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Investigations of the glow phase in high pressure spark discharges Lee, Chi-Sun 1971

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INVESTIGATIONS OF THE GLOW PHASE IN-HIGH PRESSURE SPARK DISCHARGES by CHI-SUN LEE B.S.j N a t i o n a l Taiwan U n i v e r s i t y , 1966 ] A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department of PHYSICS We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1971 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of Brit ish Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or. publication of this thesis for financial gain shall not be allowed without my written permission. Department of • P h y s i c s The University of Brit ish Columbia Vancouver 8, Canada Date A p r i l s 29, 1971 ABSTRACT The glow phase of high pressure spark d i s c h a r g e s i n i t i a t e d by the Townsend mechanism of breakdown i n hydrogen, n i t r o g e n and carbon d i o x i d e i s s t u d i e d u s i n g 16 ohm c o a x i a l c a b l e d i s c h a r g e techniques. Before the establishment of the spark channel i n these low o v e r - v o l t a g e d spark d i s c h a r g e s , there e x i s t two d i s t i n c t t r a n s i t i o n stages, namely, a d i f f u s e glow phase and a f i l a m e n t a r y glow-to-channel t r a n s i t i o n phase. The o p t i c a l appearance of the d i f f u s e glow i s s i m i l a r to that of a normal d.c. glow d i s c h a r g e . I t c o n s i s t s of a negative glow, a Faraday dark space, and a p o s i t i v e column. From our experimental r e s u l t s i t i s concluded that f o r the glow phase the cathode f a l l v o l t a g e and the r a t i O : i o f the a x i a l p o t e n t i a l g r a d i e n t to the gas pressure i n the p o s i t i v e column are f i x e d f o r each gas. These val u e s are independent of the impedance of the e x t e r n a l c i r c u i t and the gas pressure. Comparisons with low pressure d.c. glow d i s c h a r g e s i n d i c a t e f u r t h e r t h a t the cathode f a l l v o l t a g e s are i n agreement i n both types of glow f o r the gases' i n v e s t i g a t e d . In a d d i t i o n , a q u a l i t a t i v e d i s c u s s i o n about the t r a n s i t i o n nature of t h i s d i f f u s e glow phase i s given on the b a s i s of r e s u l t s o b tained from s p e c t r o s c o p i c i n v e s t i g a t i o n s . - i i i -TABLE OF CONTENTS Page Ab s t r a c t i i Table of Contents i i i L i s t of Tables v L i s t of Figures v i Acknowledgements v i i Chapter 1. I n t r o d u c t i o n 1.1 The High Pressure Spark Discharges 1 1.2 O u t l i n e of the Thesis 4 1.3 Some Remarks on Low Pressure D.C. Glow Discharges 5 2. C o a x i a l Cable Discharge Techniques and Experimental Apparatus 2.1 Spark Discharge Techniques 7 2.2 The C o a x i a l Cable Discharge Arrangement 10 2.3 Experimental Appratus a. Spark Chamber 14 b. Impedance-Matched Adaptor 16 3. E l e c t r i c a l Parameters 3.1 Experimental and Measuring Techniques a. E l e c t r o d e Cleaning Procedures 19 b. C a l i b r a t i o n of the Charging Voltage Measurement 19 c. C a l i b r a t i o n of the Current Measurement 19 3.2 A n a l y s i s of the O s c i l l o g r a p h i c I n v e s t i g a t i o n s 20 - i v -4. Plasma Parameters 4.1 Experimental Arrangement f o r Temperature Measurement 27 4.2 A n a l y s i s of the S p e c t r o s c o p i c I n v e s t i g a t i o n s 29 4.3 E l e c t r o n D e n s i t y i n the P o s i t i v e Column 33 5. D i s c u s s i o n s and C o n c l u s i o n s 5.1 Comparison with 50 Ohm C o a x i a l Cable Discharges 35 5.2 Comparison with Low Pre s s u r e D.C. Glow Discharges 36 5.3 The T r a n s i t i o n Nature of the D i f f u s e Glow Phase 37 5.4 C o n c l u s i o n s 38 B i b l i o g r a p h y 40 LIST OF TABLES T i t l e E/P of P o s i t i v e Column and Cathode F a l l V o l t a V of Hydrogen, N i t r o g e n , and Carbon Di o x i d e Glow Phase Cathode F a l l V o l t a g e s f o r Copper and Zi n c E l e c t r o d e s i n Hydrogen, N i t r o g e n , and Carbon D i o x i d e Low Pressure D.C. Glow Dis c h a r g e s - v i -LIST OF FIGURES No. T i t l e Page 1. C i r c u i t Diagrams of the Condenser and the C o a x i a l Cable Spark Discharges 8 2. C u r r e n t O s c i l l o g r a m s of the Condenser and the C o a x i a l Cable Spark Discharges under Low Over-Voltage 9 3. C o a x i a l Cable Discharge Arrangement 11 4. C u r r e n t O s c i l l o g r a m s of Spark Discharges and A r r e s t e d Glow Discharges i n Hydrogen, N i t r o g e n , and Carbon D i o x i d e ; Time-Resolved L i g h t I n t e n s i t y V a r i a t i o n ( X = 3580 A ) i n N i t r o g e n T r a n s i e n t Glow 13 5. Spark Chamber 15 6. Impedance-Matched Adaptor 17 7. A x i a l P o t e n t i a l D i s t r i b u t i o n s of the Hydrogen, N i t r o g e n , and Carbon D i o x i d e T r a n s i e n t Glow 24 8. Experimental Arrangement f o r S p e c t r o s c o p i c I n v e s t i g a t i o n s 28 9. Observed Sp e c t r a of N i t r o g e n Second P o s i t i v e Band System - 30 10. Band Strengths of N i t r o g e n Second P o s i t i v e Band Spe c t r a 32 ACKNOWLEDGEMENT I wish to s i n c e r e l y thank my s u p e r v i s o r , Dr. J . Meyer, f o r h i s advic e and h e l p throughout these i n v e s t i g a t i o n s and the p r e p a r a t i o n of the t h e s i s . The a s s i s t a n c e of the f a c u l t y members and students of the plasma p h y s i c s group i s deeply a p p r e c i a t e d as w e l l . My s p e c i a l thanks are a l s o due to Mr. D. Stonebridge and Mr. R. P. Haines f o r t h e i r p a r t s i n the c o n s t r u c t i o n of the apparatus. -1-CHAPTER 1 INTRODUCTION 1.1 THE HIGH PRESSURE SPARK DISCHARGES A gas i s almost a p e r f e c t i n s u l a t o r i n i t s normal s t a t e . However i f an e l e c t r i c f i e l d of s u f f i c i e n t s t r e n g t h i s e s t a b l i s h e d between two e l e c t r o d e s , the gas can become cond u c t i n g and the e l e c t r i c c u r r e n t f l o w i n g through the gas can range from b a r e l y measurable values up to s e v e r a l m i l l i o