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Investigations of a spark gap with mercury-jet electrodes Ngo, Frank Quoc-Hai 1970

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INVESTIGATIONS OF A SFARK GAP WITH MERCURY-JET ELECTRODES by FRANK QUOC-HAI NGO B.Sc., 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 a c c e p t t h i s t h e s i s as c o n f o r m i n g t o the r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA May, 1970 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British 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 The University of British Columbia Vancouver 8, Canada ABSTRACT A spark gap with mercury j e t s as electrodes has been developed. Investigations show that the gap can be triggered r e l i a b l y by photons from a trigger spark with a j i t t e r i n the formative time lags less than 0.2 usee* To overcome uncertainties i n the breakdown conditions produced by surface waves on the j e t , i t has been found necessary to trigger the spark no l a t e r than 0.5 sec. a f t e r s t a r t i n g the jets„ By suitably designing the geometry o l the j e t s undesirable surface i n s t a b i l i t i e s excited by e l e c t r i c f i e l d s have been eliminated. Since the spark gap i s is o l a t e d e l e c t r i c a l l y from the trig g e r i n g source very l i t t l e noise i s produced i n neigh-bouring measuring c i r c u i t s when the main spark i s f i r e d . The continual regeneration of the electrode surfaces also eliminates the e r r a t i c features observed i n normal spark gaps which are f i t t e d with s o l i d metal electrodes. i i i TABLE OF CONTENTS Page ABSTRACT i i TABLE OF CONTENTS i i i LIST OF TABLE v LIST OF FIGURES v i ACKNOWLEDGEMENTS v i i CHAPTER I INTRODUCTION 1.1 I n t r o d u c t i o n 1 1.2 The Purpose o f The Experiment 2 1.3 B a s i c Problems 4 1.4 O u t l i n e o f The T h e s i s 5 CHAPTER I I APPARATUS AND EXPERIMENTAL PROCEDURE 2.1 L i q u i d Mercury E l e c t r o d e s 6 2.2 UV T r i g g e r i n g Technique 9 2.3 E x p e r i m e n t a l P r o c e d u r e 12 2.4 The D e l a y System 17 2.5 The V o l t m e t e r 18 2.6 The H i g h P o t e n t i a l D i v i d e r s 18 2.7_ The Mercury Vapour Pumping D e v i c e 19 CHAPTER I I I MEASURING TECHNIQUES AND RESULTS 3.1 Measurements o f J i t t e r i n F o r m a t i v e Time Lags 20 3.2 S e p a r a t i o n o f Mercury E l e c t r o d e s 21 3.3 Breakdown V o l t a g e V i 0 of Mercury E l e c t r o d e s 21 3.4 D i s c h a r g e L o c a t i o n o f Mercury E l e c t r o d e s 24 i v Page CHAPTER IV DISCUSSIONS AND CONCLUSIONS 4.1 C o n c l u s i o n s 2,6 4.2 D i s c u s s i o n s 27 4.2(a) S p l i t t i n g o f the mercury j e t s i n t o d r o p l e t s 27 4.2(b) C l e a n i n g t e c h n i q u e 29 4.3 F u t u r e Work 30 BIBLIOGRAPHY 32 V LIST OF TABLE Page Comparison o f D e l a y T r i g g e r Times t 16 v i LIST OF FIGURES Page 1 Geometries of s o l i d e l e c t r o d e s and l i q u i d e l e c -t r o d e s 3 2 C o n s t r u c t i o n o f l i q u i d mercury e l e c t r o d e s 7 3 The e l e c t r o s t a t i c a t t r a c t i o n f o r c e of the j e t s 8 4 UV t r i g g e r i n g c i r c u i t 10 5 B l o c k diagram o f e x p e r i m e n t a l p r o c e d u r e 13 6 O v e r - a l l e x p e r i m e n t a l c i r c u i t 14 7 D e l a y system s e t up 17 8 J i t t e r i n f o r m a t i v e time l a g s 22 9 D i s c h a r g e l o c a t i o n of mercury e l e c t r o d e s 25 10. S p l i t t i n g e f f e c t o f Hg j e t 28 v i i ACKNOWLEDGEMENTS I would l i k e t o e x p r e s s my deepest a p p r e c i a t i o n t o Dr. F. L. Cu r z o n , my s u p e r v i s o r , f o r h i s c o n s t a n t e n c o u r a ^ gement and gu i d a n c e , and h i s a s s i s t a n c e i n the w r i t i n g of t h i s t h e s i s . The s u g g e s t i o n s of Dr. M. P h i l l i p s a t the b e g i n n i n g o f t h i s work a r e g r a t e f u l l y acknowledged. The t e c h n i c a l a s s i s t a n c e of Mr. R. H a i n e s , Mr. P. Knopp and Mr. A. F r a s e r i s v e r y much a p p r e c i a t e d . My thanks a r e a l s o due t o Mr. J . Lees and Mr. E. W i l l i a m s f o r t h e i r h e l p w i t h the g l a s s w a r e . S p e c i a l thanks a r e due t o Mr. D. S i e b e r g , Mr. J . Aazam and Mr. R. Da C o s t a f o r t h e i r a s s i s t a n c e i n the maintenance of the equipment. I am i n d e b t e d t o a l l members of the Plasma P h y s i c s Group f o r t h e i r h e l p i n someway or the o t h e r . -1-CHAPTER I INTRODUCTION 1.1 INTRODUCTION In many plasma experiments, high voltage spark gap switches play a very important r o l e i n the equipment. When the gap i s triggered, i t acts as a switch through which a condenser bank discharges into the main discharge, for