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UBC Theses and Dissertations

Some investigations into spark gap recovery in air and hydrogen Burnett, Neal Harvey 1967

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SOME INVESTIGATIONS INTO SPARK GAP RECOVERY IN AIR AND HYDROGEN BY NEAL HARVEY BURNETT B . S c , U n i v e r s i t y of B r i t i s h Columbia, 1966 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE IN THE DEPARTMENT OF PHYSICS We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA AUGUST, 1967 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced deg ree a t t he U n i v e r s i t y o f B r i t i s h C o l u m b i a , I ag r ee t h a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and S t u d y . I f u r t h e r ag r ee t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by the Head o f my Depar tment o r by h.iJs r e p r e s e n t a t i v e s . It i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l no t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Depar tment o f The U n i v e r s i t y o f B r i t i s h C o l u m b i a Vancouve r 8, Canada Date Sou^JT \ \ ° i k 7 ABSTRACT Studies were conducted into seve ra l aspects o f the recovery o f a spark gap cons i s t i ng o f 6 mm. diameter, f l a t tungsten e l ec t rodes , a f t e r an i n t i a l discharge o f 32 kamp. maximum, and durat ion l l ^ s e c . The c o n t r o l l i n g e f f e c t o f e lect rode heat t ransport on the intermediate recovery o f the spark gap was demonstrated us ing a i r at 760 mm. Hg as a discharge medium, and gap lengths of 3,4, and 5 mm. In the intermediate stages of recovery , the increase i n breakdown s t rength o f the gap was found to be p ropor t i ona l to t 2 , and to be near ly independent o f gap l eng th . These resu l t s are expla ined on the basis o f un iaxu lar heat f low, through the e l e c t rodes . A p rev ious l y observed long-time or delayed recovery i n a hydrogen spark gap between c lean tungsten e lectrodes was invest igated to e s t ab l i sh the poss ib l e ro l e o f hydrogen-tungsten adsorpt ion i n th i s e f f e c t . The temp-eratures o f the e lectrodes were va r i ed by means o f smal l heat ing c o i l s , a n d the recovery cha r a c t e r i s t i c s f o r heated e lectrodes measured. By comparing these r esu l t s to the normal recovery c h a r a c t e r i s t i c , the phenomana o f d e -layed recovery was shown to be at l eas t p a r t i a l l y a t t r i bu t ab l e to the adsorpt ion of hydrogen onto the e lect rode su r f aces . i i TABLE OF CONTENTS Page ABSTRACT i i INDEX OF FIGURES i v INDEX OF TABLES v ACKNOWLEDGEMENTS v i INTRODUCTION 1 CHAPTER 1 ~ APPARATUS L 1) High Current Discharge C i r c u i t 5 2) The Triggering C i r c u i t 7 3) The -14-kv Trigger Generator 7 L) The Re s t r i k i n g Generator 7 CHAPTER 11 — Spark Gap Recovery i n Air..- 11 HtSS^ lX'bS • • • e o 0 « . « « * » * » » o o e o o o e o o o « o o o 9 O o o o o « o e o o 0 o » » "12 T ilS 0 » f l © O » O O O O « « » » O O © O O O © O O O O O O O O 0 O « O O O O « « O » O O « « « "1 6 CHAPTER 111 — S p a r k Gap Recovery i n Hydrogen 19 CHAPTER 1V — Conclusions and Suggestions f o r Future Research 33 B jLB LIOGRAPHY" o * t t o « o o o * « « « « « o o o o Q o o o * 0 o o * o * o * « « 6 * 9 « a * « o * o o o 9 * o o « 35 i i i INDEX OF FIGURES Figure Rage 1.1 Basic Components of the Apparatus .....»....».».... 4 *1 s2 TtlQ DlSCh£LI*^8 V©SS©1 Q g O O O O O O O e O O O O O O O O O O O Q O O O O O O O O ^ 1.3 The Trigger Generator 7 1.4 The R e s t r i k i n g Generator .......................... 8 1.5 Discharge Current Monitor ......................... 9 1.6 Output of Current Monitor ......................... 9 1.7 High Current Discharge ............................ 10 2.1 Recovery i n A i r at 3 mm. Gap Length ............... 13 2.2 Recovery i n A i r at 4 mm. Gap Length „„„„........... 13 2.3 Recovery i n A i r at 5 mm. Gap Length ............... 14 2.4 Figures 2.1-2.4, Superimposed ..................... 14 2.5 Percentage Recovery i n A i r „........„„,........ b... 15 2.6 Electrode Parameters .............................. 17 3.1 Heated Electrode ............. „.................... 20 3.2 Electrode Cooling Curve ..„.„...................... 21 3.3 E f f e c t of Electrode Heating on Recovery i n Hydrogen. 25 3.4 Recovery i n Hydrogen at 525 mm. Hg ................ 27 3.5 E f f e c t of Cathode Heating on Recovery i n A i r ...... 29 3.6 E f f e c t of Cathode Temperature on Breakdown Voltage i n the Late Recovery Period ............... 32 i v INDEX OF TABLES Table Page 2.1 Recovery C h a r a c t e r i s t i c s i n A i r , at 3, U, and 5 mm. Electrode Seperations ........... 12 3.1 Recovery C h a r a c t e r i s t i c s i n Hydrogen, with Electrodes Heated to 120°C 23 3.2 Recovery C h a r a c t e r i s t i c i n Hydrogen at 525 mm. Hg .. 