As mentioned p r e v i o u s l y B (P) i s very d i f f i c u l t to determine with s u f f i c i e n t accuracy from Evans' curves. The value of n can be estimated w i t h i n a few per cent from F i g u r e 5-4. However due to the s t r c n g temperature dependence of U , even s m a l l e r r o r s i n n can r e s u l t i n very i n a c c u r a t e values f o r B (P). Since the g r a p h i c a l technique t o determine (r Q , T ) does not r e l y on n and B (P) , the values of ( r Q , T ) determined g r a p h c i a l l y can be used to determine B(P) using equation (5-8). Hence the g r a p h i c a l method d e s c r i b e d above not only a l l o w s r Q a n d T to be determined but B (P) as w e l l , so t h a t (5-8) and (5-9) can then be used to determine r Q a n d T 135 a n a l y t i c a l l y . The r a d i u s and temperature of the DC arc column have been c a l c u l a t e d as a f u n c t i o n of c u r r e n t using eguations (5-8) and (5-9) and i s shown i n F i g u r e s 5-6 and 5-7. At low c u r r e n t both the r a d i u s and temperature of the a r c column i n c r e a s e with I. Above £ 350 A the arc r a d i u s i s e s s e n t i a l l y constant while the temperature c o n t i n u e s t o i n c r e a s e . Photographs of the arc column were taken (presented i n S e c t i o n 4.6) and the r a d i u s of the luminous r e g i o n was found to be e s s e n t i a l l y constant at high c u r r e n t . The r a d i u s of the luminous core of the arc measured from the photographs was t y p i c a l l y 8 mm which i s i n good agreement with the p r e d i c t e d r e s u l t s from Fi g u r e 5-7. At high c u r r e n t the column behaves as i f i t was c o n s t r a i n e d by a w a l l at r=8 mm, when i n f a c t no s o l i d w a l l e x i s t e d at t h i s r a d i u s (the c o n f i n i n g w a l l was at a much l a r g e r r a d i u s (r w=13.5 mm)). The w a l l heat t r a n s f e r continued to i n c r e a s e when the r a d i u s of the a r c was e s s e n t i a l l y constant, so t h a t h i g h l y e f f i c i e n t heat t r a n s f e r processes must be t a k i n g place o u t s i d e the c e n t r a l core of the a r c . As mentioned p r e v i o u s l y laminar r a d i a l heat t r a n s f e r can not e x p l a i n the l a r g e values of the w a l l heat t r a n s f e r . Turbulent mixing processes must occur near the edge of the hot c e n t r a l arc core so as to enhance the r a d i a l heat t r a n s f e r t o the w a l l . The c o n s t a n t r a d i u s r e s u l t s at high c u r r e n t i n d i c a t e t h a t the edge of the luminous a r c core a c t s l i k e a s o l i d w a l l (or 136 F i g u r e 5-6: DC Arc Radius v s . C u rrent 137 250 300 350 400 450 CURRENT (A) F i g u r e 5-7: DC Arc Temperature v s . Current 138 heat sink) and i s e v i d e n t l y s t r o n g l y coupled to the water co o l e d guartz wall which i s a primary heat sink f o r the a r c . The presence of the quartz wall i s then e s s e n t i a l l y communicated t o the arc column through the non-luminous r e g i o n (which surrounds the arc channel). It i s i n t e r e s t i n g t o compare the temperature-r a d i u s s c a l i n g r e s u l t (eguation 5-8) with the e m p i r i c a l l y determined r e s u l t of Morris (1969) f o r wall s t a b i l i z e d argon a r c s . f o r 2 atm he f i n d s T = 5020 ( - V - )°- 1 5 2 (5-10) T h i s r e s u l t i s q u i t e s i m i l a r to the r e s u l t g i v e n by eguation (5-8) f o r P -2 atm which i s shown below 0.132 t - 5230 1 % = ^ - 4 6 5 (5-1 1) The r e s u l t s p r e d i c t e d by Morris and those p r e d i c t e d by equation (5-9) are p l o t t e d i n F i g u r e 5-8. The r e s u l t s agree reasonably well from 10000-14000 K. T h i s r e s u l t f u r t h e r i n d i c a t e s t h a t f o r I >350 A the vortex s t a b i l i z e d arc behaves l i k e a w a l l s t a b i l i z e d a r c . io 5 LU c r Z) h- 4 < 10 c r LU CL LU < X < io 3 MORRIS EQUATION 5 -9 ' 1 i « i t i I I 1 l i m n J — J L j i 100 1000 10000 CURRENT DENSITY (A /cm 2 ) Figure 5 - 8 : A x i a l Arc Temperature vs. Current Density co 140 AC Energy Balance M) AC R a d i a t i o n Losses The r a d i a t i o n l o s s e s from the AC arc were found to s c a l e l i n e a r l y with I . T y p i c a l r e s u l t s are shown i n F i g u r e 5-9. Th i s i s g u i t e d i f f e r e n t from the r a d i a t i o n 2 l o s s e s from the DC arc which s c a l e d with I as shown i n Figur e 5-1. The t o t a l i n p u t power t o the AC a r c was l i n e a r with I above 180 A , a s i m i l a r r e s u l t t o that o b t a i n e d f o r the DC a r c . T y p i c a l r e s u t s are shown i n Fig u r e 5-10. The r a d i a t i v e e f f i c i e n c y ( nAC^ : ' - s o f t i i e form n = M i l + bi AC M 2I + b 2 T y p i c a l values of M, b^/H^ and b 2/M 2 f o r 2 atm are 11=0.36, b 1/M 1 = -84 and b 2/M 2=-34. If the r e s u l t f o r n A C given i n (5-12) can be e x t r a p o l a t e d to higher c u r r e n t s i t i s c l e a r t h a t n A C has a maximum value ( w i l i c h i s £ 36% f o r P=2 atm) . In F i g u r e (5-11) i s shown as a f u n c t i o n of arc pressure. I t i s c l e a r that n x s a t u r a t e s at about 40% f o r P £5 atm. For a l l pressures up t o 5 atm n i s e s s e n t i a l l y l i n e a r up to 400 A RMS. The slope of t h i s l i n e a r r e g i o n of n A C i s comparable to t h a t f o r the DC a r c . n A C was a l s o s t r o n g l y a f f e c t e d by arc chamber pressure as RADIATION L O S S E S (kW) to O) CO o INPUT POWER (kW) -a c n ro Ln I > o H 3 C rt 0 z ro < n c ro 3 rt o o O cz 73 73 rv> —I o o O O • t> • o I I I I TJ 3D (ji u) ro r 4 , n (/) QJ C 3 m 143 F i g u r e 5-11: Maximum AC R a d i a t i v e E f f i c i e n c y v s . Pressure 144 shown e a r l i e r i n F i g u r e s 4-1 and 4-2. The i n c r e a s e i n n^c with P was again caused by an i n c r e a s e i n the i n t e r a c t i o n s per u n i t volume due to the i n c r e a s e d d e n s i t y that accompainies a pressure i n c r e a s e . (2) R a d i a l Heat T r a n s f e r ' The wall l o a d i n g f o r the AC a r c was a l s o found to s c a l e l i n e a r l y with the r a d i a t i o n l o s s e s , ( F i g u r e 5-12), a s i m i l a r r e s u l t to t h a t obtained f o r the DC a r c . The heat t r a n s f e r to . the exhaust gas was a l s o found to s c a l e l i n e a r l y with the r a d i a t i o n l o s s e s (Figure 5-12). Again laminar r a d i a l heat t r a n s f e r can not e x p l a i n the l a r g e value f o r PT,. As with the DC a r c i t i s b e l i e v e d t h a t t u r b u l e n t mixing processes at the edge of the luminous r e g i o n of the a r c g r e a t l y enhance the heat t r a n s f e r to the w a l l . The edge of the luminous r e g i o n was very i r r e g u l a r ( r i p p l y s t r u c t u r e ) as observed i n photographs of the a r c column (Figure 4-35). This s t r u c t u r e would seem to i n d i c a t e t u r b u l e n t processes at the edge of the arc column. (3) Channel Model f o r AC Arc As shown i n S e c t i o n 5.5.2 a simple channel model can be used t o p r e d i c t the r a d i u s and temperature of the DC a r c column. In a s i m i l a r f a s h i o n we would l i k e to be able to p r e d i c t the average temperature and r a d i u s f o r the 145 F i g u r e 5-12: AC Wall Heat T r a n s f e r v s . R a d i a t i o n Losses 146 AC arc column. For the DC arc i t was shown that a graphical and a n a l y t i c a l technigue could eguivalently be used to determine r and T. For the AC arc we would l i k e to use the graphical technigue to predict the average radius and temperature of the luminous core of the arc column. From the energy balance r e s u l t s presented in Section 4.2 the average input power per unit length normalized with respect to current has been determined for the AC arc. As with the DC arc the values of 2 P I N/LI =constant have been plotted on the graph of 2 P I N / L I vs radius (see Figure 5 -5) . Osing the measured output radiation from the AC arc the radius and temperature have been determined. In a l l cases th i s r e s u l t s i n values for r that are much too large (r >11 mm) when compared to time integrated photographs of the AC arc which y i e l d r a d i i for the luminous core of £ 7-8 mm. The question now arises as tc why t h i s method predicts an average radius for the AC arc that i s much too large. The radius and temperature of the AC arc column are most c e r t a i n l y time dependent. In attempting to use the steady state DC analysis to predict the average AC r e s u l t s we are es s e n t i a l l y assuming that at any instant the AC arc behaves l i k e an equilibrium DC arc at the same current. If t h i s was the case then t h i s method should give reasonable values for the arc radius. Since the method f a i l s then we must conclude that the AC arc column can not 147 be represented as j u s t the s u p e r p o s i t i o n of steady s t a t e DC a r c columns. In order t o model the AC a r c column i n a s i m i l a r f a s h i o n to t h a t used f o r the DC a r c the temporal v a r i a t i o n of the arc i n p u t power and power l o s s mechanisms must be known. U n f o r t u n a t e l y t h i s i n f o r m a t i o n i s not known as our measurements of the power l o s s mechanisms from the a r c were average values over the c u r r e n t c y c l e . 5_._3_-J P r a c t i c a l C o n s i d e r a t i o n s : R a d i a t i v e E f f i c i e n c y The vortex s t a b i l i z e d AC arc c o n s i d e r e d i n t h i s work was p r i m a r i l y intended f o r use as a high i n t e n s i t y l i g h t source. To be u s e f u l as a p r a c t i c a l d e v i c e the r a d i a t i v e e f f i c i e n c y must be reasonably high but net n e c e s s a r i l y as l a r g e as t h a t f o r the DC VSA. Comparing F i g u r e s 4-1 and 4 - 3 , n A C i s s u r p r i s i n g l y comparable to n . D C over the e n t i r e c u r r e n t and pressure range c o n s i d e r e d . Hence from an e f f i c i e n c y s t a n d p o i n t the AC arc c e r t a i n l y seems to be u s e f u l as a p r a c t i c a l high i n t e n s i t y l i g h t s o u r c e . The guestion a r i s e s as to why n i s comparable to ri even though the behavior of the AC arc column i s DC q u i t e d i f f e r e n t from t h a t of the DC a r c . The behavior of the AC a r c eclumn can approximately be c l a s s i f i e d by two regimes. Around c u r r e n t zero the r a d i a t i o n from the arc i s q u i t e s m a l l . Since the average w a l l and gas l o a d i n g s c a l e s with P then the i n s t a n t a n e o u s values of Pr, and RAD W 148 P G A S should a l s o s c a l e i n seme manner with PR^D* Hence and P G A s h o u l d be much s m a l l e r than t h e i r averge value i n the neighbourhood of c u r r e n t zero, so t h a t the input power to the arc w i l l t e smal l during t h i s p a r t of the c y c l e . For the r e s t c f the AC c y c l e the arc w i l l be s t r o n g l y r a d i a t i n g . During t h i s time period the ins t a n t a n e o u s behavior of the AC arc should be s i m i l a r t o a DC arc at the same c u r r e n t . I f the assumption i s made t h a t the c o n t r i b u t i o n to Pj^j) and P I N made from the r e g i o n around c u r r e n t zero i s n e g l i g i b l e then the AC r a d i a t i v e e f f i c i e n c y should be comparable t o a DC a r c o p e r a t i n g at the same average c u r r e n t . I f t h i s i s v a l i d we would expect n A C to be comparable to n D C while a t the same time the AC values f o r P and P should be smaller at the IN RAD same average c u r r e n t s i n c e around c u r r e n t zero the i n p u t power t o the AC a r c i s very s m a l l . In f a c t i t was shown p r e v i o u s l y that even though n a r ,£ n n r , when I = 1 the A U U L - RMS DC i n p u t power and r a d i a t i o n l o s s e s from the AC a r c were t y p i c a l l y 40% sm a l l e r than those f o r the DC arc. As shown p r e v i o u s l y n D C I - i n c r e a s e s l i n e a r l y with I while e x t r a p o l a t i n g the AC r e s u l t s n A C reaches a constant value at high c u r r e n t . I t i s c l e a r t h a t n D C can not i n c r e a s e i n d e f i n i t e l y with I . At higher c u r r e n t s (I > 500 A ) i t i s p o s s i b l e t h a t n could be s u b s t a n t i a l l y DC l a r g e r than n A C . However any advantage i n r a d i a t i v e e f f i c i e n c y t h a t might occur f o r the DC arc a t higher c u r r e n t s w i l l be more than o f f s e t by i n c r e a s e s i n the 149 anode heat t r a n s f e r . Heat T r a n s f e r t o the Arc E l e c t r o d e s 5i4-.J. E l e c t r o d e Heat T r a n s f e r S c a l i n g From the r e s u l t s presented i n S e c t i o n 4.3 (F i g u r e s 4-5 - 4-19) i t can be noted that the heat t r a n s f e r t o the a r c e l e c t r o d e s s c a l e s l i n e a r l y with I f o r both AC and DC e l e c t r o d e s under a l l flow c o n d i t i o n s . The c u r r e n t s c a l i n g i n a l l of these graphs i s of the form Q E L - v i + c { 5 . 