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

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INVESTIGATIONS OF THE GLOW PHASE INHIGH PRESSURE SPARK DISCHARGES by  CHI-SUN L E E B.S.j  National  Taiwan U n i v e r s i t y ,  1966  ]  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE  REQUIREMENTS  FOR THE DEGREE OF  MASTER OF SCIENCE in  t h e Department of PHYSICS  We a c c e p t required  THE  this  thesis  as conforming  t o the  standard  UNIVERSITY OF B R I T I S H COLUMBIA April,  1971  In presenting this thesis  in p a r t i a l fulfilment of the requirements for  an advanced degree at the University of B r i t i s h Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this  thesis  for scholarly purposes may be granted by the Head of my Department or by his representatives.  It  is understood that copying or. publication  of this thesis f o r financial gain shall not be allowed without my written permission.  Department of  •  Physics  The University of B r i t i s h Columbia Vancouver 8, Canada  Date  A p r i l s 29, 1971  ABSTRACT  The  glow p h a s e o f h i g h  initiated nitrogen cable  by  low  carbon  discharge  filamentary appearance d.c.  the  spark  of  the  and  the  diffuse  cathode  gradient  the  further  cathode  t h a t the  from  fall  to the  f o r each gas.  low  diffuse  coaxial  pressure fall  spectroscopic  a  glow, a  the  pressure  and  the  ratiO:iof  the  positive  independent gas  glow d i s c h a r g e s  v o l t a g e s are  Faraday  that for  i n the are  normal  pressure.  indicate  i n agreement  in  gases' i n v e s t i g a t e d . I n a d d i t i o n ,  d i s c u s s i o n about i s given  distinct  optical  i t i s concluded  These v a l u e s  d.c.  these  to t h a t of a  v o l t a g e and gas  in  two  p h a s e . The  external circuit  o f glow f o r t h e  glow p h a s e  channel  It c o n s i s t s of a negative  results  fixed  a qualitative  ohm  exist  glow i s s i m i l a r  experimental  impedance o f  types  16  i n hydrogen,  glow p h a s e and  transition  column.  the  spark there  namely, a d i f f u s e  Comparisons with  both  the  a positive  potential  column a r e  of  discharges,  glow-to-channel  glow phase  axial  of  establishment  stages,  From o u r the  dioxide i s studied using  glow d i s c h a r g e .  dark space,  discharges  techniques.  over-voltaged  transition  spark  t h e Townsend mechanism o f breakdown  and  Before  pressure  on  the  transition  the  b a s i s of r e s u l t s  investigations.  nature  of  this  obtained  -iii-  TABLE OF CONTENTS  Page Abstract Table of Contents L i s t of Tables L i s t of Figures Acknowledgements  i i i i i v vi v i i  Chapter 1. I n t r o d u c t i o n 1.1 The High P r e s s u r e Spark D i s c h a r g e s  1  1.2 O u t l i n e o f t h e T h e s i s  4  1.3 Some Remarks on Low P r e s s u r e D.C. Glow Discharges  5  2. C o a x i a l Cable D i s c h a r g e Techniques and Experimental Apparatus 2.1 Spark D i s c h a r g e Techniques  7  2.2 The C o a x i a l C a b l e D i s c h a r g e Arrangement  10  2.3 E x p e r i m e n t a l A p p r a t u s a. Spark Chamber  14  b. Impedance-Matched Adaptor  16  3. E l e c t r i c a l  Parameters  3.1 E x p e r i m e n t a l and Measuring  Techniques  a. E l e c t r o d e C l e a n i n g P r o c e d u r e s b. C a l i b r a t i o n o f t h e C h a r g i n g V o l t a g e Measurement c. C a l i b r a t i o n o f t h e C u r r e n t Measurement 3.2 A n a l y s i s of t h e O s c i l l o g r a p h i c Investigations  19 19 19 20  -iv-  4. P l a s m a P a r a m e t e r s 4.1  4.2  Experimental Measurement  Arrangement  f o r Temperature 27  A n a l y s i s of the S p e c t r o s c o p i c Investigations  4.3  Electron Density  5. D i s c u s s i o n s and  5.4  Bibliography  i n the P o s i t i v e  Column  33  Conclusions  5.1 C o m p a r i s o n w i t h Discharges 5.2 C o m p a r i s o n w i t h Discharges 5.3  29  50 Ohm  C o a x i a l Cable 35  Low  P r e s s u r e D.C.  Glow 36  The T r a n s i t i o n N a t u r e o f t h e D i f f u s e Glow P h a s e  37  Conclusions  38  40  L I S T OF  TABLES  Title E/P o f P o s i t i v e Column and C a t h o d e F a l l V o l t a V o f H y d r o g e n , N i t r o g e n , and C a r b o n D i o x i d e Glow P h a s e C a t h o d e F a l l V o l t a g e s f o r C o p p e r and Z i n c E l e c t r o d e s i n H y d r o g e n , N i t r o g e n , and C a r b o n D i o x i d e Low P r e s s u r e D.C. Glow Discharges  -vi-  L I S T OF  No.  FIGURES  Title  Page  1. C i r c u i t D i a g r a m s o f t h e C o n d e n s e r and Cable Spark D i s c h a r g e s  the C o a x i a l  2. C u r r e n t O s c i l l o g r a m s o f t h e C o n d e n s e r and C o a x i a l C a b l e S p a r k D i s c h a r g e s u n d e r Low Over-Voltage 3. C o a x i a l C a b l e  Discharge  8 the 9  Arrangement  11  4. C u r r e n t O s c i l l o g r a m s o f S p a r k D i s c h a r g e s and A r r e s t e d Glow D i s c h a r g e s i n H y d r o g e n , N i t r o g e n , and C a r b o n D i o x i d e ; T i m e - R e s o l v e d L i g h t I n t e n s i t y V a r i a t i o n ( X = 3580 A ) i n N i t r o g e n T r a n s i e n t Glow  13  5. S p a r k Chamber  15  6.  Impedance-Matched  Adaptor  17  7. A x i a l P o t e n t i a l D i s t r i b u t i o n s o f t h e H y d r o g e n , N i t r o g e n , and C a r b o n D i o x i d e T r a n s i e n t Glow  24  8. E x p e r i m e n t a l A r r a n g e m e n t Investigations  28  9. O b s e r v e d S p e c t r a System 10. Band S t r e n g t h s Spectra  f o r Spectroscopic  of N i t r o g e n -  of N i t r o g e n  Second P o s i t i v e  Second P o s i t i v e  Band 30  Band 32  ACKNOWLEDGEMENT  I wish  t o s i n c e r e l y t h a n k my s u p e r v i s o r , D r . J . Meyer,  for  h i sadvice  the  preparation  and h e l p  of the t h e s i s .  members and s t u d e n t s appreciated  throughout  these  i n v e s t i g a t i o n s and  The a s s i s t a n c e o f t h e f a c u l t y  of t h e plasma p h y s i c s  a s w e l l . My s p e c i a l  group i s deeply  t h a n k s a r e a l s o due t o  Mr.  D. S t o n e b r i d g e  and Mr. R. P. H a i n e s f o r t h e i r  the  c o n s t r u c t i o n of the apparatus.  parts i n  -1-  CHAPTER 1  INTRODUCTION  1.1  THE HIGH PRESSURE SPARK DISCHARGES A  gas i s a l m o s t  a perfect insulator  However i f a n e l e c t r i c established conducting can  field  i n i t s normal  of s u f f i c i e n t  state.  