example, a Z-pinch. The spark electrodes are generally made of a high melting point metal so as to reduce electrode wear. (1) However, even with tungsten, the useful l i f e of the spark gap i s limited by electrode wear. In other words, after a comparatively short period of time i n operation the breakdown voltage of the electrodes s t a r t s to vary. The longer the electrodes are used, the larger the v a r i a t i o n . Such changes i n breakdown voltage are, as a matter of fact, very inconvenient for an experimenter. Further,the general tr i g g e r i n g method which connects an a u x i l i a r y triggering switch i n series with the main (2 ) electrodes, w i l l r e s u l t i n large coupling noises. These e l e c t r i c a l noise signals can be eliminated by means of a UV trig g e r i n g technique i n which an intense UV l i g h t i s used to i n i t i a t e the main switch but the UV l i g h t switch i s e l e c t r i c a l l y i s o l a t e d from the electrodes. Unfortunately, the trig g e r i n g technique only works i f the potential difference across the gap i s within a (7) few hundred vo l t s of the normal breakdown voltage V^* Fluctuations i n Vp caused by electrode damage are there-fore i n t o l e r a b l e . As we know the breakdown voltage between two ele c -trodes i s a function of the electrode-separation when the ( 3 ) working pressure i s kept constant. Therefore the v a r i a -tion i n breakdown voltage i s e s s e n t i a l l y due to the ero-(4) sion of the inner-electrode surfaces. caused by the high current discharges through the condenser bank. In order to get r i d of these variations and obtain a spark gap with a reproducible breakdown voltage, the surfaces of the electrodes must not be i r r e v e r s i b l y damaged when discharge i s taking place. However, t h i s i s impossi-ble for a s o l i d metal surface as we are working at high energies. 1 . 2 THE PURPOSE OF THIS EXPERIMENT It i s the purpose of t h i s experiment to develop " s e l f -healing" electrode surfaces consisting of l i q u i d metal j e t s rather than the usual s o l i d metal. A diagram i s given i n F i g . 1 to show the geometry of the newly developed l i q u i d electrodes and. that of s o l i d electrodes. - 3 -L i q u i d mercury Two Views o f L i q u i d E l e c t r o d e s F i g . 1 G e o m e t r i e s o f S o l i d E l e c t r o d e s arid L i q u i d E l e c t r o d e s Liquid electrodes are continually replaced and there-fore the ef f e c t s of erosion w i l l be automatically e l i m i -nated. We chose mercury for- the electrodes as i t serves as a convenient material (sparking w i l l not re s u l t i n chemical decomposition), thereby affording the p o s s i b i l i t y of manu-facturing a closed, spark.gap requiring l i t t l e s e r v i c i n g . 1.3 BASIC PROBLEMS From an operational point of view, there i s one over-a l l question: Having high voltage applied, w i l l the j e t s be stable enough so that the gap can be consistently t r i g -gered? As a high voltage i s applied to the j e t s , they are attracted towards each other, thereby reducing the break down voltage. E l e c t r o s t a t i c i n s t a b i l i t i e s may also roughen (5) the surface of the j e t s } again causing fluctuations i n the breakdown voltage of the spark gap. F i n a l l y , mercury vapours which are created during repe-t i t i v e operation must be removed because th e i r presence w i l l also a f f e c t the operation of the spark gap. * By "to be consistently triggered" we mean that the j e t s can be triggered with n e g l i g i b l e f l u c t u a t i o n i n the time i n t e r v a l between trigge r i n g , and eventual breakdown of the spark gap. -5-In t h i s thesis, we present the r e s u l t we obtained, i n a search of the answers to these problems and prove the r e l i a b i l i t y of t h i s l i q u i d switch. 1.4 OUTLINE OF THE THESIS The design and construction of l i q u i d Hg electrode switch can be found i n Section 2 .1 , where some advantages of the electrode-geometry are noted. A UV trigg e r i n g techni-que has been used to i n i t i a t e the Hg electrodes to break-down. This technique i s demonstrated i n Section 2.2, while the experimental procedure and equipments are given at the rest of Chapter 2. In Chapter-3, several techniques used i n investigations of the r e l i a b i l i t y of the l i q u i d e l e c -trodes, together with th e i r r e s u l t s , are given. In Chapter 4, conclusions are drawn from experimental r e s u l t s . Also, some notes i n maintaining the Hg j e t s are mentioned thereafter. -6-CHAPTER II APPARATUS AND EXPERIMENTAL PROCEDURE 2.1 LIQUID MERCURY ELECTRODES The construction of the l i q u i d mercury electrodes i s shown i n F i g . 