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A new method for switching off a mercury arc. Fjarlie, Earl John 1958

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A NEW METHOD FOR SWITCHING OFF A MERCURY ARC by EARL JOHN FJARLIE B. A. Sc., University of B r i t i s h Columbia, 1955 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n the Department of E l e c t r i c a l Engineering We accept this thesis as conforming to the standards required from candidates for the degree of Master of Applied Science Members of the department of E l e c t r i c a l Engineering The University of B r i t i s h Columbia A p t i l v 1958 i i Abstract Continuous current control, so f a m i l i a r i n the operation of high-vacuum tubes, has not been possible, except under special circumstances, f o r gas tubes. Even current interruption has been awkward, except for low currents, for the usual manner of interrupting the current i s to decrease the anode poten-t i a l to zero. The time to switch off the gas tube has been of the order of the deionization time for the gas employed. Now a method i s developed for switching off mercury-pool arcs using a t h i r d electrode. There i s no interference with the main power c i r c u i t and, i n f a c t , the potential on the anode causing the e l e c t r i c f i e l d aids the dispersal of the charge ca r r i e r s when the arc has been interrupted. The switching-off time i s much decreased because th i s anode-to-cathode voltage sweeps a l l the charge c a r r i e r s out of the tube. Switching off i s effected by passing a reverse current of equal or greater magnitude than the arc cathode current through the tube f o r a time long enough to interrupt the cathode spot. A technical d i f f i c u l t y arises i n that the t h i r d electrode introducing the reverse current has to have an already formed or an easily formed cathode spot since t h i s t h i r d electrode i s a cold cathode. Many methods for forming the cathode spot are discussed. The method f i n a l l y used i s probably not the best one but i t hag the virtue of being e a s i l y effected. There appears to be no l i m i t as to the current that can ,be interrupted i f the spot-forming mechanism i s altered. Energy used i n not an important factor. The amount varies with the time to switch off and does not influence the actual switching process. In presenting t h i s thesis i n p a r t i a l fulfilment of the requirements f o r an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the Head of my Department or by his representative. It i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of E l e c t r i c a l Engineering The University of B r i t i s h Columbia, Vancouver B, Canada. Date 10 October, 1957 i i i Table of Contents page •Akii) S t r O » C t » * * » » » » o o o o o o o o o o o o o o o o o o * e o o « o o e o o o « o o o o o o o » Q o X X AclCXIOWl © d ^ G I f l G I l t o • e o e o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o V I X o » o o « o o o o o o o o o o o o o o o o o o o Lxs"fc of S y n i b o l s o o o o « o o o o c o o o o o o 1 • X H * t > X * O d l X C ~ b i . O Z X O 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 9 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 4 P a r t A, The Mercury-pool Tube 2 o p f l » 3 T & " t U S o o * * o « o e o o o o o o o * e o o o o o o o o o o o o 6 o o o o o o o o o o o » 7 2 . 1 . The Tube 2 . 2 . The I g n i t i o n Method. 2 . 3 . V a r i a t i o n of Pressure 3 . I n i t i a l Attempts to Switch O f f c o o . . . . . . . . . . . . . . . . . . 11 3 . 1 . E x t e n s i o n of the Space Charge 3 . 2 . Use of a Strap 3 . 3 . Use of a Ca p a c i t o r 4 o .Cfl»"tilOd©*" Spot 1?0 3TIll£L"bX O H o O 0 * « o o o o o o o o o o o o o o o o o o o o c o o o 18 5c 0 B * p O * C X"kOr S W X *trC l l X X l £ £ o • © • • • o o « o o o « o o o o o o o o o o o o o o o o o o o 2 1 5 . 1 . The S o l i d Probes 5 . 1 . 1 . N a t u r a l I r r e g u l a r i t i e s A i d Spot Formation 5 . 1 . 2 . P r o v i d i n g the Co n d i t i o n s f o r Spot Formation ' 5 . 1 . 3 . Comments on Solid-probe Switching 5 . 2 . The Mercury-pool Probe 5 . 2 . 1 . Spot Formation by a Band I g n i t e r 5 . 2 . 2 . The Other Attempts at Cathode-spot Formation 5 . 2 . 3 . An A u x i l i a r y - a r c Discharge Provides the Spot 6. Experimental D i f f i c u l t i e s . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3 6. 1 . The O s c i l l o s c o p e 6.2. The Camera 6 . 3 . The Relays and Contacts 6.4. The Contamination Problem 6 . 5 . The S t r u c t u r a l Problem 6.6. The Connecting-channel Arcs 6.7. The Pumping System 7. S e l e c t i n g the Best C o n n e c t i o n . . . . . . o . 0 0 . . . . . o . . . . o . 3 7 page 8 c The Optimum Conditions for Switching Off.............. 3 9 8 0 L The Current-interrupting C i r c u i t 8<,2„ Analysis of the C i r c u i t 9« Increasing the Current and Voltage.,»o a«<> a <> o <> 0 o<>« <= <> <> <> <> o 4 8 9 o l o High Voltage 9o2o High Current 9 o 3 o Comments on Switching Off High Currents and Voltages 10c. Conclusions About Switching Off a Mercury Arc,,,....'.. 52 1 1 o Projects That May Be Examined0 <> 0 o o 0•>o o o <= » o „ 0 0 o <. o 0 o o o 0 » 5 4 Part Bo The Hydrogen Tube JL2 O Tli 6 T\ib 6 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 55 13 o SWl *t C l l X O f f » o o o o o o o a o o o o o v o o o * o o o o o o o o o o o o o o o o o o o o o o 56 14» Conclusion About Switching Off the Hydrogen Tube,.,... 59 Part Co References S W X h i l l § * " O f f 1 6^ "th.0 CL S o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o 60 1 ^ X 1 X ^ X 0 X 1 M O * t f l X O d . S o o o o o o o o o o o o o o o o o o o o o o o o o o c o o o o o c o o o o o o o 0 0 o 60 C Q*"fcllO CL©"™ S J30 "fc F O riUQ^'t 1 O H O 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 » 0 8 0 0 0 6X General Information About Gas D i s c h a r g e s . „ . „ < > s 0 o o 0 o . » » < > . 6 1 H3fXld.l3 0 0l&. V S L I U ^ S o o o o o o o o o o o o o o o o o o o o o o o o o o o o c o o o o o o o o o o o o o o o 62 Hx ^2[il~ V 3 » C \ X V U J 1 JDO.C l l X l X ^ U G S O 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 62 V L i s t of T a b l e s Table page 5ol« V a r i o u s Values Important f o r S w i t c h i n g O f f . o . o . . . . . 22 8 o l . The Minimum Values f o r S u c c e s s f u l S w i t c h i n g 0 . „. ° °. <> 41-8 » 2 . T h e o r e t i c a l L i m i t s on Cathode-spot Stability-time<>» 46 9ol» S w i t c h i n g O f f a t High A rc C u r r e n t o . l > . . I . . . I . . I . o . . . 50 v i F i g u r e page 2 o l o The M e r c u r y - p o o l Tube "bO f o l l O W 00000000 o o o o o o o o o o o o o o o o o o o o o o o o o o o o e o IT 2 . 2 . A S e c t i o n a l View of 3.1. The F i r s t S w i t c h i n g - o f f Method.................... 13 3 o 2 . C a p a c i t o r S w i t c h i n g on a S o l i d Probe.............. 1 5 5 . 1 . Graphs of the S w i t c h i n g - o f f P r o b a b i l i t y "to f o i l OUT o o o o « * e o o o o o o o e o o o o o o o o o o o o o o o o c o o o o o o 2 1 5 . 2 . Cathode-spot I n i t i a t i o n by a Band I g n i t e r . . . o . . . . . .27 5 . 3 . An A u x i l i a r y - a r c D i s c h a r g e P r o v i d e s the Spot...... 3 0 7 . 1 . The Con n e c t i o n s and C u r r e n t E s t i m a t e s tO f OllOWo o o o o o o o o o o o o o o o o o o o o o o ' o o o o o o o o o o o o o o o .37 7.2„ Some of the C u r r e n t and V o l t a g e Waveforms "bO f o i l O W o o o o o o o o o o o o o o o o o o o o o o o O o o o o o o o o o o o o o o ,38 8 o 1 . The S w i t c h i n g — o f f C i r c u i t . . . . o o o o o o . o o o . . o o . o o o . o . 4 0 8.2. The M o d i f i e d M e r c u r y - p o o l Tube "tO f o l l O W o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o • 4 0 8.3. The Complete C i r c u i t f o r S w i t c h i n g Off "tO f o l l O W o o o o o o o o o « o c o o o o o o o o o o * o e o o o o o c o o o o o o « 4 0 8.4. A Graph of the Optimum S w i t c h i n g C o n d i t i o n s "tO f o i l OW o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o 4.1 8.5. The T h e o r e t i c a l and A c t u a l A r c - c u r r e n t Waveforms.. 42 806. The S w i t c h i n g — o f f Networks o o 0 o. 0 o o <> .... o. . o 0 o. . . o o 4 4 1 2 . 1 . The Hydrogen Tube "tO f OllOWo o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o 5 5 1 3 . 1 . S w i t c h i n g O f f the Hydrogen Tube "tO f o l l O W o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o 5 6 v i i Acknowledgement Many thanks a re due Dr. W0 A D Gambling, a N a t i o n a l R e search C o u n c i l p o s t - d o c t o r a t e f e l l o w a t the U n i v e r s i t y of B r i t i s h Columbia, f o r t a k i n g time t o h e l p and encourage the auth.r i n his studies. . Acknowledgement i s g i v e n to the Defense Research Board f o r e d u c a t i o n a l l e a v e from the Canadian Armament Research and Development E s t a b l i s h m e n t a t Quebec, P. Q., and f o r con-t i n u a t i o n of r e m u n e r a t i o n f o r p a r t of the time n e c e s s a r y to complete the p o s t - g r a d u a t e s t u d i e s 0 S p e c i a l thanks a re g i v e n tos Dr. F. Noakes, head of the Department of E l e c t r i c a l E n g i n e e r i n g , f o r h i s i d e a s and c o n s t a n t i n t e r e s t } Mr. J . Lees, of the Department of P h y s i c s , f o r r e p a i r i n g and m o d i f y i n g the mercury-pool tube on f o u r d i f f e r e n t o c c a s i o n s ; members of the E l e c t r o n P h y s i c s Group of the Radio and E l e c t r i c a l E n g i n e e r i n g D i v i s i o n of the N a t i o n a l Research C o u n c i l , Ottawa, Ont., f o r b u i l d i n g the hydrogen tube; t e c h n i c i a n s of the E l e c t r i c a l 9 M e t a l l u r g i c a l , and P h y s i c s Department f o r s u g g e s t i o n s , t o o l s , m a t e r i a l s and equipment. \ L i s t of Symbols a = ammeter A = c o n n e c t i n g p o i n t , s w i t c h c i r c u i t A s = s c r e e n - g r i d s u r f a c e a r e a B = c o n n e c t i n g p o i n t , s w i t c h c i r c u i t C = s w i t c h i n g - o f f c a p a c i t o r D = a f u n c t i o n of tube geometry E = c o n n e c t i n g p o i n t , s w i t c h c i r c u i t f = ( a p e r t u r e d i a m e t e r ) - f ( f o c a l l e n g t h ) G A = g e n e r a t o r , 100 v o l t s , a u x i l i a r y a r c G l = g e n e r a t o r , 120 v o l t s G 2 = generator,\100 v o l t s G 3 - g e n e r a t o r , 140 v o l t s GND = c o n n e c t i n g p o i n t , s w i t c h c i r c u i t ) = ground I = a r c c u r r e n t *A = a u x i l i a r y - a r c c u r r e n t I a = anode c u r r e n t = cathode c u r r e n t I s = s w i t c h c u r r e n t = peak s w i t c h c u r r e n t J P = i o n c u r r e n t - d e n s i t y k = Boltzmann's c o n s t a n t m e e l e c t r o n i c mass m P = i o n i c mass M = atomic-mass r a t i o N = random-ion c o n c e n t r a t i o n s = e l e c t r o n i c or i o n i c charge Q s < d t s w i t c h i n g - o f f charge r a t e of change of s w i t c h i n g - o f f charge 2 r = t o t a l resistance i n the switching network r ^ = measuring r e s i s t o r , anode current, 2 d ohms r^ = measuring r e s i s t o r , cathode currents 1°0 ohm r = plasma-edge radius of the sheath r y = probe-edge radius of the sheath r = charging r e s i s t o r , switch c i r c u i t , 27,000 ohms s R = switching-off r e s i s t o r R = arc resistance^ anode to cathode a * = current l i m i t i n g rheostat, a u x i l i a r y arc RQ = current l i m i t i n g rheostat, arc Rp = plasma resistance, anode to switch probe t = time T = random-ion temperature v = voltmeter v = ion thermal-velocity P V = potential difference, anode to cathode V = switching-off voltage s Z = an impedance function = an impedance function Zg = an impedance function 6 = a function of r and r ^ p r V = relay contacts, anode current 6 = relay contact, i g n i t i o n bond GQ = perraitivity for free space y = relay contact, spark-coil primary 7[ = relay contact, spot formation 0 = relay contact, sopt formation ^ = relay contact, switch current a = relay contact, spark-coil primary switching-off time, anode current deionization time of the plasma minimum spot-interruption time switching-off time, cathode current switching-off time, switch current relay contact, auxiliary-arc current 4 A NEW METHOD FOR SWITCHING OFF A MERCURY ARC l o I n t r o d u c t i o n In a high-vacuum t h e r m i o n i c tube t h e r e a re o n l y e l e c t r o n s present,, These e l e c t r o n s t r a v e l a l o n g the e l e c t r i c -f o r c e l i n e s a c r o s s the tube to the anode when a p o s i t i v e v o l t a g e i s a p p l i e d t o the anode. Because the c u r r e n t i s due t o p a r t i c l e s of one s i g n o n l y ? a n e