1 4 ) where Q EL =heat t r a n s f e r t o the e l e c t r o d e V = e f f e c t i v e e l e c t r o d e v o l t a g e drop C=current independent heat t r a n s f e r term These r e s u l t s could a l s o be expressed as Q = (V + + H E L 1 (5-15) = V * ( I ) + H where V , =V' (I) = (V* (C-H) /I) H=current independent heat t r a n s f e r term In t h i s i n t e r p r e t a i o n the e f f e c t i v e v o l t a g e drop V' c o n t a i n s a term t h a t i s i n v e r s e l y p r o p o r t i o n a l to I. Both i n t e r p r e t a t i o n s are c o n s i s t e n t with the experimental r e s u l t s . U n f o r t u n a t e l y from the heat t r a n s f e r measurements i t i s n ' t p o s s i b l e to determine which i n t e r p r e t a t i o n f o r the e f f e c t i v e v o l t a g e drop i s the c o r r e c t one. 150 The l i n e a r r e l a t i o n s h i p between Q E L and I has been reported by other investigators with short (electrode stabilized) DC arcs (Eeider (1956) and Dorodnov ( 1973)) but has not been reported previously f o r vortex s t a b i l i z e d arcs. 5.4.2 Anode Heat Transfer In Section 2.3 the measured anode heat transfer was expressed i n terms of the i n d i v i d u a l heat transfer terms as QA - I- ( * + f K_Te + V A F ) + Q p R - 1^ + Q + Q e = AI_ + B (5-16) where A=coefficient of I =1 (at the anode) B = Q P R - V C C V + Q C O Comparing eguations 5-16 and 5-14 i t i s evident that the e f f e c t i v e voltage drop V A can be i d e n t i f i e d with the co e f f i c i e n t of I_ i n equation (5-14) since the remaining terms w i l l net be s i g n i f i c a n t l y affected by I_. The current dependence of B w i l l now be discussed to substantiate t h i s claim. The radiative heat transfer Q^R to the anode from the arc column and the thermal radiation (L ) emitted R from the anode w i l l of course be dependent on I. However 151 the difference of these terms i s quite small compared to the t o t a l anode heat transfer. Using the arc temperature and radius calculated in Section 5.2 and the anode temperature and arc attachment area, QpR - 1*^ A ** a s keen calculated over the current range examined in t h i s work. UQr| was found to be £ 100 watts at 200 A and 240 watts at 400 A so that AQ i s dependent on current but can be viewed as a constant since i t i s so much smaller than the other heat transfer terms that are e x p l i c i t l y dependent on I_ . The change i n the thermal conduction term 10^, Q ) due to changes i n I over the current range considered should be small. i s the conduction heat transfer due to neutral argon atoms. If thermal eguilibrium exists then the neutral temperature (T o) should follow the electron temperature (T^) with increasing I. In the simple model presented i n Section 5.2 T^  was calculated and increased by less than 10% over the current range considered in these experiments. Hence should e s s e n t i a l l y be constant over the current range considered. The convective heat transfer term i s given by hAg^T^-Tg^) where the variables have been defined i n Section 2.3. For flow towards the anode heat should be transferred from the hot gas of the arc column to the anode while for flow away from the anode i t should be cooled by the cold gas from the i n l e t j e t s . This indicates that would be larger for flow towards than 152 f o r flow away from the anode. The q u e s t i o n a r i s e s as to whether Q i s a cons t a n t term or i s dependent on I . The cv temperature d i f f e r e n c e ( tQ~ t E I j) shouldn't i n c r e a s e much with I as T_ i n c r e a s e s when T _ T i n c r e a s e s . The e f f e c t of I on the co n v e c t i v e heat t r a n s f e r c o e f f i c i e n t (h) i s much more d i f f i c u l t t o p r e d i c t . (h) w i l l be r e l a t e d t o the flow v e l o c i t y (v) through the Reynolds' number. I f v i s a f f e c t e d by I through p r e s s u r e changes i n the flew f o r i n c r e a s i n g I , then i t would be p o s s i b l e f o r h to be dependent on I. A d i r e c t check on the c u r r e n t dependence of Q c v# however, has been performed by examining Q c v as a f u n c t i o n of I f o r d i f f e r e n t gas flow r a t e s . I f Q c v was a f u n c t i o n of I then the e f f e c t i v e voltage drop V should change with gas flow r a t e . In F i g u r e s 4-16 and 4-19 i t was shown t h a t changing the flow r a t e by a f a c t o r of 2.25 had no e f f e c t on V f o r both flow towards and away from the anode. Hence Q c v i s not s i g n i f i c a n t l y a f f e c t e d by the c u r r e n t . In a d d i t i o n i f the change i n V f o r d i f f e r e n t flow d i r e c t i o n s was a t t r i b u t i b l e t o Q_cv then V should change with flew r a t e . Since no measurable changes i n 7 were observed t h i s i n d i c a t e s t h a t Q c v can't be the cause of the d i f f e r e n c e i n V when the flow d i r e c t i o n i s r e v e r s e d . Seme other mechanism must be r e s p o n s i b l e and one strong p o s s i b i l i t y i s changes i n due t o the flew d i r e c t i o n , which w i l l be d i s c u s s e d i n d e t a i l l a t e r i n t h i s c h a p t e r . Since B i n equation (5-16) i s only weakly 153 dependent on I i f a t a l l then the c o e f f i c i e n t of I i s j u s t the measured voltage V and i s given by V A = * + f ^ + V A F ( 5 _ 1 7 ) e I t can be argued that the c o e f f i c i e n t of l _ i s i n f a c t dependent on I through changes i n T g as shown i n S e c t i o n 5.2. only i n c r e a s e s by % 5% when the c u r r e n t i s i n c r e a s e d from 240 t o 480 amps. Since 5/2KT £ 2 .5 v o l t s e and i s much s m a l l e r than (A *V ) then changes i n the T AF c o e f f i c i e n t of I over the c u r r e n t range c o n s i d e r e d are n e g l i g i b l e . Using the experimental r e s u l t s f o r V (Table IV) f o r flow towards and away from the anode, V can be 1 AF determined f o r both c a s e s . T y p i c a l l y <t> =4. 5 v o l t s ( E a z n j e v i c (1975)) and KT £l.O eV as c a l c u l a t e d i n S e c t i o n 5.2. For both flow towards and away from the anode the e l e c t r o n temperature w i l l be assumed to be the same. For flow towards the anode t h i s t y p i c a l l y y i e l d s V ^ £ 3 . 5 v o l t s and f o r flow away V £ 0.5 v o l t s . Hence the changes A F i n Vfl due t o flow d i r e c t i o n can be e x p l a i n e d by changes i n V with flow d i r e c t i o n . A F Other i n v e s t i g a t o r s (Schoek and Eckert (1961)) have found t h a t V i s a f f e c t e d by the a r c flow A F 1 c o n d i t i o n s . They i n v e s t i g a t e d s h o r t argon a r c s i n which the cathode plasma j e t was i n c o n t a c t wirh the anode s u r f a c e . The flow v e l o c i t y i n the j e t r e g i o n was the same 154 o r d e r or l a r g e r than the i o n d r i f t v e l o c i t y so t h a t t h e r e would be a net movement of i o n s towards the anode. T h i s would reduce the n e c e s s i t y f o r i o n p r o d u c t i o n i n the anode f a l l r e g i o n and as a r e s u l t V A F would be reduced. I f t h i s was the case f o r the a r c s c o n s i d e r e d i n t h i s work then V A F would be l a r g e r f o r flow away from the anode than f o r flow towards the anode. Since j u s t the o p p o s i t e i s observed, then any e f f e c t s to the anode heat t r a n s f e r caused by changes i n i o n d r i f t v e l o c i t y due to the net gas flow must be n e g l i g i b l e . In S e c t i o n 5.4.5 a p o s s i b l e e x p l a n a t i o n of the e f f e c t of gas flow d i r e c t i o n on V A p w i l l be presented. The maximum value f o r V ( ^3.5 v o l t s ) AF determined above i s c o n s i d e r a b l y lower than the values found by other i n v e s t i g a t o r s who examined s h o r t low c u r r e n t argon a r c s . Busz-Peuckert and F i n k l e n b u r g (1956) i n v e s t i g a t e d 10mm argon a r c s at 200 A and measured anode vol t a g e drops from 4.5 to 8.5 v o l t s . Schoeck (1961) ob t a i n e d a value of 5.0 v o l t s i n a 6 mm arc a t 150 A C l e a r l y these values f o r V are i n a p p r o p r i a t e f o r a long high c u r r e n t vortex s t a b i l i z e d a r c . More r e c e n t work (Eberhart (1965)) on h i g h i n t e n s i t y a r c s has i n d i c a t e d much lower values f o r V,^ ( £l v o l t ) . 155 5.4.3 Cathode Heat T r a n s f e r In S e c t i o n 2.3 the measured cathode heat t r a n s f e r Q c was expressed i n terms of the i n d i v i d u a l heat t r a n s f e r terms as KTj + a n (U ± - <j>) + V C F ) - I_ e (5-18) + ( Q P R - L R ) + Q cv Qco = B I _ (a+ | KT^ + a n ( U i - $ ) + V C F - I - <J> + B e where B = Q P R - L R + Q C V + Q C O 0 < B < 1 I=I_+I +=(1+ B)I. Q c = WI_ + B = W(l + B ) " 1 I + B where W = B ( a+ y KTj + a n ( U i - <f>) + V C F ) - * (5-19) e E x p e r i m e n t a l l y i t was found under a l l flow c o n d i t i o n s that Q c s c a l e d l i n e a r l y with the c u r r e n t as shown i n equation (5-14). Comparing eguation (5-14) and (5-19) i t i s e v i d e n t t h a t the e f f e c t i v e cathode v o l t a g e drop (V c) can be i d e n t i f i e d with the c o e f f i c i e n t of I i n equation (5-19) s i n c e B should e s s e n t i a l l y be independent of I as was the 156 case f o r the anode. The e f f e c t of Q_cv on 0^ , was determined by examining how the gas flow r a t e a f f e c t e d the 0_c vs I c a r v e s . These r e s u l t s were presented i n S e c t i o n 4.3 (Fi g u r e s 4-17 and 4-18). For flow towards the cathode Q^, as a f u n c t i o n of I i s independent of gas flow r a t e . For flow away from the cathode i n c r e a s e s with flow r a t e (Figure 4-20). Not only does the c u r r e n t independent p a r t of i n c r e a s e with flow r a t e but the e f f e c t i v e cathode v o l t a g e drop a l s o i n c r e a s e s . T h i s i s j u s t the o p p o s i t e of what would be expected i f t h i s i n c r e a s e i n was due t o a c o n v e c t i v e heat t r a n s f e r e f f e c t . For flow away from the cathode, c c l d gas passes by the cathode b e f o r e i t e n t e r s the arc chamber so t h a t i n c r e a s i n g the flow r a t e should decrease Q_c r a t h e r than i n c r e a s e i t . Hence the i n c r e a s e i n f o r flow away from the cathode can not be e x p l a i n e d by a c o n v e c t i v e heat t r a n s f e r process. S i n c e V c d i d n ' t i n c r e a s e f o r flow towards the cathode with i n c r e a s i n g flow r a t e and the i n c r e a s e i n V c f o r flow away from the cathode can not be caused by a c o n v e c t i v e p r o c e s s , then we can conclude t h a t C c v must be e s s e n t i a l l y c o n s t a n t independent of I. The e f f e c t of gas flow r a t e on Q a l s o i n d i c a t e s t h a t the d i f f e r e n c e observed i n V when G C the flow d i r e c t i o n i s r e v e r s e d can not be due t o the c o n v e c t i v e heat t r a n s f e r term. Since B i n eguation (5-19) i s only weakly dependent on I i f a t a l l , then the c o e f f i c i e n t of I i s 157 j u s t the measured cathode v o l t a g e drop and i s given by V c = (1 + B) 1 {[ a + y K T i + a n ( U ± -<),)+ V C F ] 6 - *] (5 - 2C ) e Again i t can be argued that the c o e f f i c i e n t of I i s i n f a c t dependent on I through changes i n T^ with I . 