26 3.3 Recovery C h a r a c t e r i s t i c i n A i r , with Cathode Heated to 120°C 28 3.4 Recovery Voltage i n Hydrogen, as a Function o f Cathode Temperature 3^ v ACKNOWLEDGEMENTS I wish to thank my supervisor, Dr. F. L. Curzon, on whose suggestions and advise, most o f th i s work i s based, and whose patience has been much appreciated. I am also indepted to Mr. J . Dooyeweerd, and other members of the tech-n i c a l s t a f f of the plasma group f o r help i n the construction and maintenance of the apparatus. v i 1 INTRODUCTION Spark gaps are widely used as switching elements i n high voltage c i r c u i t s , and thus the study of the recovery of the breakdown strength of these devices a f t e r discharge i s of importance, e s p e c i a l l y i n applications where spark gaps are discharged at high r e p e t i t i o n r a t e s . Immediately a f t e r a discharge, there remains between the spark gap electrodes, a column of hot ionized gas, thus f o r a period of time, up to sever a l seconds a f t e r a discharge, the resistance o f the gap to r e - i g n i t i o n w i l l be less than i t s normal sparking voltage. The recovery process i s p h y s i c a l l y very complex, i t s r a p i d i t y depending on, the rate of recombination of electrons and ions i n the spark channel, the rate o f cooling of the gas i n the channel, the rate o f cooling of the electrode surfaces, and i n some cases, on other electrode e f f e c t s . I t i s known that electron-ion recombination w i l l r e s u l t i n a neutral gas between the gap electrodes w i t h i n a few hundred microseconds of the passage o f a high current discharge. Because of acoustic wave propogation, the pressure i n the channel w i l l also r a p i d l y reach equilibrium. Thermal e f f e c t s i n the recovering spark channel are, however, of a somewhat larger time constant. The cooling of the inter-electrode gas i s somewhat d i f f i c u l t to t r e a t exactly; and depends on the gap geometry. Curzon & Gautam [ij have presented a one dimensional model of heat flow, i n a recovering spark channel with the same electrode geometry as that which was used here, taking into account the e f f e c t of electrode heat transport. Other electrode e f f e c t s , such as adsorption o f the discharge gas onto the electrode surfaces may be important, i f the layer of adsorbed gas were removed during a discharge, the re-adsorption of gas would modify the e l e c t r o n emission c h a r a c t e r i s t i c s 2 of the cathode. In t h i s report, the term recovery c h a r a c t e r i s t i c , pertaining to spark gap i s to be taken to mean a pl o t of breakdown voltage versus the delay time a f t e r an i n i t i a l discharge of the gap. Because of the random nature of spark breakdown , there are several c r i t e r i a that can be used to define the breakdown voltage. The most usual i s the s o - c a l l e d 50% breakdown v o l -tage, that i s , the amplitude of a voltage pulse which causes the gap to breakdown i n more than 50% of the number of t r i a l s . As could be expected, the breakdown voltage defined i n t h i s manner depends on the shape of the r e s t r i k i n g pulse. In this experiment a step function r e s t r i k i n g pulse was used, and the 50% c r i t e r i o n adopted, with the a d d i t i o n a l requirement that breakdown occur before 100/isec. a f t e r applying the r e s t r i k i n g pulse to the gap. A j u s t i f i c a t i o n of t h i s c r i t e r i o n i s given by M. S. Gautam [2JJ . Several aspects of the recovery problem have been investigated i n t h i s experiment, f o r a spark gap of diameter c y l i n d r i c a l tungsten electrodes, through which was i n i t i a l l y passed an arc discharge of 32 kamp. maximum, and of 11yucsec duration. In chapter one, the experimental setup i s describ-ed i n d e t a i l . In chapter two r e s u l t s of recovery measurements at various electrode separations, i n a i r at 760 mm. Hg, are presented, and the r e s u l t s compared to those expected on the basis of Curzon's model [ l ] of one dimensional heat flow. The main features of the recovery c h a r a c t e r i s t i c s are found to be explained quite s a t i s f a c t o r i l y on the basis of t h i s model. In chapter three, r e s u l t s are presented of investigations into the phen-omenon of long time (>100ms) recovery i n hydrogen spark gaps, f i r s t noted by M. S. Gautam [2] . The dominant r o l e i n t h i s e f f e c t , of hydrogen adsorp-3 t i o n onto the electrode surfaces, i s established by comparing the recovery c h a r a c t e r i s t i c s of gaps i n which the electrode surfaces were heated by means of a u x i l i a r y heating c o i l s , to the recovery c h a r a c t e r i s t i c i n the normal case. Chapter four contains a discussion o f the r e s u l t s and some suggestions f o r future research. u CHAPTER ONE APPARATUS The equipment used i n t h i s experiment was very s i m i l a r to that described by M. S. Gautam [2] > a n d C h u r c h i l l , Parker, and Craggs [3] , consequently a somewhat abridged d e s c r i p t i o n w i l l be presented here. The basic components were; 1. The main discharge c i r c u i t , which produced a u n i d i r e c t i o n a l current pulse of 32 kamp maximum, and o f 11yu. sec duration, through the t e s t gap. 2. A delay unit with some associated t r i g g e r i n g c i r c u i t r y . 