2. Clean l i q u i d mercury i s contained i n two glass r e s e r v o i r s . Their outlets are separately connected by f l e x i b l e tygon tubes to st a i n l e s s s t e e l ones. A brass clamp i s placed across the tygon tubes and acts as an i n s -tantanuous stopper of the mercury. The j e t s are i n i t i a t e d by opening this stopper. As soon as i t i s open, mercury passes through the tygon tubes, and upper and lower s t a i n -less s t e e l tubes, f i n a l l y emerging as a pair of electrode-j e t s underneath. The upper st a i n l e s s s t e e l tubings are used as conduc-tors for charging purpose, and are hard soldered to two brass plates which form part of a p a r a l l e l plate transmis-sion l i n e connected to a capacitor bank. The lower s t e e l tubings are used to construct the l i q u i d electrode geometry. They are bent to approximately 45° with respect to the ver-t i c a l axis and are so oriented such that the j e t s coming out from these tubes w i l l pass by each other at a lower distance with a desired separation (see F i g . 3). In such a way, the j e t s are act u a l l y a pair of continually replaced, electrodes with electrode-distance equal to the minimum -7-Luc/fe / 6 5 " + „J1 1 rzzzz. *•• TZ Glass heservo/r Liquid HCJ mm Tygon tubing (O.D. 6.S , 1 . 0 . 3 . 2 5 " ™ r n ) V Clamp Lucite switch handle. Brass plate O-Ririg Upper stainless steel tubing ( QD. S > /. D. 2 . 8 w m J 'l/apor outlet 3.r Lucite centre ujalL Rubber gasket • Lower Stainless steel tubing C O.D. s~ w w , /. D. 2.fi 7" >") Insula, tor -O- /?/nc/ Tungsten U V e lectrodes ( electrode - separa ti'on = U Itlylar sheet THylor sheet roller Tygon tubing F i g . 2 Construction of Liquid Mercury Electrodes s e p a r a t i o n between them. The e l e c t r o d e - s e p a r a t i o n i n t h i s e x periment i s 3.97 i C%16 mm, a r e s u l t o f measurements g i v e n i n S e c t i o n 3.2. The v o l t a g e on t h e s e e l e c t r o d e s i s s i m p l y e q u a l t o the v o l t a g e a p p l i e d t o the b r a s s p l a t e s . The p r e -s e n t g e o m e t r i c a l c o n s t r u c t i o n o f e l e c t r o d e j e t s (see F i g . 3) has the advantage o f r e d u c i n g the e f f e c t s of e l e c t r o s t a t i c a t t r a c t i o n f o r c e between the j e t s . T h i s can be seen by n o t i n g t h a t the a t t r a c t i o n f o r c e i s some i n v e r s e f u n c t i o n o f the d i s t a n c e between the j e t s . Thus the f o r c e s which have c o n s i d e r a b l e e f f e c t a r e those a t the r e g i o n around the minimum e l e c t r o d e - s e p a r a t i o n . But s i n c e the j e t s are moving w i t h a v e r y h i g h v e l o c i t y , t hey e x p e r i e n c e the l a r g e s t a t t r a c t i v e f o r c e f o r a c o m p a r a t i v e l y s m a l l time i n t e r v a l and t h e i r geometry i s t h e r e f o r e not s i g n i f i c a n t l y a f f e c t e d by the e l e c t r i c f i e l d . F i g . 3 The e l e c t r o s t a t i c a t t r a c t i o n f o r c e i s overcome by the c o n s t r u c t i o n geometry and the v e l o c i t y o f the j e t s . - 9 -The empty space between the upper and lower s t e e l tubings serves the purpose of reducing some reverse mer-cury impulses which might occur immediately a f t e r the spark occurring between the j e t s . The j e t s with opposite charges are f i n a l l y c o l l e c t e d by two l u c i t e pools which are is o l a t e d from each other by a ce n t r a l l u c i t e w all. To prevent any accident d i s -charges from occuring due to splashing when the j e t s im-pinge on s o l i d surfaces, a mylar sheet i s placed across the central wall as shown i n the diagram of F i g . 2 and i t s two ends are joined into two r o l l e r s on opposite sides. When the used mylar surface i s contaminated with too many mercury droplets, we can obtain a new surface by simply turning the r o l l e r s . The whole mercury electrode system i s enclosed i n a gas tight l u c i t e container, to prevent mercury vapour from escaping into the atmosphere. 2.2 THE UV TRIGGERING TECHNIQUE The aim of trigg e r i n g a spark gap, generally speaking, i s to control the timing of the e l e c t r i c a l breakdown of the gap. In order to eliminate the e l e c t r i c a l noises which u M P C I s T D E T D £ /2.S KSl I t—1 o 1 UV trigger electrodes Hg electrodes . . . High potential power supply ( 50 mA, 20 KV ) Hg jet Opening tap Capacitor ( 0.32 uT ), Ci=C2^°« 6 4' |iF ( 12 KV ) Isolating switch _ . • . . . A i r gap switch Thyratron trigger unit ( 16 KV neg. pulse, 40 nsec r i s e time ) Tektronix type 160 delay unit ( delay range 0.1 to 10000 msec, output pulse 25 V ) Pulse generator ( 16 V neg. pulse ) F i g . 