strength i s  between two e l e c t r o d e s , t h e gas c a n become and the e l e c t r i c  c u r r e n t f l o w i n g through  t h e gas  range from b a r e l y measurable v a l u e s up t o s e v e r a l m i l l i o n  amperes o r more.  D e p e n d i n g on t h e d u r a t i o n o f c u r r e n t c o n d u c t i o n , t h e discharges In s p a r k highly  discharges  conducting  achieved of  are either  of a steady  s t a t e or a t r a n s i e n t  the t r a n s i t i o n  from  the i n s u l a t i n g  nature. t o the  s t a t e o f t h e gas between t h e e l e c t r o d e s i s  v e r y r a p i d l y . A s h o r t time  the c u r r e n t pulse,  the i n i t i a l  later,  after  insulating  termination  state i s  recovered. Early  i n v e s t i g a t i o n s of high pressure  have been a l m o s t following the  entirely  the establishment  l a c k of f a s t  photographic  low  intensities  development  to the events  of the spark  rise-time electrical  sensitive light  limited  of these  techniques from  channel  instruments  necessary  the e a r l i e r  transient  spark  discharges immediately due t o and  t o detect the  stages  i n the  discharges. Theories of  -2-  the  spark  channel  expansion  based  on t h e e l e c t r i c a l l y d r i v e n  s h o c k wave model have been i n t r o d u c e d by D r a b k i n a and B r a g i n s k i i ( l , 2 ) and e x p e r i m e n t a l confirmed  some o f t h e i r  measurements l a t e r  predictions  have  (3,4).  Due t o t h e r e c e n t d e v e l o p m e n t o f new t e c h n i q u e s fast  r i s e - t i m e p h o t o m u l t i p l i e r s , image c o n v e r t e r s and image  intensifiers, stages  i t i s p o s s i b l e to study  f o l l o w i n g the spark  preceding channel  breakdown p r o c e s s  channel  e x t e n s i v e l y not only the  formation  but a l s o the  and t h e i n i t i a t i o n o f t h e s p a r k  itself.  The  investigations  responsible  of the e l e c t r o n  discharges  ( 5 ) who some t h i r t y  c l o u d chamber t e c h n i q u e s  the e l e c t r o n  which a r e have  o u t , f o r e x a m p l e , by a r e s e a r c h g r o u p i n Hamburg  h e a d e d by P r o f . H. R a e t h e r utilized  avalanches  f o r t h e breakdown o f t h e s p a r k  been c a r r i e d  avalanches.  breakdown p r o c e s s e s  y e a r s ago  f o r the e a r l i e s t  studies of  Most o f t h e • f e a t u r e s o f t h e i n i t i a l  a r e now w e l l u n d e r s t o o d  mechanisms c a n be d i s t i n g u i s h e d : s t r e a m e r  and two breakdown  mechanism and  Townsend mechanism. Which one t h e d i s c h a r g e the  such as  f o l l o w s d e p e n d s on  o v e r - v o l t a g e a p p l i e d a t t h e d i s c h a r g e gap.  a.  S t r e a m e r mechanism: A t v o l t a g e f a r i n e x c e s s  breakdown v o l t a g e a c r o s s a g a p o f s e v e r a l c e n t i m e t e r spark  channel  avalanche  i s formed v e r y q u i c k l y .  i s formed moving toward  Initially  of the length a  an e l e c t r o n  t h e anode u n d e r t h e i n f l u e n c e  -3-  of the h i g h e l e c t r i c avalanche  If the e l e c t r o n d e n s i t y i n the  head i s a m p l i f i e d beyond a c e r t a i n value the  of the avalanche fields,  field.  changes, mainly due  and a streamer  to the space charge  of e l e c t r o n s r e s u l t s which b r i d g e s  e l e c t r o d e s and produces a pre-channel c o n d u c t i v i t y . The  nature  spark channel  a c r o s s the gap c o l l a p s e s very  of f a i r l y  the  high  soon develops and  the v o l t a g e  rapidly.  b: Townsend mechanism ( g e n e r a t i o n mechanism ): For  low  to moderate o v e r - v o l t a g e s and s h o r t e r gap d i s t a n c e s , a s i n g l e avalanche  cannot cause the e l e c t r i c a l breakdown. Instead,  successor e l e c t r o n s are produced at the cathode by  secondary  e f f e c t s such as p o s i t i v e - i o n bombardment and p h o t o - e l e c t r i c e f f e c t . Generation avalanches  are thus formed and  r e s p o n s i b l e f o r the breakdown. The  they are  f o r m a t i v e time l a g of t h i s  type of d i s c h a r g e i s much longer as rtineeds a l a r g e number of successor The  avalanches. spark d i s c h a r g e s i n i t i a t e d by the Townsend mechanism  have been i n v e s t i g a t e d , f o r example, by Schroder, Meyer i n d i f f e r e n t gases ( 6 , 7 , 8 ) .  I t i s observed  Doran and that p r i o r  to the complete c o l l a p s e of the gap v o l t a g e , there are t r a n s i t i o n stages which do not e x i s t i n the case of breakdown d i s c h a r g e s . At f i r s t  streamer  a d i f f u s e glow-like discharge  of low c o n d u c t i v i t y i s formed a f t e r breakdown, which e x h i b i t s i n g e n e r a l a b r i g h t negative glow, a Faraday dark space and uniform p o s i t i v e column. Then a t h i n f i l a m e n t a r y  a  -4-  glow-to-channel channel  transition  phase d e v e l o p s  and  t h e breakdown p r o c e s s  and  the spark  d e v e l o p m e n t have been s t u d i e d e x t e n s i v e l y and has  contraction  been a c h i e v e d , process  unclarified.  1.2  understanding  I t has  encouraging  glow s t i l l  the  remains  I t seems a p p r o p r i a t e t h e r e f o r e , t o p r o v i d e some  OUTLINE OF  THE  this  glow p h a s e i n o r d e r  of t h i s  transient  been shown by C a v e n o r and  linearly  with  Meyer  (9) t h a t f o r t h e  i n a 50 ohm  torr  coaxial'  pressure,  the  z e r o - l e n g t h v o l t a g e i s i n agreement w i t h  cathode  v o l t a g e o f low  discharges.To  a r r a n g e m e n t has during  pressure  d.c.  a 16  been s e t up  ohm  and  coaxial cable  the e l e c t r i c a l  t h e glow p h a s e o f h y d r o g e n s p a r k s  gas  p r e s s u r e s and  The  same measurements have been e x t e n d e d  two  gases —  are presented  hydrogen  examine w h e t h e r t h e s e c h a r a c t e r i s t i c s  dependent or not,  different  n i t r o g e n and i n Sec.  3.2,  gap  t h e e l e c t r o d e d i s t a n c e and  extrapolated fall  a  phase.  