n amperes or more. Depending on the d u r a t i o n of c u r r e n t conduction, the d i s c h a r g e s are e i t h e r of a steady s t a t e or a t r a n s i e n t nature. In spark d i s c h a r g e s the t r a n s i t i o n from the i n s u l a t i n g to the h i g h l y c o n ducting s t a t e of the gas between the e l e c t r o d e s i s achieved very r a p i d l y . A s h o r t time l a t e r , a f t e r t e r m i n a t i o n of the c u r r e n t p u l s e , the i n i t i a l i n s u l a t i n g s t a t e i s recov e r e d . E a r l y i n v e s t i g a t i o n s of high pressure spark d i s c h a r g e s have been almost e n t i r e l y l i m i t e d to the events immediately f o l l o w i n g the esta b l i s h m e n t of the spark channel due to the l a c k of f a s t r i s e - t i m e e l e c t r i c a l instruments and s e n s i t i v e photographic techniques necessary to d e t e c t the low l i g h t i n t e n s i t i e s from the e a r l i e r stages i n the development of these t r a n s i e n t d i s c h a r g e s . T h e o r i e s of -2-the spark channel expansion based on the e l e c t r i c a l l y d r i v e n shock wave model have been i n t r o d u c e d by Drabkina and B r a g i n s k i i ( l , 2 ) and experimental measurements l a t e r have confirmed some of t h e i r p r e d i c t i o n s (3,4). Due to the r e c e n t development of new techniques such as f a s t r i s e - t i m e p h o t o m u l t i p l i e r s , image c o n v e r t e r s and image i n t e n s i f i e r s , i t i s p o s s i b l e to study e x t e n s i v e l y not onl y the stages f o l l o w i n g the spark channel formation but a l s o the pr e c e d i n g breakdown process and the i n i t i a t i o n of the spark channel i t s e l f . The i n v e s t i g a t i o n s of the e l e c t r o n avalanches which are r e s p o n s i b l e f o r the breakdown of the spark d i s c h a r g e s have been c a r r i e d out, f o r example, by a r e s e a r c h group i n Hamburg headed by P r o f . H. Raether ( 5 ) who some t h i r t y years ago u t i l i z e d c l o u d chamber techniques f o r the e a r l i e s t s t u d i e s of the e l e c t r o n avalanches. Most of t h e • f e a t u r e s of the i n i t i a l breakdown processes are now w e l l understood and two breakdown mechanisms can be d i s t i n g u i s h e d : streamer mechanism and Townsend mechanism. Which one the d i s c h a r g e f o l l o w s depends on the o v e r - v o l t a g e a p p l i e d at the d i s c h a r g e gap. a. Streamer mechanism: At v o l t a g e f a r i n excess of the breakdown v o l t a g e a c r o s s a gap of s e v e r a l c e n t i m e t e r l e n g t h a spark channel i s formed very q u i c k l y . I n i t i a l l y an e l e c t r o n avalanche i s formed moving toward the anode under the i n f l u e n c e -3-of the high e l e c t r i c f i e l d . If the electron density i n the avalanche head i s amplified beyond a c e r t a i n value the nature of the avalanche changes, mainly due to the space charge f i e l d s , and a streamer of electrons r e s u l t s which bridges the electrodes and produces a pre-channel of f a i r l y high conductivity. The spark channel soon develops and the voltage across the gap collapses very r a p i d l y . b: Townsend mechanism ( generation mechanism ): For low to moderate over-voltages and shorter gap distances, a single avalanche cannot cause the e l e c t r i c a l breakdown. Instead, successor electrons are produced at the cathode by secondary e f f e c t s such as p o s i t i v e - i o n bombardment and photo-electric e f f e c t . Generation avalanches are thus formed and they are responsible for the breakdown. The formative time lag of t h i s type of discharge i s much longer as rtineeds a large number of successor avalanches. The spark discharges i n i t i a t e d by the Townsend mechanism have been investigated, for example, by Schroder, Doran and Meyer i n d i f f e r e n t gases ( 6 , 7 , 8 ) . It i s observed that prior to the complete collapse of the gap voltage, there are t r a n s i t i o n stages which do not exist i n the case of streamer breakdown discharges. At f i r s t a diffuse glow-like discharge of low conductivity i s formed afte r breakdown, which exhibits in general a bright negative glow, a Faraday dark space and a uniform pos i t i v e column. Then a thin filamentary -4-glow-to-channel t r a n s i t i o n phase develops and f i n a l l y a spark channel comes i n t o e x i s t e n c e . Although the breakdown process and the spark channel development have been s t u d i e d e x t e n s i v e l y and encouraging success has been achieved, some u n c e r t a i n t y about the c o n t r a c t i o n process of the d i f f u s e glow s t i l l remains u n c l a r i f i e d . I t seems a p p r o p r i a t e t h e r e f o r e , to provide some more i n f o r m a t i o n about t h i s glow phase i n order to achieve a b e t t e r understanding of t h i s t r a n s i e n t phase. 1.2 OUTLINE OF THE THESIS I t has been shown by Cavenor and Meyer (9) that f o r the glow phase i n hydrogen sparks produced i n a 50 ohm c o a x i a l ' c a b l e d i s c h a r g e arrangement at 500 t o r r p r e s s u r e , the gap v o l t a g e v a r i e s l i n e a r l y with the e l e c t r o d e d i s t a n c e and the e x t r a p o l a t e d z e r o - l e n g t h v o l t a g e i s i n agreement with the cathode f a l l v o l t a g e of low pressure d.c. hydrogen glow dis c h a r g e s . T o examine whether these c h a r a c t e r i s t i c s are c u r r e n t dependent or not, a 16 ohm c o a x i a l c a b l e d i s c h a r g e arrangement has been s e t up and the e l e c t r i c a l parameters d u r i n g the glow phase of hydrogen sparks over a wide range of gas p r e s s u r e s and d i f f e r e n t gap s e p a r a t i o n s have been measured. The same measurements have been extended to i n c l u d e another two gases — n i t r o g e n and carbon d i o x i d e . The r e s u l t s o b tained are presented i n Sec. 3.2, together with a d e s c r i p t i o n of the -5-a n a l y z i n g method which has been used to measure these q u a n t i t i e