g a t i v e space charge i s always p r e s e n t and the r e p u l s i v e f o r c e s on the e l e c t r o n s about t o l e a v e the cathode l i m i t the c u r r e n t . The c u r r e n t f l o w can be p r e v e n t e d , and may be i n t e r r u p t e d a t any t i m e , by making a g r i d i n the tube s u f f i -c i e n t l y n e g a t i v e . Continuous c u r r e n t c o n t r o l i s q u i t e u s u a l . In a gas tube, on the o t h e r hand, the e l e c t r o n s p r e s e n t make c o l l i s i o n s w i t h the gas m o l e c u l e s on a p p l i c a t i o n of a p o s i t i v e v o l t a g e t o the anode. The e l e c t r o n s , t h e r e f o r e , a l t h o u g h d r i f t i n g towards the anode, no l o n g e r move a l o n g the e l e c t r i c - f o r c e l i n e s a c r o s s the tube. I f the anode v o l t a g e i s s u f f i c i e n t l y p o s i t i v e , i o n i z a t i o n of the gas m o l e c u l e s o c c u r s and the p o s i t i v e i o n s formed l a r g e l y n e u t r a l i z e the n e g a t i v e space charge. The v o l t a g e drop due t o space charge i s then much l e s s than i n a c o r r e s p o n d i n g high-vacuum tube. The c u r r e n t f l o w can be p r e v e n t e d by a p p l y i n g a n e g a t i v e v o l t a g e t o a g r i d , but because of the presence of p o s i t i v e i o n s the tube cannot n o r m a l l y be e x t i n g u i s h e d i n the same manner. Continuous c u r r e n t c o n t r o l i s not p o s s i b l e except f o r s p e c i a l l y - c o n s t r u c t e d tubes and then o n l y a t low c u r r e n t s ( 2 6 ) . The g r i d p r e v e n t i n g c o n d u c t i o n i n the gas tube i s p l a c e d between the anode and the cathode. When the tube i s i n the n o n - c o n d u c t i n g s t a t e a n e g a t i v e g r i d v o l t a g e p r e v e n t s the e l e c t r o n s from e n t e r i n g the a n o d e - g r i d r e g i o n where they would be a c c e l e r a t e d t o the energy r e q u i r e d t o cause i o n i z a t i o n and produce breakdown of the t u b e The i g n i t i o n c o n d i t i o n may be reached by b r i n g i n g the g r i d to a more p o s i t i v e v a l u e . When the tube i s c o n d u c t i n g , the g r i d does n ot e x e r t any f u r t h e r c o n t r o l . The re a s o n f o r t h i s i s t h a t i f ; f o r example, the g r i d i s a g a i n made n e g a t i v e , p o s i t i v e i o n s a re a t t r a c t e d t o i t and e l e c t r o n s are r e p e l l e d so t h a t a p o s i t i v e - i o n space charge forms around the g r i d . T h i s p o s i t i v e space charge e f f e c t i v e l y n e u t r a l i z e s the n e g a t i v e g r i d v o l t a g e so t h a t the plasma o u t s i d e the sheath i s u n a f f e c t e d . S i m i l a r l y , i f the g r i d i s made p o s i t i v e w i t h r e s p e c t t o the cathode, a n e g a t i v e space-charge i s formed. Thus, once c o n d u c t i o n has s t a r t e d , the g r i d v o l t a g e : has v e r y l i t t l e e f f e c t on the c u r r e n t f l o w i n g between the anode and cathode The c u r r e n t can o n l y be i n t e r r u p t e d by r e d u c i n g the anode v o l t a g e t o z e r o . Once t h i s i s done, a n e g a t i v e g r i d v o l t a g e a g a i n p r e -v e n t s c o n d u c t i o n . T h i s i s a severe l i m i t a t i o n t o the use of gas t u b e s , p a r t i c u l a r l y a t h i g h powers, but many t e c h n i q u e s have been d e v i s e d t o o b v i a t e the d i f f i c u l t y . Some attempts have been made t o cause c u r r e n t i n t e r r u p t i o n i n gas tubes by e x t e n d i n g the space change around the g r i d t o c l o s e o f f c o m p l e t e l y the a n o d e - g r i d r e g i o n from the g r i d - c a t h o d e r e g i o n . Because of the l a r g e n e g a t i v e g r i d v o l t a g e r e q u i r e d and the r e s u l t i n g h i g h g r i d c u r r e n t t h i s method, i n g e n e r a l , can o n l y be used a t low c u r r e n t s ( 2 ) or under p a r t i c u l a r c o n d i t i o n s of p r e s s u r e , geometry, and gas ( 1 ) . T h i s has been c o n f i r m e d by the au t h o r u s i n g a s p e c i a l l y - c o n s t r u c t e d tube. In the p r e s e n t i n v e s t i g a t i o n , a t e c h n i q u e has been developed f o r s w i t c h i n g out mercu r y - p o o l a r c s u s i n g a t h i r d 6 e l e c t r o d e o The method i s s i m p l e and r e l i a b l e and i n v o l v e s no i n t e r f e r e n c e w i t h the main c i r c u i t , c u r r e n t i n t e r r u p t i o n i s e f f e c t e d w i t h the f u l l anode v o l t a g e s t i l l a p p l i e d 0 The method has the a d d i t i o n a l advantage of r e d u c i n g the d e i o n i z a t i o n time by a c o n s i d e r a b l e f a c t o r 0 The f i r s t p a r t of the t h e s i s , P a r t A, d e a l s w i t h the development of the above t e c h n i q u e and forms the main p a r t of the worko Experiments performed u s i n g the space-charge-sheath method of s w i t c h i n g o f f are d e s c r i b e d i n P a r t B<, 7 P a r t Ao The M e r c u r y - p o o l Tube The mercury-pool tube was o p e r a t e d i n the a r c r e g i o n of the c h a r a c t e r i s t i c curve f o r g a s - d i s c h a r g e t u b e s . The c u r r e n t was h i g h and the v o l t a g e a c r o s s the tube was low. 2. Apparatus Some of the p h y s i c a l measurements of the mercury-p o o l tube and v a r i o u s f e a t u r e s of the a s s o c i a t e d c u r c u i t r y and c o n n e c t i o n s are d e s c r i b e d below. 2.1. The Tube The tube was a mercury-pool type of s i m p l e d e s i g n c o n t a i n i n g two mercury p o o l s , see F i g u r e 2.1. The t h r e e probes and the connections to the anode and c a t h o d e ? o f 0.75-mm r a d i u s . p r o j e c t e d from 0.6 to 1.4 cm i n t o the tube. These probes were of t u n g s t e n and were s e a l e d i n t o the 0 . 4 0 - i n . - t h i c k p y r e x - g l a s s w a l l s . The double-probe t u n g s t e n e l e c t r o d e s , of 0o75-mm d i a m e t e r , were covered by a. t h i n g l a s s i n s u l a t i n g l a y e r t o w i t h i n 0.3 cm of the ends which pro j e c t e d ^ i o n g the 'centre l i n e i n the o p p o s i t e d i r e c t i o n t o the n o r m a l - c u r r e n t f l o w . They were spaced 0.28 cm 7 a p a r t . The l a r g e main channel was of g - i n . d i a m e t e r and the c o n n e c t i n g channel was of ^ - i n 0 d i a m e t e r . F o r an i d e a of the o v e r a l l s i z e of the tube and the l e n g t h of the d i s c h a r g e p a t h see the dimensions i n the f i g u r e . About 27 ml of d i s t i l l e d mercury f i l l e d b o t h the anode and cathode p o o l s . The c o n n e c t i n g channel was f o r convenience i n r e p l e n i s h i n g the anode p o o l from the cathode p o o l s i n c e the p o s i t i v e m e r c u r y - i o n c h a r g e - c a r r i e r s g r a d u a l l y f i l l e d the cathode p o o l as they condensed. The double p r o b e , i n c o r p o r a t e d t o make e l e c t r o n temperature and o t h e r measurements, was e v e n t u a l l y employed f o r s w i t c h i n g o f f . The probes were to be used e i t h e r Figure 2,1. The Mercury-pool Tube 8 f o r s w i t c h i n g o f f or f o r o b t a i n i n g measurements 0 S t r u c t u r a l changes were made s e v e r a l times and a photograph of the f i n a l l y m o d i f i e d tube i s shown i n F i g u r e 8.2. 2 , 2 . The I g n i t i o n Method T h e r m i o n i c 8 c o l d - c a t h o d e , and mercury-pool gas tubes are s w i t c h e d on i n a v a r i e t y of ways. A method s i m i l a r t o t h a t f o r i g n i t i n g Cooper H e w i t t mercury-vapour lamps ( 3 , 5 ) was used here. The output from a h i g h - v o l t a g e s p a r k - c o i l was a p p l i e d to a band p l a c e d o u t s i d e the tube a t the cathode (7) as shown i n F i g u r e 2 9 2 . Most of the p o t e n t i a l drop was i n the m e r c u r y - m e n i s c u s - t o - g l a s s gap^ s i n c e g l a s s has a h i g h r e l a t i v e d i e l e c t r i c c o n s t a n t , 5 . 4 - 9 . 9 ( 3 4 ) . The f r e e e l e c t r o n s from the cathode, produced by the e l e c t r i c f i e l d , s t a r t e d i o n i z a t i o n when they were a c c e l e r a t e d by the p o s i t i v e v o l t a g e on the anode, mercury p o o l • i g n i t i o n band «"i to « v 2 0 a 0 0 0 v g l a s s w a l l cathode c o n n e c t i o n F i g u r e 2.2. A S e c t i o n a l View of t h e Cathods 9 an a r c i g n i t e d and the cathode spot was formed,, Once the e l e c -t r o n s were produced, breakdown o c c u r r e d almost i n s t a n t a n e o u s l y , see a l s o S e c t i o n 5.2.1. 2.3. V a r i a t i o n of P r e s s u r e I n i t i a l l y , the tube was c o m p l e t e l y s e a l e d and the p r e s s u r e was ~(10) mm Hg. Then, v a r y i n g the p r e s s u r e by changing the temperature (39) would have g i v e n d i f f e r e n t e x p e r i -mental c o n d i t i o n s . A c c o r d i n g l y 9 an e n c l o s u r e f o r the tube was b u i l t and h e a t e r s i n s t a l l e d , but a u n i f o r m temperature c o u l d not be e a s i l y m a i n t a i n e d due to the a i r c o n v e c t i o n streams. I t was u n c e r t a i n t h a t the l o w e s t temperature of the tube was b e i n g measured. F i n d i n g the p o s i t i o n i n the e n c l o s u r e of t h i s l o w e s t temperature would have been d i f f i c u l t , but t h i s temperature v a l u e was i m p o r t a n t s i n c e i t determined the p r e s s u r e . As the d i s c h a r g e operated- the o v e r a l l temperature of the tube i n c r e a s e d because of k i n e t i c - e n e r g y h e a t i n g l o s s e s , the p r e s s u r e would t h e n i n c r e a s e , and the a r c would e x t i n g u i s h . H e a t i n g was d i s c o n -t i n u e d because of t h e s e l i m i t a t i o n s and the tube was r e d e s i g n e d so t h a t a pump, to e l i m i n a t e the temperature dependence, and a p r e s s u r e gauge c o u l d be i n s t a l l e d . H e a t i n g would have been an advantage i f i t c o u l d be ensured t h a t the p o o l s s t a y e d a t the l o w e s t t e m p e r a t u r e . Mercury vapour condenses a t the p o s i t i o n of l o w e s t temperature i n the tube. An unwanted c o n d u c t i n g p a t h c o u l d be caused by the condensed mercury i f the l o w e s t temperature were a t the w a l l s . C u r r e n t would f l o w a l o n g the g l a s s s u r f a c e s and the breakdown v o l t a g e would be i n c r e a s e d to c r e a t e an i g n i t i o n problem. Such a c o n d i t i o n a c t u a l l y happened, see S e c t i o n 6.4. An o i l - f i l l e d r o t a r y fore-pump connected to a 10 mercury d i f f u s i o n - p u m p was chosen f o r the pumping system. The f o r e pump was of a s t a n d a r d t y p e 5 the d i f f u s i o n - p u m p d e s i g n was a l s o q u i t e s t a n d a r d , i t had a 365-watt h e a t e r and a maximum b a c k i n g - p r e s s u r e r a t i n g of 0 0 3 mm Hg. Pumping was c o n t i n u o u s on most of the t e s t s made. 11 3= I n i t i a l Attempts to Switch Off 3.1. Extension of the Space Charge The f i r s t attempt at switching off the mercury-pool tube was by biassing a probe negatively,, Extending the space-charge sheath across the main channel by application of a high voltage on a probe was an obvious method of trying to cause conduction to cease. Electrons coming from the cathode would not pass through the grid sheath and no further ionization would result. The accelerating potential on the anode would sweep the existing charge c a r r i e r s out of the anode-grid region, The positive ions would disperse i n the grid-cathode region and the current would cease. No influence of the anode would be f e l t i n the grid-cathode region. Biassing a probe p o s i t i v e l y would have had no interrupting effect since ionization would probably have been aided, not stopped, and the probe would take over as the anode. Since the current to the probe c i r c u i t , supplied by the plasma, i s limited by the space charge surrounding the probe, i t follows the Child-Langmuir T^-power law (13, 28); 1 3 A /2q \ 2V 2 J = |e -§ . (3.1) P 9 °\ rap/ D 2 In t h i s notations J i s the ion-current density: c i s the P 0 permitivity for free space, — — ™ — j r farad/meter; q i s the 4 n 9 ( l 0 r i g P electronic or ionic charge, 1 D6(10) coulomb/particle; ra i s 3? the mass of the ion; V i s the potential difference across the s sheath; and D i s a quantity which depends on the geometry of the tube. The mass of the ion i s calculated from: m = 1840Mm , (3.2) 12 where % M i s the atomic-mass r a t i o (oxygen i s 1 6 ) ; and m i s e —31 th e e l e c t r o n i c mass, 9.11(10)"" k g o P r e l i m i n a r y c a l c u l a t i o n s were made u s i n g c y l i n d r i c a l geometry. Then (28, 29)s D = r p 8 , ( 3 . 