3. A r e s t r i k i n g generator which provided a step function voltage s i g n a l of 3-12 kv amplitude across the tes t gap, when triggered from the delay unit A schematic diagram of these elements i s presented below; FIG. 1.1 Unit 6*3 3 G-jit "R = T = C ipoox Probe. BASIC COMPONENTS OF THE APPARATUS The high current discharge was i n i t i a t e d by applying a -14 kv pulse to electrode 2, causing the capacitor C to discharge through the non-linear 5 r e s i s t o r R, and the t e s t gap . ^ a v a r : i a D l e delay time, a f t e r the i n -i t i a t i o n of the high current discharge, the r e s t r i k i n g generator was t r i g g e r -ed, and a step function voltage s i g n a l applied across the t e s t gap. The amplitude o f the step function was varied systematically to determine the breakdown voltage, V-^ , as a function of delay time, t^„ THE HIGH CURRENT DISCHARGE CIRCUIT; The high current discharge c i r c u i t consisted of a capacitor bank, a non-linear damping r e s i s t o r , and a discharge v e s s e l , to which was attached the t r i g g e r i n g spark gap assembly shown i n fi g u r e 1.1. The capacitance was provided by two NRG type 201 capacitors, connected i n p a r a l l e l , g i v i n g a t o t a l o f 10/*,F. These could be charged to 20kv by means of a high voltage power supply, y e i l d i n g a maximum charging current of 50 ma. The non-linear r e s i s t o r was a type 801/22, Y6535333 AX, man-ufactured by the Morganite Company of Canada. The use of the non-linear r e s i s t o r allowed a shorter, u n i - d i r e c t i o n a l current pulse than could have been obtained using a normal damping r e s i s t o r . The device had a resistance of 17.5_n.at 4kvj more d e t a i l e d s p e c i f i c a t i o n s can be found .in [V | . The t e s t gap, G a / , was mounted inside a sealed, c y l i n d r i c a l , brass 34 chamber, with four protruding viewing ports, and l u c i t e windows. The t e s t electrodes were -5-" diameter tungsten rods, mounted i n brass holders, and generally spaced 5mm apart. The switching gaps G , 1 9 and G , consisted of ' I * 23 5/8" diameter tungsten rods, mounted on top of the discharge vessel as shown i n f i g u r e 1.2. The switching gaps G ^ and &23' w e r e s o arranged that applying -14kv pulse to electrode 2 (Fig 1.1), broke down G.^, thus applying the f u l l v o l -tage of the capacitor bank to G ? o and G,,, whiGh subsequently broke down, discharging the bank. 6 FIG. 1.2 m 15* — A., f i VTTA L S3 to high current generator to -Kkv trigger generator to restriking circuit - test gap 3 30 cm r -20 cw THE DISCHARGE VESSEL THE TRIGGERING CIRUITS. The high current discharge, and the r e s t r i k i n g pulse were triggered from a delay unit which provided a 300v pulse when triggered manually, and another one at a delay time, variable from 10CyAs to 10 sec. The f i r s t pulse was fed into a 300v-10kv pulse transformer, and then into a -1/Vkv trigger generator to be described i n the next section. The delay pulse was fed v i a another pulse transformer to the r e s t r i k i n g gen-erator. THE -Ukv TRIGGER GENERATOR; The -14.kv pulse used to i n i t i a t e the high current discharge, was prov-ided by the following c i r c u i t , constructed c o - a x i a l l y , i n the manner des-cribed by Medley [A~j $ and shielded by a metal container. FIG. 1.3. 3oo v puAfie*-J , 6o + IS kv • oi.s/j^F THE TRIGGER GENERATOR The output pulse was fed into electrode 2 of figure 1.1. THE RESTRIKING GENERATOR; A voltage step function impulse of 0-12kv amplitude was applied across the test gap G34 by means of the following c i r c u i t which provided an impulse with a r i s e time of 0.2 / X s , and a droop i n amplitude of the order of i n 10CyjLsec. FIG. 1.4. 8 5 lav 50 N~ V W A A - r - 5oo pF To De.