s . These r e s u l t s are d i s c u s s e d i n Sec. 5.1 and 5.2 where they are compared with 50 ohm c o a x i a l c a b l e d i s c h a r g e measurements on the one hand and with low pressure d.c. glow dis c h a r g e measurements on the other.. A monochromator-photomultiplier system has been used to analyze the l i g h t e mitted d u r i n g the glow phase from the p o s i t i v e column i n n i t r o g e n sparks. The experimental arrangement i s b r i e f l y o u t l i n e d i n Sec. 4.1. In Sec. 4.2 an estimate of the gas temperature i s deduced from the s p e c t r a l d i s t r i b u t i o n of the n i t r o g e n second p o s i t i v e band s p e c t r a . The e l e c t r o n d e n s i t y i s eval u a t e d by the methods d e s c r i b e d i n Sec. 4.3. A d i s c u s s i o n of the t r a n s i t i o n nature of the d i f f u s e glow phase based on these r e s u l t s i s given i n Sec. 5.3. The f o l l o w i n g chapter d e s c r i b e s the p r i n c i p a l and the experimental apparatus of the c o a x i a l c a b l e d i s c h a r g e techniques. I t s advantages over the condenser d i s c h a r g e techniques are noted i n Sec. 2.1. 1.3 SOME REMARKS ON LOW PRESSURE D.C. GLOW DISCHARGES Since the t r a n s i e n t glow of the spark d i s c h a r g e s i s q u i t e s i m i l a r to the d.c. glow d i s c h a r g e s i n some as p e c t s , the jfltil) c h a r a c t e r i s t i c s of the l a t t e r w i l l be b r i e f l y mentioned here f o r the purpose of l a t e r comparison. The g e n e r a l o p t i c a l appearance of the d.c. glow d i s c h a r g e s - 6 -shows d i f f e r e n t b r i g h t and dark r e g i o n s , i . e . cathode da r k space, n e g a t i v e glow, Faraday dark space, p o s i t i v e column e t c . The a x i a l p o t e n t i a l d i s t r i b u t i o n i s i n g e n e r a l not l i n e a r e x c e p t i n the p o s i t i v e column, and t h e r e the p o t e n t i a l g r a d i e n t i s of the o r d e r o f s e v e r a l v o l t s per c e n t i m e t e r . The anode f a l l v o l t a g e and the v o l t a g e d r o p a c r o s s the p o s i t i v e column are u s u a l l y v e r y s m a l l compared w i t h the cathode f a l l v o l t a g e . The l a t t e r i s the p o t e n t i a l drop measured from the cathode t o the anode end of the Faraday dark space. F o r the same gas and the same e l e c t r o d e m a t e r i a l the cathode f a l l v o l t a g e i s a c o n s t a n t and so i s the p r o d u c t of the t h i c k n e s s of cathode f a l l r e g i o n and gas p r e s s u r e (10). The ratiOQ»of the a x i a l p o t e n t i a l g r a d i e n t of the p o s i t i v e column t o the gas p r e s s u r e depends v e r y l i t t l e on the c u r r e n t but i s a f u n c t i o n of the p r o d u c t of i t s r a d i u s and the gas p r e s s u r e . - 7 -CHAPTER 2 COAXIAL CABLE DISCHARGE TECHNIQUES AND EXPERIMENTAL APPARATUS 2.1 SPARK DISCHARGE TECHNIQUES In r e c e n t y e a r s two d i f f e r e n t t e c h n i q u e s have been used t o s t u d y s p a r k d i s c h a r g e s , namely, t h e condenser d i s c h a r g e t e c h n i q u e s and the c o a x i a l c a b l e d i s c h a r g e t e c h n i q u e s . The c i r c u i t diagram f o r the condenser d i s c h a r g e s i s shown i n F i g . l a . The main d i s a d v a n t a g e of t h i s k i n d of d i s c h a r g e arrangement i s t h a t the c u r r e n t r i s e depends on the e x t e r n a l c i r c u i t as w e l l as the d i s c h a r g e . As an example, a c u r r e n t o s c i l l o g r a m o f such a d i s c h a r g e i s i l l u s t r a t e d i n F i g . 2a.(8) On the o t h e r hand, the o n l y e f f e c t o f the e x t e r n a l c i r c u i t i n c o a x i a l c a b l e d i s c h a r g e s i s the l i m i t a t i o n o f the maximum magnitude of the d i s c h a r g e c u r r e n t . F o r s u f f i c i e n t l y h i g h p r e s s u r e s , f o r example, the d i s c h a r g e o p e r a t e s a l m o s t l i k e an i d e a l s w i t c h and the d i s c h a r g e c u r r e n t r i s e s i n s t a n e o u s l y ( i n a f r a c t i o n of one p i c o s e c o n d ) t o i t s maximum v a l u e . T h e r e f o r e i n c o a x i a l c a b l e d i s c h a r g e s from the measurement of the d i s c h a r g e c u r r e n t i t i s p o s s i b l e t o c a l c u l a t e o t h e r i m p o r t a n t e l e c t r i c a l q u a n t i t i e s such as the v o l t a g e a c r o s s the gap, the gap r e s i s t a n c e and the energy i n p u t i n t o the d i s c h a r g e . F o r t h i s . r e a s o n we use the c o a x i a l c a b l e d i s c h a r g e t e c h n i q u e s -8-for our investigations rather than the condenser discharge techniques. A t y p i c a l discharge current oscillogram of coaxial cable spark discharges i s sketched i n Fig . 2b. Fi g . 1 C i r c u i t Diagrams of the Condenser and the Coaxial Cable Spark Discharges a. Condenser Spark Discharge L Spark Gap 77/77 Capacitor Bank 6 H.T. b. Coaxial Cable Spark Discharge Spark Gap 777777 'I O H.T, L£ : Inductance per Unit Length of Cable C^ : Capacitance per Unit Length of Cable F i g . 2 Current O s c i l l o g r a m s of the Condenser and the C o a x i a l Cable Spark Discharges under Low Over-Voltage Condenser Spark Discharge Current C o a x i a l Cable Spark Discharge Current Time 2T tA< t < t b t b < t < t c tc^ t < t d t_ < t < 2T T Glow Formation Region D i f f u s e Glow Phase Glow-to-Channel T r a n s i t i o n Phase Spark Channel Stage P u l s e T r a n s i t Time of Pulse-Forming Cable -10-2.2 THE COAXIAL CABLE DISCHARGE ARRENGEMENT The p r i n c i p a l of the experimental arrangement i s i l l u s t r a t e d i n F i g . 