3 ) 6 . l a i - f ( l n ^ ) 2 - + !M(m^)3 - 3 3 ^ ( 1 ^ [ + .... (3 0 4) where? i s the plasma-edge r a d i u s , and r y the probe-edge r a d i u s of the space-charge s h e a t h . The v a l u e s used were: M = 201 f o r mercury vapour; and J = 1 ma/cm , a t y p i c a l v a l u e f o r the XT random-ion c u r r e n t - d e n s i t y f o r mercury vapour (21, 2 7 ) . See r 2 S e c t i o n 2.1 f o r the geometry measurements. S i n c e —^ = 29.6, 8 r was t a k e n as 1.09 ( 2 9 ) . T h e r e f o r e , the s w i t c h i n g v o l t a g e , found froms ~ 0 Q i 4 r ^enrj V 2 = — E _ R from (3.1) . s 5.43(10) a 2 was 12,500 v o l t s . A l s o , u s i n g a v a l u e of <v0.48 cm f o r the s u r f a c e a r e a of a t y p i c a l probe, the power r e q u i r e m e n t of the V power—supply was found t o be 6 w a t t s , s The c a l c u l a t i o n of V was not e n t i r e l y v a l i d . s J I t was assumed t h a t j u s t e x t e n d i n g the space-charge s h e a t h from the end of a probe t o the g l a s s w a l l o p p o s i t e would be a l l t h a t was needed t o c l o s e o f f the tube. The r e s u l t i n g s h e a t h would be t h i c k a l o n g the i n t e r s e c t i o n of the main-channel and probe c e n t r e - l i n e s , but would be t h i n towards the w a l l i n a d i r e c t i o n m u t u a l l y p e r p e n d i c u l a r t o t h e s e c e n t r e l i n e s . I n o t h e r words, even h i g h e r v o l t a g e would p r o b a b l y be n e c e s s a r y t o c l o s e o f f the tube w i t h c e r t a i n t y . The c r i t i c a l p arameters, J and r ^ , were d i f f i c u l t t o e s t i m a t e c o r r e c t l y . Because the c a l c u l a t i o n 13 was o n l y f o r o r d e r - o f - m a g n i t u d e , the l i m i t a t i o n s were n ot e x p l o r f u r t h e r o The. a r c c i r c u i t c o n s i s t e d of the g e n e r a t o r , G^, p r o d u c i n g 1 2 0 v o l t s ; the r h e o s t a t , R^; and the tube. When the d i s c h a r g e was r u n n i n g , the anode-to-cathode p o t e n t i a l - d i f f e r e n c e , V, was 2 3 v o l t s . Once c o n d u c t i o n had s t a r t e d , the c u r r e n t was l i m i t e d o n l y by the impedance, i n t h i s case R^, i n s e r i e s w i t h the anode The p r o b e - v o l t a g e power-supply, V , was connected to the v a r i o u s s p r o b e s ! 1, 2, and/or 5 , Probe v o l t a g e s of up to - 1 , 4 0 0 v o l t s , w i t h r e s p e c t t o the cathode, were t e s t e d , but the tube d i d n o t s w i t c h o f f , as was expected from the p r e l i m i n a r y c a l c u l a t i o n s 0 Note t h a t the n e g a t i v e v o l t a g e w i t h r e s p e c t to the plasma was a c t u a l l y the s w i t c h i n g ~ o f f v o l t a g e v a l u e . No d i s t i n c t i o n has V been made between the power s u p p l y , V , and the s w i t c h i n g v o l t a g S" V , a l t h o u g h t h i s becomes s i g n i f i c a n t l a t e r , see P a r t B, The c i r c u i t was connected as shown i n F i g u r e 3.1c 2 4 \ t o probes 1 5 \ I V s I anode cathode F i g u r e 3 » L The F i r s t S w i t c h i n g - o f f Method 14 Because the v o l t a g e n e c e s s a r y to i n t e r r u p t the c u r r e n t by e x t e n d i n g the probe sheath would be so l a r g e , >12,500 v o l t s by c a l c u l a t i o n , the method was d i s c a r d e d . The c a l c u l a t e d power r e q u i r e d , however, was not a l i m i t a t i o n on the method. The a c t u a l energy employed would depend on how q u i c k l y the d i s -charge was e x t i n g u i s h e d . I t was thought the tube would s w i t c h q u i t e q u i c k l y and the energy used t h e n would be s m a l l . I f a s c r e e n - l i k e g r i d were to be employed w i t h v e r y s m a l l h o l e s i n i t , the s w i t c h i n g - o f f v o l t a g e c o u l d p r o b a b l y be reduced v e r y markedly. 3.2. Use of a S t r a p An attempt was n e x t made a t s w i t c h i n g o f f the m ercury-pool tube i n a s i m i l a r manner to t h a t employed f o r i g n i t i o n . A s t r a p o u t s i d e the tube was p l a c e d below the double-probe p o s i t i o n . The c i r c u i t was the same as i n F i g u r e 3.1 except t h a t the power s u p p l y , V , was connected t o the s t r a p . s V o l t a g e s up to 1,400 v o l t s , p o s i t i v e and n e g a t i v e w i t h r e s p e c t to the cathode, were a p p l i e d . The tube d i d n o t s w i t c h o f f . The v o l t a g e was a g a i n p r o b a b l y too low. However, t h e r e i s doubt whether i n c r e a s i n g the v o l t a g e would be any more s u c c e s s f u l , see S e c t i o n 5.1.2, and, i n any event, u s i n g a l a r g e r v o l t a g e would n o t be p r a c t i c a l . 7 Note t h a t , i f a s t r a p of y g — i n . d i a m e t e r were p l a c e d i n s i d e the tube and the s pace-charge-sheath method were t o be used, the V c a l c u l a t e d v a l u e t o s w i t c h o f f would be s d e c r e a s e d to -4,800 v o l t s . T h i s s w i t c h i n g - o f f v o l t a g e was s t i l l l a r g e , but the advantage of a s u i t a b l e e l e c t r o d e c o n s t r u c -t i o n was a p parent, see P a r t B f o r f u r t h e r d e t a i l s . 1 5 Use of a C a p a c i t o r The p r e v i o u s methods had not worked a t low v o l t a g e s and would be, a t b e s t , not p r a c t i c a l when the v o l t a g e was i n c r e a s e d . Perhaps an e n t i r e l y d i f f e r e n t method of s w i t c h i n g o f f the mercury-pool tube by u s i n g p u l s e s of c u r r e n t r a t h e r t h a n the steady space-charge method would be p r a c t i c a b l e . P u l s e a m p l i t u d e c o u l d be double the v a l u e used i n the s t e a d y - s t a t e experiment and s t i l l t he average-power requirement would be the same or l e s s depending on the p u ] s e - r e c u r r e n c e - f r e q u e n c y . i s by d i s c h a r g i n g a c a p a c i t o r . A c c o r d i n g l y , c a p a c i t o r s were connected to the d i s c h a r g e by e i t h e r or a l l of the probess 1, ' 2, and/or 5. The c i r c u i t i s shown i n F i g u r e 3.2. The a r c c i r c u i t c o n s i s t e d of the g e n e r a t o r , Gg, p r o v i d i n g 100 v o l t s ; t h e c u r r e n t - l i m i t i n g r h e o s t a t , R^J the ammeter, a; the v o l t m e t e r , v; and the tube. I n the s w i t c h i n g - o f f c i r c u i t were the power An obvious way of o b t a i n i n g a s h o r t c u r r e n t p u l s e •i t o j probe F i g u r e 3.2. C a p a c i t o r S w i t c h i n g on a S o l i d Probe 1 16 supply, V , the charging r e s i s t o r , r , 27,000 ohms; the capacitor, s s C; and the tube. Capacitor values of 0.25 to 15 microfarads were t r i e d . The capacitors were separately charged, 6 <r gC 4400 milliseconds, and then connected to a probe. Various V values • 1 c s to a maximum of 1,400 v o l t s , positive and negative with respect to the cathode, were t r i e d . The arc current, I, was «»2.5 amperes'. With the largest capacitor and various negative switching-off voltages, the arc could be extinguished sometimes, but with a smaller capacitor current interruption did not occur. Positive charging of the capacitor^ C s caused the arc to switch off only very occasionally when C was connected. Placing a r e s i s t o r , B, i n series with the capacitor increased the switching-off probability for negative switching-off voltages only. Charg-ing the capacitor negatively was a procedure that worked often enough to warrent studying further. A test made to determine the optimum switching-off voltage for various r e s i s t o r s with constant arc current gave inconclusive r e s u l t s . There were two reasons for t h i s . F i r s t , the arc current was *>2.5 amperes. This value was below that of three amperes usually quoted as the minimum for cathode-spot s t a b i l i t y on a mercury surface (14). Thus, the arc some-times extinguished of i t s own accord even though conditions were not conducive to switching o f f . This possibly explained why switching occasionally happened when positive pulses were employed. Second, i n order to feed a negative pulse from the probe to the cathode, a spot had to be formed at the probe. In other words, the switching-off pulse had to have i t s own cathode spot. The conditions for cathode-spot formation were not the same on each switching attempt, sometimes the spot would not form, e i t h e r the c a p a c i t o r s l o w l y d i s c h a r g e d or d i d n o t change a t a l l , and s w i t c h i n g o f f was n o t e f f e c t e d . E a r l i e r s w i t c h i n g s u c cesses caused s p u t t e r i n g of the e l e c t r o d e m a t e r i a l so t h a t the probe s u r f a c e was c o n t i n u a l l y changing. The i m p o r t a n t r e s u l t was t h a t a c u r r e n t a t the cathode i n the o p p o s i t e d i r e c t i o n to t h a t of the normal c u r r e n t f l o w f o r c e d a r c - c u r r e n t i n t e r r u p t i o n . E s t a b l i s h i n g t h i s r e v e r s e c u r r e n t r e q u i r e d e s t a b l i s h i n g a cathode spot on the e l e c t r o d e i n t r o d u c i n g the p u l s e . C o n d i t i o n s n e c e s s a r y f o r f o r m a t i o n of a cathode spot and more d a t a c o l l e c t e d to i s o l a t e the reasons f o r a r c - c u r r e n t i n t e r r u p t i o n are g i v e n below. 18 4o Cathode-spot Formation Discharges are c l a s s i f i e d by examining the cathode region 0 In an arc, the cathode has a f a l l of potential of the order of the minimum ionizing potential of the gas, a very high current density, and the emitted l i g h t shows the spectrum of the cathode material (17) 0 There are several theories of the func-tioning of the arc cathode i n a cold-cathode tube, they a l l require somewhat the same conditions for cathode-spot formation,, When the cathode spot did not form on the cold-cathode switching probe, the capacitor sometimes discharged slowly, but most often did not change i n e l e c t r i c a l conditions, and the arc would not extinguish. The current at which t r a n s i -tion takes place from a glow, which probably would be the mode used by the switching pulse without interrupting the current, to an arc switching pulse i s not easily predictable. In general, the unstable t r a n s i t i o n region i s between 0.01 and 1.0 ampere (20). Then the reverse current has to be greater than one ampere for a cathode spot to form. Some of the other conditions for spot formation w i l l now be stated. There are several requirements (10, 12) for cathode-spot formation of a cold cathode. F i r s t , low-work-function impurities cause the current density to increase i n a l o c a l region on the cathode. Adsorbed ion and gas layers also have the same effe c t . The l o c a l temperature then increases and electron emission i s enhanced, due to ionization more positive: ions are formed and the e l e c t r i c - f i e l d strength i s increased causing even more electron emission, and so for t h . Impurities i n the cathode enable steady evaporation to result and the formation and ejection of evaporation droplets which 19 act as c h a r g e - c a r r i e r recombining s u r f a c e s and can cause i n t e r -r u p t i o n of the arc i s avoided. Second, microscopic i r r e g u l a r i -t i e s ( l l ) on the cathode surface cause high l o c a l e l e c t r i c -f i e l d g r a d i e n t s which a i d emission. Normally, the e l e c t r i c -f i e l d s t r e n g t h i s not great enough to cause e l e c t r o n emission and a cathode spot would not form on a very c l e a n and smooth e l e c t r o d e . Note t h a t the l o c a l c u r r e n t s have to be s m a l l , otherwise the i r r e g u l a r i t i e s can not s u r v i v e the high temperature a s s o c i a t e d with the spot. T h i r d , i f i t i s assumed t h a t the mechanism forming the spot i s thermionic, a low thermal conduc-t i v i t y of the cathode i s a h e l p . The high temperature i n the f i r s t two or three atomic l a y e r s must be maintained. Fourth, oxide l a y e r s on the e l e c t r o d e r e s u l t i n poor e l e c t r i c a l c ontact between the plasma and the cathode. High e l e c t r i c - f i e l d g r a d i e n t s r e s u l t when the p o s i t i v e ions r e s t on the oxide i n s u l a t i n g l a y e r . Emission i s i n c r e a s e d then and due to i o n i -z a t i o n more p o s i t i v e ions are formed i n c r e a s i n g the e l e c t r i c f i e l d s t r e n g t h and the process c o n t i n u e s . F i f t h , when the c o l d cathode i s a mercury