1 would be expected t o f o l l o w T^ which i n c r e a s e s by only % 5% over the cu r r e n t range c o n s i d e r e d , so t h a t the term a (5/2JKT. i s e s s e n t i a l l y constant, U n f o r t u n a t e l y V c can not be determined from eguation (5-20) s i n c e B , a + and a.^ are unknown. However we b e l i e v e t h a t the change i n when the flow d i r e c t i o n i s r e v e r s e d i s due to changes i n when the gas flow d i r e c t i o n i s r e v e r s e d . In S e c t i o n 5.4.5 a p o s s i b l e e x p l a n a t i o n f o r the changes i n V and when the flow d i r e c t i o n i s reversed w i l l be presented. 5.4.4 Model of AC E l e c t r o d e Heat Tr a n s f e r In t h i s s e c t i o n a simple model of the heat t r a n s f e r to the e l e c t r o d e s of the AC a r c i s considered. I t i s assumed t h a t the AC e l e c t r o d e s behave e s s e n t i a l l y l i k e a cathode f o r h a l f a c y c l e and an anode f o r the other h a l f . From the DC e l e c t r o d e heat t r a n s f e r r e s u l t s presented i n S e c t i o n 4-3 we have the f o l l o w i n g r e l a t i o n s 158 between Q E L and I QCt = V + Q 2 (5-21) f o r flow towards the e l e t r o d e s . e l e c t r o d e s we have ^Aa = V A a J + ^3 For flow away from t h e (5-22) where Q A t=anode heat t r a n s f e r (flow towards anode) Q =anode heat t r a n s f e r (flow away from the A a anode) Q =cathode heat t r a n s f e r (flow towards the c t cathode) Q =cathode heat t r a n s f e r (flow away from C a the cathode) V s = e f f e c t i v e v o l t a g e drop ( n o t a t i o n the same as f o r the Q's) Now i t i s assumed t h a t the heat t r a n s f e r to the AC e l e c t r o d e s can be con s i d e r e d as a s u p e r p o s i t i o n of the DC r e s u l t s . With t h i s assumption the heat t r a n s f e r Q to the AC e l e c t r o d e s w i l l be given by % = f w tf V I dt + ; V I dt] + [*L±3l t 2TT & At o c t  K j i Q, 0) rill = 27 [ ' Aa I dt + V ca I dt] + ( Q3 + Qk •) (5-23) 159 where Q =AC e l e c t r o d e heat t r a n s f e r {flow away a from the e l e c t r o d e to =AC e l e c t r o d e heat t r a n s f e r (flow towards) t=time oj=freguency of AC waveform The c u r r e n t waveforms f o r the AC arc are n e a r l y s i n u s o i d a l f o r I >_ 300 A (Befer to S e c t i o n 3.2) so i n t h i s simple model the AC c u r r e n t waveform w i l l be assumed p e r f e c t l y s i n u s o i d a l ( i e I = I o s i n w t ) . The heat t r a n s f e r to the AC e l e c t r o d e s i s then given by V A t + V C t ) 2 x 2 '* ° (5-24) The average c u r r e n t f o r h a l f the AC c y c l e i s given by I = ( 2 / 7 r ) I Q so t h a t i n eguation (5-24) the heat t r a n s f e r to the AC e l e c t r o d e s can be c a l c u l a t e d using the experimental r e s u l t s f o r the DC a r c . In F i g u r e s 5-13 and 5-14 Q i s p l o t t e d as a f u n c t i o n of I /fT based on equa t i o n (5-24) and i s compared with the experimental r e s u l t s . The agreement i s q u i t e good f o r the e f f e c t i v e v o l t a g e drop (V) f o r both flow towards and away from the e l e c t r o d e , while the agreement i s not n e a r l y as good f o r the c o n s t a n t , c u r r e n t independent terms ( i n t e r c e p t s ) of Q . The experimental and p r e d i c t e d r e s u l t s are summarized 1 6 0 F i g u r e 5-13: P r e d i c t e d AC E l e c t r o d e Heat T r a n s f e r v s . I 0A/2 161 162 i n Table V I I I . I t should be noted that the DC a r c was operated with a f l a t c y l i n d r i c a l e l e c t r o d e t i p while the o AC e l e c t r o d e r e s u l t s are f o r 45 c o n i c a l t i p s . The con s t a n t terms (ie Q1 ,Q 2,Q 3 r ) i n equation ( 5 - 2 1) and ( 5 - 2 2 ) are probably dependent cn e l e c t r o d e geometry. T h i s might e x p l a i n the r e l a t i v e l y poor agreement between the measured and p r e d i c t e d constant terms i n Q. Some of the measured AC heat t r a n s f e r (Q ) vs I graphs had constant terms t h a t were n e g a t i v e . These r e s u l t s mean th a t there i s a net l o s s of heat from the AC e l e c t r o d e due to the c u r r e n t independent terms. These ne g a t i v e terms occurred only f o r flow towards the e l e c t r o d e , so that the constant terms i n Q should always be p o s i t i v e . Since these negative i n t e r c e p t s were not encountered with the DC e l e c t r o d e s then the simple model used to p r e d i c t the AC e l e c t r o d e heat t r a n s f e r i s inadequate to p r e d i c t these r e s u l t s . C l e a r l y the constant terms i n the AC e l e c t r o d e heat t r a n s f e r must be a f f e c t e d by the time dependence of the AC c u r r e n t . Jj. 4_. 5 A P o s s i b l e E x p i a t i o n of the Flow. Dependence of the E l e c t r o d e Heat T r a n s f e r In what f o l l o w s we present a p o s s i b l e e x p l a n a t i o n of the e l e c t r o d e heat t r a n s f e r r e s u l t s . In p a r t i c u l a r a p o s s i b l e e x p l a n a t i o n of the d i f f e r e n c e s i n Table Viii<: Values of the AC Electrode Voltage Drop (V) and Current Independent Heat Transfer Terra (C) Predicted V C Flow Towards 6.95 0.3 Experiment V C 6.90 -0.05 Flow Away 5.0 1.25 5.45 0.66 164 the e f f e c t i v e v o l t a g e drop ( V ) 1 when the flow d i r e c t i o n i s r e v e r s e d w i l l be given f o r both the anode and cathode as w e l l as the heat t r a n s f e r r e s u l t s f o r d i f f e r e n t flow r a t e s . Observations of the ancde r e g i o n u s i n g the e l e c t r o d e imaging system d e s c r i b e d i n S e c t i o n 3 . 4 . 3 have shown t h a t f o r flow towards the anode, arc plasma was observed down the s i d e of the e l e c t r o d e while f o r flow away from the anode t h i s i s l e s s n o t i c e a b l e ( r e f e r ro F i g u r e 5-15). C o n s i d e r i n g the d i f f e r e n c e s i n c u r r e n t d e n s i t y d i s t r i b u t i o n t h a t must e x i s t between these two cases, i t seems d i f f i c u l t t o b e l i e v e t h a t the anode f a l l p o t e n t i a l would be the same f o r both c a s e s . He b e l i e v e the d i f f e r e n c e i n V when the flow d i r e c t i o n i s reversed A i s the r e s u l t of changes i n when the f l o w d i r e c t i o n i s r e v e r s e d . The plasma temperature would be expected to be higher when the arc attachment r e g i o n i s s m a l l e r (flow away) due to the higher c u r r e n t d e n s i t i e s i n v o l v e d . The anode f a l l p o t e n t i a l drop (V ) f o r the a r c used i n t h i s work r e s u l t s p r i m a r i l y from two f a c t o r s . P a r t of \? r 1 AF r e s u l t s from the need t o produce a d d i t i o n a l charge c a r r i e r s near the anode to maintain c u r r e n t c o n t i n u i t y . The anode used i n t h i s work a c t s as an a d d i t i o n a l heat s i n k . Near the anode the plasma temperature decreases l o c a l l y r e s u l t i n g i n an i n c r e a s e i n the e l e c t r i c f i e l d . 1 The e f f e c t i v e e l e c t r o d e v o l t a g e drop i s j u s t the slope of the heat t r a n s f e r vs c u r r e n t graph. 165 A N O D E FLOW TOWARDS A N O D E FLOW AWAY L A R C PLASMA F i g u r e 5-15: A r c Column Anode Attachment Region 166 T h i s i n c r e a s e i n the e l e c t r i c f i e l d due to the l o c a l r e d u c t i o n i n the plasma c o n d u c t i v i t y i s another c o n t r i b u t i o n to V A f , as pointed out by Hoyaux (1968). For low i n t e n s i t y ( ie low current) DC argon a r c s Busz-Peuckert and F i n k l e n b u r g (1956) have found t h a t V . ^ i s t y p i c a l l y A r 4.5-8.5 v o l t s . For high c u r r e n t a r c s (Eberhart (1966)) i s much s m a l l e r (^  1 v o l t ) . The plasma temperature near the anode of a high i n t e n s i t y a r c l i k e t h a t used by Eber h a r t should be l a r g e r than t h a t of a low i n t e n s i t y a r c (Busz-Peuckert e t al) so th a t V „ decreases with ' AF i n c r e a s i n g plasma temperature near the anode s u r f a c e . As pointed out before the a r c plasma s t r u c t u r e at the anode i s much d i f f e r e n t f o r flow towards and away from the anode. For flow away the temperature of the plasma near the anode should be higher than f o r flow towards the anode. Hence the two c o n t r i b u t i o n s to IT AF w i l l be reduced f o r hi g h e r temperature and V,„ should be 3 AF s m a l l e r f o r flow away than f o r flow towards the anode. When the flow r a t e i s i n c r e a s e d f o r flow towards the anode, Q A was u n a f f e c t e d . Since the l e n g t h of the arc plasma down the s i d e of the anode i s l i m i t e d by the gas e x i t assembly, once a c e r t a i n l i m i t i n g flow i s reached f u r t h e r i n c r e a s e w i l l have no e f f e c t on the arc attachment r e g i o n . Hence f o r l a r g e enough flow r a t e s towards the anode v,_ remains co n s t a n t . AF When the flow r a t e i s i n c r e a s e d f o r flow away from the anode, Q R i n c r e a s e s s l i g h t l y but i s unchanged. 167 As remarked p r e v i o u s l y f o r flow i n t h i s d i r e c t i o n the arc plasma only extends a few m i l l i m e t r e s down the s i d e of the anode. Flow r a t e i n c r e a s e s w i l l probably cause t h i s r e g i o n t o decrease s l i g h t l y which could cause i n c r e a s e s i n the c o n s t a n t (current independent) terms, but i t i s c l e a r from the r e s u l t s t h a t no s i g n i f i c a n t change i n V occurs. A F Hence i t seems t h a t the arc attachment r e g i o n on the f l a t anode s u r f a c e i s e s s e n t i a l l y screened from i n c r e a s e s i n the flow v e l o c i t y . As presented i n S e c t i o n 4 . 3 the e f f e c t i v e cathode voltage drop V c i s l a r g e r f o r flow towards the cathode than f o r flow away. For both cases an i n t e n s e plasma j e t extends from the t i p of the cathode i n t o the arc column f o r roughly 10 mm (Refer to S e c t i o n 4 . 