3. A c a b l e of c h a r a c t e r i s t i c impedance Z c ( 16 ohms i n our i n v e s t i g a t i o n s ) . i s charged through a high r e s i s t a n c e R c and then d i s c h a r g e d i n t o a p r o p e r l y terminated c a b l e of the same impedance. The spark chamber i s s p e c i a l l y designed i n order to maintain the same c h a r a c t e r i s t i c impedance throughout the whole di s c h a r g e c i r c u i t . Furthermore the chamber can be evacuated and p r e s s u r i z e d , and the gap d i s t a n c e of the e l e c t r o d e s can be changed from o u t s i d e . During the c h a r g i n g p e r i o d of the p u l s e - f o r m i n g c a b l e , i . e . the p e r i o d from the i n s t a n t at which the c h a r g i n g power supply i s switched on to the i n s t a n t at which the d i s c h a r g e s t a r t s , there are repeated r e f l e c t i o n s from both ends s i n c e the input end i s mismatched and the f a r end i s o p e n - c i r c u i t e d , The v o l t a g e E ( t ) at the f a r end v a r i e s a c c o r d i n g to: -i(.t+)/2T' E ( t ) =U { 1 -R c + Z Q when t = T,3T,5T, = U 0 £ l - exp(-t/CR c ) J , when t>> T where U 0 i s the c h a r g i n g v o l t a g e , R c the c h a r g i n g r e s i s t a n c e , T the t o t a l pulse t r a n s i t time of the c a b l e together with that p a r t of the chamber to which the c a b l e connects, C the t o t a l c a p a c i t a n c e of t h i s p u l s e - f o r m i n g s e c t i o n . When E ( t ) exceeds the breakdown v o l t a g e , breakdown takes p l a c e . -11-U.V. L i g h t T16/T50 Adaptor To O s c i l l o s c o p e To Pump 16 Ohm Cable M77 -AAAA-R o Current Meter 6 H . T . F i g . 3. C o a x i a l Cable Discharge Arrangement -12-T h i s causes the c o l l a p s e of the gap v o l t a g e and dis c h a r g e of the c a b l e , a f t e r which the power supply charges up the c a b l e a g a i n . Thus we have r e p e a t i n g d i s c h a r g e s . The r e p e t i t i o n r a t e can be c o n t r o l l e d to some extent by choosing an a p p r o p r i a t e c h a r g i n g r e s i s t a n c e f o r c a b l e s of d i f f e r e n t l e n g t h s . I f the dis c h a r g e were to operate as an i d e a l s w i t c h , a r e c t a n g u l a r c u r r e n t pulse of h e i g h t I Q = U 0 / 2 Z o and d u r a t i o n 2T would be produced. However due to the f i n i t e time.:required to a t t a i n h i g h c o n d u c t i v i t y i n the gap, the c u r r e n t r i s e s at a f i n i t e r a t e : Uo K t ) = 2Zo + Z ( t ) where Z ( t ) = U ( t ) / I ( t j i s the gap r e s i s t a n c e at time t . For a pu l s e s t a r t i n g at t=0, the above r e l a t i o n holds f o r t ^ 2T. The c u r r e n t pulse i s / r e c o r d e d with a T e k t r o n i x 519 o s c i l l o s c o p e of r i s e - t i m e 0 .30 nanosecond. The maximum value of 2T of the c a b l e s used i n our i n v e s t i g a t i o n s i s 197 ns. T y p i c a l c u r r e n t o s c i l l o g r a m s are presented i n F i g . 4a, 4b, 4c. They a l l i n d i c a t e three main stages as mentioned e a r l i e r ( F i g . 2b ). The c u r r e n t pulse f o r 2T^t^4T i s merely the double r e f l e c t i o n of the o r i g i n a l c u r r e n t pulse f o r 04t^2T. I t appears due to to the f a c t that a f i n i t e time i s r e q u i r e d to a t t a i n high c o n d u c t i v i t y . There are i n f a c t numerous r e f l e c t i o n s o c c u r i n g at t = 4T,6T,8T,...., and the -13-F i g . 4 Current Oscillograms of Spark Discharges a. Hydrogen 2360 V 2T = 197 ns P « 700 t o r r d = 1.0 mm b. Nitrogen U 0 = 2780 V 2T 197 ns p = 200 t o r r d = 2.0 mm c. Carbon Dioxide U 0 = 2750 V 2T = 197 ns p = 200 t o r r d = 1.75 mm d. Hydrogen 2080 V 2T = 95 ns p - 1200 t o r r d 0.5 mm •rent Oscillograms of A r r e s t e d Glow Discharges e. Hydrogen Uo = 2100 V 2T = 95 ns P = 600 t o r r d = 1.0 mm f. Nitrogen Uo = 2070 V 2T = 48 ns p - 300 t o r r d = 1.0 mm g. Carbon Dioxide 1880 V 2T = 95 ns p = 200 t o r r d = 1.0 mm o Time-Resolved L i g h t I n t e n s i t y V a r i a t i o n ( X = 3580 A ) Nitrogen Transient Glow h. Time Scale : 2 ns per d i v i s i o n -14-magnitude of these r e f l e c t i o n s d i m i n i s h e s very r a p i d l y . The r a t e of r i s e of the c u r r e n t d u r i n g the d i s c h a r g e depends on the gas, the pressure and the gap s e p a r a t i o n . However f o r any gap d i s t a n c e , the gap r e s i s t a n c e f a l l s very q u i c k l y a f t e r the d i s c h a r g e s t a r t s i f the gas pressure i s h i g h enough ( F i g . 4d ) T h i s f a c t w i l l be used l a t e r f o r the c a l i b r a t i o n of c u r r e n t measurements ( Sec. 3.Ic ). 2.3 EXPERIMENTAL APPARATUS a. Spark Chamber The chamber i s made of brass and p o l y e t h y l e n e , and c o n s i s t s of two separate p a r t s which are j o i n e d together by a h o l l o w : — c y l i n d e r connecter ( Fig7 5"). There are l e f t - h a n d and r i g h t - h a n d threads r e s p e c t i v e l y on the i n n e r s i d e s at each end of t h i s connecter. In t h i s way the e l e c t r o d e s e p a r a t i o n can be changed by r o t a t i n g the hollow c y l i n d e r . The gap d i s t a n c e can be s e t with an accuracy of 0l\015 mm. In order to prevent s i g n a l r e f l e c t i o n s the g e o m e t r i c a l s t r u c t u r e of the chamber must s a t i s f y the f o l l o w i n g equation: ZQ = 138 l o g i Q ( D b/D a) k where k i s the d i e l e c t r i c c onstant of p o l y e t h y l e n e , and are i n n e r and outer diameters of the brass coductors as i n d i c a t e d i n F i g . 5, and Z Q i s 16 ohms here. Between the brass conductors and the p o l y e t h y l e n e tubes ffl Polyethylene • Brass • 0- r i n g Quartz Window "^.^^ | Hole F i g . 5 Spark Chamber -16-s e v e r a l s e t s of O-rings are i n s e r t e d s o l t h a t the gap i s i s o l a t e d from the atmosphere. Four h o l e s are d r i l l e d through the chamber. One of them, serves the o u t l e t f o r e v a c u a t i n g and p r e s s u r i z i n g purposes. The others are f i t t e d with quartz windows. The e l e c t r o d e s can be i l l u m i n a t e d through these by u l t r a - v i o l e t l i g h t . They a l s o a l l o w the i n v e s t i g a t i o n of l i g h t emitted from the d i s c h a r g e . , The e l e c t r t o d e s are a l s o made of b r a s s . One of them i s p l a n e - s u r f a c e d . The other i s s l i g h t l y s p h e r i c a l i n order to l o c a l i z e the d i s c h a r g e i n the v i c i n i t y of the e l e c t r o d e s and yet keep the e l e c t r i c f i e l d s u f f i c i e n t l y uniform. b. Impedance-Matched Adaptor Since the input impedance of the Tek. 519 o s c i l l o s c o p e i s 125 ohms, the whole d i s c h a r g e c i r c u i t w i l l be mismatched when the o s c i l l o s c o p e i s connected d i r e c t l y t o the 16 ohm d i s c h a r g e system. A T-type T 16 / T 50 adaptor has been made, which can p r o p e r l y terminate a 16 ohm and a 50 ohm c i r c u i t s . Another adaptor, e.g. T 50 / T 125 adaptor, i s then connected and the d i s c h a r g e c u r r e n t pulses can be p r o p e r l y recorded with the o s c i l l o s c o p e . The s t r u c t u r e of the T 16 /T 50 adaptor i s shown i n F i g . 