\o-^ U»nV THE RESTRIKING GENERATOR Low inductance carbon r e s i s t o r s were used throughout i n the construction of t h i s c i r c u i t . The voltmeter, (2} , used to set the step function amplitude was a 25/^amp meter, i n s e r i e s with a 500rA-"L, high voltage m u l t i p l i e r . The output of the r e s t r i k i n g generator was displayed on a Textronix type 551 o s c i l l o s c o p e , using a Textronix P6014, 1000x, high voltage a t t e n -uating probei placed across gap O^W A r e s t r i k i n g of the t e s t gap was seen as a collapse of the step f u n c t i o n . The current waveform of the main discharge could be monitored by means of a small search c o i l , mounted on the leads to the capacitor bank. This c o i l was made from 15 turns of type AWG-20 wire, 1.4 inches i n diameter. The output of the c o i l was integrated by a simple RG combination. 9 FIG. 1.5. 100- Ccfele- Wk-rv - A A <) i , A A — = L o t r F C.Ft.O. DISCHARGE CURRENT MONITOR The 150Aresistance in parallel with the coil, damped out the reson-ance responce of the coil, due to its stray capacitance. A Ulsx. resistor terminated the RG58/U cable at the integrating circuit. The output o f the coil is pictured below: 6 y*tm > OUTPUT OF CURRENT MONITOR This signal can be calibrated in adsolute current units by employing the requirement that, CO CV= j"ldt 0 where C is the capacitance of the main discharge bank (10y*-F), and V is the charging voltage (I9kv). Thus the calibrated current waveform became. s 10 FIG. 1.7. 11 CHAPTER 11 SPARK RECOVERY IN AIR An analysis of the problem of uniaxular heat flow i n a recovering spark channel, of the geometry used i n t h i s experiment [ l ] , has shown that early i n the recovery period, one would expect the breakdown voltage, V^, to i_ increase proportionately to t „ Although a t 2" dependence of the recovery c h a r a c t e r i s t i c has been ver-i f i e d by Gautam [VJ , f o r a spark i n a i r at 760mm„ Hg., between f l a t tung-sten electrodes at 5mm separation: , i t was decided to examine the e f f e c t of electrode separation on the e a r l y recovery period. Measurements were made i n a i r at electrode separation of 3, U> and 5mm. The experimental procedure was to increase the amplitude of the r e -s t r i k i n g pulse i n steps of 0.1 kv, at a p a r t i c u l a r delay time, u n t i l the lowest voltage was found at which the gap would r e s t r i k e before 100yus a f t e r the i n i t i a t i o n of the r e s t r i k i n g pulse, i n at l e a s t 6 out of 11 t r i a l s . Before taking measurements on f r e s h electrodes, they were conditioned by f i r i n g 75 shots. DISCHARGE PARAMETERS s Discharge Mediums a i r at 760mm„ Hg Peak Discharge Current; 32 kamp Duration of_Current Pulses 11yusec. Electrodes; diameter, f l a t tungsten rods 12 Delay Time, tp Recovery Voltage Percentage Recovery msec. VR.(t) + 0.1kv ( V v n x 1 0 ° ) Gap lengths 3mm 4mm 5mm 3mm 4mm 5mm .635-.002 2.3 kv 2.2 2.1 25.6 22.0 20.7 .710-.002 2.9 2.9 2.5 32.3 29.6 24.3 .81ol.002 3.6 3.7 3.0 40.0 27.8 29.1 .890-.002 A»6 4.3 3.7 51.2 43.8 35.9 1.02 1.01 5.5 5.5 4.9 61.2 56.2 47.6 1.10 1.01 5.9 - 5.7 65.6 - 55.3 1.31 1.01 6.2 6.5 6.8 68.9 66.3 66.0 1.75 1.01 6.7 7.6 7.7 74.5 76.0 74.8 3.15 1.01 7.4 8.3 9.1 82.3 84.7 89.2 5.30 1.01 7.8 8.8 9.6 86.7 89.8 93.2 8.95 1.01 8.4 • 9.3 9.8 93.4 95.0 95.2 1 *4o i+ — © 1 8.7 9-4 10.1 96.8 96.0 98.1 AO.8 1.1 8.9 9.5 10.2 99.0 97.0 99.0 89.0 1.1 9*0 9.7 10.3 100 99.0 100 SPARKING VOLTAGE v G ( t D 9.0 9.8 10.3 The nature of the triggering in the restriking generator did not permit measurements with a restriking pulse of less than 2 kv amplitude. In figures 2.1 - 2.4* the recovery characteristics of the spark gap at 3,4* and 5mm electrode separation are plotted against a time scale prop-ortional to t ^ 2 . It can be seen that from .5 - 1.5 msec after the i n i t i a l x_ discharge, the recovery characteristics rise very nearly as t p 2 . Further-more, the rate of increase of the breakdown voltage i s nearly the same for a l l three gap lengths in this period (fi g . 2.4). FIG. 2.1 13 10 tew 0k* «k<r t • a n M i l T • • ' ' • 1 — I 1 _ — -ti RECOVERY IN AIR ( 3mm.GAP L E N G T H ) FIG. 2.2 lokvr 8b 6kv I I II J I I I I I I I L 5*\i 10 « M RECOVERS IN AIR C 4mm G A P L E N G T H ) FIG. 2.3 tokvf 4 6kv 4kv 2kv l l M i l J L I m i s m* 10 m« t, RECOVERY IN AIR ( G A P LENGTH 5mm) FIG. 24 iok\rr 8kvh 6kv 4Kv I I I I I ' ' I I J -J L •/ma im* FIGS. ZI-24 SUPERIMPOSED 16 Figure 2,5 shows the percentage recovery ( V ^ / V Q x 100 ) of the gap as a function of delay time f o r the three cases. I t can be seen that t h i s quantity i s only s l i g h t l y a f f e c t e d by gap length over the range 3-5mm, The percentage recovery i s seen to decrease with gap length from 0.6-1,5 msec a f t e r the discharge, and increase with gap length f o r times greater than t h i s . These r e s u l t s can be explained on the basis of the thermal recovery of the interelectrode gas. Curzon and Gautam ' C"G show that, neglecting con-vective cooling and l a t e r a l heat conduction, one would expect the gas temper-ature i n the column to be given bys T Is the equilibrium temperature o f the electrodes and gas. S , S , are the s p e c i f i c heats at constant pressure, per u n i t volume, LT 6 of the gas and electrodes r e s p e c t i v e l y . k,, k s are the thermal c o n d u c t i v i t i e s of the gas and electrodes r e s -G e p e c t i v e l y . S3, i s the distance to the nearest electrode ( f i g . 