4 UV Triggering C i r c u i t -11-occur i n the g e n e r a l t r i g g e r i n g method mentioned i n S e c t i o n 1.1, we used, i n s t e a d , a UV t r i g g e r i n g technique. In t h i s technique, a s t r o n g photon f l u x from s m a l l t r i g g e r i n g e l e c t r o d e s i s e s s e n t i a l l y u t i l i z e d to i n i t i a t e the main gap to breakdown. But, the UV t r i g g e r i n g e l e c t r o d e s are e l e c t r i c a l l y i s o l a t e d from the main e l e c t r o d e s . The UV t r i g g e r i n g e l e c t r o d e s are simply made of two tungsten wires of 1 mm i n diameter with e l e c t r o d e - s e p a r a -t i o n of 4 mm. A diagram of the t r i g g e r c i r c u i t appears i n F i g . 4. A c a p a c i t o r C, formed by two 0.64 uF (12 KV working p o t e n t i a l d i f f e r e n c e ) c a p a c i t o r s i n s e r i e s , i s charged by a hig h v o l t a g e power supply P through a 260 K A r e s i s t a n c e c h a i n . The UV e l e c t r o d e s U are connected be-(1) tween the c a p a c i t o r C and an a i r gap switch S- which i s (6) t r i g g e r e d by a T h y r a t r o n t r i g g e r u n i t T. The T h y r a t r o n t r i g g e r u n i t i s i n t u r n i n i t i a t e d by a pulse generator E through a T e k t r o n i x type 160 delay u n i t D. In t h i s way, the breakdown on the UV e l e c t r o d e s immediately f o l l o w s the breakdown on the a i r gap s w i t c h . Note that the p o t e n t i a l (7) on the a i r gap switch has to be w i t h i n 4% of i t s break-down v o l t a g e before the a i r gap can be s u c c e s s f u l l y t r i g -gered by the T h y r a t r o n u n i t . In t h i s experiment, the c a p a c i t o r C was charged up to about 8,000 v o l t s , thus p r o v i d e d an i n t e n s e UV l i g h t when the gap U brokedown. -12-Originally, a quartz test tube, 6.35 ram in inner-diameter, was used to cover the UV electrodes so as to isolate the triggering unit from the main electrode dis-charges. Unfortunately,, mercury droplets caused by the main spark, collected on the tube, rendering i t opaque. Further, the contaminations can not be completely removed from the quartz by mechanical method. We f i n a l l y abandoned the quartz isolation and simply shifted the tungsten elec-trodes of gap U far enough from the main mercury gap for air to provide good insulation. Fortunately, the distance (3.5 cm) was small enough for the UV photon flux from U to trigger the main spark gap, 2,3 EXPERIMENTAL PROCEDURE The experimental procedure i s analysed by a block diagram shown in Fig. 5, while an overall experimental ci r c u i t appears in Fig. 6. A capacitor bank of maximum operating potential of 20 KV is connected in parallel to the main electrodes. In operation, this bank and the capacitor of the UV t r i g -ger cir c u i t were charged in parallel up to 9 KV and 8 KV respectively by a single high potential power supply. - 1 3 -High Po t e n t i a l Power Supply (20 KV) UV Liquid UV Triggering Trigger "Hg Electrode Electrode (9 KV) (8 KV) 2 * ^ H g Open Tap / Pulse Generator Tektronix •* 160 Series Delay Unit Thyratron Trigger Unit Tektronix P6013A High Pot. D i v i -der(1000 : 1) Tektronix P6013A High Pot. D i v i -der(1000 • 1) Tektronix Type 545A Oscilloscope Ext. Trig, •° Input Operation Procedure: 1 to 6 i n order F i g . 5 Block Diagram of Experimental Procedure 260KO. - s / W V ^ — IZ KV I L/OO "] Clomd sy*rem I yvj , 5, T D .Eg e l e c t r o d e system: M,Hg e l e c t r o d e s j S i , i s o l a t i n g s w i t c h ; C,50|j. A ammeter; UV t r i g g e r system: U,UV. e l e c t r o d e s ; S 2 , i s o l a t i n g s w i t c h ; S , a i r gap s w i t c h ; u n i t (16 KV neg. p u l s e , 40 nsec r i s e t i m e ) ; D , T e k t r o n i x ( d e l a y range 0.1 t o 10,000 msec); E , p u l s e g e n e r a t o r (16 Energy s o u r c e : P,high. p o t e n t i a l power s u p p l y (20 KV, 50 mA) -M e a s u r i n g system: A and B , T e k t r o n i x P6013A 1000 : 1 p o t e n t i a l d i v i d e r s ; C e R . O . , T e k t r o n i x 545A o s c i l l o s c o p e with T e k t r o n i x t ype G p l u g i n u n i t S 3 , s h o r t i n g s w i t c h T , T h y r a t r o n t r i g g e r type 160 d e l a y u n i t V neg. p u l s e ) --ype F i g . 