6 . The outer envelope i s made of two t h i n copper cones s o l d e r e d together and f i l l e d w ith epoxy to provide s a t i s f a c t o r y F i g . 6 Impedance-Matched A d a p t o r -18-i n s u l a t i o n . The values of the resistances Rt , Rz and R3., as indicated i n the figure, are 11.0, 5.3, and 45.8 ohms respectively. It gives an attenuation factor of 6.1 times from 16 ohm to 50 ohm end. F i n a l l y the geometrical structure of the adaptor must also s a t i s f y the equation mentioned i n the preceding section, with d i f f e r e n t Z a for each end. -19-CHAPTER 3 ELECTRICAL PARAMETERS 3.1 EXPERIMENTAL AND MEASURING TECHNIQUES a. E l e c t r o d e C l e a n i n g Procedures In order that the di s c h a r g e s occur at c o n s i s t e n t breakdown v o l t a g e to produce r e p r o d u c i b l e c u r r e n t p u l s e s , s l i g h t o v e r - v o l t a g e i s a p p l i e d a c r o s s the gap. T h i s g i v e s s a t i s f a c t o r y d i s c h a r g e s t r i g g e r e d by the e l e c t r o n s emitted from the cathode which i s i l l u m i n a t e d by u l t r a - v i o l e t l i g h t . B efore resuming experiments, the e l e c t r o d e s are p o l i s h e d and washed i n a l c o h o l , and a low pressure glow di s c h a r g e at about 20 t o r r i n d i f f e r e n t gases i s run between them f o r about twenty minutes to reduce, the absorbed l a y e r of oxygen on the e l e c t r o d e s u r f a c e s . These procedures are repeated over and over a g a i n to assure the r e l i a b i l i t y of c o n s i s t e n t breakdown. b. C a l i b r a t i o n of the Charging Voltage Measurement A c u r r e n t meter i s connected i n s e r i e s with a r e s i s t o r and then c a l i b r a t e d to i n d i c a t e the v o l t a g e a c r o s s t h i s combination. Connecting t h i s i n p a r a l l e l with the power supply, the c h a r g i n g v o l t a g e U Q can be measured. c. C a l i b r a t i o n of the Current Measurement -20-The d i s c h a r g e c u r r e n t t r a c e s recorded by the o s c i l l o s c o p e are a t t e n u a t e d by d i f f e r e n t combinations of a t t e n u a t o r s and adaptors. T h e r e f o r e the a t t e n u a t i o n f a c t o r s of d i f f e r e n t combinations have to be known i n order to c a l c u l a t e the magnitude of the d i s c h a r g e c u r r e n t . I t has been mentioned that f o r s u f f i c i e n t l y high gas pressures the d i s c h a r g e c u r r e n t w i l l r i s e and approach i t s maximum value I 0 = U 0/2Z 0 very r a p i d l y but nerer reach i t . As i l l u s t r a t e d i n F i g . Ad, n e v e r t h e l e s s , I(2T) + I(4T) i s approximately equal to I Q s i n c e the c u r r e n t s at 6T, 8T,... are n e g l i g i b l y s m a l l . S i m i l a r c u r r e n t o s c i l l o g r a m s a t d i f f e r e n t p r e s s u r e s and gap s e p a r a t i o n s are recorded and measured. The a t t e n u a t i o n f a c t o r s of d i f f e r e n t combinations of a t t e n u a t o r s and adaptors can thus be determined. 3.2 ANALYSIS OS THE OSCILLOGRAPHIC INVESTIGATIONS I t i s observed that the c u r r e n t o s c i l l o g r a m s of the spark d i s c h a r g e s i n the three gases i n v e s t i g a t e d ( hydrogen, n i t r o g e n and carbon d i o x i d e ) Hhave the s i m i l a r g e n e r a l c h a r a c t e r i s t i c appearance of three d i s t i n c t stages, i f the t o t a l pulse t r a n s i t time T i s long enough and the gas pr e s s u r e s ? a r e not so high as to cause the spark channel to develop too e a r l y . To a v o i d the appearance t o f the l a t t e r two stages, s h o r t e r c a b l e s with 2T •<Ctc are used and the d i s c h a r g e s are ". a r r e s t e d " i n the sense that no subsquent stages can develop and only the glow phase appears. -21-The f e a t u r e of these g l o w phases c o n s i s t s of a negative glow, a Faraday dark space and a p o s i t i v e column. The c u r r e n t o s c i l l o g r a m s of the a r r e s t e d t r a n s i e n t glow d i s c h a r g e s at d i f f e r e n t gap s e p a r a t i o n s and gas pressures f o r a l l these gases are reco r d e d and three t y p i c a l ones are presented i n F i g . 4e,4f,4g. By measuring the c u r r e n t at the time when the d i f f u s e glow s t a r t s to appear f o r each gas ( t^ as i n d i c a t e d i n F i g . 2b ), and u s i n g the r e l a t i o n : U ( t ) = U 0 - 2 Z 0 I ( t ) , the gap v o l t a g e s at d i f f e r e n t gas pr e s s u r e s and gap s e p a r a t i o n s can then be c a l c u l a t e d . These val u e s o b t a i n e d f o r the three gases are p l o t t e d a g a i n s t e l e c t r o d e s e p a r a t i o n i n F i g . 