2.6). d, i s the electrode separation. 1, i s the depth to which the electrodes are heated by the i n i t i a l discharge. Tj,, i s the temperature of the electrode surfaces and the inter-electrode gas, immediately a f t e r the discharge, (assumed to be constant) ( D providing that; (2) Where s FIG. 2.6 17 Y77A H ELECTRODE PARAMETERS Now, i f i t i s assumed that the breakdown voltage of the gap i s r e l a t e d l i n e a r l y to the product yS d,. where ys i s the average density (over z) of the inter-electrode gas. one has: (3) VR(t,d^ = J\ /(t)d + B (This i s equivalent to assuming that the Paschen curve i s l i n e a r i n the region o f in t e r e s t £5^ J .) Now, f o r T _ » T _ , i t follows from (1) that i n the region where (2) i s G K s a t i s f i e d ; (4) v B ( t , d ) - * A W + B ,LL P 2 A _ Assuming that only a t h i n surface layer of the electrodes i s heated by the i n i t i a l discharge, so that i t i s safe to neglect 1 (S /k i n comparison e e with d ( S ^ k ^ ) 2 , * one obtains; (5) 4 - A ' U + B that i s , the recovery c h a r a c t e r i s t i c i n the i n i t i a l stage should r i s e prop-i_ o r t i o n a t e l y to t ^ 2 , and be nearly independent of the electrode separation. (T^ may depend somewhat on d). r 18 *This i s equivalent to assuming that the heat flow from the gas i s l i m -i t e d by the "thermal i n e r t i a " of the gas, rather than the rate at which heat can be removed through the electrodes. 19 CHAPTER 111 SPARK GAP RECOVERY IN HYDROGEN M. S. Gautam [2~] has noted a pec u l i a r y long recovery period, f o r a spark gap i n hydrogen, between tungsten electrodes. The continued r i s e i n break-down strength beyond 100 msec, up to delay times of several seconds, he has a t t r i b u t e d to the re-adsorption o f hydrogen onto the tungsten electrode surfaces, which had been cleaned by the i n i t i a l discharge, thus changing the work function of the cathode surface. The adsorption o f hydrogen onto tungsten i s a w e l l established f a c t , t h i s combination was used i n the e a r l y adsorption i n v e s t i g a t i o n s , conducted by Langmuir j^ T| . Although several adsorbed layers may be present at high pressures, the i n i t i a l layer i s thought to consist of chemi-sorbed hydrogen atoms [ 7 j . This i n i t i a l layer of adsorbed hydrogen was found by J . K. Roberts [V] , to become unstable at temperatures of the order of 700°K, at -3 a gas pressure of about 10 mm. Hg. The time required f o r the amount of adsorbed gas to saturate was found to be temperature dependent at temperatures below t h i s . Although very l i t t l e work has been done on adsorption at pressures 3 above 10 mm. Hg, i t seems l i k e l y that at high pressures, se v e r a l layers of p h y s i c a l l y , or Van der Waals-adsorbed hydrogen molecules w i l l also be present. I f the delayed recovery e f f e c t observed by Gautam was a consequence o f adsorption, the nature of the recovery c h a r a c t e r i s t i c i n the delayed recovery region should be a l t e r e d i f the electrodes were heated, since i n t h i s case, the adsorption would be delayed. The f a c t that the delayed r e -covery he observed d i d not begin u n t i l about 100 msec, a f t e r the i n i t i a l discharge, by which time the electrodes would be quite c o o l , suggests that heating to a r e l a t i v e l y low temperature would be e f f e c t i v e . 20 Thus to attempt to establish the role adsorption in the recovery of this type of spark gap system, the electrodes were fitted with small heating coils capable of heating them to about 200*C. Figure 3.1 shows an electrode with heating c o i l installed. FIG. 3.I v8-T tungsten electrode * Sauereisen", porcelain heating coil brass — thermoj unction HEATED ELECTRODE The heating coil consisted of 1.5 f t . of .0159" diameter Copper-nickel alloy wire of 1.19-ft-/ft. This would dissipate a maximum of 30 watts, when installed on the electrode. The heating coil power supply was an a.p. vol-tage source, variable from 0-1 Ov. The temperature of the electrode was meas-ured by means of a copper-constantun thermo-junction, soldered directly into the brass electrode holder. The thermocouple emf. was measured by a 21 potentiometer and standard c e l l , using the freezing point of water for a referenoe junction,, The device was calibrated at the boiling points of water and naphthalene. :-E;.SUREMESTS» To obtain recovery characteristics for heated electrodes, the procedure adopted in the first instance was to heat the electrodes at 20 watts until an equilibrium temperature