p o o l , p h y s i c a l a g i t a t i o n causes t r a n s i e n t d i s c o n t i n u i t i e s i n c r e a s i n g l o c a l e l e c t r i c - f i e l d strengths and formation of a cathode spot i s a i d e d . Of the c o n d i t i o n s j u s t d e s c r i b e d , the l a s t three may be c o n t r o l l e d by design. P h y s i c a l a g i t a t i o n r e s u l t s i n spot formation at random time which i s not d e s i r e d . The tube must be shock mounted to stop the e f f e c t of v i b r a t i o n s from a n c i l l a r y equipment and thus e l i m i n a t e one cause of random v a r i a t i o n s i n the r e s u l t s . Next, u n f o r t u n a t e l y , tungsten has a r a t h e r high c o e f f i c i e n t of thermal c o n d u c t i v i t y , 1.99 watt/cm-'C at room temperature (36), which makes forming a spot on a probe 20 d i f f i c u l t . This l i m i t a t i o n was tolerated 0 Several techniques for cathode-spot formation were devised and these are described belowo It should be made clear that no attempt was made to determine which theory explained the functioning of the cathode spot. A l l that was of interest was how to form and maintain a spot. 21 5. Capacitor Switching Once i t had been discovered that the mercury-arc current was interrupted by a pulse of reverse current, various tests were made on the tube to obtain the necessary and s u f f i c i e n t conditions for switching o f f o 5 d . The Solid Probes 5.1.1. Natural I r r e g u l a r i t i e s Aid Spot Formation The f i r s t method of attempting to switch off the mercury-pool tube depended on the physical cha r a c t e r i s t i c s of the switching-electrode material for establishing a cathode spot. The same c i r c u i t as i n Figure 3.2 was employed and more data were taken with the arc current at various values. The independent variables were varied according to Figure 5.1; that i s , various R values were chosen for certain switching voltage and arc-current values, and the number of current interruptions were counted for 20 switching attempts. A maximum value of 1,000 volts for the switching voltage, V , was t r i e d s and the arc current, I, was increased to 10 amperes. Even though the probability of current interruption was not great, certain results could be noted. On examining the graphs i t was seens f i r s t , that a higher switching-off voltage gave a greater l i k e l i h o o d of switching; second, that a high arc current caused uncertain switching; and t h i r d , that there was a p o s s i b i l i t y of an optimum value for R. Again, using the same c i r c u i t , more data were collected and these are summarized i n Table 5.1. No significance should be given to the pa r t i c u l a r values of R, V , and C used s as these values were chosen only to bring about current i n t e r -probability- probability 2 2 0 Table 5.1. Various Values Important f o r Switching Off I -- a R - -A 1C - uf | switch' (I ) ' ^ • s m 1.5 | 5 15 yes 15.3 91 ! M..0 1.5 1 5 15 yes 8.2 1400 •-1.8 1.5 i 10 15 yes 9.4 1350 *2.0 1.5 10 15 yes 4.0 1900 1.1 1.5 15 15 yes 2.1 I960;:. 3.1 1.5 i 15 15 yes 4.5 1900' 2.2 1.5 i 31 19 yes 3.3 2100 v3 o0 1.5 i 50 15 yes 2.5 2450 3.1 1.5 50 15 yes 1.7 2600 • v2 . 5 1.5 j 100 19 no 1.1 1400 d . 0 2 . 2 : 5 15 yes 14.7 1300 A>3.0 2.2 10 15 yes 9.1 2100 -.4.5 2.2 15 15 yes 6.7 2400 •v4.2 2.2 31 15 yes 3 o 6 2300 v 2 o 5 2.2 100 15 no 1.3 400 -0.4 5.0 5 15 yes 16.5 360 ••J2.2 5.0 5 15 yes 10.6 330 ••-'1.7 5.0 5 19 yes 21.7 2210 <L2.7 5.0 5 30 yes 17.1 2000 12.0 5.0 10 15 yes 9.7 420 2.3 5.0 10 15 yes 7.6 420 -.1.7 5.0 10 19 y,es 9.4 1800 "9.9 5„0 10 30 yes 10.0 1800 -10.0 5.0 15 15 yes 7.5 540 r*2 o 8 5.0 15 15 no 5.2 240 -1.0 5.0 15 19 yes 7.3 1400 ^7.1 5.0 15 30 yes 7.4 1400 •v6.0 5.0 31 15 no 3.3 120 ^0.2 5.0 31 15 no 2.8 480 ^0.9 5.0 50 15 no 2.9 1800 -3.3 5.0 50 15 no 2.9 1200 ~J3 o 1 5.0 100 15 no 1.3 900 0.6 V was 700 v except f o r switching attempts; s and 8, when the value was 400 v. ( I ) and T are not very accurate; s m s 0, was obtained b s under the I waveform t r a c e . s 1 Q_ was taken as TJ-IT which w i l l be seen to be g r e a t e r than x h e s c o r r e c t value The tube was co n t i n u o u s l y pumped. +15$. rough c a l c u l a t i o n of the area mq. 0< 1, 1< 1< l c 1< 1< 1< 2.0 1.1 1.4 2.3 2.6 2.5 0.4 0.9 0.8 5.5 5C 1< l c 4c 4 t l c Oc 3.5 3.5 0.3 1< 4c 3C 2c 3, 5S r u p t i o n with a reasonable degree of r e g u l a r i t y . I t was found from the t a b l e t h a t the charge, Q , on the c a p a c i t o r was not an important f a c t o r , but — j y was. The peak switch c u r r e n t , (* s) m'. had to be g r e a t e r than the arc c u r r e n t to cause c u r r e n t i n t e r -23 ruption. The few times switching off occurred when (I ) m was not greater than I were dismissed, since I <3.0 amperes on those attempts and would extinguish of i t s own accord because of i n s t a -b i l i t y (14) when any small e l e c t r i c a l disturbance was made. With R i n series with C, the time to discharge the capacitor, T , s increased and, therefore, increased the time ( I g ) m ^ » On repeating the switching tests, the p r o b a b i l i t i e s had decreased s i g n i f i c a n t l y . This indicated that v :raost of the i r r e g u l a r i t i e s on a l l the probes had been burned off by the high temperature associated with the spot. The probes were now smooth and clean having no i r r e g u l a r i t i e s for cathode-spot formation. Probe 5, see Figure 2.1, gave the best control because mercury, splashing from the arc cathode, caused temporary di s c o n t i n u i t i e s aiding the formation of a spot. A f i n a l attempt at using electrode i r r e g u l a r i t i e s or contaminating layers for cathode-spot formation was made by connecting a l l the probes together. The switching-off voltage was applied to the resultant "probe," but no new information was obtained. The switching-off probability remained quite small. Depending on the microscopic physical i r r e g u l a r -i t i e s for cathode-spot formation was unsatisfactory. After a few operating cycles, the i r r e g u l a r i t i e s were worn smooth and impurities sputtered away so that switching off became random. Provided the cathode spot would form, arc-current interruption would be successful as long as the reverse-current-pulse magnitude was greater than the arc-current value. However, the cathode spot would not form on every switching attempt. This was due to a randomness i n the necessary conditions for 24 cathode-spot formation. 5.1.2. Providing the Conditions for Spot Formation Some method of ensuring that a cathode spot would always be formed when V was applied had to be found. s The f i r s t way of increasing the probability of switching was by introducing a i r into the tube. The tungsten probes, contaminated very quickly by a new oxide layer, again had l o c a l areas where cathode spots could form. However, the increase i n switching probability proved to be only temporary. The oxide layers were worn away too quickly. In any event, the procedure was not p r a c t i c a l , periodic opening and closing the tube stop-cock and repumping was clumsy. It was then remembered that one of the s o l i d probes had given s l i g h t l y better performance because splashing mercury had provided temporary di s c o n t i n u i t i e s upon which spots could form. Since the double probe, electrodes 3 and 4 i n Figure 2.1, was i n the discharge path, i t was hoped the mercury ions would condense easily on the electrodes, cause discontin-u i t i e s , and aid spot formation. The fact that the double probe was i n a region of high ion concentration (the probes employed previously were out of the dir e c t discharge path) would also help spot formation. Operation was changed to the double probe and switching off became regular. Unfortunately, a varying delay from about 0.1 millisecond to i n f i n i t e time, between the application of V g and the time of establishment of the cathode spot, was measured. The tube would switch off on every attempt provided (I ) >I, but the time to actual switching off was r s m ' random. This delay was probably partly a function of the size of the condense1i'*1inercury-vapour p a r t i c l e s since i t was possible 25 that mercury droplets of a c r i t i c a l size were necessary, with large p a r t i c l e s not affecting the e l e c t r i c - f i e l d strength and small p a r t i c l e s causing the gradient to be small c The delay was probably also dependent on the physical i r r e g u l a r i t i e s on the electrodes. As the discharge operated, the overall tempera-ture increased and mercury vapour was less l i k e l y to condense at the double-probe position, for the double probe would be no longer at a low temperature. Note that the physical i r r e g u l a r -i t i e s here s t i l l played a role i n cathode-spot formation., The delay had to be eliminated. It was decided to try forming a spot by sparking between each electrode on the double probe. If the time of sparking could be controlled, the time of current interruption could also be controlled. The breakdown voltage was 250 volts without the discharge. When the arc was running, the voltage was increased to greater than 450 volts and s t i l l no l o c a l breakdown occurred between the two electrodes. This was because a space-charge sheath, the same as described i n Section 3 . 1 , surrounded the electrodes i s o l a t i n g the voltage applied and the plasma. These electrodes were too f a r apart on the double probe to make extending the sheath p r a c t i c a l since the voltage would have to be increased greatly. During the tests, one of the double-probe electrodes completely sputtered away. The surface area a v a i l -able for co ndensed-mercury-vapour deposits was decreased and the switching-off probability also decreased. This, and the fact that the delay-time problem was not solved caused abandon-ment of the mothod. In another attempt to force spot formation, the 2 6 switching electrode again was made a probe. The high-voltage spark c o i l , also employed for i g n i t i o n , was connected to a strap placed i n turn around the various probes outside the tube. A spot did not form on any probe on application of the strap voltage because the current from the spark c o i l would take the discharge as a conducting path to the cathode to ground. No influence on spot formation was f e l t at the probes, 5 . 1 o 3 , Comments on Solid-probe Switching The tube could be switched off by using any s o l i d probe, but the performance was e r r a t i c . The character-i s t i c s of the solid-electrode material determined the action. The physical variations at the switching-off electrode were the main reason for the randomness of current interruption; for example, the probes were always becoming smoother leaving no i r r e g u l a r i t i e s upon which a cathode spot could be i n i t i a t e d . Creating an oxide layer on the probes aided spot formation, but the layer was quickly worn away and the i r r e g u l a r i t i e s again caused the random behaviour. Depending on mercury-vapour condensation to provide di s c o n t i n u i t i e s gave results that also were random. The exact cause of the randomness was not known. Condensation effects and p h y s i c a l - i r r e g u l a r i t y wear probably each played a part. The double probe was not much better than the single probes for switching o f f . It was c l e a r l y demonstrated, however, that once the cathode spot was established and the e l e c t r i c a l condition f u l f i l l e d , switching-off would ensue. 