6 O v e r - a l l E x p e r i m e n t a l C i r c u i t A g e n e r a l o p e r a t i o n p r o c e d u r e was the f o l l o w i n g : (1) Charge the c a p a c i t o r banks, (2) Open the mercury s t o p -p e r . (3) Wait u n t i l the h i g h v o l t a g e l i q u i d e l e c t r o d e s . were formed. (4) F i r e the UV t r i g g e r e l e c t r o d e s . The d e l a y t between t u r n i n g on the mercury j e t s and t r i g g e r i n g ( i t e m (3) above) i s r e q u i r e d f o r the f o l l o w i n g r e a s o n s . I f t i s too s m a l l (a few m i l l i s e c o n d s ) the UV gap i s f i r e d l o n g b e f o r e the mercury j e t s are c l o s e enough t o each o t h e r f o r t r i g g e r i n g t o be s u c c e s s f u l ; I f t i s t o o l a r g e ( s e v e r a l seconds) the j e t s are w e l l formed but s u r -f a c e waves e x c i t e d by the e l e c t r i c f i e l d s , o r t r a n s m i t t e d from where the j e t s impinge on the m y l a r , cause breakdown even though the UV s p a r k i s not a c t i v a t e d . W i t h t h i s s e t up, we found t h a t a 0.45 second d e l a y time ( t ) p r o v i d e d t h e " b e s t " performance. The b e s t p e r -formance was d e f i n e d as the c o n d i t i o n which e n a b l e d the mercury s p a r k t o be t r i g g e r e d s u c c e s s f u l l y e v e r y time the UV s p a r k was a c t i v a t e d . I n a d d i t i o n I r e q u i r e d t h a t the j i t t e r i n the breakdown time l a g ( t * ) of the mercury gap be a minimum. A l i s t o f d a t a i s shown i n T a b l e I where we compared the r e s u l t s from s e v e r a l d i f f e r e n t chosen d e l a y s . -16-DELAY t X . - ( s e c ) EXPT. ^ > s ^ RESULTS 0.40 0.45 0.50 ' T o t a l no. o f e x p e r i m e n t s '20 '20 20 Hg gap p r e b r e a k -down 0 1 4 Hg gap breakdown by t r i g g e r i n g J i t t e r > 0.2usec 8 2 2 Be s t performance J i t t e r __ 0.2usec 12 !7 14 V o l t a g e o f Hg gap = 9 KV, v o l t a g e o f UV gap - 8 KV T a b l e .1 Comparison o f D e l a y T r i g g e r Times t The r e l a t i v e j i t t e r s o f the f o r m a t i v e time l a g s * were found c o n s i s t e n t l y w i t h i n the o r d e r of 0.2 u.sec. The mea-s u r i n g t e c h n i q u e can be found i n S e c t i o n 3.1. Some t y p i c a l r e s u l t s a r e shown i n F i g . 8. F o r m a t i v e time l a g i s d e f i n e d as time i n t e r v a l between i n i t i a t i o n o f the l i q u i d e l e c t r o d e gap, and breakdown of the l a t t e r . -17-2.4 DELAY SYSTEM The d e l a y t was c o n t r o l l e d by a T e k t r o n i x type 160 d e l a y u n i t and a m e c h a n i c a l s e t up by which the handle of the mercury s t o p p e r w i l l s t a r t the p u l s e g e n e r a t o r i m m e d i a t e l y f o l l o w i n g an o p e n i n g a c t i o n . The s e t up i s shown d i a g r a m i c a l l y below: ///////// j r a r n * P u l s e G e n e r a t o r D e l a y U n i t T h y r a t r o n T r i g g e r U n i t I —Hg stopper To Hg E l e c t r o d e s ~*"To UV T r i g g e r i n g E l e c t r o d e s F i g . 7 D e l a y System Set Up Now, as mentioned i n S e c t i o n 2.2, the p u l s e g e n e r a -t o r was c o n n e c t e d by the 160 d e l a y u n i t t o the T h y r a t r o n t r i g g e r u n i t . An a p p r o p r i a t e t r i g g e r t i m i n g c o u l d t h u s be chosen by s i m p l y a d j u s t i n g the d e l a y u n i t . 2.5 THE VOLTMETER By virtue of the fact that the main capacitor bank and that of the trigger electrodes were connected i n para-l l e l to the high potential power supply ( see F i g . 6 ), only one voltmeter was needed for measuring the voltages of both the main and the trigger capacitor banks because they were of the same po t e n t i a l . A c a l i b r a t e d ammeter i n series with two p a r a l l e l avo resistances was used as a voltmeter ( see F i g . 6 ). The f u l l range of the ammeter, consisting of 50 d i v i s i o n s , cor-responded to a voltage range from zero to 11 KV. 2.@ THE HIGH POTENTIAL DIVIDERS Two Tektronix P6013A probes of attenuation factor 1000 : 1 and risetime 14 nsec were used as high potential dividers for the measurements of r e l a t i v e j i t t e r i n forma-ti v e time lags. These probes stand an input voltage up to 12 KV, thus they can be used to pick up the pulse ( 8 KV J due to the breakdown of the UV trig g e r i n g electrodes or the pulse ( 9 KV from the main l i q u i d electrodes without damage to the probes or the oscilloscope. -19-The accuracy of the output waveform of the probe should be frequently checked by adjusting f i v e variable trimmer capacitors i n i t s compensating box to match the input ca-pacitance of the oscilloscope. The method i s c l e a r l y des-cribed i n the manual. 2 .7 Hg VAPOUR PUMPING DEVICE Mercury vapour created at the time the mercury el e c -trodes discharged was large enough to change the working conditions ( i . e . d i e l e c t r i c strength) of the electrode gap. Therefore, i t i s important to-remove the mercury vapour from the system a f t e r "every" discharge so as to keep the working conditions constant. To remove the mercury vapour, an e f f i c i e n t vacuum device has been made from a f i l t e r i n g f l a s k , rubber stopper, several pieces of tygon and glass tubings, and a small Cenco-Hyvac vacuum pump. By means of t h i s device, a large f r a c -t i o n of Hg vapour w i l l condense into the fl a s k where some water was f i l l e d . The pressure i n the electrode system i s balanced by introducing dry a i r i n (see F i g . 