5 ) . We b e l i e v e the i n t e r a c t i o n between the flow f i e l d and t h i s j e t i s r e s p o n s i b l e f o r the change i n V . For flow towards C the cathode a low l u m i n o s i t y region ( r e l a t i v e to the cathode j e t l u m i n o s i t y ) was observed over the e n t i r e s u r f a c e of the 4 5 c o n i c a l cathode t i p . For flow away from the cathode a low l u m i n o s i t y r e g i o n over the e n t i r e cathode s u r f a c e d i d not e x i s t , probably due to the c o l d gas p a s s i n g by the e l e c t r o d e t i p . The g e n e r a l shape of the two e l e c t r o d e r e g i o n s f o r d i f f e r e n t flow d i r e c t i o n s i s i l l u s t r a t e d i n F i g u r e 5-16. For flow towards the cathode the a r c i s much l e s s c o n s t r i c t e d than f o r flow away from i t . The cathode f a l l p o t e n t i a l f o r these two cases c o u l d be g r e a t l y i n f l u e n c e d by the d i f f e r e n t plasma s t r u c t u r e s 168 CATHODE FLOW TOWARDS CATHODE FLOW AWAY •-ARC PLASMA L C A T H O D E J E T F i g u r e 5-16 : A r c Column Cathode Attachment Region 169 near the cathode. i e b e l i e v e the d i f f e r e n t plasma s t r u c t u r e and probably c o n d i t i o n s (temperature and dens i t y ) are r e s p o n s i b l e f o r the d i f f e r e n c e s i n V c when the flow d i r e c t i o n i s r e v e r s e d . In F i g u r e 4-18 i t was shown that f o r flow towards the cathode, Q was independent of flow r a t e , a s i m i l a r r e s u l t t o t h a t o b t a i n e d f o r flew towards the anode. The s i z e of the low l u m i n o s i t y plasma r e g i o n d i d not seem to vary much over the flow range c o n s i d e r e d . As s t a t e d b e f o r e we b e l i e v e V C F * S dependent on the plasma s t r u c t u r e at the cathode. Since the observed s t r u c t u r e was not s e v e r e l y a f f e c t e d by the flow r a t e one would expect V t o be e f f e c t i v e l y independent of flow r a t e . For flow away from the cathode the heat t r a n s f e r was s i g n i f i c a n t l y a f f e c t e d by i n c r e a s i n g the gas flow r a t e (see F i g u r e 4-17). Not only was the i n t e r c e p t i n the graph changed but the e f f e c t i v e cathode voltage drop i n c r e a s e d as w e l l , with V c i n c r e a s i n g by £ 35% when the flow r a t e i s i n c r e a s e d from 0.48 to 1.2 l i t r e s / s e c o n d . We b e l i e v e these e f f e c t s are due to the i n j e c t i o n of c o l d argon i n t o the cathode attachment r e g i o n which c o n s i s t s of the i n t e n s e plasma j e t p l u s a l e s s luminous r e g i o n surrounding i t . I n c r e a s i n g the i n j e c t i o n r a t e (flow rate) of c o l d gas w i l l r e s u l t i n l o c a l c o o l i n g of t h i s r e g i o n , p a r t i c u l a r l y at the i n t e r f a c e between the c o o l incoming gas and the arc plasma. Be b e l i e v e t h a t the i n c r e a s e i n V„ with i n c r e a s i n g flow r e s u l t s from the i n c r e a s e d 170 i n j e c t i o n r a t e of c o l d gas i n t o the arc attachment r e g i o n . U n f o r t u n a t e l y d e t a i l s o f the a r c s t r u c t u r e i n t h i s region which are necessary to understand these r e s u l t s f u l l y , are not known. The e f f e c t o f the flow r a t e on the heat t r a n s f e r f o r the AC e l e c t o d e s should e s s e n t i a l l y be the s u p e r p o s t i o n of the DC r e s u l t s . For flow towards e i t h e r the anode or cathode was independent of flow r a t e . Hence f o r flow towards an AC e l e c t r o d e the heat t r a n s f e r would be expected t o be independent of flow r a t e . As shown i n F i g u r e 4-15 t h i s was e s s e n t i a l l y what o c c u r r e d . In the DC a r c , f o r flow away from the cathode V c i n c r e a s e s with flow r a t e , while f o r flow away from the anode the s l o p e i s u n a f f e c t e d by flow r a t e . Hence f o r flow away from an AC e l e c t r o d e 2L_ would be expected t o show some i n c r e a s e i n V with flow r a t e . The i n c r e a s e i n ^  shouldn't be as l a r g e as those observed f o r the cathode because each AC e l e c t r o d e i s a cathode f o r onl y one h a l f of the c u r r e n t c y c l e . As shown p r e v i o u s l y i n F i g u r e 4-14 V f o r flow EL away from an AC e l e c t r o d e i n c r e a s e s s l i g h t l y with flow r a t e ( £10% i n c r e a s e when the flow r a t e i s i n c r e a s e d from 0.48 t o 1.2 l i t r e s / s e c ) . The constant p a r t of ^ a l s o EL i n c r e a s e s with flow r a t e as would be expected i f the AC e l e c t r o d e heat t r a n s f e r i s e s s e n t i a l l y a sup e r p o s t i o n of the EC r e s u l t s . 171 5.4.6 AC E l e c t r o d e Heat T r a n s f e r : P r a c t i c a l C o n s i d e r a t i o n s From the viewpoint of a p r a c t i c a l arc l i g h t source the r e s u l t s presented i n Chapter 4 have c o n s i d e r a b l e s i g n i f i c a n c e . The l i f e t i m e of high i n t e n s i t y v o r t ex s t a b i l i z e d a r c s can be l i m i t e d by c a t a s t r o p h i c anode f a i l u r e (Camm (1977)). I t has been shown i n the r e s u l t s presented i n Chapter 4 t h a t the AC e l e c t r o d e heat t r a n s f e r was up to 50% lower than t h a t f o r a DC anode o p e r a t i n g a t the same a r c c o n d i t i o n s . Hence the AC e l e c t r o d e s should be much more durable. The r e d u c t i o n i n ft f o r the AC e l e c t r o d e s a l l o w s t h i c k e r e l e c t r o d e t i p s to E L be used. For high i n t e n s i t y DC arcs s i m i l a r to those of Vortek I n d u s t r i e s (100kW i n p u t power, £ 10 kW anode loading) the e l e c t r o d e t i p must be very t h i n t o avoid l o c a l m e l t i n g . T h i s r e s u l t s i n a l a r g e temperature g r a d i e n t and the accompanying high thermo-mechanical s t r e s s . I t i s b e l i e v e d t h a t t h i s high s t r e s s i s p a r t l y to blame f o r anode f a i l u r e . Hence by using an AC arc i t i s p o s s i b l e t o reduce the e l e c t r o d e heat t r a n s f e r and thermo-mechanical s t r e s s which occur i n the anode of high i n t e n s i t y DC a r c s . 172 5 i 5 E l e c t r o d e Surface Temperature Measurements As presented i n S e c t i o n 4.4.2 the AC e l e c t r o d e s u r f a c e temperature f o r both e l e c t r o d e t i p geometries c o n s i d e r e d was constant throughout the AC c y c l e . An e x p l a n a t i o n of these r e s u l t s can be found from the temperature decay curves a f t e r c u r r e n t i n t e r r u p t i o n ( F i g u r e s 4-25 and 4-26). For both e l e c t r o d e s the s u r f a c e temperature decayed e x p o n e n t i a l l y with time. The thermal decay time constants f o r the f l a t and c o n i c a l e l e c t r o d e t i p s were £ 600 and 80-100 ms r e s p e c t i v e l y , g r e a t l y exceeding the p e r i o d of the a p p l i e d AC waveform (16.6 ms . Hence the e l e c t r o d e s , due to t h e i r long thermal time c o n s t a n t s , can not r e a c t f a s t enough to f o l l o w the a p p l i e d AC waveform. I t must be noted t h a t the r e g i o n c o n s i d e r e d i n these temperature measurements was a 2 mm diameter zone of the a r c attachment area. The main attachment r e g i o n f o r the c o n i c a l e l e c t r o d e c o n s i s t e d of a c i r c u l a r r e g i o n a t the e l e c t r o d e t i p with a diameter of approximately 2-3 mm. The cathode j e t always s t a r t e d a t the a r c attachment r e g i o n and extended f o r n e a r l y 10 mm i n t o the a r c plasma. The arc attachment r e g i o n f o r the f l a t AC e l e c t r o d e d i f f e r e d c o n s i d e r a b l y from t h a t which occurs on the c o n i c a l e l e c t r o d e . When the e l e c t r o d e was a cathode the j e t always s t a r t e d from the edge of the e l e c t r o d e t i p . The f l a t s u r f a c e of t h i s e l e c t r o d e was always i n c o n t a c t with 173 the a r c column. However the plasma near the s u r f a c e of the e l e c t r o d e was much l e s s luminous than the cathode j e t emanating from the edge of the e l e c t r o d e t i p . For the c o n i c a l e l e c t r o d e t i p hot s p o t s of extremely s m a l l s i z e were observed ( S e c t i o n 4.5) w i t h i n the a r c attachment r e g i o n t h a t had time v a r y i n g l u m i n o s i t i e s . I f the l u m i n o s i t y of these e l e c t r o d e r e g i o n s v a r i e d throughout the AC c y c l e then i t i s c l e a r t h a t t h e i r temperature must a l s o vary with time. The d e t e c t i o n system used t o measure the s u r f a c e temperature however, d i d not i n d i c a t e the presence of these hot spots. The e l e c t r o d e t i p s were examined a f t e r running the a r c and very s m a l l r e g i o n s of l o c a l melting were observed i n the a r c attachment r e g i o n , which undoubtedly corresponded t o the high l u m i n o s i t y r e g i o n s observed i n the e l e c t r o d e attachment r e g i o n . The s i z e of these melted s p o t s was q u i t e s m a l l . They were t y p i c a l l y s p h e r i c a l i n shape with a diameter of ^  0.2-0.4 mm. The temperature of the a r c attachment r e g i o n was t y p i c a l l y found t o be £ 2600 K throughout the AC c y c l e , much below t h a t o f the s m a l l hot spots which were at l e a s t a t t h e . m e l t i n g p o i n t of tungsten (3653 K). Although these hot s p o t s were much more luminous than the r e s t of the e l e c t r o d e when the a r c was running they decayed very r a p i d l y a f t e r c u r r e n t i n t e r r u p t i o n . The temperature measuring system begins measurements 10 ms a f t e r c u r r e n t i n t e r r u p t i o n and by t h i s time the e l e c t r o d e hot spots had decayed t o the 174 temperature of the r e s t of the e l e c t r o d e t i p . The f o l l o w i n g simple model shows, i n f a c t , that the decay time o f these hot spots would not exceed % 1 ms. The geometry used i n t h i s c a l c u l a t i o n i s shown below. Figure 5-17: Electrode Hot Spot Geometry The s m a l l hot spot i s assumed to be s p h e r i c a l i n shape and at the melting temperature. The i n f l u e n c e of the hot spot extends out to a r a d i u s r which i s at the average o e l e c t r o d e temperature i n the a r c attachment r e g i o n . At some i n s t a n t during the AC c y c l e the h e a t i n g mechanism r e s p o n s i b l e f o r the hot spot stops. I t i s assumed t h a t the hot spot c o o l s predominantly by thermal conduction to the r e s t of the e l e c t r o d e . The d i f f e r e n c e i n heat content per u n i t volume between the hot spot and the arc attachment r e g i o n i s g i v e n by P C (T^-T ) . . I f the heat t r a n s f e r r a t e can be estimated then an order of magnitude f o r the time constant can be determined. For steady s t a t e heat t r a n s f e r i n s p h e r i c a l geometry the heat t r a n s f e r r a t e 175 i s given approximately by = 2TT K ( f ) ( T j - Tp)  q s 1 1 r~. " r ~ (5-25) 1 o where T*= (T^ *-TQ) /2 T^=temperature of hot spot T Q =temperature of e l e c t r o d e arc attachment r e g i o n f o r the geometry shown i n F i g u r e 5-20. I f the average heat t r a n s f e r r a t e f o r the c o o l i n g o f the hot spot i s given approximately by (1/2)g s then the time r e g u i r e d f o r the hot spot t o c o o l down to T o i s given by % P CP VHs(*l + 0 TTk(T) (5-26) where V =volume of the hot spot An upper l i m i t f o r t H S can be found by l e t t i n g r Q with r i equal t o the hot spot r a d i u s ( 0.2 mm). Using a p p r o p r i a t e values f o r C p and K(T) (fiaz n j e v i c (1975)) we f i n d an upper bound f o r t H g . t 'v —3 HS ^ 10 seconds « T A C Where T A C = p e r i o d of the AC waveform=16.6 ms This r e s u l t i s c o n s i s t e n t with the e x p l a n a t i o n 176 g i v e n above that the hot s p o t s c o c l b e f o r e they can be observed by the temperature measuring system. The s u r f a c e temperature measurements and the o b s e r v a t i o n s of the e l e c t r o d e t i p r e g i o n (Section 4.5) show t h a t the AC e l e c t r o d e attachment r e g i o n f o r the c o n i c a l e l e c t r o d e t i p s c o n s i s t s of a r e g i o n whose temperature i s time independent with time v a r y i n g hot spots superimposed on i t . The time v a r y i n g hot s p o t s were predominantly observed on the cathode h a l f of the c y c l e . Me b e l i e v e t h a t these hot spots are cathode sp o t s which ar e r e q u i r e d to produce s u f f i c i e n t e l e c t r o n s by some emi s s i o n process predominantly t h e r m i o n i c , to maintain the a r c column. The average temperature on the f r o n t s u r f a c e of the f l a t e l e c t r o d e was found t o be e s s e n t i a l l y c o n s t a n t , independent of p o s i t i o n . O c c a s i o n a l l y much higher temperatures were observed on t h i s f l a t e l e c t r o d e near the edge of the t i p . As w e l l as having h i g h e r temperatures than the f r o n t s u r f a c e o f the e l e c t r o d e , the decay time c o n s t a n t was much s h o r t e r as was shown i n F i g u r e 4-25. He b e l i e v e t h a t these higher temperatures corresponded to the main attachment r e g i o n l o c a t e d a t the edge of the e l e c t r o d e . T h i s r e g i o n moved from run to run as evidenced by a s m a l l molten r e g i o n around the edge of the e l e c t r o d e t i p , so t h a t i t would not be observed on every run. This main attachment r e g i o n was b e