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Submitted on 1 Jan 1979
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EXPERIMENTAL DETERMINATION OF THE SPATIO-TEMPORAL DISTRIBUTION OF THE
SPACE CHARGE FIELD IN A BREAKDOWN
P. Bayle, Maxime Bayle, E. Morales
To cite this version:
P. Bayle, Maxime Bayle, E. Morales. EXPERIMENTAL DETERMINATION OF THE SPATIO-
TEMPORAL DISTRIBUTION OF THE SPACE CHARGE FIELD IN A BREAKDOWN. Journal
de Physique Colloques, 1979, 40 (C7), pp.C7-249-C7-250. �10.1051/jphyscol:19797122�. �jpa-00219094�
JOURNAL DE PHYSIQUE CoZZoque C7, suppldment au n07, Tome 40, JuiZZet 1979, page C7- 249
EXPERIMENTAL DETERMINATION
W
THE SPATIO-TEMPORAL DISTRIBUTION OFTHE
SPACE CHARGE FIELD IN A BREAKDOWNP. Bayle, M. Bayle, E. Morales.
Centre de Physique Atomique, Laboratoire associd au C.N.R.S. n o 277, Universitd Paul Sabatier, TouZouse.
Ionizing processes occuring during a breakdown c i s the mean decay time constant of excited s t a t e s a r e generally analysed through optical devices,
e F q [ - m,]
G Pphotomultipliers o r image converters. Though l e s s i s the light excitation coefficient by electrons (4) sensitive than photomultipliers, the s t r e a k image
the drift velocity converters have the advantage of giving a conti-
P
ne(%,t)
i s the electron density distribution nuous spatio temporal representation of the dis-charge ; this i s particularly interesting in the case n a ~ t ~ r n e e r r [ ~ ~ ( ~ . ~ ) ~ e ~ ~ . ~ ) ~ ~ l ( 1 1
cw(*,t) I p = A ecp I- a P / E 1%.
t )I
of non r e c u r r e n t discharges. We have thus deve- As the macroscopic coefficients a r e functions of lopped a technique to interpret streak c a m e r a r e - the e l e c t r i c field, we obtain from relations (1) and cords that give us the parameters governing the (2), the electric field. 1f E
(*.&I[$*
we obtain.$D z
3%
b, + 3 bdischarge, and in particular the space charge field.
39- - b ) zI~S(+?
f - +
It i s known that a breakdown i s essentially gover
- a t - P & t qg,+
f(BPI * a t
ned by the field resulting from the superposition of Equation (3) shows the dependency of the electric the applied field and the space-charge field (1)(2). field on the f i r s t derivatives of the optical density
of the s t r e a k c a m e r a record. This dependency indi- It i s possible to s o r t out two kinds of informations
cates important perturbations of the field that will from the streak camera records :
appear a t the bounderies of the light and d a r k a- a ) information of geometric order that gives the di-
r e a s . mension, the position of the discharge and i t s pro-
pagation speed (3). These a r e essentially qualitati- A diagram of the s t r e a k c a m e r a r e c o r d i s given in
ve informations. fig. 1 ;analysis of the record was c a r r i e d out by
b) information about the energies related to the means of scanning microdensitometer that gives blackening density of the film. F o r a film emulsion, the m a t r i x D(x, t). The isodensity curve a r e shown the characteristic curve links the image density in fig. 1. One can s e e the propagation of a cloud of D(x, t ) to the luminous energy received. The blac- electrons between the cathode and the anode. The kening density D ( x , t) i s related to the number+ of cloud widens out and i s then followed by a second photons present at the corresponding point (x, t ) in discharge canal of lower speed. The impact of the the discharge. As a f i r s t approximation this r e l a - electrons cloud on the anode gives r i s e to a r e t u r n tion can be expressed by D(x, ~ ) = V L ~ { T + (x, tj) front in the direction of the cathode, where i t pro- where
r
i s the film contrast ratio and T i s the duces a second luminous front that goes back to- transmission function of the experimental s e t up. wards the anode.The number of photons emitted between time t and Equation (3) i s solved with respect to time for each t
+
dt at a distance x from the cathode i s given by value of x. This give us the continuous evolutiont
of the electric field with position and time. In fig.d#,k)i$
$ ~ , & ) & X , P ) ~( ~ , ~ ) e ~ ~ ~ $ ! ' ] d t ]
2 we show the analysis corresponding to the line in-o
'
dicated by a r r o w s on fig. 1. The field i s derived,Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797122
which amounts to heglecting m e m o r y effects on the
CATHODE recorded picture due to the lifetime of the excited
s t a t e s . We have for example in fig. 2 the evolution E ( x ~ , t ) of the field i n time a t a given point in the gap. The optical density i n c r e a s e s in a n almost monotonous manner. The f i r s t luminous front c o r - responds to a maximum in optical density, while the two other fronts, the anode and cathode r e t u r n waves give a modification of the gradient of the op-
Fig. 1 . Microdensitogram of s t r e a k photograph tical density a s a function of time (5). of discharge in N2+CH4(2. 5 %), Each luminous front can be associated to the pro- P = 130 t o r r , E ~ / P = 118 ~ / c m . t o r r . pagation of a strong perturbation of the e l e c t r i c
fields a s one can s e e on fig. 2. T h e propagation of
I
D ( x , t ) a luminous' front indicates the propagation of an io-nizing wave. The perturbation of the field happens in the following way : in front of the wave t h e r e
exists a strong field that corresponds to a zone of 50
high optical density gradient, this wave leaves a 0
zone of almost neutral plasma characterized by a 40
50 weak e l e c t r i c field behind. The wave after reflec-
tion on the anode reinforces the e l e c t r i c field. The 20.
a r r i v a l of the ionizing wave a t anode gives r i s e to
r y strong space charge field due to the electron
2 , , J r
an intense anodic spot. The spot i s related to a ve- 0 --.-
10 15 20 t
Ins 1
.
,cloud just in front of the anode. The gradual absorp-
Fig. 2. E l e c t r i c field (1) and optical density (2) tion of the electrons by the anode modifies the net a t x = 33. 5 m m f r o m the cathode.
charge n e a r the electrode. Once the electrons a r e absorbed, the cloud of remaning positive ions lo- w e r s the field a t the anode, while the field towards the cathode ,become m o r e strong fig. 3. T h i s p e r t u r - bation goes towards the cathode. T h e propagation speed of the luminous front and that of the ionizing wave a r e little different, because the propagation i s , in fact governed by the ionizing wave. The stron- ger the ionization of the gas the f a s t e r i s the propa- gation. The l a s t wave can c r o s s the gap (5 c m ) in one nanosecond.
(1) Yoshida (K. ), Tagashira (H.) : J. Phys. D.
Appl. Phys. 9, 485-490 (1976).
(2) Davies (A. J. ), Evans
(c.
J.), Townsend (P. ), Wbodison (P.M. ) : P r o c . IEE, Vo1.124, n o 2.179-182 (1977). Fig. 3. Perturbation of e l e c t r i c field a t the anode.
(3) Bayle (P. ), Bayle (M. ), Crokaert (M. ) : J. (x = distance f r o m the cathode).
Phys. D. Appl. Phys. 8,2181-2189, (1975).
(4) L e g l e r (W. ) : Zeit. Phys. 143, 173-190, (1955).
(5) Bayle (P. ), Bayle (M. ), Morales (E. ) : J.
Phys. D. Appl. Phys. T o be published.