2 ) . -20-CHAPTER I I I MEASURING TECHNIQUES AND RESULTS 3.1 MEASUREMENTS OF JITTER IN FORMATIVE TIME LAGS To measure the r e l a t i v e j i t t e r i n t h e breakdown time l a g s t ' o f the main gap, v/e employed the s e t up shown i n the b l o c k diagram o f F i g . 5 S e c t i o n 2,3. An 8 KV n e g a t i v e p u l s e appeared a t the e l e c t r o d e s of the UV gap. T h i s p u l s e was a t t e n u a t e d by a 1000 : 1 p o t e n t i a l d i v i d e r A ( a T e k t r o n i x P 6013 A probe J and t r i g g e r e d the time base o f a T e k t r o n i x 545 o s c i l l o s c o p e . The 9 KV n e g a t i v e p u l s e produced by the breakdown o f the main gap was a t t e n u a t e d by a n o t h e r 1000 : 1 probe B, and d i s p l a y e d on the o s c i l l o s c o p e which was f i t t e d w i t h a T e k t r o n i x type G p l u g - i n a m p l i f i e r . The time i n t e r v a l t ' between s t a r t i n g the time base and the p u l s e from the main s p a r k gap was the r e q u i r e d time l a g . The s t a n d a r d d e v i a t i o n of t * about i t s mean v a l u e was ta k e n as a mea-s u r e of the " j i t t e r " i n t ' . In the p r e s e n t i n v e s t i g a t i o n s , a l l work was done w i t h the t r i g g e r c a p a c i t o r bank C charge d t o 8 KV and t h a t of the main gap t o 9 KV. The l i q u i d e l e c t r o d e - s e -p a r a t i o n was s e t t o 3 . 9 6 + 0.16 mm thro u g h o u t the whole e x p e r i m e n t . -21-I t was found e x p e r i m e n t a l l y t h a t the j i t t e r o f the f o r m a t i v e t ime l a g s was n u m e r i c a l l y w i t h i n 0 . - 2 . u s e e and t h a t the d a t a were s a t i s f a c t o r i l y r e p r o d u c i b l e . Some t y p i c a l o s c i l l o g r a p h s a r e shown i n F i g . 8 . 3 . 2 LIQUID MERCURY ELECTRODE-SEPARATION d As we mentioned b e f o r e the e f f e c t i v e mercury e l e c -t r o d e - s e p a r a t i o n d i s the minimum d i s t a n c e between the i n t e r - e l e c t r o d e s u r f a c e s of the mercury j e t s (see F i g . 3). T h i s e l e c t r o d e - s e p a r a t i o n d was measured by means o f a t r a v e l l i n g m i c r o s c o p e and i t s mean v a l u e was c a l c u l a t e d t o be 3.97 mm t o an a c c u r a c y o f 4%. 3.3 THE BREAKDOWN VOLTAGE V b OF THE MERCURY ELECTRODES I n measurement of the minimum v o l t a g e which i s r e -q u i r e d f o r t h e l i q u i d mercury e l e c t r o d e s t o breakdown w i t h o u t UV t r i g g e r i n g , we used the f o l l o w i n g p r o c e d u r e : I s o l a t e the UV t r i g g e r i n g c i r c u i t from the h i g h p o t e n t i a l power s u p p l y by s w i t c h i n g on the i s o l a t i n g s w i t c h s z ( s e e F i g . 6 J . Charge up the main capae/itor bank t o d i f f e r e n t v o l t a g e s a t i n c r e m e n t s of 5 0 0 v o l t s per t r i a l . Open the mercury j e t s f o r each of t h e s e v o l t a g e s . C o n t i n u e -22-Time (1) No breakdown on Hg e l e c t r o d e s (2) Breakdown on Hg e l e c t r o d e s , but l o n g f o r m a t i v e time l a g (3) Breakdown on Hg E l e c t r o d e s , b e s t performance ... ( 4 ) Breakdown cn Kg e l e c t r o d e s , b e s t performance ( 5 ) No breakdown on Hg e l e c t r o d e s (6) Breakdown on Hg e l e c t r o d e s , b e s t performance (7) Breakdown on Hg e l e c t r o d e s , b e s t performance " —*• " : Zero p o t e n t i a l l i n e (1) (2) ( 3 ) ( 4 J : 2 u s e c / d i v h o r i z o n t a l , 10 V / d i v ^ v e r t i c a l (5) (6) (7) : 1 u s e c / d i v h o r i z o n t a l , 10 V / d i v v e r t i c a l V o l t a g e waveforms w i t h o s c i l l o s c o p e t r i g g e r e d e x t e r n a l l y by UV e l e c t r o d e s breakdown F i g . 8 J i t t e r i n F o r m a t i v e Time Lags 0 --23-t h i s p r o c e s s u n t i l t he f i r s t s p a r k o c c u r r e d between the j e t s . Around the v o l t a g e o f the f i r s t s p a r k , choose a narrow range i n i n c r e m e n t s (about 150 v o l t s ) and r e p e a t the p r o c e s s . A v o l t a g e a t which the j e t s would breakdown ( 8 ) a t 90% of the a p p l i c a t i o n s - was c o n s i d e r e d as the br e a k -down v o l t a g e V b of the l i q u i d e l e c t r o d e s . The breakdown v o l t a g e V b of the p r e s e n t gap d i s t a n c e ( S J 4 mm) was thus found t o be 7.7 KV by a r e a d i n g on the v o l t m e t e r which was d e s c r i b e d i n S e c t i o n 2.5. I t s h o u l d be p o i n t e d out t h a t t h i s breakdown had a c o m p a r a t i