produced  c a b l e d i s c h a r g e a r r a n g e m e n t a t 500 voltage varies  to achieve  THESIS  glow phase i n hydrogen s p a r k s  current  spark  channel  some u n c e r t a i n t y a b o u t  of the d i f f u s e  more i n f o r m a t i o n a b o u t better  a  comes i n t o e x i s t e n c e .  Although  success  finally  gap  carbon  over  the  the  glow are  discharge  parameters a wide r a n g e  of  s e p a r a t i o n s have been m e a s u r e d . to include  d i o x i d e . The  together with  another  results  a description  obtained of  the  -5-  analyzing  method w h i c h h a s been u s e d  quantities. where t h e y  These r e s u l t s  t o measure  a r e d i s c u s s e d i n S e c . 5.1 and 5.2  a r e compared w i t h  50 ohm c o a x i a l c a b l e  measurements on t h e one hand and w i t h discharge  positive  the l i g h t  Sec.  emitted  i s briefly  during  outlined  o f t h e gas temperature  distribution electron  density i s evaluated  techniques. techniques  experimental  i n S e c . 4.1. I n S e c . 4.2 an i s deduced  from  apparatus  band s p e c t r a . T h e  nature  i s given  of the d i f f u s e  i n S e c . 5.3.  describes the p r i n c i p a l  of the c o a x i a l cable  I t s advantages over are noted  the s p e c t r a l  by t h e methods d e s c r i b e d i n  results  f o l l o w i n g chapter  experimental  i n Sec.  and t h e  discharge  the condenser  discharge  2.1.  SOME REMARKS ON LOW PRESSURE D.C. GLOW DISCHARGES Since  similar  the t r a n s i e n t  of the l a t t e r  the purpose of l a t e r  The  glow o f t h e s p a r k  t o t h e d . c . glow d i s c h a r g e s  characteristics for  The  o f the n i t r o g e n second p o s i t i v e  glow p h a s e b a s e d on t h e s e  1.3  t h e glow p h a s e f r o m t h e  4.3. A d i s c u s s i o n o f t h e t r a n s i t i o n  The  d . c . glow  s y s t e m h a s been u s e d t o  column i n n i t r o g e n s p a r k s .  arrangement estimate  low p r e s s u r e  discharge  measurements on t h e o t h e r . .  A monochromator-photomultiplier analyze  these  general  optical  will  discharges  i n some a s p e c t s , be b r i e f l y  i s quite t h e jfltil)  mentioned  here  comparison.  a p p e a r a n c e o f t h e d . c . glow  discharges  - 6 -  shows d i f f e r e n t b r i g h t space, negative The  and d a r k r e g i o n s , i . e . c a t h o d e  glow, Faraday dark space, p o s i t i v e column e t c .  axial potential  d i s t r i b u t i o n i s i n general  e x c e p t i n t h e p o s i t i v e column, and t h e r e is  of the order  not l i n e a r  the p o t e n t i a l  of s e v e r a l v o l t s per centimeter.  voltage  and t h e v o l t a g e d r o p a c r o s s  usually  very  latter  dark  gradient  The anode  the p o s i t i v e column a r e  s m a l l compared w i t h t h e c a t h o d e f a l l  v o l t a g e . The  i s t h e p o t e n t i a l d r o p measured from t h e cathode t o t h e  anode e n d o f t h e F a r a d a y d a r k For  space.  t h e same g a s a n d t h e same e l e c t r o d e m a t e r i a l t h e  cathode f a l l  voltage  i s a constant  and so i s t h e product  t h i c k n e s s of cathode f a l l  The  ratiOQ»of t h e a x i a l p o t e n t i a l g r a d i e n t o f t h e p o s i t i v e  column t o t h e gas p r e s s u r e  r e g i o n and gas p r e s s u r e  of  the  but  fall  depends v e r y  i s a f u n c t i o n of the product  pressure.  little  (10).  on t h e c u r r e n t  o f i t s r a d i u s and t h e gas  -7-  CHAPTER 2 COAXIAL CABLE DISCHARGE TECHNIQUES AND EXPERIMENTAL APPARATUS  2.1  SPARK DISCHARGE TECHNIQUES In recent years  to study  spark  techniques The  two d i f f e r e n t  techniques  d i s c h a r g e s , namely, t h e condenser  and t h e c o a x i a l c a b l e d i s c h a r g e  circuit  discharge  techniques.  diagram f o r the condenser discharges  i n F i g . l a . The m a i n d i s a d v a n t a g e  i s shown  of t h i s kind of discharge  arrangement i s t h a t the c u r r e n t r i s e circuit  have been u s e d  d e p e n d s on t h e e x t e r n a l  as w e l l as t h e d i s c h a r g e . As an example, a c u r r e n t  o s c i l l o g r a m of such a discharge  i s illustrated  i n F i g . 2a.(8)  On t h e o t h e r h a n d , t h e o n l y e f f e c t o f t h e e x t e r n a l c i r c u i t in c o a x i a l cable discharges  i s t h e l i m i t a t i o n o f t h e maximum  magnitude of the d i s c h a r g e c u r r e n t . F o r s u f f i c i e n t l y p r e s s u r e s , f o r example, the discharge  operates  i d e a l s w i t c h and t h e d i s c h a r g e c u r r e n t r i s e s a f r a c t i o n o f one p i c o s e c o n d in c o a x i a l cable discharges discharge  a l m o s t l i k e an  instaneously ( i n  ) t o i t s maximum v a l u e .  Therefore  f r o m t h e measurement o f t h e  current i t i s p o s s i b l e to c a l c u l a t e other  e l e c t r i c a l q u a n t i t i e s such as t h e v o l t a g e a c r o s s gap  high  important  the gap,the  r e s i s t a n c e and t h e e n e r g y i n p u t i n t o t h e d i s c h a r g e . F o r  t h i s . r e a s o n we u s e t h e c o a x i a l c a b l e d i s c h a r g e  techniques  -8-  f o r our i n v e s t i g a t i o n s r a t h e r than the condenser d i s c h a r g e techniques. A t y p i c a l d i s c h a r g e c u r r e n t o s c i l l o g r a m of c o a x i a l c a b l e spark d i s c h a r g e s i s sketched i n F i g . 2b.  Fig. 1 C i r c u i t Diagrams of the Condenser and the C o a x i a l Cable Spark D i s c h a r g e s  a. Condenser Spark Discharge  L  Spark Gap  Capacitor Bank  6 H.T.  77/77  b. C o a x i a l Cable Spark Discharge  Spark Gap 'I  O H.T,  777777  L£ :  Inductance per U n i t Length of Cable  C^  Capacitance per U n i t Length of Cable  :  Fig.  Condenser  2  Spark  C u r r e n t O s c i l l o g r a m s of the Condenser and t h e C o a x i a l C a b l e S p a r k D i s c h a r g e s u n d e r Low O v e r - V o l t a g e Discharge  Current  C o a x i a l C a b l e Spark  Discharge  Current  Time 2T  tA<  t t < t tc^ t t_ < t T b  < t < t < t < 2T b  c  d  Glow F o r m a t i o n R e g i o n D i f f u s e Glow P h a s e Glow-to-Channel T r a n s i t i o n Phase Spark Channel Stage P u l s e T r a n s i t Time o f P u l s e - F o r m i n g  Cable  -10-  2.2  THE  COAXIAL CABLE DISCHARGE ARRENGEMENT  The  principal  illustrated (  16  the e x p e r i m e n t a l  i n F i g . 