of about 150*0 was reached. The heating coils were then disconnected and the main bank charged. The high current discharge was then triggered 15 seconds after disconnecting the heating c o i l power supply. The cooling curve of the electrodes is shown in figure 3.2. It can be seen that at the time of firing, the electrode temperature was 120110*C. FIG. 3.2 ELECTRODE COOLING CURVE 22 99.8% pure, commercial hydrogen was used i n these i n v e s t i g a t i o n s , and the tank was flushed and r e f i l l e d s e v e r a l times with the heating c o i l power on, before taking measurements. The electrode surfaces were polished f l a t using f i n e g r a i n abrasive p o l i s h i n g paper, and i t was found necessary to f i r e about 150 "conditioning" shots to make the breakdown voltages i n the recovery period reproducible. When working with hydrogen, i t was found that a f t e r about 500 shots, the main bank could not be triggered i n the configur-a t i o n described i n chapter one. The sparking voltage of the tes t gap was s t i l l only about 13 kv , as could be v e r i f i e d by connecting a power supply, v i a a large r e s i s t o r , d i r e c t l y across 6-34, charging. Thus, t h i s f a i l u r e to f i r e must have been due to an increase i n the formative time l a g of the gap. Therefore, a 25,000 ~ r e s i s t o r was connected from electrode 2 of figure 1.1 to ground. This increased the duration of high voltage pulse applied across when was tr i g g e r e d . This arrangement proved s a t i s f a c t o r y and no change i n the current waveform of the main discharge was detected. Results: Recovery c h a r a c t e r i s t i c s were measured, heating the cathode, and anode separately, as w e l l as with both electrodes "cold". The electrodes were conditioned as described above. Measurements were made at successive delay times: a) with both electrodes cold b) with the anode at 120°C c) with the cathode at 120°C d) with both electrodes c o l d The r e s u l t s are tabulated below; DISCHARGE PARAMETERS! .. . Discharge Gas: . Hydrogen at 760mm. Hg Peak Discharge Current: 32 kamp. Duration of Current Pulses 11 JLK.sec E l e c t r o d e s : diameter tungsten rods TABLE 3.1 Delay Time "Cold" Electrode V Rl.1kv Hot Anode (120°C) V R±.1kv Hot Cathode (120 «C) V R±.1kv "Cold" Electrodes V Rl.1kv ,380l.005ms ', 3.3 kv - - -.4751.005 - -.605-.005 4.9 - - -.790i.005 ; 5.4 - - -1,26i,01 ! 5.8 - - -1.7ll.01 6.2 - - -3.081.01 6.8 - - -5.2Q1.01 \ 7.3 (6.9) -• - -8.6ol„05 7.4 (7.3) 7.1 6.7 (6.7) 7.4 13.8-.1 7.9 (7.8) 7.3 (7.3) 6.8 (6.8) 7.9 23.21.2 8.2 (8.7) 7.3 (7.3) 6.8 (6.9) 8.2 40.21.2 8.9 (9.3) 7.5 (7.3) 7.1 (7.0) 8.9 67.01.5 (10.3) (9.6) (7.4) (7.4) -109 l l 9.7 7.6 (7.7) 7.8 9.3 235 -2 ! 10.2 (10.2) 7.9 8.5 (8.2) 10.1 520 -5 ! 11.51.5(11.2) 8.7 (8.2) 9.4 (9.1) 11.51.5 875 -5 I >12 kv 9.5 (8.7) 11.2 (10.4) >12 kv 1.41-.01s i >12 kv 11.1 (10.5) >12 kv >12kv 2U The apparatus d i d not permit measurement of V R > 1 3 kv, since at t h i s point, the gap Gpj ( f i g 1.1) broke down under the a p p l i c a t i o n of the step f u n c t i o n r e s t r i k i n g voltage to electrode 3 . ( f i g 1 . 1 ) . With col d electrodes, at recovery times greater than a second, the breakdown strength of the gap was not very w e l l defined, and recovery measurements became quite e r a t i c . Formative time lags > 1 0 0 ^ , sec were common. Two t r i a l s were made with d i f f e r e n t sets of electrodes i n which just the cathode was heated, or just the anode was heated. As the recovery charac-t e r i s t i c s seemed to change somewhat when d i f f e r e n t sets o f electrodes were used (probably due to the alignment or conditioning of the electro d e s ) , these r e s u l t s are included i n brackets i n table 3 . 1 . The recovery points tabulated i n table 3 . 1 are p l o t t e d i n fig u r e 3 . 3 . FIG. 3.3 1 i i I i i I i I I I I I I I I 1 I I I I I | | I M i l l ) 1 I ms 10ms lOOms I sec t.—-EFFECT OF ELECTRODE HEATING ON RECOVERY IN HYDROGEN 26 I t can be seen that heating e i t h e r electrode to 120 C has a s i g n i f i c a n t e f f e c t on the recovery c h a r a c t e r i s t i c at delay times greater than 10 msec. To investigate the p o s s i b i l i t y that the observed e f f e c t may have been due to a heating, and consequently a density reduction of the inter-electrode gas, i t was decided to examine the worst possible case. Assuming that the i n t e r -electrode gases were a l l at ^O^C, i t s density would correspond to a pressure of 525 mm. Hg, at room temperature. Therefore, the recovery c h a r a c t e r i s t i c of the spark gap, i n hydrogen, at 525 mm. Hg, was measured, and compared to the c h a r a c t e r i s t i c s with heated electrodes. The r e s u l t s are presented i n table 3.2, and p l o t t e d i n f i g u r e 3.4. Table 3.2: DISCHARGE PARAMETERS: Discharge Medium: El e c t r o d e s : Peak Discharge Currents hydrogen at 525 mm. Hg j-u diameter tungsten rods 32 kamp. Duration of Current Pulses 11yU- sec Delay Time t n Recovery Voltage V R 1.77-.02 msec. 4.5± .1 kv 3.15±.02 .1 5,20±.02 8.60±.05 5.4* .1 5.8t .1 23.5-.2 6.4± .1 6.2± . 