7a,7b,7c. In the normal d.c. glow d i s c h a r g e s the a x i a l p o t e n t i a l d i s t r i b u t i o n of the p o s i t i v e column can be determined by measuring the gap v o l t a g e s f o r v a r i o u s e l e c t r o d e s e p a r a t i o n s while keeping the d i s c h a r g e c u r r e n t constant (10, 11J . However i t has been shown by Cavenor and Meyer (9) t h a t f o r a hydrogen t r a n s i e n t glow produced i n 50 ohm c o a x i a l c a b l e d i s c h a r g e s at 500 t o r r the d i s t r i b u t i o n can be determined by simply measuring the gap v o l t a g e s at s t a t i c breakdown. Even the c u r r e n t s f o r d i f f e r e n t gap s e p a r a t i o n s are not the same. In t h e i r i n v e s t i g a t i o n s they f i r s t measure the gap v o l t a g e s at time tb f o r d i f f e r e n t e l e c t r o d e s e p a r a t i o n s under s t a t i c breakdown c o n d i t i o n s ( low o v e r - v o l t a g e ~ 1 % ). Then they perform the same measurements by keeping the c u r r e n t s at time -22-t b c o n s t a n t . In order to keep the c u r r e n t s a t tt, equal, they must apply a p p r o p r i a t e o v e r - v o l t a g e s as high as 30%. They f i n d t h a t the gap v o l t a g e s measured under these two d i f f e r e n t c o n d i t i o n s are c o n s i s t e n t . Furthermore, framing photos show t h a t the d i f f u s e glow does not change even though the over-v o l t a g e has been i n c r e a s e d . They t h e r e f o r e conclude t h a t f o r the c u r r e n t range of t h e i r i n v e s t i g a t i o n s the gap v o l t a g e i s independent of c u r r e n t . By p l o t t i n g the gap v o l t a g e s measured at low o v e r - v o l t a g e a g a i n s t gap d i s t a n c e a s t r a i g h t l i n e i s obtained. The s l o p e of t h i s l i n e t h e r e f o r e r e p r e s e n t s the a x i a l p o t e n t i a l g r a d i e n t i n the p o s i t i v e column and the e x t r a p o l a t i o n to zero gap s e p a r a t i o n r e s u l t s i n a p o t e n t i a l drop. I t has been suggested by Gambling and E d e l s (11) that t h i s . z e r o - l e n g t h v o l t a g e i s equal to the cathode f a l l v o l t a g e . We assume that the c o n c l u s i o n s of Cavenor and Meyer can be extended to higher c u r r e n t range as i n the case of our i n v e s t i g a t i o n s . Then the graphs p l o t t e d i n F i g . 7 r e p r e s e n t the a x i a l p o t e n t i a l d i s t r i b u t i o n s of the p o s i t i v e column d u r i n g the d i f f u s e t r a n s i e n t glow phase f o r these gases i n v e s t i g a t e d . The z e r o - l e n g t h v o l t a g e s and the r a t i o s of the a x i a l p o t e n t i a l g r a d i e n t to the gas pressure c a l c u l a t e d are l i s t e d i n the f o l l o w i n g t a b l e . -23-Table 1 E/P of Posi t i v e Column and Cathode F a l l Voltage V c of Hydrogen, Nitrogen, and Carbon Dioxide Glow Phase Gas E/P (V/cm-torr) V (V) Hydrogen 21.7±0 . 5 220+ 5 Nitrogen 36.7+1.0 240 ± 5 Carbon Dioxide 38.0+1.3 470 ± 10 I - 2 4 -F i g . 7 A x i a l P o t e n t i a l D i s t r i b u t i o n s of the Hydrogen, Nitrogen, and Carbon Dioxide Transient Glow E l e c t r o d e Separation (mm) 1 0.5 1.0 1.5 E l e c t r o d e Separation (mm) 2.0 2.5 -26-c. Carbon Dioxide P ( t o r r ) A 200 X 150 o 100 0.5 1.0 1.5 2;0 2.5 E l e c t r o d e Separation (mm) -27-CHAPTER 4 PLASMA PARAMETERS The gas temperature d u r i n g the glow phase i n n i t r o g e n sparks i s estimated from the s p e c t r o s c o p i c measurements. The corr e s p o n d i n g average e l e c t r o n d e n s i t y i s c a l c u l a t e d by measuring the magnitude of the d i s c h a r g e c u r r e n t and the diameter of the p o s i t i v e column. 4.1 EXPERIMENTAL ARRANGEMENT FOR TEMPERATURE MEASUREMENT The b l o c k diagram of the experimental arrangement i s shown i n F i g . 8. The l i g h t p u l s e s from the spark d i s c h a r g e column are focused and pass through the entrance s l i t of a low d i s p e r s i o n monochromator ( Bausch and Lomb 50 cm }. A R.C.A. type 931-A f a s t r i s e - t i m e p h o t o m u l t i p l i e r tube i s pl a c e d at the e x i t s l i t of the monochromator. The l i g h t p u l s e s are converted i n t o c u r r e n t p u l s e s which are then f e d i n t o the Tek. 1S1 sampling u n i t of a Tek. 549 storage o s c i l l o s c o p e . The o p t i c a l system i s a l i g n e d with the a i d of a He-Ne l a s e r . To c a l i b r a t e the r e l a t i v e s p e c t r a l response of the monochromator-photomultiplier system a tungston r i b b o n lamp i s used as the l i g h t source. By approximating the lamp as a blackbody, the r a t i o s of the r e l a t i v e s p e c t r a l r a d i a n c y of the l i g h t source t o the r e l a t i v e p h o t o m u l t i p l i e r response -28-F i g . 8 Experimental Arrangement f o r S p e c t r o s c o p i c I n v e s t i g a t i o n s Power Supply Monochromator j "--4:----Spark Chamber P h o t o m u l t i p l i e r Power Supply O s c i l l o s c o p e -29-c u r r e n t s f o r the same wavelengths can be c a l c u l a t e d . 4.2 ANALYSIS OF THE SPECTROSCOPIC INVESTIGATIONS The spark d i s c h a r g e s i n n i t r o g e n at 200 t o r r pressure and 1.0 mm gap d i s t a n c e are c a r r i e d out r e p e a t e d l y . To assure that the glow d i s c h a r g e i s a r r e s t e d and no subsquent 'discharges of high e r i n t e n s i t y can occur, a very s h o r t cable with t o t a l p u lse t r a n s i t time T = 5 ns i s used. The t i m e - r e s o l v e d r e l a t i v e i n t e n s i t y v a r i a t i o n of the l i g h t of d i f f e r e n t wavelengths emitted from the p o s i t i v e column i s i n d i c a t e d by the c u r r e n t t r a c e d i s p l a y e d on the o s c i l l o s c o p e s c r e e n . o F i g . 4h shows the c u r r e n t o s c i l l o g r a m f o r = 3580 A. I t i s noted that the time d u r a t i o n from the i n s t a n t the l i g h t i n t e n s i t y begins to r i s e to the i n s t a n t the i n t e n s i t y reaches i t s h i g h e s t value i s equal to ten nanoseconds as expected. The r e l a t i v e l i g h t i n t e n s i t i e s f o r d i f f e r e n t wavelengths at t = 10 ns are e v a l u a t e d and p l o t t e d i n F i g . 