3. A c a b l e  ohms i n our  resistance R  of  cable of  the  designed  i n order  then  t h e chamber c a n of  During i.e.  the  supply  to maintain  be  and  the c h a r g i n g  p e r i o d from  same  on  instant  t o the  there are repeated  the  i n p u t end  The  v o l t a g e E ( t ) a t the  be  circuit.  Furthermore the  gap  outside.  pulse-forming  cable,  a t w h i c h the c h a r g i n g a t which  reflections  f a r end  high  characteristic  changed from  instant  i s m i s m a t c h e d and  a  the  c  terminated  p r e s s u r i z e d , and  p e r i o d of the  the  through  Z  chamber i s s p e c i a l l y  the whole d i s c h a r g e  evacuated  impedance  into a properly spark  the  the e l e c t r o d e s c a n  i s switched  starts,  discharged  same impedance. The  impedance t h r o u g h o u t  distance  of c h a r a c t e r i s t i c  i n v e s t i g a t i o n s ) . i s charged  and  c  arrangement i s  the  from both  f a r end  power  discharge ends s i n c e  i s open-circuited,  varies according  to:  -i(.t+)/2T' E ( t ) =U  {1 -  when t = T,3T,5T, R  +  c  Z  Q  =U £l - exp(-t/CR )J where U T  the  that the  0  i s the c h a r g i n g  total p a r t of  total  , when t > >  c  0  pulse  transit  voltage, R  E ( t ) exceeds the  of  the c h a r g i n g r e s i s t a n c e ,  t i m e o f the c a b l e  the chamber t o w h i c h  capacitance  c  this  T  together  with  the c a b l e c o n n e c t s ,  pulse-forming  C  s e c t i o n . When  breakdown v o l t a g e , breakdown t a k e s  place.  -11-  U.V.  Light  To Pump  16 Ohm Cable  T16/T50 Adaptor  M77  To  Oscilloscope  -AAAAR  o  Current Meter  6  H.T.  Fig.  3.  Coaxial  Cable Discharge  Arrangement  -12-  T h i s c a u s e s t h e c o l l a p s e o f t h e gap v o l t a g e and d i s c h a r g e o f t h e c a b l e , a f t e r w h i c h t h e power s u p p l y  c h a r g e s up t h e c a b l e  a g a i n . Thus we have r e p e a t i n g d i s c h a r g e s . The r e p e t i t i o n can  be c o n t r o l l e d  t o some e x t e n t  by c h o o s i n g  charging r e s i s t a n c e f o rcables of d i f f e r e n t If  an a p p r o p r i a t e lengths.  t h e d i s c h a r g e were t o o p e r a t e a s an i d e a l  rectangular  I = U /2Z  switch, a  o  and d u r a t i o n  2T w o u l d be p r o d u c e d . However due t o t h e f i n i t e  time.:required  to  current pulse of height  rate  0  Q  a t t a i n h i g h c o n d u c t i v i t y i n t h e gap, t h e c u r r e n t r i s e s a t  a finite  rate:  K t )  =  Uo  2Zo + Z ( t )  Z(t) = U(t)/I(tj  where a pulse  starting  t ^ 2T.  The c u r r e n t p u l s e i s / r e c o r d e d w i t h  oscilloscope of  at  i s t h e gap r e s i s t a n c e a t t i m e t . F o r  t=0, t h e above r e l a t i o n  o f r i s e - t i m e 0.30  2T o f t h e c a b l e s u s e d  holds f o r  a Tektronix  n a n o s e c o n d . The maximum  4b,  earlier  4c. T h e y a l l i n d i c a t e ( F i g . 2b  ).  required  t h r e e main s t a g e s  The c u r r e n t p u l s e  merely the double r e f l e c t i o n 04t^2T.  i n Fig. as mentioned  f o r 2T^t^4T  o f the o r i g i n a l  I t a p p e a r s due t o t o t h e f a c t to attain  value  i n o u r i n v e s t i g a t i o n s i s 197 n s .  T y p i c a l current oscillograms are presented 4a,  519  is  current pulse f o r  that a f i n i t e  time i s  high c o n d u c t i v i t y . There a r e i n f a c t  numerous r e f l e c t i o n s  occuring at  t = 4 T , 6 T , 8 T , . . . . , and t h e  -13Fig. 4 C u r r e n t O s c i l l o g r a m s of Spark D i s c h a r g e s a. Hydrogen b.  Nitrogen  c. Carbon D i o x i d e d. Hydrogen  2360 V P « 700 t o r r U = 2780 V p = 200 t o r r U = 2750 V p = 200 t o r r 2080 V p - 1200 t o r r 0  0  2T d 2T d 2T d 2T d  = 197 ns = 1.0 mm = = = =  197 ns 2.0 mm 197 ns 1.75 mm 95 ns 0.5 mm  •rent O s c i l l o g r a m s of A r r e s t e d Glow D i s c h a r g e s e. Hydrogen f.  Nitrogen  g. Carbon D i o x i d e  Uo = 2100 V  P = 600 t o r r Uo = 2070 V p - 300 t o r r 1880 V p = 200 t o r r  2T d 2T d 2T d  = = = = = =  95 ns 1.0 mm 48 ns 1.0 mm 95 ns 1.0 mm o  Time-Resolved L i g h t I n t e n s i t y V a r i a t i o n ( X = 3580 A ) N i t r o g e n T r a n s i e n t Glow h.  Time S c a l e : 2 ns per d i v i s i o n  -14-  magnitude of these rate  of r i s e  of  the  gas,  pressure  gap  d i s t a n c e , the  discharge  starts  This  will  measurements  2.3 a.  diminishes  the c u r r e n t d u r i n g  the  fact  reflections  and  gap  the  gas  used  later  ( Sec.  the d i s c h a r g e  3.Ic  The  depends  on  s e p a r a t i o n . However f o r  resistance f a l l s  i f the  be  gap  very r a p i d l y .  pressure  very q u i c k l y a f t e r  i s h i g h enough  f o r the  any  calibration  the  ( F i g . 4d  of c u r r e n t  ).  EXPERIMENTAL APPARATUS S p a r k Chamber The  chamber  consists  of  two  i s made o f b r a s s separate  and  right-hand  end  of  t h i s connecter.  can  be  c h a n g e d by r o t a t i n g  distance  can  be  In o r d e r structure  ZQ  =  of  ( Fig7  5").  respectively In t h i s way  set with  to prevent  on the  the hollow  an  accuracy  log ( i Q  and by  a  There are l e f t - h a n d the  inner s i d e s at  each  electrode separation cylinder.  o f 0l\015  signal reflections  t h e chamber must s a t i s f y  138  polyethylene,  p a r t s which are j o i n e d together  hollow:—cylinder connecter threads  and  the  the  The  gap  mm. geometrical  f o l l o w i n g equation:  D /D ) b  a  k where k i s t h e are  i n n e r and  indicated  dielectric outer  i n F i g . 5,  Between  the  constant  diameters and  brass  of  Z i s 16 Q  conductors  of polyethylene,  the ohms  and  brass  coductors  and as  here.  the  polyethylene  tubes  )  ffl P o l y e t h y l e n e  Quartz Window " ^ . ^ ^ |  Fig. 5  •  Brass  •  0-ring  Hole  Spark Chamber  -16-  several  s e t s of O-rings  isolated  from  the  The be  the  others  outlet are  the  , The  localize yet  f i t t e d with  electric  the  oscilloscope  i n the field  p r e s s u r i z i n g purposes.  windows. The  emitted  electrodes  light.  from  the  recorded  of the  circuit  i s connected  with  outer  together  the  the  envelope and  can  They a l s o  discharge. of  them i s  i n order  electrodes  to and  uniform.  e.g.  a 16  519  will  directly  T 16 / T 50  the d i s c h a r g e  s t r u c t u r e of  F i g . 