High voltage breakdown initiated by particle impact across electrode ga

Friichtenicht, J. F.

Hansen, D. O.

Slattery, J. C.

English

NASA Technical Reports Server

4158-6009-TU-000 * 
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TECHNICAL REPORT 
HIGH VOLTAGE BREAKDOWN INITIATED BY PARTICLE IMPACT 
1 
ITHRUI ‘April 1965 
(ACCESSION NUMBER) 
N65 
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HIGH VOLTAGE BREAKDOWN INITIATED BY PARTICLE IMPACT* 
J. C. S l a t t e r y ,  J. F. F r i i c h t e n i c h t  and D. 0. Hansen 
T R W  Space Technology Laboratories,  Redondo Beach, Ca l i fo rn ia  
This l e t t e r  desc r ibes  the techniques employed and r e p o r t s  re- 
s u l t s  of explora tory  experiments on t h e  p a r t i c l e  i n i t i a t e d  high 
vol tage breakdown hypothesis  due t o  Cranberg!. 
made t h a t  smal l  p a r t i c l e s  a r e  t o r n  from an e l e c t r o d e  by e l e c t r i c a l  
stress and a r e  acce le ra t ed  across  the  gap. Upon impact a t  t h e  
opposi te  electrode, gas  and metal vapor a r e  evolved from t h e  sur -  
f ace ,  c r e a t i n g  l o c a l  condi t ions  conducive t o  the  i n i t i a t i o n  of a 
discharge between the  e l ec t rodes ,  No at tempt  is made t o  e x p l a i n  
t h e  mechanism of p a r t i c l e  formation nor t o  j u s t i f y  the ex i s t ence  
of such p a r t i c l e s  on fundamental grounds. However, an a n a l y s i s  
by Slivkov a p p l i e s  c o n s t r a i n t s  on p a r t i c l e  s ize .  The upper l i m i t  
is e s t a b l i s h e d  by r equ i r ing  t h a t  t h e  k i n e t i c  energy of t h e  p a r t i c l e  
be s u f f i c i e n t  t o  vaporize i t s  own mass. The lower l i m i t  is es- 
t a b l i s h e d  by r e q u i r i n g  t h a t  the gas  cloud be l a r g e  enough t o  per- 
m i t  charge m u l t i p l i c a t i o n  while simultaneously s a t i s f y i n g  t h e  
cond i t ions  a t  t h e  Paschen minimum. 
The assumption is 
2 
3 Yait land has compiled data  from a number of experimenters 
and f i n d s  t h a t  t h e  r e s u l t s  are  compatible with t h e  p a r t i c l e  h i -  
t i a t e d  breakdown concept e Rozanova4 has shown conclus ive ly  t h a t  
p a r t i c l e  impacts are capable of i n i t i a t i n g  discharge by p lac ing  
loose ly  bound p a r t i c l e s  on t h e  su r f ace  of condi t ioned electrodes. 
Appl ica t ion  of pulsed vol tages  r e s u l t e d  i n  breakdown a t t r i b u t e d  
t o  t h e  p a r t i c l e  impacts. Resul ts  of work in our l abora tory  on t h e  
e f f e c t s  of micrometeoroid bombardment of ion engines5 suggested a 
s imi la r  mechanism. This l e d  t o  the  experiments descr ibed below, 
* 
This work supported by the  NASA under Contract No. NASw-936. 
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In  these  experiments, p a r t i c l e s  of known mass and energy a r e  
i n j e c t e d  through an ape r tu re  i n  one e l e c t r o d e  of a plane p a r a l l e l  
electrode conf igura t ion .  The p a r t i c l e  parameters a r e  completely 
independent of e l e c t r o d e  spacing and vol tages  and t h e  experiments 
a r e  pr imar i ly  concerned with the i n v e s t i g a t i o n  of t h e  e f f e c t s  of 
vo l t ages  and e lec t r ic  f i e l d s  on t h e  developing discharge.  The ex- 
periments provide no information on the  mechanism of p a r t i c l e  
formation r equ i r ed  f o r  t h e  Cranberg hypothesis .  However, t h e  
parameters of p a r t i c l e s  requi red  t o  i n i t i a t e  discharge can be 
determined and compared t o  what might be a n t i c i p a t e d  i n  an a c t u a l  
case 
The STL e l e c t r o s t a t i c  hypervelocity acce lera tor '  was used a s  
a source of p a r t i c l e s .  I n  t h i s  device,  smal l  i r o n  p a r t i c l e s  are 
charged by a process  descr ibed elsewhere7 and a r e  i n j e c t e d  i n t o  
the  a c c e l e r a t i n g  f i e l d  of a 2-mill ion v o l t  Van de Graaff gener- 
a t o r .  The p a r t i c l e  mass and energy a r e  determined p r i o r  t o  i m -  
pac t  by techniques descr ibed i n  Ref. 7. 
A diagram of the  experimental conf igu ra t ion  is shown i n  