9. The s p e c t r a are i d e n t i f i e d as p a r t s of the second p o s i t i v e band system which u s u a l l y occurs i n the p o s i t i v e column of low pressure n i t r o g e n d i s c h a r g e tubes (12). T h i s p a r t i c u l a r band system i s emitted when n i t r o g e n molecules change from a higher e l e c t r o n i c s t a t e C % to a lower one B 3 i r . Each l i n e spectrum corresponds to a v i b r a t i o n a l s t a t e t r a n s i t i o n v — > v a . The observed s p e c t r a can be grouped i n t o f o u r bands with v-v' = 1,2,3,and 4. -OC-From the theory of diatomic molecular s p e c t r a , the l i g h t i n t e n s i t y I(v,v») of a v—=>vT s p e c t r a l l i n e i n an p(v,v') the v i b r a t i o n a l t r a n s i t i o n p r o b a b i l i t y (13). Thus we have: where G(v) i s the v i b r a t i o n a l energy of the v - l e v e l . The t r a n s i t i o n p r o b a b i l i t i e s of the n i t r o g e n second p o s i t i v e band system have been c a l c u l a t e d by Jarmain and N i c h o l l s (14). Using t h e i r r e s u l t s and e v a l u a t i n g the r e l a t i v e l i g h t i n t e n s i t y of each observed s p e c t r a l l i n e , we p l o t the band s t r e n g t h l o g e (IX/p) f o r each group with the same v-v.' and l o g ^ (Zlt IX^/ZItP ) f o r each v i n F i g . 10. e v' y From the nature of these graphs we can conclude that the e m i s s i o n of the second p o s i t i v e band system d u r i n g the t r a n s i e n t glow phase i s a process which i s dependent upon the s t a t i s t i c a l e q u i l i b r i u m of the n i t r o g e n molecules i n the i n i t i a l s t a t e at one d e f i n i t e temperature. T h i s e f f e c t i v e v i b r a t i o n a l temperature as c a l c u l a t e d from the s t r a i g h t l i n e i n F i g . 10b i s 2 7 0 0 ± 1 0 0 °K. U n f o r t u n a t e l y no i n f o r m a t i o n about the e f f e c t i v e r o t a t i o n a l temperature can be obtained, as the l i n e i n t e n s i t i e s are too weak tor.make p o s s i b l e an e l e c t r o n i c band system i s p r o p o r t i o n a l t o "4 N(v) p(v,v') X(v,v') where N(v) i s the p o p u l a t i o n of the molecules i n v - l e v e l , -32-F i g . 10 Band Strengths of N i t r o g e n Second P o s i t i v e Band Spectra G(v) ( 1000 1/cm ) 1 3 G(v) ( 1000 1/cm ) -33-a n a l y s i s of the r o t a t i o n a l s t r u c t u r e . However even i n n o n - e q u i l i b r i u m s i t u a t i o n s the gas temperature does not d e v i a t e much from the v i b r a t i o n a l temperature and we can t h e r e f o r e conclude t h a t our measurement i n d i c a t e s the gas temperature. 4.3 ELECTRON DENSITY IN THE POSITIVE COLUMN The d i s c h a r g e c u r r e n t o s c i l l o g r a m of the n i t r o g e n glow i s recorded with the Tek. 519 o s c i l l o s c o p e and the c u r r e n t magnitude at t = 10 ns i s measured. The c u r r e n t d e n s i t y can then be c a l c u l a t e d i f the c r o s s - s e c t i o n a l area of the di s c h a r g e column i s known. The measurement of the l a t t e r i s done by two d i f f e r e n t methods. One method i s to take the photographs of the di s c h a r g e column and then measure the l e n g t h of the image on the f i l m s . The other method i s to measure the outermost diameter of the anode spot l e f t by the d i f f u s e glow. The c u r r e n t d e n s i t y c a l c u l a t e d i s found to be 50+5 A/cm* The c u r r e n t d e n s i t y i s gi v e n by the e x p r e s s i o n : J = n_e_v_+ n + e + v + where n i s the number d e n s i t y , e the charge of the charge-c a r r i e r , v the d r i f t v e l o c i t y , with - and + s i g n s r e f e r r i n g to e l e c t r o n s and p o s i t i v e ions r e s p e c t i v e l y . For the n i t r o g e n t r a n s i e n t glow the value of E/P i s 36.7 V/cm-torr as measured i n chapter 2. The e l e c t r o n d r i f t v e l o c i t y at t h i s value i s - 3 4 -1.1X10 cm/sec and that of the p o s i t i v e i o n s i s o n l y about 1% of v_ ( 5 ) . T h e r e f o r e the c o n t r i b u t i o n of the p o s i t i v e i ons can be n e g l e c t e d ( n_«-*n +): J*»n_e_v_ . The average e l e c t r o n d e n s i t y i n the p o s i t i v e column thus e v a l u a t e d i s of the order of 3 X 1Q/3 e l e c t r o n s per c u b i c c e n t i m e t e r . -35-CHAPTER 5 DISCUSSIONS AND CONCLUSIONS 5.1 COMPARISON WITH 50 OHM COAXIAL CABLE DISCHARGES The cathode f a l l v o l t a g e and E/P of the p o s i t i v e column d u r i n g the hydrogen glow phase as measured i n our i n v e s t i g a t i o n s are 220 V and 21.7 V/em-torr r e s p e c t i v e l y . The c o r r e s p o n d i n g v a l u e s obtained by Cavenor and Meyer (9) f o r a hydrogen glow produced i n a 50 ohm c o a x i a l c a b l e d i s c h a r g e arrangement at 500 t o r r are 220 V and 20 V/cm-torr. Although the d i s c h a r g e c u r r e n t has been i n c r e a s e d by more than a f a c t o r of two by changing from a 50 to a 16 ohm impedance, i t i s seen that these values agree w e l l . T h i s f a c t i n d i c a t e s that d u r i n g the d i f f u s e glow phase of hydrogen spark d i s c h a r g e s i n i t i a t e d by the Townsend mechanism of breakdown, the c h a r a c t e r i s t i c s as d e s c r i b e d by constant values of E/P and cathode f a l l v o l t a g e are independent of the impedance of the e x t e r n a l c i r c u i t . Our experimental r e s u l t s show that f o r the glow phase of n i t r o g e n sparks and carbon d i o x i d e sparks, the p o t e n t i a l d i s t r i b u t i o n s e x h i b i t s i m i l a r c h a r a c t e r i s t i c s , i . e . constant E/P and constant cathode f a l l v o l t a g e . T h e r e f o r e i t i s reasonable to assume that these q u a n t i t i e s are c u r r e n t independent f o r n i t r o g e n , carbon d i o x i d e and any gas i n which the d i s c h a r g e passes through a glow phase. -36-5.2 COMPARISON WITH LOW PRESSURE D.C. GLOW DISCHARGES The o p t i c a l appearance of the d i f f u s e glow i s very s i m i l a r to that of normal d.c. glow discharges. The l i n e a r dependence on the gap distance of the a x i a l p o t e n t i a l d i s t r i b u t i o n of p o s i t i v e column i s another property i n common f o r these two types of glow. Nevertheless, the p o t e n t i a l gradient and the gas temperature i n the p o s i t i v e column of the former are much higher. In low pressure d.c. glow discharges, these q u a n t i t i e s are of the order of a few v o l t s per centimeter and s e v e r a l hundred degrees K e l v i n . Furthermore E/P f o r low pressure d.c. glow discharges i s not constant. Another i n t e r e s t i n g r e s u l t