27 5.2. The Mercury-pool Probe It was evident from the above results that the switching delay would be much reduced i f a cathode spot could be farmed at the switching probe when the switching-off voltage was a p p l i e d o 5.2.1. Spot Formation by a Band Igniter One method of spot formation was to employ a mercury pool as the .switching electrode and a band i g n i t e r to s i n i t i a t e the spot when V was applied; that i s , a similar s technique as used for i g n i t i n g the arc was devised for i n i t i a t i n g a cathode spot at the switching pool. The new. c i r c u i t i s i n Figure 5,2. Probe 2 was i made the arc anode and the former mercury-pool anode became the v ^3 vi on on jfc— a. V -.t -"-period = 2 sees. Automatic switching sequence Figure 5,2, Cathode-spot I n i t i a t i o n by a Band Igniter 28 switching-off electrode. A second band was placed around th i s pool and a second high voltage spark c o i l was provided to i n i t i a t e the switching-off cathode-spot. The other connections remained the same. Minor modifications to the c i r c u i t were made to f a c i l i t a t e the automatic control of the operating cycle. An automatic switching sequence was constructed to prevent over-heating of the anode which was now a tungsten probe. The period of operation lasted two seconds. If the tube did not switch o f f . the maximum conduction time, controlled by relay 3, was one second; when switching occurred, the conduction time was 200 milliseconds. Care was taken to ensure that relays: tf» a n < l °% opened and closed i n the proper order. The closing of relay g determined the time of application of V . The relay operating sequence i s shown i n the figure. Current interruption was now successful every time when similar c i r c u i t values as employed previously were used. The randomness aspect of the problem had been eliminated. However, there was s t i l l a delay, much reduced now, of a maximum of *>1 millisecond. This delay was caused by the frequency, ^1 kilocycle/second, of the spark c o i l . The cathode spot only appeared when the band voltage was s u f f i c i e n t l y positive with respect to the switch pool. Since no control was had over the phasing of the band voltage when V was applied, the band could be at the correct voltage instantaneously or as much as 1 millisecond l a t e r . The main result obtained was that the switching-off method was sound. Given suitable e l e c t r i c a l conditions an arc current could be interrupted regularly. 29 5.2o2= The Other Attempts at Cathode-spot Formation The small varying delay between application of V and the beginning of the switching-off process when the s cathode spot formed was undesirable. Several d i f f e r e n t tech-niques were attempted i n order to eliminate the delay. F i r s t , the spark-coil i g n i t e r was replaced by a Tesla c o i l . This would not have eliminated the delay time, but would have reduced i t to microseconds. Unfortunately, the Tesla c o i l had too high an internal impedance and not enough useful voltage was available to furnish a spot. The apparatus was discarded. The contacts on the spark-coil primary were next screwed together i n an attempt at providing a "one-shot" pulse. Even though the peak magnitude was probably very great, unre-l i a b l e operation resulted. The short time duration of the pulse no doubt accounted for the low probability of success. The band was then grounded i n an attempt to have 7 , i t s e l f , generate a spot. As was expected, the experiment did not work. The voltage was much too low. F i n a l l y , a capacitor charged to 1,400 vo l t s , positive with respect to the cathode, was applied to the band. The resultant potential difference between the band and the mercury pool could be as much as 1,900 volts depending on the value of V . The method was somewhat similar to one already s developed (8), but i t did not work. Probably the voltage was too small. Other band-igniter techniques (4) could have been t r i e d as could e n t i r e l y d i f f e r e n t methods, such as the ignitron (6) or the excitron (9) methods. A l l that was 30 required was that the spot be formed immediately the switching-off voltage was applied. However, these procedures require special construction and modifications to the tube. To expedite the research, the manner of furnishing the cathode spot described below was accepted. 5.2.3. An Auxiliary-arc Discharge Provides the Spot An instantaneous beginning of the current-interruption process on application of the switching voltage was required. The cathode spot had to be available when V g was applied. One way of establishing t h i s would be to have a permanent cathode spot or at least one that was always present before connecting the capacitor, C. An auxiliary-arc discharge was ignited to provide the cathode spot. The c i r c u i t i s i n Figure 5.3. The auxiliary o n :V////Z//\ .^.t on on .'7 on _ £ZZ22L or. ! 0*1 on a ! on on -'•period = 2 sees.-*— Automatic switching sequence Figure 5.3. An Auxiliary-arc Discharge Provides the Spot 31 arc connections were to the switch pool, as the auxiliary-arc cathode, and to probe 1, as the auxiliary-arc anode. The rheostat, R^ , was present i n order to l i m i t the auxiliary-arc current to ^3 amperes. The generator, G^, produced 100 volts and the cathode spot was present at the switch pool at a l l times that the auxiliary-arc discharge was ignited. The switch-ing c i r c u i t remained the same as i n Figure 5.2. The arc c i r c u i t remained similar to previous connections, only R^ was moved to the cathode side of the generator. Having a higher positive potential on the anode, with respect to ground, might increase the switching off probability when there was a greater potential difference between the plasma and the switch pool. The arc c i r c u i t values were s l i g h t l y changed because the generator, G Q, was increased to 140 v o l t s , o P a r t i c u l a r attention had to be given to the automatic switching sequence. A relay, 6, connecting the switch pool to ground was necessary to provide a return path for the auxiliary-arc band-igniter-current. This relay had to be disconnected before V was applied, v i a relay ^, or else a s short c i r c u i t of V would r e s u l t . Also, the auxiliary-arc s band-voltage had to be removed before V was applied, to ensure s that the influence of the switching-off voltage alone was responsible for current interruption. This was done by opening relay 9„ Since only one spark c o i l was being used, relay sequence was important i n order to apply the auxiliary-arc i g n i t i o n voltage only to the auxiliary-arc band. This was controlled by relays 8 and ";. No i g n i t i o n current would then be passed by the low-impedance path provided by the arc already ignited. 32 The c i r c u i t was a success, switching off occurred every time and the reverse-current pulse began to r i s e immedi-ately V was applied; no delay was discovered. The a u x i l i a r y -s arc discharge apparently did not influence the switching-off c i r c u i t behaviour. The method of ensuring certain and almost instantaneous current interruption had been developed. Exhaustive tests were next made to determine the optimum switching conditions. 33 \ 60 Experimental D i f f i c u l t i e s At various stages of the research, certain experimental d i f f i c u l t i e s were experienced. Most of these were circumvented i n various ways and were not, therefore, too interesting. Other problems, li m i t a t i o n s , and curious features of the c i r c u i t s and apparatus w i l l now be noted. 6.1. The Oscilloscope There were several problems associated with obtaining accurate waveforms from the oscilloscope. Short persistence of the cathode-ray-tube screen necessitated reading the transient picture very quickly. Accordingly, accuracy suffered on the e a r l i e r visual measurements. A wide band oscilloscope was required i n order that a correct picture of the signal would be given. A Tektronix Type-531 oscilloscope with a Type-53/54A v e r t i c a l - a m p l i f i e r having a 10-megacycle/ second pass-band was f i n a l l y used for a l l important measurements^ 6.2. The Camera Photographs of certain waveforms at various component and independent variable values were taken. The camera was an Exacta reflex-type having an f-2.8 lens. The f i l m was Kodak Tri-X. The photographs were taken by leaving the camera shutter open for one cycle of the automatic control and the exposures were regulated by varying the oscilloscope trace-intensity and screen-brightness controls. 6.3. The Relays and Contacts The automatic control sequence was regulated by a motor having mechanical contacts on the rotor. Poor f r i c t i o n contact here and at the relays made frequent repairs and clean-34 ing necessary. This was an equipment cause for the randomness of switching o f f . The entire randomness problem had to be studied quite c a r e f u l l y to make certain that limitations i n the tube were the ultimate cause of randomness and not equip-ment f a u l t s . Unfortunately, some of the contacts were not rated to control the current or withstand the potential d i f f -erences required. These contacts then were employed to control other relays which were of higher rating, but which themselves could not be governed by small currents. The reasons for some of the connections i n Figure 8.3 should now be apparent. Arcing at some contacts necessitated placing several relays i n o i l baths. Contact chatter occurred i n 200 microsecond in t e r v a l s . It w i l l be seen that the interesting current pulses are finished before the f i r s t contact opens because of chatter. 6.4. The Contamination Problem Due to the sputtering of electrode material and entrance of impurities and oxides, amalgams formed on the tube walls. Occasional attempts were made to have the tube clean i t s e l f up by continuously running the discharge between the two pools. This was not ef f e c t i v e . D i f f i c u l t i e s were met i n i g n i t i o n because of the amalgam near the i g n i t i o n band (15) and the tube had to be cleaned. The procedure was as follows. N i t r i c acid was f i r s t introduced to clean off any metallic material. Potassium hydroxide was next added to dissolve the amalgam. Repeated cycles of washings with these two chemicals, interspersed with d i s t i l l e d water washings, were carried out to clean thoroughly. A heated sodium hydroxide and methyl 35 alcohol mixture was used to eliminate the stop-cock grease which had entered the tube. Care was taken not to allow t h i s mixture to reach any ground-glass surfaces as these surfaces would be attacked extremely rapidly. F i n a l l y , an iron slug was introduced to the tube and the l a s t isolated p a r t i c l e s were scraped off by using a magnet to control the slug. After cleaning, opera-tion again became satisfactory. 6.5. The Structural Problem Twice probe seals cracked due to the stress produced when the probe and glass expanded d i f f e r e n t amounts because of the high temperature reached when the tube did not switch o f f . Operating the tube at a nominal value of I, 5.0 amperes, and V, 23 vo l t s , resulted i n the probe temperature r i s i n g to ^ 2000 C on these occasions. The temperature i s calculable using values easily obtained from handbooks. The glass was then stressed to ^23,000 psi which i s i n the region of the maximum compressive stress permissible for glass, 20,000 - 50,000 psi (35). Temporary seals of the cracks were made with Glyptal varnish, but both times the glass had to be reworked for f i n a l repairs. 6.6. The Connecting-channel Arcs Sometimes the discharge ignited through the connecting channel. When this happened, the arc which should have ignited around the main channel did not operate. Of course, this only happened when a mercury pool served as the anode. Both arcs would not ignite since two arcs w i l l not work i n p a r a l l e l except at very high currents (23). Since the small-diameter channel was a shorter length path, arcing there 36 must have occurred because of increasing pressure. As the discharge was running the temperature increased, the pressure increased then (39) and the mean-free-path of the electrons decreased (25). This increased the probability of breakdown i n the shorter-length path, there could be s u f f i c i e n t c o l l i s i o n s for self-maintenance of the arc. Whenever th i s arcing occurred, the discharge was stopped and the tube allowed to cool. The experiment was resumed after the tube had cooled enough to give normal behaviour. 6.7. The Pumping System The pumping system had rubber connections throughout, even from the d i f f u s i o n pump to the tube. These rubber f i t t i n g s would outgas at ^(10) -^mm-Hg vapour-pressure (40) making i t impossible to attain any lower pressure. Several d i f f e r e n t o i l - f i l l e d fore-pumps were used of varying ratings, but did not affe c t the pressure l i m i t . Pressure gauges used only to detect leaks i n the system, were a McLeod gauge (37) and several thermocouple gauges (38). A mercury-vapour trap was inserted between the di f f u s i o n pump and the fore pump. Packing the connection i n ice here, prevented, to a certain extent, mercury from contami-nating the fore pump. 3 7 7o Selecting the Best Connection Once i t was known that the capacitor switching method worked every time as long as a cathode spot was formed, i t remained to f i n d the best connection for the switching-current path. The independent variables of interest which would determine the best connection were the time to switch off and the necessary magnitude of the switching voltage. There were three p o s s i b i l i t i e s for connecting the switching c i r c u i t return path. These were obtained, see Figure 7 . 