Pig.  1. P a r t i c l e s  from t h e  a c c e l e r a t o r  pass  through a d e t e c t o r  
used f o r  t h e  measurement of p a r t i c l e  mass and ve loc i ty .  Most of 
them pass through a 0.75 mm diameter de f in ing  ape r tu re  which is 
a l i g n e d  with a 1 mm ape r tu re  i n  t h e  ground e l ec t rode .  F ina l ly ,  
t h e  p a r t i c l e s  impact upon t h e  high vol tage  e l e c t r o d e .  Another 
p a r t i c l e  d e t e c t o r  is interposed between the  t w o  ape r tu re s  and s e r v e s  
t o  i d e n t i f y  which of t h e  p a r t i c l e s  passing through t h e  main de- 
tector eventua l ly  s t r i k e  the  t a r g e t .  In  p r a c t i c e ,  s l i g h t  m i s -  
alignment r e s u l t s  i n  l e s s  than 100 per  cen t  t ransmission through 
both ape r tu re s .  For each set  of condi t ions ,  t h e  t ransmission was 
determined and t h e  observed frequency of d i scharge  was weighted 
accordingly 
Both of t he  e l e c t r o d e s  were made of polished s t a i n l e s s  s t ee l ,  
The ground plane was i n  t h e  form Qf t runca ted  cy l inde r  about 3.8 c m  
i n  diameter .  The high vol tage e l e c t r o d e  was machined t o  a Rogowski 8 
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s u r f a c e  having a f l a t  c e n t e r  por t ion  of about 1 c m  diameter and 
an o v e r a l l  diameter of 2.5 c m .  The gap between e l e c t r o d e s  was 
a d j u s t a b l e  over a r e l a t i v e l y  wide range a s  was the  vol tage  appl ied  
t o  t h e  e l e c t r o d e ,  However, no vol tages  exceeding p o s i t i v e  or 
negat ive 30 k i l o v o l t s  were used. 
The electric f i e l d  l i n e s  a t  t he  impact po in t  a r e  somewhat 
d i s tor ted  by t h e  presence of the  ape r tu re  i n  t h e  ground plane. 
To reduce t h i s ,  t he  electrodes were i n c l i n e d  by 45' t o  t he  p a r t i c l e  
beam. The e f f e c t s  of d i s t o r t i o n  have not  been eva lua ted  completely, 
but it is assumed t h a t  the discharge occurs between t h e  impact 
po in t  and a smooth po r t ion  of t h e  ground plane.  Discharges were 
sensed by observing the voltage developed across  a 0.1 ohm re- 
sistor in te rposed  between t h e  ground plane and t r u e  e l e c t r i c a l  
ground. 
Resu l t s  obtained under t y p i c a l  condi t ions  a r e  shown i n  Fig.  
2. The x ' s  denote t h e  parameters of p a r t i c l e s  which i n i t i a t e d  
discharge whi l e  t he  poin ts  represent  p a r t i c l e s  t h a t  d i d  not .  For 
t h e  case i l l u s t r a t e d ,  a 15 kv p o s i t i v e  vol tage was app l i ed  t o  t h e  
alectrode zrnd the gap was 0.5 mn. The p robab i l i t y  of discharge is 
ev iden t ly  much g r e a t e r  for p a r t i c l e s  w i t h  ene rg ie s  and r a d i i  i n  
excess  of about 0 . 8  x lom7 j o u l e s  and 0.9 microns, r e spec t ive ly .  
The r e l a t i v e l y  sharp  threshold between discharge and no discharge 
is exh ib i t ed  f o r  a l l  vol tages  and f i e l d s  used, This d e f i n e s  a 
lower l i m i t  i n  p a r t i c l e  energy (or s ize)  below which discharge i s  
not  i n i t i a t e d .  No upper l i m i t s  were observed, rH"hus Sl ivkov*s  
p r e d i c t i o n  of p a r t i c l e  s ize  l i m i t s  is only p a r t i a l l y  v e r i f i e d  by 
these experiments. However, the  requirement imposed by Slivkov 
t h a t  t h e  p a r t i c l e  possesses  enough k i n e t i c  energy t o  vaporize its 
own mass is an ove r s impl i f i ca t ion  of the  case.  P a r t i a l  vagori-  
z a t i o n  of a l a r g e  p a r t i c l e  would r e l e a s e  more gas  than  complete 
vapor i za t ion  of a small  one. The f r a c t i o n  of p a r t i c l e  ma te r i a l  
vaporized is a s t rong  func t ion  of ve loc i ty ,9  but Slivkov does not  
cons ider  t h i s  aspec t .  A r e v i s i o n  of S l ivkov ' s  c r i t e r i a  t o  include 
I .  
3 x  lo-‘ 
2 x  lo-’ 
1 x lo-’ 
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v e l o c i t y  e f f e c t s  may be i n  order.  
The th re sho lds  i n  p a r t i c l e  energy and size requi red  t o  i n i -  