v e l y l o n g d e l a y . U s u a l l y , i t took s e v e r a l seconds when the j e t s were w e l l formed t i l l a s p a r k took p l a c e . T h e r e f o r e i n c o n s i d e r i n g V^, we have t o ta k e i n t o a ccount the e f f e c t of a t t r a c t i o n f o r c e betv/een the j e t s and t h a t of the s u r f a c e waves e x c i t e d by the e l e c t r i c f i e l d s , o r by the my l a r s h e e t on which the j e t s were i m p i n g i n g . As a r e s u l t o f t h e s e e f f e c t s , the a c t u a l e l e c t r o d e - s e p a r a t i o n was r e d u c e d . I t i s c l e a r t h a t t h i s v o l t a g e V^ i s lo w e r than the breakdown v o l t a g e o f the j e t s when th e y a r e i n i -t i a l l y formed. F or t h i s r e a s o n , the v o l t a g e ( 9 KV) we a p p l i e d t o the j e t s f o r UV t r i g g e r i n g purpose was h i g h e r t h a n Vjj (7.7 KV) s i n c e the j e t s were s t i l l a t i n i t i a l s t a g e when they were t r i g g e r e d . -24-3.4 DISCHARGE LOCATION OF MERCURY ELECTRODES To i n s u r e m i n i m a l damage i t i s e s s e n t i a l t h a t the s p a r k between the j e t s be as f a r as p o s s i b l e from i m p o r t -ant i n s u l a t i n g s u r f a c e s . The l o c a t i o n o f the s p a r k was o b s e r v e d p h o t o g r a p h i c a l l y . A t y p i c a l photograph i s shown i n F i g . 9 where two vie w s o f a s p a r k d i s c h a r g e were t a k e n on the same p i c t u r e by s e t t i n g up a m i r r o r b e s i d e the mercury e l e c t r o d e system. T h i s p i c t u r e demonstrated t h a t the mercury e l e c t r o d e gap was l o c a t e d a t the minimum s e p a r a t i o n between the j e t s , f a r from i m p o r t a n t i n s u l a t o r s . -25-Two Views of A Spark D i s c h a r g e Diagram c o r r e s p o n d s t o R.H.S. of above photograph F i g . 9 D i s c h a r g e L o c a t i o n of Hg E l e c t r o d e s - 2 6 -CHAPTER IV DISCUSSIONS AND CONCLUSIONS 4 . 1 CONCLUSIONS The b a s i c problems, which were proposed a t the b e g i n -n i n g of t h i s t h e s i s , c o n c e r n i n g the o p e r a t i o n o f the l i q u i d mercury e l e c t r o d e s , have been s o l v e d . We have found t h a t the Coulomb f o r c e and the s u r f a c e i n s t a b i l i t i e s have n e g l i g i b l e i n f l u e n c e on the l i q u i d e l e c -t r o d e geometry p r o v i d e d t h a t the e l e c t r o d e s a r e t r i g g e r e d a t an e a r l y formed s t a g e . An a p p r o p r i a t e t r i g g e r e d t i m i n g has been worked out by means o f a d e l a y system and a mecha-n i c a l s e t up. An i n t e n s e p u l s e d u l t r a v i o l e t l i g h t s o u r c e has been used t o t r i g g e r the l i q u i d e l e c t r o d e s , but the q u a r t z b u l b i n s u l a t o r p r e v i o u s l y used by Curzon and Smy has been shown t o be s u p e r f l u o u s . F o r our purposes a i r i s a s u f f i c i e n t l y s a f e i n s u l a t o r . In o r d e r t o a v o i d mercury vapour p o i s o n i n g , we have e n c l o s e d the e l e c t r o d e system i n a g a s - t i g h t l u c i t e h o u s i n g . The w o r k i n g c o n d i t i o n s a r e m a i n t a i n e d unchanged by removing -27-mercury vapours from the e n c l o s e d system and f i l l i n g i n w i t h d r y a i r , a f t e r e v e r y d i s c h a r g e e x p e r i m e n t . A l t h o u g h e r o s i o n may o c c u r on the UV t r i g g e r e d t u n g s -t e n e l e c t r o d e s , no e f f e c t which s i g n i f i c a n t l y a f f e c t s i t s u s e f u l n e s s has been fo u n d . E x p e r i m e n t a l r e s u l t s show t h a t the newly d e v e l o p e d l i q u i d mercury e l e c t r o d e s w i t c h i s a r e l i a b l e s p a r k gap s w i t c h which i s f r e e from the problems u s u s l l y a s s o c i a t e d w i t h e l e c t r o d e e r o s i o n . F u r t h e r m o r e i t has been demons-t r a t e d t h a t the j i t t e r i n the f o r m a t i v e time l a g s i s neg - l i g i b l e , and t h a t the low n o i s e photon t r i g g e r i n g t e c h n i q u e works v e r y w e l l . 4.2 DISCUSSIONS In t h i s s e c t i o n , we p r e s e n t and d i s c u s s some problems and o b s t a c l e s which a r o s e , and su g g e s t f u r t h e r t o p i c s which need t o be i n v e s t i g a t e d . 4.2 (a) S p l i t t i n g o f the mercury j e t s i n t o d r o p l e t s As the p r e s e n t l i q u i d e l e c t r o d e s a r e formed by -28-w e l l c o n f i n e d mercury j e t s of d i a m e t e r s 2.8 mm, i f the j e t s p l i t s up i n t o d r o p l e t s , the e l e c t r o d e - g e o m e t r y w i l l be c o m p l e t e l y r u i n e d . U n f o r t u n a t e l y , the s p l i t t i n g e f f e c t i s u n a v o i d a b l e once gas b u b b l e s appear i n s i d e the tygon t u b e s . These b u b b l e s become more p r e v a l e n t as the mercury g e t s d i r t y . Clean Hg Tycjon. Tubes Dust-!