6 . The  spherical  vicinity  properly terminate  and  them,  Adaptor  Another adaptor,  connected  soldered  of  sufficiently  system. A T-type  circuits.  The  the chamber. One  i n p u t impedance o f t h e Tek.  made, w h i c h c a n  properly  is  is slightly  ohms, t h e w h o l e d i s c h a r g e  discharge  then  gap  by u l t r a - v i o l e t  of l i g h t  other  the d i s c h a r g e  Since  when t h e  the  e l e c t r t o d e s a r e a l s o made o f b r a s s . One  Impedance-Matched  125  solthat  and  quartz  these  investigation  keep the  b.  through  for evacuating  p l a n e - s u r f a c e d . The  is  drilled  i l l u m i n a t e d through  allow  inserted  atmosphere.  Four h o l e s are serves  are  oscilloscope be  mismatched  t o the  16  adaptor  has  ohm  a 50  and  T 50 / T 125  ohm  been ohm  adaptor,  current pulses  can  is  be  oscilloscope.  T 16 /T  50 a d a p t o r  i s made o f two  filled  with  thin  i s shown i n copper  epoxy t o p r o v i d e  cones  satisfactory  Fig.  6  Impedance-Matched  Adaptor  -18-  i n s u l a t i o n . The  values of the r e s i s t a n c e s R , R t  i n d i c a t e d i n the f i g u r e , are 11.0, respectively. 16 ohm  5.3,  z  and 45.8  and R., 3  ohms  I t g i v e s an a t t e n u a t i o n f a c t o r of 6.1  to 50 ohm  as  times  from  end. F i n a l l y the g e o m e t r i c a l s t r u c t u r e of the  adaptor must a l s o s a t i s f y the equation mentioned i n the preceding s e c t i o n , with d i f f e r e n t Z  a  f o r each  end.  -19-  CHAPTER 3 ELECTRICAL  3.1  PARAMETERS  EXPERIMENTAL AND MEASURING  a. E l e c t r o d e C l e a n i n g  In o r d e r  from  i s applied across  discharges  the cathode which  Before  occur  at consistent  t o produce r e p r o d u c i b l e c u r r e n t  over-voltage  satisfactory  Procedures  that the discharges  breakdown v o l t a g e slight  TECHNIQUES  the gap. T h i s  t r i g g e r e d by t h e e l e c t r o n s  and a low p r e s s u r e  i n different  about  t w e n t y m i n u t e s t o reduce, t h e a b s o r b e d  g a s e s i s r u n between them f o r  the e l e c t r o d e s u r f a c e s . These procedures and o v e r  consistent  again  to assure  of the Charging  calibrated  combination. supply,  o f oxygen  are repeated  the r e l i a b i l i t y  Voltage  A c u r r e n t meter i s connected then  layer  of  breakdown.  b. C a l i b r a t i o n  and  light.  glow d i s c h a r g e a t  20 t o r r  over  emitted  t h e e l e c t r o d e s a r e p o l i s h e d and  about  on  gives  i s i l l u m i n a t e d by u l t r a - v i o l e t  resuming experiments,  washed i n a l c o h o l ,  pulses,  to indicate  Connecting  the charging  c. C a l i b r a t i o n  this  Measurement  i n s e r i e s with  the voltage across  i n p a r a l l e l with  voltage U  Q  of the Current  a resistor  t h e power  c a n be m e a s u r e d .  Measurement  this  -20-  The are  discharge  attenuated  adaptors.  current traces recorded  by d i f f e r e n t  Therefore  combinations  combinations  by t h e o s c i l l o s c o p e  o f a t t e n u a t o r s and  the a t t e n u a t i o n f a c t o r s  have t o be known  i n order  of d i f f e r e n t  to c a l c u l a t e the  magnitude o f the d i s c h a r g e  c u r r e n t . I t h a s been m e n t i o n e d  that  gas p r e s s u r e s  for sufficiently  current very  will  rise  rapidly  since  and a p p r o a c h  but nerer  nevertheless,  high  reach  i t s maximum v a l u e  i t . As i l l u s t r a t e d  I(2T) + I(4T) i s approximately  t h e c u r r e n t s a t 6T, 8T,...  Similar  of d i f f e r e n t  combinations  3.2  ANALYSIS OS THE OSCILLOGRAPHIC i s observed  discharges  pulse  to I  Q  small.  pressures  and gap factors can  INVESTIGATIONS  that the c u r r e n t o s c i l l o g r a m s of the  dioxide  ) Hhave t h e s i m i l a r  appearance of three d i s t i n c t  transit  time  not so h i g h as t o cause  too e a r l y .  stages,  shorter cables with  To a v o i d  stages,  the spark  the appearance  i fthe  channel  to  t o f the l a t t e r  two  2T •<Ct a r e u s e d and t h e d i s c h a r g e s  ". a r r e s t e d " i n t h e s e n s e and o n l y  general  T i s l o n g enough and t h e gas  develop  develop  i n F i g . Ad,  i n the three gases i n v e s t i g a t e d ( hydrogen,  and c a r b o n  pressures?are  are  0  determined.  characteristic total  0  o f a t t e n u a t o r s and a d a p t o r s  be  nitrogen  = U /2Z  0  and m e a s u r e d . The a t t e n u a t i o n  thus  spark  I  equal  are n e g l i g i b l y  current oscillograms at different  separations are recorded  It  the d i s c h a r g e  c  t h a t no s u b s q u e n t  t h e glow p h a s e  appears.  stages can  -21-  The  f e a t u r e of these  g l o w phases c o n s i s t s o f a  glow, a F a r a d a y d a r k s p a c e and a p o s i t i v e oscillograms different  gap s e p a r a t i o n s and g a s p r e s s u r e s  in  and t h r e e  4 e , 4 f , 4 g . By m e a s u r i n g  diffuse  glow s t a r t s  U(t)  = U  0  the  three  Fig.  f o r a l l these  ones a r e p r e s e n t e d i n  t h e c u r r e n t a t t h e t i m e when t h e f o r e a c h gas ( t^ as i n d i c a t e d  the r e l a t i o n :  - 2Z I(t) , 0  gap v o l t a g e s  separations  typical  t o appear  F i g . 2b ), and u s i n g  the  c o l u m n . The c u r r e n t  o f t h e a r r e s t e d t r a n s i e n t glow d i s c h a r g e s a t  gases a r e recorded Fig.  at d i f f e r e n t  can then  gas p r e s s u r e s  and gap  be c a l c u l a t e d . T h e s e v a l u e s  gases a r e p l o t t e d a g a i n s t  7a,7b,7c.  distribution  of the p o s i t i v e  m e a s u r i n g t h e gap v o l t a g e s while  keeping  the a x i a l  f o r various electrode  the d i s c h a r g e  current constant  a h y d r o g e n t r a n s i e n t glow p r o d u c e d discharges simply  a t 500 t o r r  measuring  currents  In t h e i r time  t h e gap v o l t a g e s  ( 1 0 , 11J .  