t i a t e  breakdown s h i f t  with voltage and e lectr ic  f i e l d .  Smaller, 
less e n e r g e t i c  p a r t i c l e s  are capable of i n i t i a t i n g  d ischarges  f o r  
h igher  vol tages  and f i e l d s .  I n s u f f i c i e n t  d a t a  a r e  a v a i l a b l e  t o  
t iefine t h e  vol tage and f i e l d  dependence but it appears t h a t  t h e  
e lec t r ic  f i e l d  is a more c r i t i c a l  f a c t o r .  This again is i n  
gene ra l  agreement with Slivkov. He impl ies  t h a t  charge mult i -  
p l i c a t i o n  with t h e  expanding gas cloud is dependent upon t h e  
p o t e n t i a l  d i f f e rence  ac ross  t h e  cloud. For a cloud w i t h  dimensions 
sma l l e r  than  the  gap spacing, t h e  t o t a l  vo l tage  across  t h e  cloud 
is determined by the  vol tage g rad ien t .  
The parameters of p a r t i c l e s  charged and acce le ra t ed  under 
cond i t ions  suggested by Cranberg and Slivkov can be computed and 
compared t o  t h e  parameters of p a r t i c l e s  used here .  Assuming 
s p h e r i c a l  iron p a r t i c l e s ,  8 charging mechanism s i m i l a r  t o  t h a t  
descr ibed  i n  Ref. 7 ,  and condi t ions i d e n t i c a l  t o  those used in 
ob ta in ing  the  results given i n  F ig ,  2> a 30 micron r a d i u s  p a r t i c l e  
Y Q G I ~  he required t o  a t t a i n  the threshold  energy shown i n  Pig.  2. 
This is obviously t o o  l a r g e  t o  be compatible with the  Cranberg- 
Slivkov formulation. However, the  p a r t i c l e  impact hypothesis  is 
no t  d i r e c t l y  app l i cab le  a t  t h e  vo l t ages  and f i e l d s  used h e m .  It 
is i n t e r e s t i n g  Lo note t h a t  the e s t ima te  i n  p a r t i c l e  s ize  i s  com- 
p a t i b l e  with t h e  experimental  observa t ions  of Rozanova. 
A d e f i n i t e  dependence on the  p o l a r i t y  of t h e  appl ied  vol tage  
was observed. S i g n i f i c a n t l y  less e n e r g e t i c  p a r t i c l e s  a r e  requi red  
t o  i n i t i a t e  discharge when t h e  p a r t i c l e  impacts on t h e  cathode. 
For example, v i r t u a l l y  a l l  of t h e  p a r t i c l e s  shown i n  Fig. 2 would 
have i n i t i a t e d  discharge if the  p o l a r i t y  had been reversed.  
In summary, the  experiments descr ibed  above provide quant i -  
t a t i v e  information on the  p a r t i c l e  i n i t i a t e d  vol tage breakdown 
hypothesis .  The ex i s t ence  of a l o w  energy l i m i t  f o r  p a r t i c l e s  
capable  of i n i t i a t i n g  discharge has been v e r i f i e d  and measured. 
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A d e f i n i t e  p o l a r i t y  dependence was noted and t h e  e f fec ts  of 
app l i ed  vol tage  and e lec t r ic  f i e l d  were observed q u a l i t a t i v e l y .  
Although t h e  v a l i d i t y  of t h e  Cranberg hypothes is  cannot be v e r i -  
f ied d i r e c t l y  by these experiments, ex tens ion  of the  measurements 
t o  higher vol tages  and f i e l d s  would enable one t o  de f ine  t h e  
parameters of p a r t i c l e s  requi red  t o  i n i t i a t e  breakdown under 
cond i t ions  where t h e  p a r t i c l e  impact hypothesis  is presumed t o  be 
v a l i d .  Evaluat ion of p a r t i c l e  formation mechanisms i n  l i g h t  of 
these r e s u l t s  could be instrumental  i n  t e s t i n g  t h e  v a l i d i t y  of the 
Cr a nbe r g  breakdown hypo t he s is. 
vw 
1. 
2. 
3. 
4. 
5. 
6. 
7. 
8. 
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REFERENCES 
L. Cranberg, J .  Appl. Phys., 23, 518 (1952). 
I. N. Slivkov, Soviet  Phys., Tech. Phys. - 2, 1928 (1957). 
A Maitland, J. Appl. Phys., 32, 2399 (1961). 
N. B. Rozanova, Izv. AN SSSR, Ser. Fiz., Trans. B u l l e t i n  26, 
1462 (1962). 
J. C. S l a t t e r y ,  F i n a l  Report, NASA Contract No.  NAS3-3569, 
NASA CR-54057, June, 1964. 
J. F. F r i i c h t e n i c h t ,  Rev. of Sci. I n s t r .  - 33, 209 (1962). 
H. Shelton, C. D. Hendricks, Jr., and R. F. Wuerker, J. Apple 
Phys., 31, 1243 (1960). 
J. D. Cobine , "Gaseous Conductors", McGraw-Hill, p. 177, 
(1941). 


High voltage breakdown initiated by particle impact

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