1 , by connecting points A to B to GNDjor A to B, E to GNDjor A to E to GND. Only the components of the arc and switching c i r c u i t s , are shown fo r c l a r i t y . The current directions are indicated and d i s t i n c t i o n i s made between arc current, I, and switching current, I . Note that the resultant currents into the tube are ca l l e d cathode current, 1^, and anode current, I . Then! * a I = I + Z, I , ( 7 . 1 ) a 1 s and! I k = -I + Z 2 I g , ( 7 . 2 ) where: Z^ and are functions dependent on the impedance i n the c i r c u i t . Estimates of the current waveforms were made i n order to better recognise the oscilloscope traces. These are drawn beside the appropriate c i r c u i t i n the figure. It was seen that only two configurations, A to B to GND and A to E to GND, would be substantially d i f f e r e n t . Grounding d i f f e r e n t parts of the c i r c u i t would only change dc-voltage levels and, in the case of A to B to GND or A to B, E to GND, the change would probably not be great enough to influence the switching-of f p r obability. The times, T , T, , and T , for the three B A 0 T ! / ! I \l 1 at Resultants \ r A to B, E to GND A to E to GND B A =^GND * 1 l \ -*° t • f < / i / !/ 1 Resultants -9 & — » H Figure 7,L The Connections and Current Estimates 38 currents, 1 , 1 , , and I , respectively, to decrease to zero a K. s would d i f f e r for the three configurations. The important switching time for any applications, the time for the anode current to f a l l to zero, was % . 9 a The connections were tested. Photographs of the waveforms and data obtained are given i n Figure 7.2. The parameter values used have no p a r t i c u l a r significance. They were chosen only to make switching a certainty. Some of the photographs show only the i n i t i a l r i s e of the trace. They were included to indicate that the i n i t i a l part of the wave-form was observable. Since the time to switch was approximately the same for the two connections of interest, i t did not matter which one was chosen for detailed study. Connection A to B to GND had a smaller resistance path for the component of I g flowing to the cathode, the major part of I would take t h i s s path. It was thought then that current interruption would happen at a lower value of V for t h i s connection and A to B to GND was chosen for switching. Legend - a p p l i e s to a l l photographs T. B. = o s c i l l o s c o p e time-base, microseconds/centimeter. Sens. = o s c i l l o s c o p e s e n s i t i v i t y i n the ap p r o p r i a t e u n i t s . The l a r g e d i v i s i o n s are centimeters, s c a l e d . Zero gi v e s x = 0 . 0 [is and y = ground l e v e l . The p o s i t i v e sense i s always towards the top of the p i c t u r e . A few p i c t u r e s have nois e and spurious t r a c e s present. Since these i n d i c a t i o n s d i d not obscure the waveform of i n t e r e s t , no attempt was made to e l i m i n a t e them. • A to B to GND C = 9 uf R = 4 . 5 ft I = 4 a x =1.1, y = 4 . 0 V = 490 v s Zero T. B. = 200 us7cm Sens. = 2.38 a/cm bk I - A to B to GND a V = 490 v C = 9 uf s ^ R = 4 . 5 -a I = 4 a Zero x = 0 . 3 , y = 3.6 T. B. = 200 us/cm Sens. = 2.38 a/cm c. I - A to B to GND d. I - A to B to GND a a V = 490 v C = 9 uf V = 490 v C = 9 uf s s R = 4 . 5 -ti. I = 4 a R = 5.2 Si I = 3 . 5 a Zero x = 1.1, y = 4 . 0 Zero x = 0 . 8 , y = 4 .2 T. B. = 100 us/cm T. B. = 200 us/cm Sens. = 2.38 a/cm Sens. = 9.52 a/cm Figure 7.2. Some of the Current and Voltage Waveform^ A t o B to GND v C = 9 uf 5.2 n. I = 3 . 5 a x = 1.1, y = 5.0 V = 40 s Zero T. B. = 100 us/cm Sens. = 0.96 a/cm -i -! = : i i i i 1 1 1 1 : i 1 t i w 1 1 1 1 i i 1 111! / - :y / '-. L L " ' f . I - A t o E to GND a V = 490 v C = 9 uf s R = 4.5 n- I = 4 a Zero x = 1.0, y = 3.5 T. B. = 20 us/cm Sens. = 4.78 a/cm , I - A to E to GND a V = 490 v C = 9 uf s ^ R = 5.2 A I = 4 a Zero x = 1.3, y = 4.0 T. B. = 200 us/cm Sens. = 11.6 a/cm h. I . - A to E t o GND 106 v C = 9 uf a R = 5.2 rv I = 4 a Zero x = 0.8, y = 4.2 T. B. = 200 us/cm Sens. =2.3 a/cm I - A t o E to GND a V g = 150 v C = 9 of R = 5.2 ^ I = 4 a Zero x = 1.4, y = 4.2 T. B. m 100 us/cm Sens. = 11.6 a/cm k. V - A to B to GND V g = 490 v C = 9 uf R = 4.5 n I = 4 a Zero x = 0.4, y = 1.5 T. B. = 500 us/cm Sens. = 50 v/cm m. V - A t o B to GND V = 490 v C = 9 uf s R = 4.5 -n- I = 4 a Zero x = 0.4, y = 5.2 T. B. = 200 us/cm Sens. = 20 v/cm p. I - A to B to GND V = 488 v C = 9 Uf s R = 5.2 -n- I = 4 a Zero x = 0.8, y = 5.8 T. B. = 20 us/cm Sens. = 23.8 a/cm r . I, - A to B to GND V = 490 v C = 9 uf s R = 5.2 *- I = 4 a Zero x = 0.8, y = 4.8 T. B. = 1 ps/cm Sens. = 47.6 a/cm R = 4.5 -n- I = 4 a Zero x = 0.4, y = 3.2 T. B. • 50 us/cm Sens. = 200 v/cm q, A to B to GND - I V = 470 v C = 9 uf s ^ R = 5.2 si I = 3.5 a Zero x = 1.0, y = 5.0 T. B. = 20 us/cm Sens. = 3.85 a/cm s. I, - A t o E t o GND V = 95 v C = 9 uf s R = 5.2 A I = 4 a Zero, x = 1.4, y = 0.0 T. B. m 100 us/cra Sens. = 0.95 a/cm t . I, - A to E to GND k V = 490 v C = 9 uf s R = 5.2 Si I = 4 a Zero x = 1.0, y = 3.6 T. B. = 20 us/cm Sens. = 2.4 a/cm v. I - A to B to GND s V = 490 v C = 9 uf s R = 4.5 Si I = 4 a Zero x = 0.5, y = 0.2 T. B. = 20 us/cm Sens. = 2.22 a/cm x. I - A to E to GND V = 232 v C = 9 uf s ^ R = 5.2 Si I = 4 a Zero x = 1.4, y = 1.8 T. B. = 100 us/cm Sens. = 3.85 a/cm u. I - A to B to GND s V = 106 v C = 9 uf s R = 5.2 n. I = 4 a Zero x = 0.8, y = 1.2 T. B. = 100 us/cm Sens. = 0.96 a/cm w. I - A to B to GND s V = 490 v C = 9 uf s R = 4.5 si I = 4 a Zero x = 1.0, y = 0.2 T. B. = 1 us/cm Sens. = 22 a/cm y. I - A to E to GND V = 490 v C = 9 uf s R = 5.2 si I = 4 a Zero x = 0.0, y = 1.8 T. B. = 50 us/cm Sens. = 9.62 a/cm 39 8o The Optimum Conditions for Switching 8»1. The Current-interrupting C i r c u i t , Using the best switching-off connection, A to B to GND, a test was carried out to f i n d i f any optimum conditions would re s u l t . The switching current was of exponential type and varying C would change x only, however, varying R would s change x and the peak value of I , This was for a constant s s value of V * In other wordsy the larger C was, the longer s time for which ( l g ) m >I» but increasing R would eventually mean the maximum switching current would decrease below the arc-current value and no current interruption would res u l t , and decreasing R would decrease % to such a small value even-e ( s t u a l l y that the arc current would probably not experience any unstabling influence. The theory was checked by experiment. Data was collected using the c i r c u i t i n Figure 8,1 and the results tabulated i n Table 8.1, Various R and C values were employed and the minimum switching voltage for each combination of R and C was measured. It should be noted again that the switching voltage value, measured with respect to the plasma, was always greater than the power-supply value, V , because of the plasma s potential with respect to the cathode. The arc current was kept at an approximately constant value, 3 amperes. The switch-ing-off times T and T , were measured and the stored energy a s i n the capacitor was calculated. The c i r c u i t i s similar to that employed i n Figure 5.3. Capacitors were connected across the generators to suppress noise. The tube had now been modified to i t s f i n a l shape, see- Figure 8.2 for a photograph. Probe 1 had been 40 Figure 8.1. The Switching-off C i r c u i t replaced by a mercury p o o l o Since the a u x i l i a r y arc had to be running for 1 second, an a u x i l i a r y arc anode of larger surface area had to be employed i n order to dissipate the heat 0 In a l l the c i r c u i t diagrams given, only the essential connections are shown for the sake of c l a r i t y . The complete switching c i r c u i t , including the co n t r o l l i n g motor, i s shown in Figure 8.3. This i s drawn only f o r general in t e r e s t . Reference w i l l not be made to this figure i n the text. Results from Table 8.1 indicated that an optimum switching-off condition existed, as was expected. The important data were graphed, see Figure 8.4. It was d i f f i c u l t to t e l l Figure 8„3o The Complete C i r c u i t - f o r Switching Off 4 1 Table 801. The Minimum Values for Successful Switching C - uf 9 9 5 2, 2, 2, 2, 2, 2, ,2 ,2 ,2 ,2 ,2 ,2 0 o 2 5 0 , 2 5 0 . 2 5 0 . 2 5 0 . 2 5 0 . 2 5 0 . 0 2 5 0 . 0 2 5 0 . 0 2 5 0 . 0 2 5 0 . 0 2 5 0 . 0 2 5 0 . 0 0 2 0 . 0 0 2 0 . 0 0 2 0 . 0 0 2 0 . 0 0 2 2 3 5 uuf 2 3 5 uuf 2 3 5 uuf 2 3 5 uuf 2 3 5 uuf R - .a 9< 5c l c 1 0 0 9c 5c l c 1 0 0 9< 5, l c 1 0 0 9 5 1 9, 5, l c 7 2 6 7 2 6 1 0 5 7 2 6 1 0 5 7 2 6 1 0 5 7 2 6 .1 .05 9.7 5.2 1.6 .1 .05 V - v s 4 5 4 0 12 2 5 0 39 3 0 17 2 4 5 0 0 2 8 0 66 3 4 2 1 2 3 4 9 4 4 5 98 6 7 58 6 5 6 5 4 3 0 3 3 0 2 0 0 2 7 5 3 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 I - a 3. 3. 3. 3. 2. 2. 2. 3. 4. 2 . 3. 2 . 2. 2. 3. 2 . 2 . 2 . 3. 3. 3. 2. 3. 3. 3. 3. 2. 3. 2. 3. 3. (I ) - a • s m 5.6 2c 2c 2c 2< 2< 4 2 32 2c 3c 1< . 5c 19 2 6 0 e l c 2c 3c 2 5 0 6 4 0 3 8 0 1 3 6 0 7 2 0 2 0 0 0 ,45 >3 ,1 ,0 no 7 6 0 7 0 0 4 3 0 1 4 0 0 3 0 0 3 0 0 3 0 0 2 7 5 switch 1 2 0 7 0 * 7 0 * 7 0 * 7 0 * 7 0 * 6 0 * 6 0 * 6 0 * 6 0 * 6 0 * 5 8 * 5 5 * 5 5 * 5 5 * 5 6 * 6 5 * switch switch 5 8 * switch switch no no no no us; fCT s - 3 4 2 5 3 2 5 1 1 0 0 3 0 0 3 0 0 3 0 0 12 2 1 2 0 4 8 4 4 4 4 10 10 2 4 6 0 2 0 18 1 5 10 8 0 4 0 8 7 6 0 6 0 3 3 9 1 . 1 ( 1 0 ) : 7 2 . 0 1 3.6 6 2 5 . 0 1 5 . 2 9 . 9 ( 1 0 ) 3.2 6.4 9 8 . 0 -4 -4 4 ( 1 0 ) 4 6 7 0 2 4 . 7 ( 1 0 ) 1.2 7 4 5 0 . 5 ( 1 0 ) 1.9 1 4 8 5, 1. 0, 0< 3, 0, 0< 0< -4 -4 -4 1, 0, 0, 0 . 9 ( 1 0 ) 0.3 -4 * O s c i l l a t e d — c r o s s e d zero at 1 8 , 4 0 , and 6 0 The values are reasonably accurate: + 1 0 $ . (is, exactly what the optimum value of R was. The value was indepen-dent of C and measured approximately 0.1 4R <2 ohms. The f a c t that V increased so quickly as R decreased from one ohm to s f r a c t i o n a l values indicated the lower l i m i t on T G was important. Another result was that the switching-off time, —6 T , of the arc current was A>60(10) second. The deionization time for mercury vapour i n thyratrons i s a function of pressure, arc current, grid voltage, and tube geometry ( 2 1 ) . For con-— 3 ventional thyratrons, the deionization time i s ^ ( 1 0 ) second R (ohms) Figure 8.4. A Graph of the Optimum Switching Conditions 42 and for inverter tubes, when special construction has been used, —6 the deionization time i s reduced to ^100(10) second (16, 33). What probably happened i n the switching-time measurement was the following, see Figure 8.5. The reverse current pulse extinguished the arc cathode-spot. A cathode —8 spot only needs to be interrupted for a time, x^, of (10) second and i t remains extinguished (18, 31). The remaining period i s the time, x^, to disperse the charged p a r t i c l e s i n the plasma. Then: x k = x f + x d, and, p r a c t i c a l l y : X, A * x,. k d (8.1) from (8.1) If the resistance, R, was too small, the time, x^, that (I ) >I would be too small, less than (10)"® second, and s i . current interruption would not happen. Unfortunately, the exponential current was superimposed on I, as shown, and t h i s picture seen, I exponential current theoretical behaviour Figure 8.5. The. Theoretical and Actual Arc-current Waveforms 43 —6 theory was not v e r i f i e d . The somewhat low result of 1/60(10) second for the f a l l to zero of the arc current could be due to one of two reasons. Either the anode-to-cathode potential difference, s t i l l causing an e l e c t r i c f i e l d , could a f f e c t the recombination process and decrease the deionization time by sweeping a l l . the charge c a r r i e r s out of the tube, or the noise l e v e l i n the c i r c u i t was such as to obscure any signal beyond the 60-raicroseeond time-to-zero, even when examining the wave-forms at high oscilloscope s e n s i t i v i t i e s . If the l a t t e r i s the case, from a p r a c t i c a l point of view the current s t i l l —6 present after AJ60(10)~ second i s so small i t may be neglected. However, the waveforms were examined c a r e f u l l y and i t i s f e l t that the former supposition i s correct. The current waveforms o s c i l l a t e d with approxi-—6 mately the same period, <v40(l0)"~ second^ for every successful current interruption at the low capacitance values. The use of inductive r e s i s t o r s and the f a c t that C discharged to zero then charged up to the anode voltage and discharged again probably accounts for the o s c i l l a t i o n s . With the large C values, the o s c i l l a t i o n s did not occur since this capacitance would be large with respect to the inherent c i r c u i t capacitance and would play the dominant role. The energy needed for switching off was small, see Table 8.1 for values. However, the energy was not an important factor for switching other than that i t was small, as