tb f o r d i f f e r e n t  breakdown c o n d i t i o n s perform  first  (9) t h a t f o r  c a n be d e t e r m i n e d by  at static  gap s e p a r a t i o n s  i n v e s t i g a t i o n s they  separations  i n 50 ohm c o a x i a l c a b l e  the d i s t r i b u t i o n  for different  potential  c o l u m n c a n be d e t e r m i n e d by  However i t h a s been shown by C a v e n o r and Meyer  at  obtained f o r  electrode separation i n  I n t h e n o r m a l d . c . glow d i s c h a r g e s  the  negative  breakdown. E v e n a r e n o t t h e same.  measure t h e gap v o l t a g e s  e l e c t r o d e s e p a r a t i o n s under  ( low o v e r - v o l t a g e ~ 1 % ). T h e n  t h e same measurements by k e e p i n g  static they  the c u r r e n t s a t time  -22-  t  b  constant.  must a p p l y find  that  In o r d e r  appropriate the  gap  c o n d i t i o n s are that  the  voltage the  has  low  axial  I t has  slope  line  this  during  voltage  the  axial  are  listed  i n the  the  gap  distance  diffuse .  i s equal  to the  conclusions  gap  different  that  of  Edels  cathode  i n the  the  voltages  to the  following table.  gas  for is  measured line  is  the  the  in a potential (11) fall  o f C a v e n o r and  r a n g e as  zero-length  over-  voltage  a straight  that voltage.  Meyer  case of  graphs p l o t t e d i n F i g . 7  p o t e n t i a l gradient  They  voltages  results  they  p h o t o s show  can  our  represent  p o s i t i v e column  t r a n s i e n t glow phase f o r t h e s e  The  i n the  two  p o s i t i v e c o l u m n and  separation  current  Then the  30%.  therefore represents  potential distributions  investigated the  the  to higher  investigations. axial  plotting  been s u g g e s t e d by G a m b l i n g and  assume t h a t  extended  framing  i n v e s t i g a t i o n s the  of  gap  as  They t h e r e f o r e c o n c l u d e  gap  to zero  this.zero-length  We  high  tt, e q u a l ,  change e v e n t h o u g h t h e  against  p o t e n t i a l gradient  drop.  the  their  o f c u r r e n t . By  The  as  at  measured under these  glow does n o t  range of  extrapolation  be  voltages  over-voltage  obtained.  over-voltages  been i n c r e a s e d .  current  currents  c o n s i s t e n t . Furthermore,  diffuse  independent at  t o keep the  and  the  pressure  gases ratios  of  calculated  -23-  Table 1 E/P of P o s i t i v e Column and Cathode F a l l Voltage V Hydrogen, N i t r o g e n , and Carbon D i o x i d e Glow Phase  Gas  E/P (V/cm-torr)  V  (V)  Hydrogen  21.7±0.5  220+ 5  Nitrogen  36.7+1.0  240 ±  Carbon D i o x i d e  38.0+1.3  470 ± 10  I  5  c  of  -24-  Fig. 7  A x i a l P o t e n t i a l D i s t r i b u t i o n s o f the Hydrogen, N i t r o g e n , and Carbon D i o x i d e T r a n s i e n t Glow  Electrode Separation  (mm)  0.5  1.0  1  1.5  E l e c t r o d e S e p a r a t i o n (mm)  2.0  2.5  -26-  c. Carbon D i o x i d e P A X  o  0.5  (torr) 200 150 100  1.0  1.5  Electrode Separation  2;0  (mm)  2.5  -27-  CHAPTER 4  PLASMA  The sparks  gas temperature  i s e s t i m a t e d from  corresponding measuring diameter  4.1  average  PARAMETERS  d u r i n g t h e glow p h a s e i n n i t r o g e n the s p e c t r o s c o p i c  electron  b l o c k diagram  p u l s e s from  column a r e f o c u s e d and pass  through  d i s p e r s i o n monochromator  R.C.A. t y p e 931-A f a s t at the e x i t  FOR TEMPERATURE  rise-time  slit  is  the spark discharge  the entrance s l i t  ( Bausch  and Lomb 50 cm  photomultiplier  of a }. A  tube i s  o f t h e monochromator. The l i g h t  pulses are converted into current into  MEASUREMENT  o f t h e e x p e r i m e n t a l arrangement  shown i n F i g . 8. The l i g h t  placed  and the  column.  EXPERIMENTAL ARRANGEMENT  low  i s c a l c u l a t e d by  the magnitude of the d i s c h a r g e c u r r e n t of the p o s i t i v e  The  density  measurements. The  t h e T e k . 1S1 s a m p l i n g u n i t  p u l s e s which  a r e then f e d  o f a T e k . 549 s t o r a g e  oscilloscope.  The laser.  optical  system  To c a l i b r a t e  i s a l i g n e d w i t h t h e a i d o f a He-Ne  the r e l a t i v e  monochromator-photomultiplier is  used  as the l i g h t  blackbody, the  light  the r a t i o s  spectral  system  response  o f the  a t u n g s t o n r i b b o n lamp  s o u r c e . By a p p r o x i m a t i n g t h e lamp a s a of the r e l a t i v e  source t o the r e l a t i v e  s p e c t r a l radiancy of  photomultiplier  response  -28-  Fig.  8  E x p e r i m e n t a l Arrangement f o r Spectroscopic Investigations  Power Supply  Monochromator  j  "--4:----  Spark  Chamber  Photomultiplier  Power Supply  Oscilloscope  -29-  currents  4.2  ANALYSIS OF THE SPECTROSCOPIC The  1.0 mm the  f o r t h e same w a v e l e n g t h s c a n be c a l c u l a t e d .  spark  discharges  higher pulse  i n n i t r o g e n a t 200 t o r r  gap d i s t a n c e a r e c a r r i e d  glow d i s c h a r g e intensity transit  relative  pressure  o u t r e p e a t e d l y . To a s s u r e  and that  i s a r r e s t e d and no s u b s q u e n t ' d i s c h a r g e s o f  can occur,  time  T =5  intensity  a very  from  short cable with  ns i s u s e d . The  variation  wavelengths emitted the c u r r e n t  INVESTIGATIONS  time-resolved  of the l i g h t  the p o s i t i v e  total  of d i f f e r e n t  column i s i n d i c a t e d  t r a c e d i s p l a y e d on t h e o s c i l l o s c o p e  by  screen.  o Fig.  4h shows t h e c u r r e n t o s c i l l o g r a m f o r  noted  t h a t the time  intensity  begins  its  highest value  The  relative  d u r a t i o n from  to r i s e  light  The positive  the i n t e n s i t y  for different  and p l o t t e d  spectra are i d e n t i f i e d  nitrogen discharge  particular  i s emitted  B ir. 3  Each  transition four  line  a  The o b s e r v e d  wavelengths a t  i n the p o s i t i v e tubes  when n i t r o g e n  state  spectrum corresponds  v—>v .  bands w i t h  electronic  expected.  as p a r t s of the second  c o l u m n o f low p r e s s u r e  change from a h i g h e r  reaches  i n F i g . 9.  band s y s t e m w h i c h u s u a l l y o c c u r s  band s y s t e m  the l i g h t  t o t e n nanoseconds as  intensities  t = 10 ns a r e e v a l u a t e d  the i n s t a n t  to the i n s t a n t  i s equal  = 3580 A. I t i s  C%  (12). This molecules  t o a lower  to a vibrational  one state  s p e c t r a c a n be g r o u p e d  v - v ' = 1,2,3,and 4.  