1 * 65.5±.5 6.8± .1 6.6± . 1 * 237 -2 7.7i .1 7.4^.1* 4-00 ±2 8.1± .1 7.71.1* 825 -5 10.3* .1 9.5i.1* 1.27±.05 sec. 10.7t .2* 2.10±.05 11.8± .5 Sparking Voltage, VQC ^ = C D ) ~13kv * These r e s u l t s were obtained using a d i f f e r e n t set o f electrodes, as a check on electrode conditioning. FIG. 34 / I I J I M I I 11 ' i * t i i i 11 J I L Ims 10 ms RECOVERY IN HYDROGEN AT 5 2 5 mm. Hg 3 ft r o NORMAL RECOVERY AT 760mm Hg HEATED CATHODE AT 760 mm Hg NORMAL REC0VEW,AT^25 mm H9 HEATED ANODE AT 760mm Hg 1 1 l 1 » 1 i l 1 l I l 1 1 I 100 ms I sec 28 Figure 3.4- shows that reducing the inter-electrode gas density has a greater effect than heating either electrode to 120°C, except for heating the anode at delay times greater than 500 msec. However, the fact that the heated electrode characteristics join the normal characteristic at delay times below 10 msec, while the characteristic at 525 mm. Hg does not, indicates that the inter-electrode gas density was not significantly affected by elec-trode heating. As a further check on this point, the effect of electrode heating, on the recovery characteristic of a similar spark gap, in a i r , was investigated. The recovery measurements for a spark gap, in a i r , in which the cathode was heated to 120°C, are presented in table 3.3, and plotted in figure 3.5. Table 3.3: DISCHARGE PARAMETERS s Discharge Mediums Electrodes s Peak Discharge Currents ai r at 760 mm. Hg diameter tungsten rods 32 kamp Duration of Current Pulses 11yU sec Delay Time "Cold" cathode Cathode heated to 120°C V ±.1kv tt V ±.1kv R 1.75±.01 msec 7.9 kv 7.8 kv 3.10±.02 9.6 9.3 5.25±.05 10.2 9.7 U.3±.1 10.3 9.7 U.0t.1 10.6 9.8 111 ±1 10.7 10.0 Sparking Voltages VQ (tp =a>), 10.7 kv FIG. 3.5 29 11 Kv > io kv o NORMAL RECOVERY • CATHODE TEMP. 120 °C I I 1 I I I I I I m» 10ms loo ms EFFECT OF CATHODE HEATING ON RECOVERY IN AIR It is apparent that although cathode heating in air does somewhat effect the breakdown strength, the result does not compare to the 3 kv difference between heated and cold cathode recovery characteristics in hydrogen. (Max-imum difference in air is .7 kv at 100 msec.) It can be seen from f i g . 3.3 that heating either electrode surface in hydrogen to 120aC greatly alters the recovery characteristic at delay times greater than 10 msec. The effect of heating to this temperature decreases at recovery times greater than one second. The only possible explanation for these results is that heating to 120°C increased the time required for an effective layer of hydrogen to be adsorbed onto the electrode surfaces. It is quite surprising, however, that heating the anode to 120*13, has nearly ' as large an effect as heating the cathode to this temperature. It is well'--, known, however, that certain anode processes, eg. photon emission, are import-30 tant i n the o v e r - a l l feedback system, leading to spark formation. Space charge e f f e c t s at the anode, associated with the la y e r of adsorbed gas, may also be of importance. , As can be seen from f i g u r e 3.3, heating the cathode to 120°C begins to lose i t s e f f e c t at delay times greater than 1 sec. This suggests that s i g -n i f i c a n t amounts of hydrogen are s t i l l adsorbed but that the time required f o r an e f f e c t i v e layer to form i s increased at t h i s temperature. This i s i n general agreement with the observations of J . K. Roberts [ 7 ] . At a delay time of one second, the e f f e c t of cathode temperature on breakdown strength was investigated. The cathode was heated to a steady state at various c o i l power d i s s i p a t i o n , and i t s steady state temperature measured with the thermojunction attached near the electrode. The thermojunction leads and heating c o i l leads were then removed and the bank f i r e d immediately (with 1 second of disconnecting heating c o i l ) . The r e s u l t s are presented belowJ DISCHARGE PARAMETERS. Discharge Gas; H 2 at 760 mm. Hg Peak Discharge Currents 32 kamp Duration of Current Pulse: 11JK sec Elect r o d e s ; <t" diameter tungsten rods 31 Table 3.4 Heating C o i l Power Cathode Temperature 14 watts 135*2 C 11.7*o3 kv 16 146*2 10.7*. 