/7<e impurities OLir sections Stain/ess 5 fee/ -tubes -&^ ^ jet sphts Smooth M o t i o n T u r b u l e n t M o t i o n F i g . 10 S p l i t t i n g E f f e c t of Hg J e t The e x i s t e n c e o f the s e b u b b l e s i n s i d e the tygon tube seems t o be due t o the presence o f d u s t - l i k e i m p u r i t i e s on the tygon w a l l . These i m p u r i t i e s may be p r o d u c t s c r e a t e d d u r i n g mercury s p a r k s . T h e i r presence may a l s o change the c h a r a c t e r i s t i c s o f mercury e l e c t r o d e s . To p r e v e n t t h e s e e f f e c t s from happening, both the mercury and the tygon tubes must be c l e a n e d from time t o t i m e . 4.2 (b) C l e a n i n g t e c h n i q u e When the l i q u i d e l e c t r o d e system has been o p e r a t e d f o r a c o n s i d e r a b l e p e r i o d o f t i m e , a l l components (e . g . m y l a r s h e e t , l u c i t e c o n t a i n e r , t u n g s t e n e l e c t r o d e s ) i n s i d e the system w i l l be c o n t a m i n a t e d by l a r g e amount o f mercury d r o p l e t s and mercury compounds. When the d i r t y mercury i s r e p l a c e d , the i n t e r i o r o f the s p a r k chamber ( i n c l u d i n g a l l p a r t s ) i s r i n s e d w i t h d i l u t e n i t r i c a c i d c o n t a i n i n g 25% a c i d and 75% d i s t i l l e d w a t e r . The mercury and i t s compounds can be c o m p l e t e l y removed from t h e s u r f a c e s o f a l l components, w i t h o u t s i g -n i f i c a n t c h e m i c a l d i s s o l u t i o n on t h e i r s u r f a c e s . F o r s a f e t y , d u r i n g the c l e a n i n g j o b i t i s n e c e s s a r y t o wear a p a i r of -30-r u b b e r g l o v e s t o p r e v e n t s k i n c o n t a c t w i t h a c i d o r mercury. ' 4 . 3 FUTURE WORK The r e l i a b i l i t y o f t h e l i q u i d e l e c t r o d e s , o p e r a t e d by the p r e s e n t UV t r i g g e r i n g t e c h n i q u e , depends m a i n l y on the t r i g g e r i n g t i m i n g t . When an a p p r o p r i a t e t has been worked o u t , i t i s n e c e s s a r y f o r t t o be a c c u r a t e l y r e p r o -d u c i b l e so as t o reduce the j i t t e r i n f o r m a t i v e time l a g . The t r i g g e r i n g t i m i n g t was c o n t r o l l e d by a d e l a y system and a m e c h a n i c a l s e t up. As the l a t t e r was o p e r a t e d by hand, a l a r g e random e r r o r would always c o n t r i b u t e t o t . I n o r d e r t o improve t h e a c c u r a c y and r e p r o d u c i b i l i t y of t , an e l e c t r i c a l d e v i c e i s needed t o r e p l a c e human h a n d l i n g . I t i s c o n v e n i e n t t o use a c o n t i n u o u s mercury system, i . e . have the o u t p u t pumped d i r e c t l y backed t o the i n p u t of t he system. Such a system e l i m i n a t e s the cumbersome t a s k s o f f i l l i n g o r w i t h d r a w i n g mercury. The p o s s i b i l i t y o f s p i l l a g e o f mercury d r o p l e t s and the escape o f po i s o n o u s mercury vapours i s a l s o r e d u c e d . In the p r e s e n t i n v e s t i g a t i o n s , o n l y one f i x e d e l e c t r o d e -31-s e p a r a t i o n ( 3 . 9 6 i 0.16 ram) has been used. F u r t h e r s t u d i e s o f t h e l i q u i d s w i t c h s h o u l d be made by a l t e r i n g the e l e c -t r o d e - s e p a r a t i o n . BIBLIOGRAPHY Chan, P. W„ Master's Thesis, Department of Physics, University of British.Columbia (1963) Medley, S. S., Curzon, F. L. and Daughney, C. C. Can. Journal of Physics .43, 1882 (1965) Paschen's law, = F(pd) (where P pressure, d e l e c -trode distance), was.discovered by Paschen experi-mentally and was proved t h e o r e t i c a l l y by Townsend, Thomson and others. For d e t a i l s , readers are referred to: Paschen, F. Wied. Ann., 37, 69 (1889) Townsend, J . S. E l e c t r i c i t y i n Gases, pp. 327, 380, Oxford (1915) Thomson, J . J . Conduction of E l e c t r i c i t y Through Gases, 3rd E d i t i o n , V ol. 2, p. 486, Cambridge (1933) See, for example, Loeb, L. B. Fundamental Processes of E l e c t r i c a l Dis-charge i n Gases, p. 452, John Wiley and Sons Inc. (1939) Melcher, J . R. Field-Coupled Surface Waves, Chap.l and Chap.6, MIT Press (1963) Theophanis, G. A. Review of S c i e n t i f i c Instruments 31, 427, (1960) Curzon, F. L. and Smy, P. R. Review of S c i e n t i f i c Instruments,32, 756, (1961) Loeb, L. B. Fundamental Processes of E l e c t r i c a l Dis-charge i n Gases, John Wiley and Sons Inc. (1939) p. 465 

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