into  -OC-  From light  the theory o f d i a t o m i c molecular s p e c t r a , the  intensity  electronic N(v)  band  I(v,v») system  p(v,v')  of a  v—=>v  spectral  T  line  i n an  i s proportional to  X(v,v')  "4  where N ( v ) i s t h e p o p u l a t i o n o f t h e m o l e c u l e s i n v - l e v e l , p(v,v') the v i b r a t i o n a l  transition  probability  ( 1 3 ) . Thus we  have:  where G ( v ) i s t h e v i b r a t i o n a l The  transition  positive  band  Nicholls  (14).  relative  light  plot  probabilities  s y s t e m have been  strength  v-v.' and  the  phase  statistical  initial  state  vibrational  log  about  equilibrium  the l i n e  we  group w i t h the  ) f o r e a c h v i n F i g . 10. we c a n c o n c l u d e band  system  that  during the  i s dependent  upon  of the n i t r o g e n molecules i n the  a t one d e f i n i t e  the e f f e c t i v e  line,  y  i s a p r o c e s s which  i n F i g . 10b i s 2 7 0 0 ± 1 0 0  as  (IX/p) f o r each  e  v'  temperature  by J a r m a i n and  and e v a l u a t i n g t h e  the nature o f these graphs  glow  second  o f each observed s p e c t r a l  e m i s s i o n o f the second p o s i t i v e  transient the  calculated  l o g ^ (Zlt IX^/ZItP  e From  of the n i t r o g e n  Using their r e s u l t s intensity  t h e band  same  energy of the v - l e v e l .  temperature. T h i s  as c a l c u l a t e d  from the s t r a i g h t  °K. U n f o r t u n a t e l y no  rotational  effective line  information  t e m p e r a t u r e c a n be o b t a i n e d ,  i n t e n s i t i e s a r e t o o weak tor.make  p o s s i b l e an  -32Fig.  10  G(v)  Band S t r e n g t h s o f N i t r o g e n P o s i t i v e Band S p e c t r a  ( 1000 1/cm  1 G(v)  )  3 ( 1000 1/cm  )  Second  -33-  analysis  o f t h e r o t a t i o n a l s t r u c t u r e . However  non-equilibrium deviate  even i n  s i t u a t i o n s t h e gas t e m p e r a t u r e d o e s n o t  much f r o m t h e v i b r a t i o n a l t e m p e r a t u r e and we c a n  therefore  conclude  that  o u r measurement  i n d i c a t e s t h e gas  temperature.  4.3  ELECTRON DENSITY IN THE P O S I T I V E COLUMN The d i s c h a r g e  is  recorded  with  magnitude a t  current  oscillogram  of the n i t r o g e n  t h e Tek. 519 o s c i l l o s c o p e and t h e c u r r e n t  t = 10 ns i s m e a s u r e d . The c u r r e n t  t h e n be c a l c u l a t e d i f t h e c r o s s - s e c t i o n a l a r e a discharge  c o l u m n i s known. The measurement  done by two d i f f e r e n t  methods. One  photographs of the discharge length  the outermost  diffuse  glow. The c u r r e n t  can  of the  of the l a t t e r i s i s to take the  The o t h e r  method  d i a m e t e r o f t h e anode s p o t density  i s to  left  c a l c u l a t e d i s found  by t h e t o be  A/cm*  The c u r r e n t  density  J  n e v  = n_e_v_+  +  +  i s given  carrier,  v the d r i f t  to e l e c t r o n s transient chapter  by t h e  expression:  +  where n i s t h e number d e n s i t y ,  in  method  density  column and t h e n measure t h e  o f t h e image on t h e f i l m s .  measure  50+5  glow  e the charge of the charge-  v e l o c i t y , with  - and + s i g n s  and p o s i t i v e i o n s r e s p e c t i v e l y .  glow t h e v a l u e  referring  For the  nitrogen  o f E/P i s 36.7 V / c m - t o r r a s m e a s u r e d  2. The e l e c t r o n d r i f t  v e l o c i t y at this  value i s  -34-  1.1X10  cm/sec and  1%  ( 5 ) . Therefore  can  o f v_ be  density of  3X  neglected i n the 1Q  /3  t h a t of the  ions i s only  the c o n t r i b u t i o n of the  ( n_«-*n ):  positive  positive  +  J*»n_e_v_  . The  centimeter.  positive  ions  average e l e c t r o n  column thus e v a l u a t e d  e l e c t r o n s per c u b i c  about  i s of the  order  -35-  CHAPTER  DISCUSSIONS AND  5.1  COMPARISON WITH 50 OHM The  during  cathode f a l l  the  corresponding  are  220  values  arrangement a t discharge two  that the  500  V and  diffuse  E/P  of the  are  V and  agree w e l l .  t o a 16  ohm  This fact  discharges  d e s c r i b e d by c o n s t a n t  Our nitrogen  experimental sparks  distributions E/P  and  independent the  values  results  cathode  fall  through  the e x t e r n a l  i t i s seen  the  q u a n t i t i e s are d i o x i d e and phase.  during  initiated  voltage  circuit. g l o w phase  of  potential i . e . constant  voltage. Therefore  a glow  factor  cathode f a l l  characteristics,  f o r n i t r o g e n , carbon passes  and  dioxide sparks,  similar  a  characteristics  show t h a t f o r t h e  t o assume t h a t t h e s e  discharge  o f E/P  impedance o f  carbon  exhibit  constant  reasonable  and  Although  indicates that  as  the  discharge  impedance,  t h e Townsend mechanism o f breakdown, t h e  The  (9) f o r a  been i n c r e a s e d by more t h a n  f r o m a 50  of  Meyer  20 V / c m - t o r r .  glow p h a s e o f h y d r o g e n s p a r k  independent  our  coaxial cable  by  are  column  V/em-torr r e s p e c t i v e l y .  ohm  220  positive  in  by C a v e n o r and  i n a 50  c u r r e n t has  values  21.7  obtained  torr  by c h a n g i n g  these  COAXIAL CABLE DISCHARGES  v o l t a g e and  h y d r o g e n glow p r o d u c e d  of  CONCLUSIONS  h y d r o g e n glow phase as measured  investigations  the  5  i t is  current  any  gas  i n which  -36-  5.2  COMPARISON WITH LOW PRESSURE D.C. GLOW DISCHARGES The o p t i c a l appearance o f the d i f f u s e glow i s v e r y  s i m i l a r t o t h a t o f normal d.c. glow d i s c h a r g e s . The l i n e a r dependence on the gap d i s t a n c e o f the a x i a l p o t e n t i a l d i s t r i b u t i o n of p o s i t i v e column i s a n o t h e r p r o p e r t y i n common f o r these two types o f glow. N e v e r t h e l e s s , t h e p o t e n t i a l g r a d i e n t and t h e gas temperature i n t h e p o s i t i v e column o f the former a r e much h i g h e r . I n low p r e s s u r e d.c. glow d i s c h a r g e s , these q u a n t i t i e s  a r e of t h e o r d e r o f a few v o l t s per  c e n t i m e t e r and s e v e r a l hundred degrees K e l v i n . Furthermore E/P f o r low p r e s s u r e d.c. glow d i s c h a r g e s i s not c o n s t a n t . A n o t h e r i n t e r e s t i n g r e s u l t i s t h a t the cathode f a l l v o l t a g e s f o r both t y p e s o f glow i n t h e same gas a r e i n agreement. These v a l u e s f o r copper and z i n c e l e c t r o d e s i n hydrogen, n i t r o g e n , and c a r b o n d i o x i d e low p r e s s u r e d.c. glow d i