2 18 152*2 10.2±.2 19 160±2 9 .8±.2 20 167*2 9.3±.2 22 180±2 8.6*.1 28 202*2 8.0±.1 These r e s u l t s are p l o t t e d i n fi g u r e 3.6. At temperatures below 135°C, the breakdown voltage of the gap became so e r r a t i c that f u r t h e r measurements were not p o s s i b l e . However, the break-down strength remained le s s than 13 kv f o r temperatures from 20-130 C. The d i f f i c u l t y was due to the i n s t a b i l i t y of the formative time lag i n t h i s region, formative time lags much l a r g e r than 100yu.sec were very common. This apparent change i n the r e l a t i o n between t ^ and overvoltage may have been due to the adsorption o f hydrogen. In reference to fi g u r e 3.6 i t can be seen that the e f f e c t of cathode heating begins to decrease noticeably from about 190°C. This suggests that most of the adsorbed hydrogen was r e -moved from the cathode at t h i s temperature. 32 FIG. 3.6 1 I l l I I I ' • 130 # C I 5 0 X 170*C I90*C ZIO'C CATHODE TEMPERATURE -EFFECT OF CATHODE TEMPERATURE ON RECOVERY VOLTAGE, IN THE LATE RECOVERY PERIOD 33. CHAPTER 1V CONCLUSIONS AND SUGGESTIONS FOR FUTURE RESEARCH CONCLUSIONS: a) The r e s u l t s on spark gap recovery i n a i r , at electrode separations of 3,4-, and 5mm, confirm the predictions of the Curzon-4}autam theory of uniaxular heat flow, on the gap-length dependence of the i n i t i a l p o r t i o n of the recovery c h a r a c t e r i s t i c . This shows that heat flow through the electrodes is a c o n t r o l l i n g f a c t o r i n the intermediate recovery of a spark gap. b) A long-time recovery e f f e c t i n hydrogen-tungsten spark gap systems has been shown to be at least p a r t i a l l y a t t r i b u t a b l e to a re-adsorption of hydrogen onto the electrode surfaces, which had been cleaned by the i n i t i a l discharge. I t i s f e l t that at l e a s t part of the observed delayed recovery i n hy-drogen may be due to a change i n the r e l a t i o n between over-voltage (V R(tp) -Vg(tj)), and the formative time lag of the gap. In the e a r l y stages of the recovery (tp <.100ms), formative time lags greater than 10Cyis were not observed, while at greater delay times, such long formative time lags were very common. Breakdown at such long formative time lags was ignored i n de-termining V R, according to the c r i t e r i o n adapted i n t h i s work. This possib-i l i t y does not i n any way c o n f l i c t with the conclusions on the r o l e of ad-sorption i n the phenomona of delayed recovery. Indeed, as discussed i n chap-t e r 8 of Meeks & Graggs [8j , surface layers (eg. oxides) i n other types of spark gap systems are known to a f f e c t the formative time l a g . The p o s s i b i l i t y that the e f f e c t i s at l e a s t p a r t i a l l y a consequence of a change i n the r e l a t i o n between over-voltage and t ^ may explain why other workers, using d i f f e r e n t c r i t e r i a to define V R, have f a i l e d to report a s i m i l a r delayed recovery e f f e c t [ 9~] . SUGGESTIONS FOR FUTURE RESEARCH a) I t would be of in t e r e s t to study the recovery c h a r a c t e r i s t i c of a s i m i l a r hydrogen-tungsten spark gap system, with electrodes heated to con-sid e r a b l y higher temperatures than those used here. At a temperature of 700°K, i t i s indicated by the work of J . K. Roberts , that at low presr-sures (I0~^mm. Hg) at l e a s t , the adsorbed l a y e r of hydrogen on tungsten be-comes unstable. Therefore, i t i s possible that at such temperatures, no delayed recovery would be present. b) T h e o r e t i c a l curves r e l a t i n g t ^ to percentage over^voltage, with the various secondary i o n i z a t i o n c o - e f f i c i e n t s as parameters, have been c a l -culated by various workers- see Llewellyn-Jones, Chapter 8 , [ 5 J . F i t t i n g experimental curves f o r heated, and cold electrodes i n hydrogen, to such the-o r e t i c a l curves, might allow changes i n the secondary i o n i z a t i o n c o - e f f i c i e n t s , due to the adsorption of hydrogen, to be studied, and thus i n d i r e c t l y to study adsorption at high gas pressures. % BIBLIOGRAPHY; [j^ j Curzon, F. L . , and Gautam, M. S . , The Influence o f E lec t rode Heat Transport i n Spark Recovery. , B r i t . Jour . A p p l . Phys . , 18, 79. (1967) [ 2 ] Gautam, M. S . , PhD. Thes i s , Un i ve r s i t y o f B r i t i s h Columbia. ( 1 9 6 6 ) Q f j C h u r c h i l l , R. J . , Parker , A . B., and Craggs, S. D. , Jour. E l e c t r o n . C o n t r o l . , J J , 17 (1961) Medley, S. M. , MSc. T h e s i s , Un i ve r s i t y o f B r i t i s h Columbia. (1964.) |jf] L lewel lyn-Jones, F . , I on iza t ion and Breakdown In Gases, Methuen Monograph. (1957) |V] Langmuir, I., Jour . Am. Chem. S o c , _2, 2269. (1916) [V] Roberts , J . K., The Adsorpt ion o f Hydrogen on Tungsten. , Proc . Roy. S o c , A 152, UU5. (1935) jjTj Meek, J . M., and Craggs, J . D. , E l e c t r i c a l Breakdown o f Gases . , Oxford P ress . (1953) C h u r c h i l l , R. J . , Re i gn i t i on o f F r ee l y recover ing Spark Channels i n Hydrogen., Can. Jour . Phys . , 41:, 610. (1962) 

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