l i e v e d to c o n s i s t of small hot spots whose temperature v a r i e d with time, superimposed 177 on a l a r g e r r e g i o n t h a t was e s s e n t i a l l y at constant temperature. The area of the main attachment r e g i o n was on l y a s m a l l f r a c t i o n of the t o t a l area of the e l e c t r o d e s u r f a c e i n c o n t a c t with the a r c . Since the temperature of the ma j o r i t y of the e l e c t r o d e was constant then the heat t r a n s f e r would be expected t c be of a one dimensional nature f o r t h i s simple e l e c t r o d e geometry. A one dimensional heat t r a n s f e r c a l c u l a t i o n i s now presented to model the temperature decay f o r the f l a t e l e c t r o d e f o l l o w i n g c u r r e n t i n t e r r u p t i o n . The r e s u l t s show t h a t the observed thermal decay can be well e x p l a i n e d by the proposed heat t r a n s f e r model. The geometry f o r t h i s c a l c u l a t i o n i s shown below. COOUNG WATER -y Electrode Tip (Tungsten) Q i — > x=0 x=l Figure 5-18: Electrode T i p Geometry 178 In a d d i t i o n t o the assumption of one-dimensional heat t r a n s f e r two f u r t h e r assumptions have been used i n t h i s c a l c u l a t i o n . When the ar c i s o p e r a t i n g a l i n e a r temperature p r o f i l e e x i s t s i n the e l e c t r o d e which i m p l i e s t h a t the thermal. c o n d u c t i v i t y i s independent of temperature. The thermal c o n d u c t i v i t y f o r tungsten, however, v a r i e s with temperature but t h i s doesn't cause l a r g e departures from a l i n e a r p r o f i l e as shown i n F i g u r e 5-19, where the temperature p r o f i l e f o r a tunsten rod has been c a l c u l a t e d f o r one-dimensional thermal conduction u s i n g the temperature dependent thermal c o n d u c t i v i t y f o r tungsten. The s o l u t i o n t o t h i s one-dimensional t r a n s i e n t heat t r a n s f e r problem i s f u r t h e r assumed t o be s e p a r a b l e i n the s p a t i a l and temporal c o o r d i n a t e s . At time t=0 the a r c c u r r e n t i s shut o f f and decays t o zero. Before c u r r e n t zero t h e r e i s a steady s t a t e l i n e a r temperature p r o f i l e i n the e l e c t r o d e t i p . A f t e r c u r r e n t zero the temperature i s determined by the one dimensional t r a n s i e n t heat t r a n s f e r equation given below. aU(x,t) = o 3 ZU(x,t) 3t 3: (5-27) where 0 (x,t)=X (x) T (t) =temperature ct=k/p Cp=thermal d i f f u s i v i t y 4.0 DISTANCE (mm) E i g u r e 5 -19 : E l e c t r o d e Temperature v s . D i s t a n c e (Input Power=5 kWatts) 180 The water c o o l e d end of the e l e c t r o d e t i p i s maintained at constant temperature T^ and the boundary and i n i t i a l c o n d i t i o n s are given below. U(o,t) = T x u(a,o) = T 2 U(x,o) = C(x) - T x + (^) X (5-28) g g ^ " U ( £ , t ) = 0 t > 0 To s i m p l i f y the a n a l y s i s the problem i s s o l v e d f o r the temperature d i f f e r e n c e <)> (x,t) =U [ x , t ) - T 1 = X (x) T ( t ) . A f t e r s u b s t i t u t i o n i n the t r a n s i e n t conduction equation and s e p a r a t i o n of v a r i a b l e s the f o l l o w i n g separate e q u a t i o n s f o r the s p a t i a l and temporal p o r t i o n s of the s o l u t i o n are obtained. Y 2 X " + ~ x = 0 (5-29a) t' + Y2T = 0 1 (5-29b) where YZ = constant Osing the boundary c o n d i t i o n s (5-28) the s o l u t i o n of (5-29a) i s obtained and i s given below. 181 X(x) = f Aj| Sin (n + ) — n- o n 2 £ From these r e s u l t s the e i g e n v a l u e s of the s o l u t i o n are g i v e n by rcn+±) f = x 2 < 5 - 3 1 ) Using these r e s u l t s a l l o w s the s o l u t i o n of (5-29b) which i s given by n=o The complete s o l u t i o n i s then g i v e n by (5-32) 0(x,t) = U(x,t) - T =nS0 (L 5_l *L_ „ o ( - D n Sin (n + % ) i r x w 2L T 2 [l KA] (n+ls)2 TT 2 ^ . e - a[(n + h) j V t (5-33) 182 I t i s c l e a r from (5-33) t h a t each F o u r i e r component of the temperature decays e x p o n e n t i a l l y with time. The higher order terms i n the temperature decay more r a p i d l y and hence the e f f e c t i v e decay c o n s t a n t Y w i l l be given by the f i r s t order term namely -1 Y = k pC. (4 (5-34) How the thermal c o n d u c t i v i t y and s p e c i f i c heat are both temperature dependent, the s u r f a c e temperature i s f a i r l y w e l l known and the time constant w i l l be determined f o r both the i n i t i a l temperature and 500 K below t h i s value. The values of k and C P were obtained from B a z n j e y i c (1975) while the measured l e n g t h of the e l e c t r o d e t i p was 9.1 mm. Using (5-34) t h i s y i e l d s Y =700 msec f o r T=2500°Cand Y=800 ms f o r T=2000 ° C . The measured value f o r Y was t y p i c a l l y 600 msec. S i n c e t h e r e i s good agreement between the measured decay constant and the c a l c u l a t e d c onstant then the heat t r a n s f e r i n the e l e c t r o d e t i p i s e s s e n t i a l l y one-dimensional i n nature. Another check of the one-dimensional nature of the heat t r a n s f e r i n the f l a t e l e c t r o d e i s provided by the c a l o r i m e t r i c a l l y determined heat t r a n s f e r and the e l e c t r o d e s u r f a c e temperature measurements. The heat t r a n s f e r t o the e l e c t r o d e was determined using the measured s u r f a c e temperature and the temperature dependent thermal c o n d u c t i v i t y . The c a l c u l a t e d heat t r a n s f e r was t y p i c a l l y 10% lower than the 183 measured heat t r a n s f e r . T h i s c a l c u l a t i o n ignores the heat t r a n s f e r from the hot spots as well as any heat t r a n s f e r from the s i d e of the e l e c t r o d e t i p . As remarked p r e v i o u s l y a r c plasma was observed down the s i d e of the e l e c t r o d e t i p , p a r t i c u l a r l y f o r gas flow towards the e l e c t r o d e . Since the c a l c u l a t e d heat t r a n s f e r (assuming no heat i n p u t to the s i d e of the e l e c t r o d e ) was always lower than the measured value then t h i s confirms that t h e r e was heat t r a n s f e r to the s i d e of the e l e c t r o d e t i p . However t h i s heat t r a n s f e r to the s i d e of the e l e c t r o d e was l e s s than 10% of the t o t a l heat t r a n s f e r so t h a t the heat t r a n s f e r to the f l a t e l e c t r o d e i s e s s e n t i a l l y one-d i m e n s i o n a l . As presented i n S e c t i o n 4.4.3, the s u r f a c e temperature f o r the 45 ° c o n i c a l e l e c t r o d e t i p under AC o p e r a t i o n was determined as a f u n c t i o n of c u r r e n t and p r e s s u r e . The attachment r e g i o n f o r t h i s e l e c t r o d e t i p geometry was always l o c a l i z e d on the t i p , u n l i k e the f l a t e l e c t r o d e where i t c o u l d be anywhere. For t h i s reason the dependence of s u r f a c e temperature on c u r r e n t and pressure was only examined f o r the 45 " c o n i c a l geometry. As shown i n F i g u r e 4-26 the s u r f a c e temperature of the 2 mm diameter r e g i o n examined was found to be l i n e a r l y r e l a t e d to the a r c c u r r e n t . P r e v i o u s l y i t has been shown from the c a l o r i m e t r i c measurements of the e l e c t r o d e heat t r a n s f e r t h a t the heat t r a n s f e r was l i n e a r l y r e l a t e d to the c u r r e n t . Combining these r e s u l t s , the heat t r a n s f e r i s 184 l i n e a r l y r e l a t e d to the e l e c t r o d e s u r f a c e temperature. T h i s i m p l i e s t h a t the heat t r a n s f e r f o r the c o n i c a l e l e c t r o d e i s e s s e n t i a l l y one-dimensional. As shown i n F i g u r e 4-27 the s u r f a c e temperature i s e s s e n t i a l l y constant with pressure up to about 3 atm. A f t e r t h i s p r essure the s u r f a c e temperature i n c r e a s e s very r a p i d l y . The heat t r a n s f e r to the e l e c t r o d e only i n c r e a s e d a s m a l l amount when the pressure was i n c r e a s e d so t h a t the r a p i d i n c r e a s e i n s u r f a c e temperature above 3 atm i s q u i t e unexpected and can not be e x p l a i n e d by a sharp i n c r e a s e i n heat t r a n s f e r . We s p e c u l a t e t h a t at pressures above 3 atm t h e r e i s c o n s t r i c t i o n of the arc attachment r e g i o n which r e s u l t s i n higher temperatures but does not s u b s t a n t i a l l y i n c r e a s e the o v e r a l l heat t r a n s f e r . As shown i n F i g u r e s 4-29 and 4-30 the s u r f a c e temperature of a c o n i c a l 45 anode was determined as a f u n c t i o n of c u r r e n t and pressure t o provide a d i r e c t comparison with the AC r e s u l t s . At high c u r r e n t the anode t i p tended t o melt and c o u l d not be used f o r the long running time r e q u i r e d f o r heat t r a n s f e r measurements. However a temperature measurement c o u l d be made a f t e r £ 30 seconds of running time without c a t a s t r o p h i c anode f a i l u r e . The s u r f a c e temperature f o r the anode was again found t o be l i n e a r l y r e l a t e d t o the c u r r e n t , i n d i c a t i n g l i n e a r heat t r a n s f e r f o r the e l e c t r o d e t i p . The s l o p e of the l i n e a r r e g i o n of F i g u r e 4-29 i s l a r g e r than the slope of the AC e l e c t r o d e s u r f a c e temperature vs I graph 185 (Figure 4-26). T h i s r e s u l t i s e n t i r e l y expected s i n c e Q i s t y p i c a l l y twice as l a r g e as the AC e l e c t r o d e heat t r a n s f e r . The anode s u r f a c e temperature again showed a sharp i n c r e a s e at p r e s s u r e s above 3 atm. Again the t o t a l measured heat t r a n s f e r d i d not s i g n i f i c a n t l y i n c r e a s e above 3 atm, which i n d i c a t e s t h a t the arc attachment r e g i o n must decrease i n s i z e above 3 atm. 186 Chapter 6 CONCLUSIONS AND SUGGESTIONS FOE FUTUSE WORK 6«.J C o n c l u s i o n s The work on AC vortex s t a b i l i z e d a r c s presented i n t h i s t h e s i s was motivated by the b e l i e f t h a t they might be s u p e r i o r i n performance ( p a r t i c u l a r l y e l e c t r o d e l i f e t i m e ) t o the w e l l researched DC vortex s t a b i l i z e d a r c s . To t e s t t h i s c o n t e n t i o n the same vortex s t a b i l i z e d a r c was operated e i t h e r with d i r e c t or a l t e r n a t i n g c u r r e n t and the f o l l o w i n g a r c p r o p e r t i e s were measured: (1) The t o t a l r a d i a t i o n l o s s e s from the a r c as a f u c t i o n of c u r r e n t and gas flow c o n d i t i o n s , and from these r e s u l t s the r a d i a t i v e e f f i c i e n c y (n = r a d i a t i o n l o s s e s / i n p u t power) was c a l c u l a t e d . (2) The heat t r a n s f e r t o the e l e c t r o d e s , to the arc c o n t a i n e r w a l l and to the exhaust gas as a f u n c t i o n of c u r r e n t and gas flow c o n d i t i o n s . (3) The e l e c t r o d e s u r f a c e temperature as a f u n c t i o n of c u r r e n t and gas pr e s s u r e . I t was found t h a t the r a d i a t i v e e f f i c i e n c y ( n ) f o r an AC ¥SA o p e r a t i n g a t 250-400 A and at a pressure 187 of f i v e atmospheres was comparable to a DC VSA o p e r a t i n g a t the same average c u r r e n t l e v e l . Both n A C and n D C i n c r e a s e d s i g n i f i c a n t l y with a r c p r e s s u r e . The t y p i c a l e f f i c i e n c y of both the AC and DC VSA was 30-35%. Hence on the b a s i s of r a d i a t i v e e f f i c i e n c y , the AC a r c showed promise as a p r a c t i c a l high i n t e n s i t y r a d i a t i o n source. With r e s p e c t t o e l e c t r o d e heat t r a n s f e r the AC a r c has s i g n i f i c a n t advantages over the DC a r c . The measurement of heat t r a n s f e r t o the e l e c t r o d e s has confirmed t h a t the average heat l o a d i s reduced by running the a r c i n a l t e r n a t i n g r a t h e r than d i r e c t c u r r e n t . The average heat t r a n s f e r to the AC e l e c t r o d e s i s comparable to the cathode heat t r a n s f e r at the same average c u r r e n t . The anode l o a d i n g i s t y p i c a l l y 1.5-2 times l a r g e r than the cathode l o a d i n g , so t h a t AC o p e r a t i o n s u b s t a n t i a l l y reduces the e l e c t r o d e heat t r a n s f e r . I t was found t h a t the heat t r a n s f e r t o both AC and DC e l e c t r o d e s was found to s c a l e l i n e a r l y with the arc c u r r e n t under a l l flow c o n d i t i o n s , f o r c u r r e n t s above 100 amps. The s l o p e of the heat t r a n s f e r vs I graph (V) f o r each e l e c t r o d e can be viewed as the e f f e c t i v e e l e c t r o d e v o l t a g e drop (V__) which i s independent of I over the EF c u r r e n t range examined i n t h i s work (the e f f e c t i v e cathode v o l t a g e drop i s d e f i n e d by (V__) = 31 while the e f f e c t i v e anode voltage drop i s d e f i n e d by t ? E F , A S S 9 V 3 I ) -Both ( V ™ ) . and (V ) were s t r o n g l y a f f e c t e d by EF A EF v_ 188 the gas flow d i r e c t i o n . V was always l a r g e r f o r flow EF towards than f o r flow away f r o n an e l e c t r o d e , f o r AC and DC o p e r a t i o n , f o r example the e f f e c t i v e anode v o l t a g e drop (V ) was ^ 4035 higher f o r flow towards the anode than f o r flow away while the e f f e c t i v e cathode voltage drop ( y E F ) c w a s ^ 30% l a r g e r f o r flow towards the cathode than f o r flow away. T h i s e f f e c t was not due to c o n v e c t i v e heat t r a n s f e r as i n c r e a s i n g the flow r a t e by a f a c t o r of 2.5 had no e f f e c t on V . I t was b e l i e v e d that changes i n EF both ( V E F ) A a n d ^ VEF^ C d U e t 0 g d S f ^ o w d i r e c t i o n r e s u l t e d from changes i n the e l e c t r o d e f a l l p o t e n t i a l ( V A F or V C F ) » For the d i f f e r e n t flow d i r e c t i o n s the observed c o n t a c t area between the a r c and e l e c t r o d e s u r f a c e was v a s t l y d i f f e r e n t . l o r both the anode and cathode the arc e l e c t r o d e attachment r e g i o n was c o n s i d e r a b l y l a r g e r f o r flow towards the e l e c t r o d e than f o r flow away from the e l e c t r o d e . The plasma c o n d i t i o n s (temperature, d e n s i t y and c u r r e n t density) immediately around the e l e c t r o d e s could not be expected t o be the same f o r both flew d i r e c t i o n s . Hence both V,_, and V „ should be dependent on AF CF e flow d i r e c t i o n . From the experimental r e s u l t s f o r ( v ~ T J a t Q e anode f a l l p o t e n t i a l ( V A F> w a s determined. For flow towards the anode V „ was t y p i c a l l y 3.5 v o l t s while f o r AF flow away from the anode V =0.5 v o l t s . These r e s u l t s AF are much s m a l l e r than those reported f o r s h o r t low c u r r e n t a r c s (Busz-Peuckert (1956)). However these r e s u l t s are 189 c o n s i s t e n t with more re c e n t work cn l o n g e r high i n t e n s i t y argon a r c s (Eberhart (1966)). The heat t r a n s f e r t o each AC e l e c t r o d e was c a l c u l a t e d by assuming that f o r h a l f the c u r r e n t c y c l e the e l e c t r o d e behaved l i k e a cathode i n a DC arc and f o r the other h a l f l i k e an anode. The heat t r a n s f e r was c a l c u l a t e d f o r both flow d i r e c t i o n s u s ing the e x p e r i m e ntal DC r e s u l t s f o r (V E F ) A and ( % F ^ C ' a n < ^ c u r r e n t independent heat t r a n s f e r terms. The p r e d i c t e d e f f e c t i v e AC e l e c t r o d e voltage drop f o r both flow d i r e c t i o n s was i n good agreement with the experimental r e s u l t s . The c u r r e n t independent heat t r a n s f e r terms p r e d i c t e d from t h i s model, however, d i f f e r e d widely from the experimental r e s u l t s . T h i s d iscrepency i n d i c a t e d t h a t the c u r r e n t independent terms f o r the AC e l e c t r o d e heat t r a n s f e r are more complicated than j u s t an average of the DC r e s u l t s . The c u r r e n t independent heat t r a n s f e r terms f o r the AC e l e c t r o d e s were g e n e r a l l y much s m a l l e r than f o r the DC e l e c t r o d e s . Hence the AC e l e c t r o d e heat t r a n s f e r was l e s s than the average of the heat t r a n s f e r r e d to the cathode and anode i n a DC a r c at the same average c u r r e n t . T h i s r e s u l t has very important s i g n i f i c a n c e f o r any p r a c t i c a l high i n t e n s i t y r a d i a t i o n source. Since the anode heat t r a n s f e r i s t y p i c a l l y twice as l a r g e as the cathode heat t r a n s f e r the anode i s u s u a l l y more l i k e l y t o f a i l than the cathode. Summarizing these l a s t r e s u l t s we conclude that the major advantage of o p e r a t i n g a high i n t e n s i t y VSA 190 under a l t e r n a t i n g c u r r e n t i s that the e l e c t r o d e thermal l o a d i n g i s s u b s t a n t i a l l y reduced from t h a t of a DC anode at the same c u r r e n t and gas flow c o n d i t i o n s , which promises a l o n g e r e l e c t r o d e l i f e t i m e f o r the a r c . In order to gain more understanding of the e l e c t r o d e heat t r a n s f e r processes the e l e c t r o d e s u r f a c e temperature was measured s p e c t r o s c o p i c a l l y using a two channel o p t i c a l system t h a t monitored the thermal r a d i a t i o n from the e l e c t r o d e . T h i s system d i d not r e g u i r e a knowledge of the a b s o l u t e value of the s u r f a c e e m i s s i v i t y at one wavelength, a g u a n t i t y t h a t depends s t r o n g l y on the s u r f a c e c o n d i t i o n s and i s not e a s i l y o b t a i n a b l e . Hence t h i s technigue was more a c c u r a t e than the normally used one channel system i n which the e l e c t r o d e s u r f a c e e m i s s i v i t y at one wavelength must be kncwn. The s u r f a c e temperature of the AC e l e c t r o d e at the a r c attachment r e g i o n was found to be at a constant temperature throughout the c u r r e n t c y c l e except f o r a few extremely small time dependent hot s p o t s . The maximum temperature of these hot s p o t s was a t l e a s t at the melting temperature o f t u n g s t e n . The temperature decay of the main e l e c t r o d e attachment region f o l l o w i n g c u r r e n t i n t e r r u p t i o n f o r v a r i o u s e l e c t r o d e geometries was examined. The s u r f a c e temperature decayed e x p o n e n t i a l l y with time constants of 80-600 ms depending on e l e c t r o d e geometry. The e l e c t r o d e thermal decay constant was 191 c a l c u l a t e d assuming a one-dimensional t r a n s i e n t heat t r a n s f e r model. There was good agreement between these c a l c u l a t e d time c o n s t a n t s and the measured time c o n s t a n t s . Hence the heat t r a n s f e r processes i n the e l e c t r o d e t i p are e s s e n t i a l l y one-dimensional i n nature. The e l e c t r o d e s u r f a c e temperature was found to s c a l e l i n e a r l y with c u r r e n t . Since the heat t r a n s f e r a l s o s c a l e d l i n e a r l y with the c u r r e n t , the former must be a l i n e a r f u n c t i o n of the s u r f a c e temperature. T h i s r e s u l t i s f u r t h e r c c n f i r m a t i o n of the one-dimensional nature of the e l e c t r o d e heat t r a n s f e r processes. As mentioned p r e v i o u s l y s m a l l time dependent hot s p o t s were observed on the AC e l e c t r o d e s . The time e v o l u t i o n of these hot spots was observed with a simple o p t i c a l system t h a t I d e v i s e d and c o n s t r u c t e d f o r these purposes. The system c o n s i s t e d of a c o n v e n t i o n a l stroboscope and synchronous AC motor with a r o t a t a b l e s t a t o r which allowed any part of the AC c y c l e t o be examined. This simple system w i l l undoubtedly have numerous a p p l i c a t i o n s f o r studying the behavior of p e r i o d i c a r c s . From the a r c energy balance r e s u l t s we s u r p r i s i n g l y found t h a t the heat t r a n s f e r to the wall s c a l e d l i n e a r l y with the r a d i a t i o n l o s s e s . The wall l o a d i n g i s not due to a b s o r b t i o n of r a d i a t i o n and i s much l a r g e r than that expected from laminar r a d i a l heat t r a n s f e r . To i n v e s t i g a t e t h i s f u r t h e r a simple channel 192 model was developed f o r the luminous a r c core. From t h i s model the r a d i u s and temperature of the luminous a r c core was determined as a f u n c t i o n of c u r r e n t . The model made use of the experimental s c a l i n g r e s u l t s f o r the v a r i o u s a r c heat l o s s terms. A unigue value of T and r f o r each c u r r e n t value was determined using the experimental r e s u l t s f o r the r a d i a t i o n l o s s e s from the a r c . The p r e d i c t e d r a d i i were i n good agreement with experimental r e s u l t s determined from time i n t e g r a t e d photographs of the luminous core of the a r c . At high c u r r e n t (I >350 A the DC arc r a d i u s was e s s e n t i a l l y constant (r ^ 8 mm). Hence the arc column behaved as i f i t was c o n s t r a i n e d by a w a l l , when i n f a c t no s o l i d w a l l e x i s t e d at t h i s r a d i u s (the c o n f i n i n g guartz w a l l was at a much l a r g e r r a d i u s Jr =13.5 mm)). The wa l l heat t r a n s f e r continued to i n c r e a s e when the r a d i u s of the a r c was e s s e n t i a l l y c o n s t a n t , so that h i g h l y e f f i c i e n t heat t r a n s f e r processes must have been t a k i n g p l a c e o u t s i d e the c e n t r a l luminous a r c c o r e . In f a c t laminar heat t r a n s f e r can not e x p l a i n the l a r g e w a l l l o a d i n g ; t u r b u l e n t mixing processes might be present t c enhance the heat t r a n s f e r . The r e s u l t s f o r high c u r r e n t i n d i c a t e t h a t the r e l e a t i v e l y c o l d r o t a t i n g gas o u t s i d e the luminous core a c t s l i k e an a d d i t i o n a l heat s i n k t h a t must be s t r o n g l y coupled to the water cooled g u a r t z w a l l . The presence of the quartz wall i s then e s s e n t i a l l y communicated to the arc column through the non-luminous region surrounding the c e n t r a l luminous arc 193 core. The arc temperature as a function of radius determined from t h i s simple channel model has been compared to empirical r e s u l t s for wall s t a b i l i z e d arcs. There i s good agreement between the two results which further indicates that the vortex s t a b i l i z e d arc behaves l i k e a wall s t a b i l i z e d arc. A survey cf the l i t e r a t u r e on arcs has shown very l i t t l e work on low current AC arcs and just one reference to an AC vortex s t a b i l i z e d arc, so that most of t h i s work on the high i n t e n s i t y AC vortex s t a b i l i z e d arc stands alone and can not be compared with other r e s u l t s . 6_. 