i s that the cathode f a l l v o l t ages f o r both types of glow i n the same gas are i n agreement. These values f o r copper and z i n c e l e c t r o d e s i n hydrogen, n i t r o g e n , and carbon d i o x i d e low pressure d.c. glow discharges are l i s t e d i n Table 2 (10). Comparison with our r e s u l t s f o r brass e l e c t r o d e s during the glow phase shows the d e v i a t i o n s are l e s s than 13%. Table 2 Cathode F a l l Voltages f o r Copper and Zinc Electrodes i n Hydrogen, Nitrogen, Carbon Dioxide Low Pressure D.C. Glow,Discharges *^~~----^ ^^  Gas E1 e c t r ode~~^^._ Hydrogen Nitrogen Carbon Dioxide Copper 214 208 460 Zinc 184 216 410 -37-T h i s agreement suggests t h a t the s i m i l a r i t y r e l a t i o n : holds f o r the t r a n s i e n t glow phase as w e l l , i . e . that the product of the t h i c k n e s s of cathode f a l l r e g i o n and gas pressure i s a constant f o r each gas. The product Pdc. , f o r example, f o r both copper and z i n c e l e c t r o d e s i n low pressure hydrogen d.c. glow d i s c h a r g e s i s 0.8 torr-cm (10). Assuming P d c f o r brass e l e c t r o d e s i n hydrogen t r a n s i e n t glow i s equal to t h i s v a l u e , then the average e l e c t r i c f i e l d i n the cathode f a l l r e g i o n Ec - V c/dc at d i f f e r e n t p ressures can be c a l c u l a t e d from our experimental r e s u l t s . The cathode f a l l f i e l d can a l s o be estimated by another independent method. We assume that d u r i n g the glow phase the space charge i n f r o n t of the cathode b u i l d s up u n t i l i t e s t a b l i s h e s the optimum f i e l d f o r ionizaioni*: For t h i s purpose we f u r t h e r assume that i o n p r o d u c t i o n i s predominately the r e s u l t of e l e c t r o n - m o l e c u l e i o n i z i n g c o l l i s i o n processes, d e f i n e d i n terms of a/P where a i s the f i r s t Townsend i o n i z a t i o n c o e f f i c i e n t . The maximum " e f f e c t i v e " value of a/P has been found to occur at E/P of the order of 300 V/cm-torr (15), and the average cathode f i e l d can be estimated. The r e s u l t s are i n agreement with the above e x p e r i m e n t a l l y determined v a l u e s . 5.3 THE TRANSITION NATURE OF THE DIFFUSE GLOW PHASE A f t e r the i n i t i a l breakdown of the gas, the d i f f u s e glow soon appears. Our a n a l y s i s i n the preceding chapter shows t h a t due to the high E/P, only a few nanoseconds l a t e r , the gas has - 3 8 -a l r e a d y been heated up to s e v e r a l thousand degrees K e l v i n and a hi g h e l e c t r o n d e n s i t y has a l r e a d y developed i n the v i c i n i t y of the di s c h a r g e a x i s . I t i s then expected that i f the di s c h a r g e i s not a r r e s t e d , the e l e c t r i c a l c o n d u c t i v i t y and the gas temperature w i l l be f u r t h e r i n c r e a s e d as the r e s u l t s of r e l a t e d i o n i z i n g and. h e a t i n g processes. High pressure and high e l e c t r o n d e n s i t y then b u i l d up along the di s c h a r g e a x i s and a spark channel soon develops whose expansion can be e x p l a i n e d by the theory based on the e l e c t r i c a l l y d r i v e n shock wave model. 5.4 CONCLUSIONS The glow phase of the high pressure spark d i s c h a r g e s i n i t i a t e d by the Townsend mechanism of breakdown i s q u a s i - s t a b l e i n nature. I t passes l a t e r through a glow-to-channel t r a n s i t i o n phase and then develops i n t o a h i g h l y conducting spark channel. T h i s d i f f u s e glow d i s c h a r g e e x h i b i t s a cathode f a l l r e g i o n and a uniform p o s i t i v e column a c r o s s which a c o n s i d e r a b l e p o t e n t i a l g r a d i e n t e x i s t s . The c h a r a c t e r i s t i c s of t h i s t r a n s i e n t glow, as d e s c r i b e d by constant E/P of p o s i t i v e column and by constant cathode f a l l v o l t a g e , are independent of the c u r r e n t l i m i t i n g impedance. C l o s e s i m i l a r i t i e s between t h i s glow phase and the normal d.c. glow d i s c h a r g e s have been noted. These s i m i l a r i t i e s suggest that many of the f e a t u r e s of the t r a n s i e n t glow, at present d i f f i c u l t to study, may be i n v e s t i g a t e d f u r t h e r by means of experiments c a r r i e d out on the d.c. -39-d i s c h a r g e s . As our s p e c t r o s c o p i c a n a l y s i s of the n i t r o g e n d i f f u s e glow i n d i c a t e s that the l i g h t emission i s of molecular o r i g i n , which i s a l s o the case f o r hydrogen (9), the r e s u l t s we obtained may be h e l p f u l f o r the study of d i s c h a r g e l a s e r s . -40-BIBLIOGRAPHY 1. Drabkina, S.I., 11 J . Exp. Theor. Phys. (USSR) " 1951, 21, p. 473 2. B r a g i n s k i i , S.I., " S o v i e t Phys. TEPT " 1958, 7, p. 1068 3. S o m e r v i l l e , J.M. and W i l l i a m s , J.F., " Proc. Phys. Soc. " 1959, 74, p. 309 4. Andreev, S.T., Vanyukov, M.P. and Kotolov, A.B., " S o v i e t Phys. Tech. Phys. " 1962, 7, p. 37 5. Raether, H., " E l e c t r o n Avalanches and Breakdown i n Gases " 1964, Butterworths, London 6. Schroder, G.A., " Proc. 7th I n t . Conf. on Phen. i n I o n i z e d Gases, Belgrad " 1966, I, p.606 7. Doran, A.A. and Meyer, J . , , r B r i t ; J . Appl. Phys. , r 1967, 18, p. 793 8. Doran, A.A. , " Z. Phys. " 1968, 208, p. 427 9. Cavenor, M.C. and Meyer, J . , " Aust. J . Phys. " 1969, 22, p. 155 10. Cobine, J.D.,," Gaseous Conductors " 1941, McGraw-Hill, V. New York 11. Gambling, W.A. and E d e l s , H., " B r i t . J . Appl. Phys. " 1954, 5, p. 36 12. Pearse, R.W.B. and Gaydon, A.G., " The I d e n t i f i c a t i o n of M o l e c u l a r S p e c t r a " 1963, Chapman and H a l l , London 13,,Herzberg, G., " M o l e c u l a r Spectra and M o l e c u l a r S t r u c t u r e , I. S p e c t r a of Diatomic Molecules " 1950, Van Norstrand, New York 14. Jarmain, W.R. and N i c h o l l s , R.W. " Can. J . Phys." 1954, 32, p. 201 15. Haydon, S.C. and Stock, H.M.P. , " Aus. J . Phys. 11 1966, 19, p. 795 

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