can be seen from the table. The energy amount varied with the capacitor employed. The only important e l e c t r i c a l require-ments were the magnitude of the reverse current and the time, that (I ) >I. x s m 44 8.2. A n a l y s i s of the C i r c u i t A m a t h e m a t i c a l a n a l y s i s of the s w i t c h i n g - c u r r e n t p a t h was made i n o r d e r to b e t t e r u n d e r s t a n d the r e s u l t s . The network i s i n F i g u r e 8,6 and the symbols are the same as used p r e v i o u s l y . I t was assumed t h a t the a u x i l i a r y - a r c d i s c h a r g e -s. r V switch probe a n ( j d e y ?f R > a ] t cathode \ k « R; E \ Q > B tzimr F i g u r e 8.6. The S w i t c h i n g - o f f Network d i d n o t a f f e c t s w i t c h i n g except i n t h a t a cathode spot was p r o v i d e d . The s w i t c h i n g c u r r e n t was; V I G = ^ -exp where: r = R + R + P R + r, a k 1 + R a + r k (8.2) (8.3) The symbols a r e ; R ^ , the r e s i s t a n c e between the probe anode and the s w i t c h i n g - o f f p o o l ; R , the a r c r e s i s t a n c e ; r, , the a H. s m a l l measuring r e s i s t a n c e f o r I^j RQ, the c u r r e n t - l i m i t i n g r e s i s t a n c e ; r„ <, the s m a l l measuring r e s i s t a n c e f o r I . tr a 45 The anode and cathode currents were; R + r V and: I k = -I + R Q + R G r ' ^ x p j - ^  ). (8.5) a K (j u The times of interest, when I = I, = 0, were: x = rCln a and; t, = rCln k a k . i ._S R G + R G . R I R a + r k , V. s 1 +K a + rk from (8.4) from (8=5) \ ~ ' R G + R G Examining these equations showed that the condition f o r a physical solution was: when r >0, (R, + r, ) <0, and |Ra + r f c| <|RG + r G | . When r >0, (Rft + r k ) >0, and when r <0, the solutions were impossible. Note that: r = Z(R, Rp, Ra, r k , RG, r G ) , from (8.3) and R and R are functions of time, t« and arc current, I. a p No calculations of T and T. were made since the functional a k relationship was not known; R& was a dynamic arc-resistance and equations were available only for a st a t i c arc-resistance (32) However, R & and R p would probably be constant for the time, T ^ , that ( l g ) m Therefore, an order-of-magnitude calculation was made using Equation 8.2. It was assumed that increasing-I = I at t =0, and decreasing~I = I at t = T„. Then: S S x v T F = rCdny 2- - ln«r). from (.8.2) It was also assumed that R =0 ohms and R = -1 ohm. When a p a general tube characteristic i s examined (22, 24), these r e s i s -46 tances w i l l be seen to be near the values used at the arc currents involved,. Note that r had to be of positive value for the solution to have physical meaning„ The calculations are given i n Table 8.2. As can be seen, the time that (I ) >I was ^(10)~ second. Analysing the equation indicated that t h i s was an u p p e r l i m i t f or T^. Comparing with the interruption time for extinction, (10)"" second, indicated that the theory possibly was correcto Table 8.2. Theoretical Limits on Cathode-spot Stability-time C - B - 12 rC - |is v a - v s I - a V 1 lny£ ln»r # 9* i T f T US 9 9,7 87,3 45 3.3 13.6 2.61 2,27 .34:29.7 9 5.2 46.8 40 3.3 12.1 2.49 1,65 ,84 39.4 5 1.6 8.0 12 3.0 . 4.0 1.39 .47 .92 7.4 2.2 100 220 250 3.0 83.4 4.42 4.61 2.2 9,7 21.5 39 2.9 13,4 2.59 2.27 .32 6.9 2,2 5.2 11.6 30 2.8 10,7 2,37 1.65 ,72 '8.4 2.2 1.3 3.6 17 2.8 6,1 1,81 .47 1,34 4.8 2.2 .1 .22 24 3,3 7,3 2,03 -2.31 4.34 .95 2.2 .05 .11 500 4.4 113.7 4.72 -2,99 7.71 ,85 0.25 100 25 280 2.8 100.0 4,60 4,61 0,25 ! 9.7 2.4 66 3.0 22.0 3 09 2,27 .82 1.97 0.25 5.2 1 o 3 34 2.6 13*1 2,57 1.65 .92 1.20 0.25 1.6 e 4 21 2.8 7.6 2.01 .47 1.54 .62 0.25 .1 ,03 23 2.9 7,9 2,07 -2,31 4.38 .13 0.25 .05 .01 49 3.1 15.8 2.77 -2,99 5,76 .06 0.025 100 2,5 445 2,9 153,7 5.03 4,61 .42 1.05 0.025 9,7 .24 98 2.8 35.0 3.56 2.27 1.29 ,31 0.025 5,2 .13 67 2,6 25.8 3.25 1.65 1,60 .21 0,025 1.6 .04 58 3 o.2 18.1 2.89 .47 1.42 .06 0,025 • ,1 .003 65 "3 o 2 20.3 3.01 -2,31 5.32 .016 0.025 ,05 .001 65 3.1 20.9 3.04 -2,99 6.03 .006 0,002 9.7 .02 430 2.8 157.2 5.06 2,27 2.79 .056 0.002 5.2 . .01 330 3.0 110.0 4,70 1,65 3,05 .031 0.002 1.6 .003 200 3,0 66.7 4.20 .47 3.73 .011 0.002 .1 .2 mus 275 3.0 91.7 4,51 -2.31 6.82 1.36 mus 0.002 .05 . 1 mus 300 3.1 96.9 4,57 -2.99 7.56 .76 mus 235 uuf 9.7 2.3 mus 500 2.6 192.2 5.26 2.27 2.99 6.87 mus 235 uuf 5.2 1.2 m^s 500 3.1 161.4 5.08 1.65 3.43 4.12 mus 235 uuf 1.6 .4 mus 500 2.9 172 ,3 5.15 .47 4.68 1.87 m^ s 235 uuf .1 .02 1 500 3.4 147.1 4.99 -2.31 7.30 .14 mus 235 uuf .05 .01 1 500 3.4 147.1 4.99 -2.99 7.98 .08 mus *# 9 i s InV /I - l n - r . The term r reduces to R when.values are substituted. The table i s arranged i n the same order as Table 8.1. 47 There were many intangibles i n the analysis that were d i f f i c u l t ( i f not impossible) to estimate. Nothing has been rigorously proved. The values were calculated i n order to show that interruption of a necessary arc cathode-spot, by the introduction of a reverse-current pulse of equal or greater magnitude than the arc current, for a short time i n t e r v a l could be a l l the mechanism required to switch off the tube. 48 9. Increasing the Current and Voltage The current interruption method had been proved successful at low currents. It now had to be checked when the arc was running at higher voltages and currents. 9 o l . High Voltage The arc supply-voltage was changed to a 25-kilowatt 500-volt mercury-arc r e c t i f i e r . Naturally, R^ was increased to keep the arc current, I, between 3.5 and 4.5 amperes. The remainder of the c i r c u i t was the same as i n Figure 8.1. When the arc was ignited, the already operating auxiliary-arc discharge extinguished. A 3-kilowatt 220-volt mercury-arc r e c t i f i e r replaced the generator, G^, i n the aux-i l i a r y - a r c c i r c u i t and R^ was appropriately increased to keep 1^ = 3 amperes. The behaviour was the same. Using the 100-vo l t and 140-volt generators, G^ and G^, respectively, resulted i n normal operation. A test was made using 240-volt and 120-vol t generators for arc and auxiliary-arc voltage-supplies, respectively. It was seen that the c i r c u i t with the greater current flowing remained ignited after the second band-ignition voltage was applied. These voltage supplies were interchanged and the same resu l t was obtained. Apparently, increasing the voltage of the arc and auxiliary-arc supplies increased the current i n s t a b i l i t y . When the 140-volt and 100-volt generators were employed, the arc current could be increased to ^10 amperes with the aux-i l i a r y arc remaining at three to four amperes and operation was normal. As soon as the 240-volt and 120-volt generators were used or the 500-volt and 220-volt r e c t i f i e r s connected, variations of one to two amperes between the currents determine 49 which c i r c u i t would stay in operation. The connections were thoroughly checked, but were found to be completely independent of each other except for the switching-off c i r c u i t . Using the procedure here to switch required three stable arcs i n the tube, the a u x i l i a r y arc provided the cathode spot, the main arc was where the normal current flowed, and the reverse-current arc extinguished the main arc. These arcs were approximately at the same conditions except for the currents flowing and the power supplies used. The arcs had to be stable i n order to obtain reproducable r e s u l t s . As has been shown, changing the power supplies so that they d i f f e r e d s i g n i f i c a n t l y caused a marked change i n the current s t a b i l i t y of the a u x i l i a r y and main arcs. It w i l l be shown that instar-b i l i t y was also involved when the arc current was increased. It i s f e l t that using a d i f f e r e n t method to provide the cathode spot necessary for immediate switching would eliminate the i n s t a b i l i t y problem. 9.2. High Current An experiment was made to determine i f the method was successful at high currents. The same c i r c u i t as i n Figure 8.1 was employed. The arc current was increased to a maximum of 30 amperes by changing R^. The capacitor method of supply-ing the reverse-current pulse for current interruption s t i l l worked. The results are tabulated i n Table 9.1. Here also, the auxiliary-arc current had to be increased as I was increased. Otherwise, the arcs were not stable. There was considerably more i n s e n s i t i v i t y to current differences at the low power-supply voltages. Currents higher than 30 amperes were not obtained because the probe anode was destroyed due to the very 50 Table 9.1 Switching Off at High Arc Current V - v I - a (I ) - a 1 I. - a s s m A 55 6.5 8,8 4 55 7o5 10.2 4 60 9.0 9.5 4 72 10.5 11.2 10 25 14.0 i 15.0 25 ! 109 19.5 19.5 25 130 23.8 24.2 25 ! 180 27.5 29.5 25 \ R = 1.6 ohms, C = 0.25 Uf. J Arc generator = Gg, auxiliary--arc = G^« high temperature reached. 9.3. Comments on Switching High Currents and Voltages To maintain the cathode spot required for the switching off pulse. 1^ had to be of the same order of magnitude as I. The two arcs would not run simultaneously unless this condition was met. This was the l i m i t a t i o n on using an aux-i l i a r y - a r c discharge to provide the cathode spot for switching. Vnfortunately, no more measurements at higher current or v o l -tage could be taken since a further increase i n the a u x i l i a r y -arc current would have been necessary and thi s increase together with the high arc-current value being tested would have meant operating the tube close to or above the temperature l i m i t for safety, see also Section 6.5. Using an auxiliary-arc discharge was not too satisfactory f or another reason.^ As has been mentioned, see Section 2.2 or Section 5.2.2, the cathode spot could be provided i n various ways. Many of these ways require only a small energy output. Using the aux i l i a r y arc to provide only a cathode spot for switching off was a rather great waste of energy. 51 The switching method was successful for currents up to 30 ampereso The theory explaining current interruption was s t i l l v a l i d , but some other manner of ensuring cathode-spot formation has to be found; for example, the ignitron method (6). Because an a u x i l i a r y arc was being run i n the tube a problem i n arc s t a b i l i t y resulted. This was not a defect i n the switching-off method, but just a poor choice i n the cathode-spot-forming techniqueo 52 10. Conclusions About Switching Off a Mercury Arc Only two requirements are the necessary and s u f f i c i e n t conditions for switching off a mercury-arc discharge without opening the main c i r c u i t . F i r s t , a pulse of current must be fed into the cathode i n the reverse d i r e c t i o n to the arc current, and this reverse current must be equal to or greater than the arc current for a minimum time. This minimum time was not measured, but i t was indicated to be less than —6 (10)~ second and probably was the minimum time, T ^ , usually quoted as ( 1 0 ) second, that a mercury-arc cathode-spot may be interrupted without r e i g n i t i o n . Once the cathode spot has been interrupted, the arc w i l l not reignite and the charge c a r r i e r s quickly disperse under the influence of the e l e c t r i c f i e l d i n the tube so that the current ceases. One switching time, T, , i s the time u n t i l I, = 0, th i s time includes T,, the ' k* k ' d' deionization time of the gas, and T ^ . The important switching-time i s T , the time u n t i l I or I f a l l s to zero. The two a' a times d i f f e r depending on how the switching c i r c u i t i s connected. The second requirement for switching off i s actually necessary only to control the exact time of switching. The reverse current "cathode" must have either a cathode spot already formed or an easily provided cathode spot at the instant the switching-off voltage i s applied. If the spot i s not instan-taneously present when V i s applied, a random delay w i l l s make i t uncertain just when the arc w i l l be switched o f f . The power required to switch off the arc i s small, and the energy i s also small since application of the —8 switching c i r c u i t need only be necessary for ^(10) second. However, from a p r a c t i c a l point of view, this time i s probably 53 d i f f i c u l t to at t a i n because of stray inductance and capacitance. Higher currents and voltages were not attempted because of the l i m i t a t i o n i n the mechanism for providing the spot. There appears to be no reason why the switching method should not work at very high current and voltage provided formation of the cathode spot i s effected i n a d i f f e r e n t manner. A better way to form the spot is necessary for another reason. The technique of the auxiliary-arc discharge was employed only for expediency. The power and energy used were very high. On the other hand, the ignitron method (6) should be very satisfactory. Only low energy output i s required and one arc would be eliminated. The main point to be made i s that there was no interference with the main c i r c u i t . Switching was effected not by open-circuiting the main power c i r c u i t , but by passing a short pulse of current i n the reverse d i r e c t i o n through the mercury-pool tube. 54 11o Projects That May Be Examined The reverse-current pulse method has been developed now for extinguishing mercury-pool tubes and the complete switching-off i s accomplished faster than by conven-t i o n a l methods. Since the smallest switching time, T , was a —6 60(10)"" second, the main l i m i t a t i o n i n the c i r c u i t now i s the spark-coil band-ignition technique. The time to ignite can be —3 as much as (10) second, but when i g n i t i o n i s changed, perhaps also to the ignitron method (6), i t w i l l probably decrease to the order of a microsecond. A project that could then be examined i s the building of a high-frequency power o s c i l l a t o r . High-power short pulses could be easily generated and a tuned c i r c u i t would give the frequency desired. The present power o s c i l l a -tors i n induction-heating apparatus are limited i n frequency because of the deionization time of the gas employed i n the tube. It should now be possible to increase the frequency markedly. High-power c o n t r o l - c i r c u i t s can also be devised. Faster on-and-off control i s now possible. 