2 Suggestions for Future Work The experiments performed on the AC VSA have shown that i t has considerable promise for use as a high i n t e n s i t y radiation source. Hence I f e e l experiments should be continued on the AC VSA i n order to gain a better understanding of the processes occurring i n i t . Suggestions for future work are given below. Since some of the electrode heat transfer processes are guite d i f f e r e n t for the AC and DC VSA, i t would be interesting to examine the heat transfer processes for AC arcs at lower frequencies than the 60 Hz used i n t h i s work. This would hopefully provide information on the time scales of the electrode heat 194 t r a n s f e r terms, p a r t i c u l a r l y the c u r r e n t independent terms which are q u i t e d i f f e r e n t f o r AC and DC o p e r a t i o n . The plasma temperature and d e n s i t y d i s t r i b u t i o n around the a r c e l e c t r o d e s should be determined. These measurements might p r o v i d e some i n s i g h t i n t o the l a r g e d i f f e r e n c e s i n the e l e c t r o d e f a l l p o t e n t i a l t h a t seem to occur when the gas flow d i r e c t i o n i s r e v e r s e d . The temperature d i s t r i b u t i o n of the main cathode j e t should be measured as well as the i n d i v i d u a l j e t s emanating from the cathode hot s p o t s . In a d d i t i o n the time e v o l u t i o n of the e l e c t r o d e hot spot temperature should be determined. These measurements w i l l have t o be made without i n t e r r u p t i n g the arc c u r r e n t which w i l l make them more d i f f i c u l t than the s u r f a c e temperature measurements performed i n t h i s work. A knowledge of the e l e c t r o d e hot spot temperature and plasma temperature immediately around the hot spots throughout the c y c l e w i l l provide more i n s i g h t i n t o the e l e c t r o d e processes f o r the AC a r c . The plasma temperature and d e n s i t y i n the arc column should a l s o te measured. The temperature p r o f i l e would provide a check on the v a l i d i t y of the simple channel model proposed f o r the luminous r e g i o n of the arc column. The temperature of the arc column o u t s i d e the luminous r e g i o n should a l s o be measured. He b e l i e v e t u r b u l e n t mixing processes, which g r e a t l y enhance the r a d i a l heat t r a n s f e r are o c c u r r i n g t h e r e . Hence a knowledge of the temperature i n t h i s r e g ion would be 195 u s e f u l to g a i n a b e t t e r understanding of the heat t r a n s f e r processes i n the arc column. The temperatures o u t s i d e the luminous column w i l l be g u i t e low ( t y p i c a l l y l e s s than 9000 K) so t h a t i t might be d i f f i c u l t t o o b t a i n the temperature using atomic s p e c t r a . A p o s s i b l e method to measure these low temperatures would be to use s m a l l amounts of helium i n the argon and examine the molecular s p e c t r a of the He* -He molecule. I t would be i n t e r e s t i n g to understand why the heat t r a n s f e r to the a r c v e s s e l w a l l s c a l e s l i n e a r l y with the r a d i a t i o n l o s s e s even though the arc o p e r a t i n g c o n d i t i o n s vary so widely. The simple channel model f o r the arc column p r e d i c t e d behavior t h a t i s g u i t e s i m i l a r to that of wall s t a b i l i z e d a r c s . We b e l i e v e the c i r c u l a t i n g c o l d gas o u t s i d e the luminous core of the arc p l a y s the r o l e of a w a l l f o r the vortex s t a b i l i z e d a r c used i n t h i s work. The e f f e c t of the vortex flow f i e l d on the p r o p e r t i e s of the a r c column should be examined by r e p e a t i n g some of the experiments performed i n t h i s work under d i f f e r e n t vortex flow f i e l d c o n d i t i o n s . Now t h a t i t has been shown t h a t the AC VSA shows promise f o r use as a high i n t e n s i t y r a d i a t i o n source, the i n p u t power t o our arc should be i n c r e a s e d to the 100 kW l e v e l which i s an a p p r o p r i a t e power l e v e l f o r many p r a c t i c a l a p p l i c a t i o n s . T h i s i n c r e a s e i n power to the arc should be a t t a i n e d by i n c r e a s i n g the a r c l e n g t h which w i l l 196 i n c r e a s e the arc v o l t a g e but maintain the same c u r r e n t l e v e l s . An i n c r e a s e i n the a r c length by a f a c t o r of two to three should accomplish t h i s r e s u l t . 197 BIBLICGRAPHY Anderson, J . , Eschenbach, R. And Troue, H., Appl. Optics 4 , 1435 (1965) Andrada, T. And E r f u r t h , K., Proceedings of the F i r s t Plasma Arc Seminar of the Thermo-Mechanics Research Laboratory, 100 (1962) Bauer, A. And Sc h u l z , P., Z. F. Physik 139 , 197 (1954) B a t u r i n , V.A. And Ulanov, I.M., S o v i e t Physics-Techn. P h y s i c s j 3 , 667 (1968) Borodin, V.S. And Rutberg, F.G., T e p l o f i z . Vys. Temp. 6 No. 3 (1968) Borodin, V.S. And Gebekhov, V.D., Opt. I Spektrosk. 2 7 , 4 (1969) Boxman, R.L. , J. Appl. Phys. 46 , 4701 (1975) Burhorn, F. And Maecker, H., Z e i t s c h r i f t f u r Physik 129 , 369 (1951) Busz-Peuckert, G. And F i n k l e n b u r g , W., Z. Phys. 144 , 244 (1956) Camra, D. And Nodwell,R., US Patent A p p l i c a t i o n No. 476, 872 (1974) Camm, 0., P r i v a t e Communication, (1977) C a s i e , M. , CIGEE Report No. 102, (1939) (unpublished) Compton, K.T. , Phys. Rev., 2\ , 266 (1923) Compton, Proc. Nat. Acad. S c i . , USA, J[8 , 705 (1932) Decker, A.J. And P o l l a c k , J . , Proceedings of a Symposium on Space S i m u l a t i o n , USA, Nat. Aeronautics and Space A d m i n i s t r a t i o n , 17 (1972) Denis, P. R. , Gates, D. H. , Smith, C R . And Bond, J.B., SP-5033, NASA, (1965) D e t l o f f , L . And Uhlenbusch, J . , Z. Angew. Phys. 28 , 205 (1970) Devoto, R.S., Phys. Of F l u i d s , J6 , 616 (1973) 193 Dorodnov, A.M., Kozlov, N.P. And Reshetnikov, N., High Temperature JL3 , 556 (1975) D r e l l i s h a k , K. S. , Knopp, C. F. And Cambel, A.B. , Phys. Of F l u i d s 6 , 1280 (1963) Eb e r h a r t , R.C. And Seban, R.A., I n t J . Heat Mass T r a n s f e r 9 , 939 (1966) Ecker, G. , Z. Physik V36 , 556 (1954) Ecker, G., Erg. Der Exakten Naturwissenschaften 33 , 1 (1961) Elenbaas, P h y s i c a 2 , 155 (1935) Evans, D.L. And Tankin, R.S., Phys. F l u i d s JO , 1137 ( 1 967) Fang, M., Fung, H. And E d e l s , H., Proc. IEEE, 120 , 709 (1 973) F i n k e l n b u r g , W. And Schluge, H., Z. Physik 122 , 714 ( 1944) Finkelnburg,W., O f f i c e of M i l i t a r y Government f o r Germany (OS), F i e l d Information Agency T e c h n i c a l Report 1052 (1950) F i n k e l n b u r g , W. And Maecker, H., E l e c t r i s c h e Bogen und ThermischePlasmen, Handbuch der Physik, ed. S. Flugge, B e r l i n , S p r i n g e r - V e r l a g (1956) F r a n c i s , V.J., Fundamentals of Discharge Tube C i r c u i t s (Meuthen, London, 1548) Froome, K. D., Nature, J57 , 446 (1946) Ghcsh Roy, D.N. And Tankin, R.S., J . Quant. S p e c t r o s c . Radiat. T r a n s f e r V2 , 1685 (1972) G i l l e t t e , M.R. And Benenson, D.M., Paper C73 324-1 IEEE Power Eng i n e e r i n g S o c i e t y Summer Meeting (1973) Goldbach, C., N o l l e z , G. And Peyturaux, R., J . Quant. Sp e c t r o s . Radiat. T r a n s f e r \2 , 1089 (1972) Hoyaux, M., Arc P h y s i c s , S p r i n g e r - V e r l a g (1968) Hugel, H. And K r u l l e , G., B e i t r a g e aus der Plasmaphysik 9 , 2 (1969) K i m b l i n , C. w. , J . A p p l i . Phys. 40 , 1744 (1969) 199 K i m b l i n , C. W. And Lowke, J . J . , J . Appl. Phys. 44 , 4545 (1973) Lowke, J . J . , Z c l l w e g , R.J. And Liebermann, R.W., J. Appl. Phys. 46 , 650 (1975) Lowke, J . J. , J . Appl. Phys. 4J , 2588 (1970) M a l l i a r i s , A . C , John, R.R. And Enos, G.R., Proceedings of the 16th Annual T e c h n i c a l Meeting, I n s t i t u t e of Envicrnmental Sciences 1 (1970) Maecker, H., Z e i t s c h r i f t f u r Physik J 2 9 , 108 (1951) Mayr, 0., Arch. E l e c t r o t e c h . 37 , 588 (1943) M o r r i s , J.C., J . Quant. S p e c t r o s c . R a d i a t . T r a n s f e r 9 , 1629 (1969) M o r r i s , J.C. And Krey, R.V., J . Quant. S p e c t r o s c . R a d i a t . T r a n s f e r 9 , 1633 (1969) Olsen, H.N., J. Quant. S p e c t r o s c . R a d i a t . T r a n s f e r 3 , 59 (1963) P h i l l i p s , R.L., B r i t . J . Appl. Phys. J 8 , 65 (1967) Pustogarov, A. V. , Kolesnichenko, A.N., Gavryushenko, B.S., Zakh a r k i n , E. And Daragan, V.D., High Temperature VI , 174 (1973) R a z n j e v i c , K., Handbook of Thermodynamic Tables and Charts (1976) (Hemisphere P u b l i s h i n g Corporation) R e i d e r , W., Z e i t s c h r i f t f u r Physik .146 , 629 (1956) Schmidt, E., Ann. Physik 4 , 246 (1949) Schoeck, P. And E c k e r t , E., Proc. Of 5th I n t . Conference on I o n i z a t i o n Phenomena i n Gases (1961) Schoenherr, 0., E l e c t r o t e c h n ZS (1909) S t r e s i n o , E. P. And Eschenbach, B.C., AFAL-TR-66-204 (1966) S t u c k e l b e r g , E-, H e l v e t i c a Phys. Acta \ . 75 (1928) Tam, S.Y.K. And Gibbs, B.W., RCA Report No. 96208-2 (1972) Ter Horst, D. And Rutgers, G., Physica _19 , 565 (1953) 200 Tuchraan, A. And Enos, G., Avco C o r p o r a t i o n T e c h n i c a l Report AVSSD-0043-67-RR (1967) Wiencke, R. , Z e i t . Fur Physik, J50 , 231 (1958) Wiencke, R. , Z e i t . Fur Physik, 15_1 , 159 (1958) Yakubov, I.T., Symposium on I n t e n s i t y and Contour Shape of S p e c t r a l L i n e s (Krasnoyarsk) 497 (1964) 201 APPENDIX A ABC POWER SUPPLY In the experiments performed on the vortex s t a b i l i z e d AC and DC a r c s the maximum e l e c t r i c a l power i n p u t to the arc was ^ 50 kW. In t h i s s e c t i o n the power s u p p l i e s used f o r the AC and DC vortex s t a b i l i z e d a r c s are d e s c r i b e d . The power used f o r our experiments was d e r i v e d from a s i n g l e phase of a th r e e phase power l i n e present i n our l a b o r a t o r y . The l i n e to l i n e voltage f o r t h i s system was 208 v o l t s EMS and the maximun c u r r e n t was £ 425 A RMS per phase. The power supply used f o r the AC a r c i s shown i n F i g u r e A-1. The transformer T^ serves as an i s o l a t i o n transformer and a l l o w s one s i d e of the a r c t c be grounded s i n c e the three phase power l i n e i n our l a b o r a t o r y i s wired i n a d e l t a c o n f i g u r a t i o n which can not be grounded. Current c o n t r o l of the AC a r c i s accomplished by a choke c o i l JL=1 mH) , b a l l a s t r e s i s t o r s R^ and and a s a t u r a b l e r e a c t o r (SR) which can be viewed e s s e n t i a l l y as a v a r i a b l e i n d u c t o r . The i n d u c t i v e r e a c t a n c e of the s a t u r a b l e r e a c t o r can be c o n t r o l l e d by the p e r m i t t i v i t y of the r e a c t o r i r o n c o r e , which i s c o n t r o l l e d by a low c u r r e n t DC c o n t r o l winding which i s wound on the same core as the AC windings. Transformer 1^ i s the secondary winding of the a r c s t a r t i n g c i r c u i t (Refer to Appendix B f o r complete d e t a i l s ) . SR 208 V RMS _ L -nrm-AC ARC F i g u r e A - l : AC Arc Power Supply to o fO 203 DC arc Power Supply The power supply used f o r the DC a r c i s shown i n F i g u r e A-2. The c u r r e n t c o n t r o l i s accomplished by the s a t u r a b l e r e a c t o r i n the primary c i r c u i t , and the b a l l a s t r e s i s t o r s i n the secondary c i r c u i t . F i l t e r i n g of the f u l l wave r e c t i f i e d v o l t a g e i s performed by the choke c o i l L . 205 APPENDIX fl S t a r t i n g C i r c u i t f o r the Vortex S t a b i l i z e d Arc The most elementary method of s t a r t i n g any e l e c t r i c arc i s to touch the e l e c t r o d e s together and then s e p a r a t e them. T h i s method was used i n the i n i t a i l stages of our work on the vortex s t a b i l i z e d AC arc. U n f o r t u n a t e l y t h i s procedure r e s u l t e d i n both e l e c t r o d e and w a l l damage. Touching the e l e c t r o d e s together caused l o c a l melting on the e l e c t r o d e s t i p s and o c c a s i o n a l l y severe damage to the i n n e r guartz tube. T h i s damage was mainly due t o the arc plasma which c o u l d o c c a s i o n a l l y extend as f a r as the guartz w a l l . For small e l e c t a r o d e s e p a r a t i o n (L£ 2 mm) the a r c i s not w e l l c o n s t r a i n e d and can reach the w a l l . Hence i t i s e s s e n t i a l to s t a r t the a r c by a technigue which e l i m i n a t e d the problems c i t e d above. DC a r c s can be s t a r t e d g u i t e e a s i l y using a high v o l t a g e breakdown pulse (£ 30 kV, pulse l e n g t h 20 ps coupled with a c a p a c i t o r bank which s u s t a i n s the i o n i z a t i o n channel u n t i l the a r c power supply takes over. The power supply used f o r these experiments responded somewhat slowly t o power demands (time constant ^ 0 . 