s c h a r g e s a r e l i s t e d i n T a b l e 2 ( 1 0 ) . Comparison w i t h our r e s u l t s f o r b r a s s e l e c t r o d e s d u r i n g the glow phase shows the d e v i a t i o n s a r e l e s s than 13%. Table 2 Cathode F a l l V o l t a g e s f o r Copper and Z i n c E l e c t r o d e s i n Hydrogen, N i t r o g e n , Carbon D i o x i d e Low P r e s s u r e D.C. G l o w , D i s c h a r g e s Carbon ^*~~----^^^ Gas Hydrogen N i t r o g e n D i o x i d e E1 e c t r ode~~^^._ Copper  214  208  460  Zinc  184  216  410  -37-  This holds  f o r the  product is  agreement s u g g e s t s transient  of the  a constant  for  both  d.c.  t h a t the  glow p h a s e as w e l l ,  t h i c k n e s s of cathode f o r each gas.  copper  and  zinc  glow d i s c h a r g e s  similarity  The  i . e . that  f a l l r e g i o n and  product  torr-cm  pressure  (10). Assuming  Pd  value,  then  i n the cathode  region  Ec  from be  our  - V /dc  at d i f f e r e n t  c  experimental  estimated  by  results.  another  builds  up u n t i l  i s predominately  ionizing  collision  a  i s the  first  "  effective  the  order  can  be  o f 300  estimated.  THE  o f a/P  V/cm-torr The  the  soon appears.  results  determined  analysis  t o t h e h i g h E/P,  field  are  fall  can  also  of  the  cathode  for  assume t h a t i o n of  electron-molecule  The  to occur  the average  where  maximum a t E/P  of  cathode  i n agreement w i t h  the  field above  values.  THE  DIFFUSE GLOW PHASE  i n i t i a l breakdown o f t h e Our  in front  been f o u n d  ( 1 5 ) , and  this  calculated  d e f i n e d i n t e r m s o f a/P  has  for  assume t h a t  ionization coefficient.  TRANSITION NATURE OF  After  due  Townsend  further  c  to  fall field  optimum  the r e s u l t  processes,  " value  experimentally  5.3  p u r p o s e we  be  method. We  charge  i t e s t a b l i s h e s the this  cathode  independent  t h e glow p h a s e t h e s p a c e  production  p r e s s u r e s can The  during  ionizaioni*: For  field  pressure  hydrogen  glow i s e q u a l  electric  the  gas  brass e l e c t r o d e s i n hydrogen t r a n s i e n t the average  :  Pdc. , f o r example,  e l e c t r o d e s i n low  i s 0.8  relation  gas,  i n the p r e c e d i n g  o n l y a few  nanoseconds  the d i f f u s e chapter later,  glow  shows t h a t t h e gas  has  - 3 8 -  already  been h e a t e d up t o s e v e r a l  a high electron of  gas  related high and  will  ionizing  electron  shock  5.4  the e l e c t r i c a l  be f u r t h e r  density  then  build  i f the  conductivity  and t h e  i n c r e a s e d as t h e r e s u l t s o f p r e s s u r e and  up a l o n g t h e d i s c h a r g e  d e v e l o p s whose e x p a n s i o n  by t h e t h e o r y b a s e d  K e l v i n and  i n the v i c i n i t y  that  and. h e a t i n g p r o c e s s e s . H i g h  a s p a r k c h a n n e l soon  explained  on t h e e l e c t r i c a l l y  axis  c a n be driven  wave m o d e l .  CONCLUSIONS The  glow p h a s e o f t h e h i g h p r e s s u r e s p a r k d i s c h a r g e s  initiated in  I t i s then expected  i s not a r r e s t e d ,  temperature  degrees  d e n s i t y has a l r e a d y d e v e l o p e d  the discharge a x i s .  discharge  thousand  by t h e Townsend mechanism o f breakdown i s q u a s i - s t a b l e  nature.  I t passes  later  p h a s e and t h e n d e v e l o p s This  diffuse  into  a glow-to-channel  a highly  glow d i s c h a r g e e x h i b i t s  a uniform p o s i t i v e gradient  through  exists.  conducting spark a cathode  fall  The c h a r a c t e r i s t i c s  v o l t a g e , are independent  impedance. C l o s e s i m i l a r i t i e s normal  of t h i s  suggest  transient  glow,  of the current  between t h i s  limiting  glow p h a s e and t h e These  t h a t many o f t h e f e a t u r e s o f t h e t r a n s i e n t  glow, a t p r e s e n t d i f f i c u l t further  r e g i o n and  c o l u m n and by c o n s t a n t  d . c . glow d i s c h a r g e s have been n o t e d .  similarities  fall  channel.  column a c r o s s which a c o n s i d e r a b l e p o t e n t i a l  as d e s c r i b e d by c o n s t a n t E/P o f p o s i t i v e cathode  transition  t o s t u d y , may be  by means o f e x p e r i m e n t s  carried  investigated  o u t on t h e d . c .  -39-  discharges.  As  diffuse  glow  origin,  which  we  obtained  our s p e c t r o s c o p i c  indicates that  a n a l y s i s of the  the l i g h t  emission  i s a l s o the case f o r hydrogen  may  nitrogen  i s of  molecular  (9), the r e s u l t s  be h e l p f u l f o r t h e s t u d y o f d i s c h a r g e  lasers.  -40BIBLIOGRAPHY 1. D r a b k i n a , S . I . , p. 473 2. B r a g i n s k i i , 3.  11  J . Exp. T h e o r . P h y s .  (USSR) " 1951,  S . I . , " S o v i e t P h y s . TEPT " 1958,  S o m e r v i l l e , J.M. and W i l l i a m s , 1959, 74, p. 309  7, p.  J.F., " Proc. Phys.  4. A n d r e e v , S.T., Vanyukov, M.P. and K o t o l o v , A.B., P h y s . T e c h . P h y s . " 1962, 7, p. 37  21,  1068 Soc. "  "  Soviet  5. R a e t h e r , H., " E l e c t r o n A v a l a n c h e s and Breakdown i n G a s e s " 1964, B u t t e r w o r t h s , London 6.  S c h r o d e r , G.A., " P r o c . 7 t h I n t . C o n f . on Phen. i n I o n i z e d G a s e s , B e l g r a d " 1966, I , p.606  7. D o r a n , A.A. 18, p. 793  and Meyer,  J.,  Brit;  ,r  8. D o r a n , A.A. , " Z. P h y s . " 1968, 9. C a v e n o r , M.C. p. 155  and Meyer,  10. C o b i n e , J.D.,," G a s e o u s New Y o r k  J . A p p l . Phys.  208,  p.  ,r  1967,  427  J . , " A u s t . J . P h y s . " 1969, C o n d u c t o r s " 1941,  11. G a m b l i n g , W.A. and E d e l s , H., 1954, 5, p. 36  " Brit.  22,  M c G r a w - H i l l , V.  J . A p p l . Phys.  "  12. P e a r s e , R.W.B. and Gaydon, A.G., " The I d e n t i f i c a t i o n o f M o l e c u l a r S p e c t r a " 1963, Chapman and H a l l , London 1 3 , , H e r z b e r g , G., " M o l e c u l a r S p e c t r a and M o l e c u l a r S t r u c t u r e , I . S p e c t r a o f D i a t o m i c M o l e c u l e s " 1950, Van N o r s t r a n d , New Y o r k 14.  J a r m a i n , W.R. 32, p. 201  15. Haydon, S.C. 19, p. 795  and N i c h o l l s , and  R.W.  " Can.  S t o c k , H.M.P. , " Aus.  J . Phys." J . Phys.  11  1954, 1966,  

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