55 Part Bo The Hydrogen Tube The hydrogen tube was operated i n the arc region of the characteristic curve for gas-discharge tubes; that i s , the cathode f u l f i l l e d the conditions for an arc. The voltage and current were r e l a t i v e l y low. 12o The Tube A description and some of the physical dimen-sions of the hydrogen tube w i l l be given below. The tube was specially constructed. It was a hot-filament tube f i l l e d —3 with hydrogen under a pressure of (10) mm Hg. The tube was baked and thoroughly outgassed before f i l l i n g and a l l water vapour and oxygen was removed from the gas. The distance between the anode and cathode was 6.0 i n . and the tube diameter was 1.0 i n . , see Figure 12.1. The remaining electrodes were spaced at one-inch intervals between the anode and cathode. The tungsten probes, 1 and 6, of 1.0-mm diameter, extended one-third of the distance across the tube. The double-probe electrodes, 3 and 4, were 0.5 mm i n diameter, spaced 2.0-inm apart, and 3.0-mm long, and lay longitudinally i n the centre of the tube. The cathode was oxide coated and the heater was rated at 9.0 amperes at 6.3 vo l t s . The anode was a nickel 3 disc of — - i n . diameter and 0.5-mm thick. The discharge could be studied by using the nickel screen-like grids, 2 and 5. The holes i n the grids were 0.1-cm square. gas r e f i l l i n g connections for.heater and cathode Figure 12'. 1. The Hydrogen Tube 56 13o Switching Off The switching-off method employed involved extending the space-charge sheath u n t i l the anode was closed off from the cathode. This was the same method as that attemp-ted i n Section 3.1, but the tube construction, with f i n e -mesh-screen grids f i l l i n g the tube cross-section, was now more propitious and the use of the space-charge switching method appeared p r a c t i c a l . Hydrogen gas was employed i n order / \-5 to make use of the shorter deionization time, x^ = (10) second (19), and switching could be expected to be much faster. The c i r c u i t i s shown in Figure 13.1. To i g n i t e the discharge i t was necessary to apply v850 volts to the anode, positive with respect to the cathode, but operating the tube required only 200 v o l t s . The current flowing was 100-150 milliamperes. The arc was interrupted by a switching-off voltage of 500 v o l t s , negative with respect to the cathode. The plasma at the grid-2 position was <vL20 vol t s , positive with respect to the cathode, so that V was s ^620 volts because V was the potential difference between the s gri d and the plasma edges of the space-charge sheath. Switch-ing off occurred and the current supplied by the switching power-supply was 21 milliamperes. The space between the tube wall and the grid was larger than any holes i n the g r i d . This space was the l i m i t i n g factor on how small V could be and s t i l l effect a s switching o f f . The discharge current flowed, v i a the plasma, through any hole that was not closed off by the space-charge sheath. Sometimes when a large value of V was applied to s the grid, a narrowing of the plasma at some random position 200 v for i g n i t i o n 700 v Figure 13 „1. Switching Off the Hydrogen Tube 57 on the grid resulted and large current was delivered by the switching-voltage power-supply. The negative gr i d voltage probably caused the grid to act as a cathode. The plasma i n the anode-grid region appeared to deionize, but the current was s t i l l flowing i n the grid-cathode region. The switching voltage was removed very quickly or the grid would have been p a r t i a l l y destroyed due to the high temperature reached at the narrowed position. Either t h i s occurred, or switching resulted, or the current continued to flow between the tube wall and the grid, each at the same value of V , ^620 v o l t s . s Whichever one of these three situations happened was unpredic-table. The behaviour was random. The measurements made soon changed due to a deterioration inside the tube. The cathode emission was found to decrease over a period of time and f i n a l l y became neg l i g i b l e , I «.(10) ampere. This could be due to either the gas gradually cleaning-up or the cathode oxide-material slowly sputtering away, or even a combination of both these a f f e c t s . Accordingly, the experiment was abandoned. A calculation was made, using Equation 3.1, to determine the current delivered by the switching power-supply. The values were; V v620 volts; and M = 2.0, for hydrogen s gas is molecular. The geometry measurements were: r p = 0.15 i n . , the largest space to cover; and r ^ = 0.125 mm. Then r 2 • j * = 30.5 and 6 was taken as 1.09 (29). Therefore, J was r 2 p calculated to be 3.78 ma/cm . Since; I = J A , (12.1) s p s o and A g, the screen-grid surface-area, was ~5.7 cm , the grid current was calculated to be <v22 milliamperes. This was an 58 excellent, but fortuitous, agreement between theoretical and experimental values. Also calculated was the random-ion concentra-tion, assuming most of I g was due to the random-ion current. This was an aid to see i f the tube was operating correctly before deterioration affected the measurements. The random-ion concentration was obtained from: Jp = % v p , (12.2) wherei N i s the random-ion concentration, and v the thermal P v e l o c i t y of the ions. Since a H V - f r v < l 2 - 3 ) —23 where: k i s Boltzmann's constant, 1.38(10)*" joule/°K> and 1^ i s the temperature of the random ions. The other symbols have been explained previously. Then: 1 2 N - 3.98(10) 1 3J . from (12.1) P Using Tp ^75 C, since the random-ion temperature would be approximately the same as the gas temperature, the random-ion 10 3 concentration was calculated to be 1.14(10) ions/cm . This was a reasonable value (30). 59 14c. C o n c l u s i o n s About S w i t c h i n g O f f the Hydrogen Tube A l t h o u g h the hydrogen tube s w i t c h e d o f f by the space-charge method, the v o l t a g e and c u r r e n t r e q u i r e d were r a t h e r h i g h . The power su p p l y had t o be r a t e d a t 1 0 w a t t s . A l s o , the random b e h a v i o u r was not good. The method was not n o v e l ( 1 , 2 ) , except f o r t h f a c t t h a t hydrogen gas was employed no new i n f o r m a t i o n would be c o n t r i b u t e d by f u r t h e r s t u d y . I t was d i s a p p o i n t i n g t h a t no measurements of time to i n t e r r u p t the c u r r e n t c o u l d be t a k e n . 60 Part Co References The bibliography i s sectionalized so that the reader i s quickly directed to the references on a given topic. Switching-off Methods lo Fetz, H„ "Uber die Beeinflussung eines Quecksilbervakuum-bogens rait einem Steuergitter im Plasma." Ann, der  Phys. , ser, 5, vol., 37 (January, 1940), pp. 1 - 40. 2. Johnson, E. 0., J . Olmstead, and ff. M. Webster. "The tacitron, a low noise thyratron capable of current interruption by grid action." Proc. I. R. E., vol, 42 (September, 1954), pp. 1350 - 1362. Ignition Methods 3. Buttolph, L. J., and D. W. Dana. "P r a c t i c a l methods for starting the conduction i n mercury arcs." Rev. S c i .  Inst., v o l . 4 ( A p r i l , 1933), pp. 206-215. 4. Germeshausen, K. J. "A new form of band i g n i t e r for mercury-pool tubes." Phys. Rev., v o l . 55 (January, 1939), p. 228. 5. Kenty, C. "On the starting of Hg vapor discharge tubes." J. App. Phys., v o l . 9 (November, 1938), pp. 705 - 713. 6. Slepian, J, and L. R. Ludwig. "A new method for i n i t i a -ting the cathode of an arc." Trans. A. I. E. E., vo l . 52, (June, 1933), pp. 693 - 700. 7. Townsend, M. A. "Cathode spot i n i t i a t i o n on a mercury pool by means of an external g r i d . " J. App. Phys., vo l . 12 (March, 1941), pp. 209 - 215. 8. Vang, A. "Self-synchronizing tube discharge control system." United States Patent Office #2,432.219, 9 December, 1947. 9. Winograd, H. "Development of excitron-type r e c t i f i e r . " Trans. A. I. E. E., v o l . 63 (December, 1944), pp. 969 - 978. 61 Cathode-spot Formation 10. Cobine, J. D. Gaseous Conductors. New York, McGraw-H i l l , 1941, pp. 300 - 316. 11. Gambling, W. A. "Cathodic glow-to-arc transit i o n s . " Can. J. Phys., v o l . 34 (December, 1956), pp. 1466 -1470. 12. Loeb, L. B. Fundamental Processes of E l e c t r i c a l Discharge in Gases" New York, John Wiley and Sons, 1939, pp. 609 - 613, 629 - 636. General Information About Gas Discharges 13. Child, C. D. "Discharge from hot CaO." Phys. Rev.„ ser. 1, v o l . 32 (May, 1911), pp. 492 - 511. 14. Cobine, J. D. op. c i t . , pp. 418 - 419. 15. Ibid., pp. 425 - 426. 16. Ibid., pp. 465 - 466. 17. v. Engel, A. Ionized Ga ses. London, Oxford University, 1955, p. 229. 18. Ibid., p. 243. 19. Germeshausen, K. J. "The hydrogen thyratron." G„ N. Glascoe and J. V. Lebacqz, ed. Pulse Generators. New York, McGraw-Hill, 1948 (L. N. Ridenour, ed. M. I. T. Radiation Laboratory Series, v o l . 5) pp. 335 - 354. 20. Gray, T 0 S. Applied Electronics. New York, John Wiley and Sons, 1954, f i g . 13, p. 159. 21. Hull, A. W. "Hot-cathode thyratrons, Part 1." Gen. Elec. Rev., v o l . 32 ( A p r i l , 1929), pp. 213 - 223. 22. Ibid., f i g . 21, p. 223. 23. Hull, A. W„, and H. D. Brown. "Mercury-arc r e c t i f i e r research." Trans. A. I. E. E., v o l . 50 (June, 1931), pp. 744 - 756. 24. Ibid., f i g . 14, p. 750. 25. Jeans, J . H. The Dynamical Theory of Gases. Cambridge, University, 1925, pp. 35 - 37. 26. Johnson, E. 0., and W. M. Webster. "The plasmatron, a continuously controllable gas discharge developmental tube." Proc. I. R. E.. v o l . 40 (Jume, 1952), pp. 645 - 659. 62 27» K i l l i a n , T. J 0 ''The uniform positive column of an elec-t r i c discharge i n mercury vapor." Phys. Rev-, v o l . (May, 1 9 3 0 ) , pp. 1 2 3 8 - 1 2 5 2 . 28o Langmuir, 1= "The effect of space charge and residual gases on thermionic currents i n high vacuum." Phys^.  Rev., v o l . 2 (December, 1 9 1 3 ) , pp. 4 5 0 - 4 8 6 . 2 9 . Langmuir, I., and K. B. Blodgett. "Currents limited by space charge between coaxial cylinders." Phys. Rev. v o l . 2 2 (October, 1 9 2 3 ) , pp. 3 4 7 - 3 5 6 . 3 0 . Langmuir, I., and H. Mott-Smith. "Studies of e l e c t r i c a l discharges i n gases at low pressure, Part 3." Gen.  Elec. Rev., v o l . 2 7 (September, 1 9 2 4 ) , pp. 6 1 6 - 6 2 3 3 1 . Maxfield, A., and R„ R. Benedict. Theory of Gaseous Conduction and Electronics. New York, McGraw-Hill, 1 9 4 1 , pp. 3 8 4 - 3 8 5 . 3 2 . Spitzer, L. Physics of F u l l y Ionized Gases. New York, Interscience, 1 9 5 6 , pp. 8 1 - 8 5 . 3 3 . Wittenberg, H. H„ "Pulse measuring of deionization time. E. E.. v o l . 6 9 (September, 1 9 5 0 ) , pp. 8 2 3 - 8 2 7 . Handbook Values 3 4 . Knowlton, A. E., ed. Standard Handbook for E l e c t r i c a l Engineers. New York, McGraw-Hill, 1 9 4 9 , table 4 - 5 9 , p. 3 4 7 . 3 5 . Ibid., sec. 5 3 1 , p. 3 9 7 . 3 6 . Ibid., table 4 - 1 1 8 , p. 4 7 4 . High-vacuum Technique 3 7 . Dushman, S. S c i e n t i f i c Foundations of Vacuum Technique. New York, John Wiley and Sons, 1 9 4 9 , pp. 2 6 4 - 2 7 2 . 3 8 . Ibid., pp. 3 1 5 - 3 1 8 . 3 9 . Yarwood, J. High Vacuum Technique. London, Chapman and Ha l l , 1 9 5 6 , table, p. 1 8 6 . 4 0 . Ibid., pp. 1 9 1 - 1 9 3 . 

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