5 seconds) . A schematic diagram of the DC s t a r t i n g c i r c u i t 206 i s shown i n Fig u r e B -1 . T h i s method however does not worX f o r an AC a r c . The temperature of the e l e c t r o d e which i s i n i t i a l l y an anode has t o r i s e high enough i n h a l f a c y c l e so t h a t when i t changes p o l a r i t y the e l e c t r o n emission w i l l be l a r g e enough to s u s t a i n the a r c . The problen of s t a r t i n g the AC a r c was sol v e d by using a s e r i e s i n j e c t e d 60 kV, 4 MHz pulse t r a i n which l a s t e d f o r up t o 0.5 seconds. A schematic diagram of the AC s t a r t i n g c i r c u i t i s shown i n F i g u r e B-2. This u n i t i s a modified s t a r t i n g c i r c u i t f o r DC xenon arc lamps manufactured by Pincus and A s s o c i a t e s and was connected i n s e r i e s with the a r c so as to break down the e l e c t r o d e gap. Using t h i s system the a r c could be s t a r t e d with e l e c t r o d e s e p a r a t i o n s of £ 20 mm. For t h i s s e p a r a t i o n the a r c plasma was well s t a b i l i z e d and no w a l l damage occu r r e d d u r i n g the s t a r t i n g procedure. E l e c t r o d e damage was a l s o completely e l i m i n a t e d . This s t a r t i n g c i r c u i t was a l s o used to i g n i t e the DC a r c with no r e s u l t i n g w a l l or e l e c t r o d e damage. S t a r t i n g Procedure f o r Vortex S t a b i l i z e d Arcs The AC arc was s t a r t e d with an i n i t i a l e l e c t r o d e s e p a r a t i o n of £ 20 mm. The b a l l a s t r e s i s t o r s (Eefer to Appendix A) were i n c l u d e d i n the AC c i r c u i t f o r the s t a r t u p sequence as w e l l as having the s a t u r a b l e r e a c t o r near i t s maximum. The arc gap was broken down by the 60 H.V PULSE TRANSFORMER • A / V N / — 1 — / " V W -0.5 0.5 • 2 00 100K 0 - 5 kV 0 UJUU D C A R C 100/»f 500 pf F i g u r e B - l : DC Arc S t a r t i n g C i r c u i t RF TRAP 6 6 115 VAC Figure B-2: AC Arc S t a r t i n g C i r c u i t AC INPUT 208 VRMS 209 kV 4 MHz pulses from the s t a r t i n g c i r c u i t which s u s t a i n e d the i o n i z a t i o n i n the gap f o r % 0.5 seconds u n t i l the power supply c o u l d take over. The s t a r t i n g c u r r e n t was t y p i c a l l y 100 A. Once the a r c had been e s t a b l i s h e d the e l e c t r o d e s were separated to the usual arc l e n g t h of 100 mm. The b a l l a s t r e s i s t o r s R^  and B 2 were then shorted out and the c u r r e n t l e v e l was then c o n t r o l l e d by the s a t u r a b l e r e a c t o r . The s t a r t i n g procedure used f o r the DC arc i s i d e n t i c a l t o t h a t used f o r the AC a r c . 210 APPENDIX C Arc C a l o r i m e t r y The heat t r a n s f e r r a t e s t o the v a r i o u s components of the vortex s t a b i l i z e d arc have been measured c a l o r i m e t r i c a l l y . In p a r t i c u l a r the heat t r a n s f e r to each e l e c t r o d e , to the w a l l and to the argon which has passed through the a r c chamber have been measured. The output r a d i a t i o n has a l s o been determined c a l o r i m e t r i c a l l y using an absorbing jacket p l a c e d around the a r c . The c o o l i n g water flow r a t e s to the v a r i o u s a r c components were measured using c a l i b r a t e d Brooks flowmeters (Model 1305) and the temperature r i s e of the c o o l i n g water was monitored by N a t i o n a l Semiconductor LX5700A temperature t r a n s d u c e r s placed i n the c o o l i n g water l i n e s . A schematic diagram of the c o o l i n g system f o r the v a r i o u s a r c components i s shown i n F i g u r e C-1. T y p i c a l flow r a t e s f o r the v a r i o u s arc components are shown i n Table 1C along with t y p i c a l temperature i n c r e a s e s i n the c o o l i n g l i n e s . The c o o l i n g water was c i r c u l a t e d by a t u r b i n e pump capable of d e l i v e r i n g flow r a t e s up to 80 l i t r e s / m i n u t e at pre s s u r e s up to 10 atm. T h i s pumping system ensured that b o i l i n g heat t r a n s f e r d i d not occur at the back of the e l e c t r o d e t i p . The temperature t r a n s d u c e r s had a c a l i b r a t i o n 211 t- i < u < hi x ID I UJ u z> O cr Ld Ld -J I— I-DC O CO z UJ co cr 0 1 < UJ id W > ^ £ J ^ O < UJ _ i > I— u_ © 0 © F i g u r e C - l : C o o l i n g System f o r the A r c Components 212 accuracy of + 4°C over the temperature range considered. This figure was substantially improved by using a biasing c i r c u i t which allowed the water temperatures to be measured to + 0. 1 °C. The output from sensor S-^  (input reference temperature) was applied to one input of an operational amplifier. The output from sensors s 2 _ s 6 could be applied to the other input of the op-amp. The output of the op-amp gave the temperature differnce between either of sensors S 2 _ S 6 the reference temperature given by S^. The output from the op-amp was li n e a r with temperature and the output was 10 mV/°C. A schematic diagram of the.temperature measuring c i r c u i t r y i s shown in Figure C-2. Arc Compnent Water Flow Rate Temperature Rise (litres/minute) (*C) Cathode Anode Wall Gas Heat Exchanger Radiation Absorber 8.2 9.1 15.7 7.6 7.6 4.4 7.8 13.0 10.2 25.0 Table 1-C: Typical Arc Components Cooling Water Flow Rates for the 213 F i g u r e C-2: D i f f e r e n t i a l Temperature Measuring System 214 A P P E N D I X D Determinate on of the AC Arc I n j 3 U t Power In order to determine the r a d i a t i v e e f f i c i e n c y of the AC arc i t was necessary to know accurately the input power to the arc and the t o t a l output radiation. The output radiation was measured c a l o r i m e t r i c a l l y as mentioned previously. The input power i s given by the product V I where V T=arc voltage and I=current, and i s ea s i l y determined for sinusoidal waveforms. Unfortunately, neither the arc voltage or current are sinusoidal. To determine the input power the instantaneous power has to be calculated and averaged over a cycle. This task was accomplished by the c i r c u i t shown schematically i n Figure D-1. The c i r c u i t e s s e n t i a l l y m u l t i p l i e s the voltage and current waveforms. The re s u l t i n g power waveform i s then integrated over a complete cycle and yields an output voltage proportional to the input power to the arc. The arc voltage waveform i s derived frcm a voltage divider across the arc which l i m i t s the peak input voltage to 5 volts. The current waveform i s derived from a calibrated shunt (50 mV/400 A ) and i s amplified by an op amp to a peak value of approximately 5 volts before i t i s applied to the mu l t i p l i c a t i o n c i r c u i t . The c i r c u i t that integrates t h i s > VOLTAGE INPUT F i g u r e D - l : AC Arc Input Power Measuring C i r c u i t 216 waveform i s on f o r e x a c l y one c y c l e and i s c o n t r o l l e d by a j u n c t i o n FET analog s w i t c h . T h i s power measuring c i r c u i t was c a l i b r a t e d using known i n p u t s i g n a l s from a Hewlett-Packard s i g n a l generator. T h i s c a l i b r a t i o n y i e l d e d the p r o p o r t i o n a l i t y constant between output v o l t a g e and i n p u t power. The output voltage then y i e l d e d the input power to the arc d i r e c t l y . 217 APPENDIX E AC Arc Time Constant In t h i s s e c t i o n the AC a r c time constant f o l l o w i n g c u r r e n t i n t e r r u p t i o n w i l l be estimated. The energy balance f o r the AC a r c was given i n S e c t i o n 2.2 by 0 B 2 . - i j _ ( r t C T ) n ) . P C P V V T , ' £ p | • »• ( s . „ F o l l o w i n g c u r r e n t i n t e r r u p t i o n (I=E=0) equation (E-1) becomes < " * . £ • - 7 ^ ) -PCpvg.VT -H R ( e - 2 ) from which the decay time constant ( T ) of the a r c column w i l l be determined. x w i l l be d e f i n e d f o l l o w i n g Kimblin and Lowke (1973) and Lowke (1975) by In t h i s simple c a l c u l a t i o n c o n v e c t i o n l o s s e s from the arc column w i l l be n e g l e c t e d . An estimate of x w i l l f i r s t be made assuming t h a t thermal conduction l o s s e s dominate, x w i l l a l s o be c a l c u l a t e d assuming r a d i a t i o n dominated l o s s e s . An upper l i m i t f o r x w i l l then be g i v e n by • Tr> where x_,_ and x_, are r e s p e c t i v e l y the time CU K CU K c o n s t a n t s f o r conduction and r a d i a t i o n dominated l o s s e s 218 from the a r c . Conduction Dominated Losses F o r conduction dominated l o s s e s equation (E-2) becomes P C P ^ = i T§- ( r k § £ ) ( E -» ) 3 t r 3r 3r Now a w i l l be assumed t o be l i n e a r l y r e l a t e d t o the thermal c o n d u c t i v i t y p o t e n t i a l S which i s d e f i n e d by T S =f k(T)dT T W For argon t h i s i s a reasonalbe approximation as shown by P h i l l i p s (1967). (E-4) then becomes 1 f a 3 ( 8S, S 9 t Sr 3 r t r 3 r J (E-5) Now i t w i l l be f u r t h e r assumed t h a t S i s s e p a r a b l e ( i e S (r,T) =E (r) T (t)) • The s o l u t i o n t o (E-5) f o l l o w i n g Lowke and Kimblin i s then given by oo _ i 2 (E—6) O u t s i d e the luminous core o f the a r c S i s q u i t e s m a l l ( s i n c e T i n t h i s r e g i o n i s < 10000 K) and r a d i a l v a r i a t i o n s i n S can be i g n o r e d . Then X R, where R=radius 219 of the luminous core are the the zeroes of J Q (RXi = 2. 4, R * 2 = 5 « 5 ) . The time c o n s t a n t f o r the f i r s t order term i s 2 thus given by T ^ = B /(5.8a ) . x C Q i s then e s s e n t i a l l y g i v e n by . In S e c t i o n 5.2 t h e average temperature and r a d i u s of the AC a r c was found t o be r =0 .76 cm and T = 11000 K. Values f o r p , Cp and k were obtained from O r e l l i s h a k (1963) and are given below. - 4 3 p ^ 8x10 go/cm C ^ 0 . 4 cal/gm k k * 10 mW/cm K x ^ (R2p C Ts)/5mSk. = 1.4 msec R a d i a t i o n Dominated l o s s e s When the l o s s e s from the a r c are dominated by r a d i a t i o n (E-2) i s given by 1 3S. = Uo; s a t s T £ S/U r (E-7) Values f o r S and 0 were obtained from E d e l s (1966) and Evans ( 1 9 6 7 ) . Osing these r e s u l t s we f i n d T _ % 0 .8 ms The AC a r c co n s i d e r e d i n t h i s work was n e i t h e r r a d i a t i o n nor conduction dominated. An upper l i m i t f o r T can be giv e n by the sum of T _ • T _ , CO R 220 T M A X * T C O + V 2 m s e c 221 APPENDIX F Saf e t y C o n s i d e r a t i o n s The experimental system used i n t h i s work employed both high c u r r e n t and high v o l t a g e . In a d d i t i o n the r a d i a t i o n from the arc was extremely intense and incl u d e d s i g n i f i c a n t u l t r a v i o l e t r a d i a t i o n . A number of s a f e t y p r e c a u t i o n s were b u i l t i n t o the system to safeguard a g a i n s t these p o t e n t i a l r i s k s and are b r i e f l y mentioned below. The s a t u r a b l e r e a c t o r and c o n t r o l c i r c u i t r y f o r the high c u r r e n t power supply were enclosed i n a grounded metal cage. Access to these components r e q u i r e d disassembly of the cage. The i s o l a t i o n transformer and choke c o i l were both mounted i n grounded boxes, while the s t a r t i n g c i r c u i t was l o c a t e d underneath the t a b l e on which the arc was mounted which s e v e r e l y r e s t r i c t e d access to t h i s high v o l t a g e u n i t . When the arc was running the r a d i a t i o n absorber was normally i n p l a c e . The end l o s s e s were t y p i c a l l y 5% so that the r a d i a t i o n l e v e l s i n the l a b o r a t o r y were reduced to a c c e p t a b l e l e v e l s . When the arc was operated for long p e r i o d s a pyrex outer tube was used i n the arc which reduced the r i s k of burns due to u l t r a v i o l e t r a d i a t i o n from the a r c . The outer pyrex tube a l s o s i g n i f i c a n t l y reduced ozone p r o d u c t i o n due to u l t r a v i o l e t r a d i a t i o n from the a r c . 

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