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Submitted on 1 Jan 1979
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USE OF EMITTED PHOTONS AS A PROBE FOR LOW PRESSURE GAS DISCHARGES
J. Fletcher, H. Blevin
To cite this version:
J. Fletcher, H. Blevin. USE OF EMITTED PHOTONS AS A PROBE FOR LOW PRES- SURE GAS DISCHARGES. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-811-C7-812.
�10.1051/jphyscol:19797391�. �jpa-00219390�
QoZloque C7, suppl6ment au n07, Tom 40, J u i l l e t 1979, page C7- 811
USE OF EMITTED PHOTONS AS A PROBE FOR LOW PRESSURE GAS DISCHARGES
J. Fletcher and H.A. Blevin.
I n s t i t u t e for Atomic Studies. The Flindors University o f South Australia, Bedford Pork, 5042, South Aus t r a l i a .
Conventional methods o f studying t h e behaviour o f i s o l a t e d e l e c t r o n swarms and e l e c t r o n streams a r e complicated by t h e e f f e c t o f e l e c t r o d e boundaries on t h e v a r i a b l e being measured. I n r e c e n t y e a r s s e v e r a l authors Lowke 1962, Lucas 1966, and Skullerud 1974) have shown t h a t t h e presence o f metal e l e c t r o d e s i n t h e region o f immediate
i n t e r e s t can modify t h e e l e c t r o n swarm parameters s u f f i c i e n t l y f o r spurious r e s u l t s t o be achieved.
Indeed a f u r t h e r non-enuilibrium e f f e c t near t o t b e cathode due t o t h e e l e c t r o n i n j e c t i o n energy d i s c t r i b u t i o n being markedly d i f f e r e n t from t h e equilbrium d i s t r i b u t i o n , does not lend i t s e l f t o u s e f u l t h e o r e t i c a l treatment and hence must be e i t h e r eliminated o r ignored i n any exneriments.
One o f t h e few methods of o b t a i n i n g informa-
a , b l . RESULTS
The c r u c i a l f a c t o r involved i n t h e s e e x n e r i - ments i s t h e r e l a t i o n s h i p between t h e nhoton f l u x and t h e e l e c t r o n d e n s i t y i n t e g r a t e d along t h e l i n e of s i g h t o f t h e c o l l i m a t o r . An exhaustive Ftonte- Carlo treatment o f t h i s problem i n hydrogen has shown t h a t , i n hydrogen, t h e photon f l u x d i s t r i b u - t i o n i s o f t h e same shape a s t h e e l e c t r o n d e n s i t y d i s t r i b u t i o n although t h e former s l i g h t l y l e a d s t h e
P a t t e r by a constant time i n t e r v a l (Hunter, 1977, Blevin e t a2 1 9 7 8 ~ ) . Hence t h e measured photon d i s t r i b u t i o n may be used t o c a l c u l a t e t h e d r i f t and d i f f u s i o n parameters o f t h e e l e c t r o n swarm.
In o r d e r t o e l i m i n a t e t h e cathode e f f e c t s an a r b i t r a r y zero p o s i t i o n (z = zo, t = t ) i s taken
0
t i o n about t h e behaviour of e l e c t r o n swarms d r i f t - a t an a x i a l z p o s i t i o n near t h e beginning o f t h e i n g through a low p r e s s u r e (m 1 t o r r ) gas under t h e equilibrium region, (z
=
3 cms). The s o l u t i o n of i n f l u e n c e of an e l e c t r i c f i e l d i s t o study t h e t h e c o n t i n u i t y equation given by Huxley (1972. a , b ) photon f l u x emitted by t h e swarm following h a s been modified t o allow f o r t h i s and g i v e s a electron-molecule e x c i t a t i o n c o l l i s i o n s . This v a l u e o f t h e i n t e g r a t e d photon f l u x along t h e l i n e technique was f i r s t suggested by Corripan and von o f s i g h t o f t h e c o l l i m a t o r N @ asEngel (1958) and f u r t h e r used by Brere and von
N exp
-
[yZ/(4Dt' + q 2 ] N@(y,z,t) = oEngel (1964). The photon counting technioues used [ (4n0t' + irR 2 ) ( 4 ~ r D ~ t ' + nz2)
+
i n t h i s e a r l y work were p r i m i t i v e , however, r e s u l t -
i n g i n poor s t a t i s t i c s and l a r g e e r r o r s . Develop- exp
[-
(" -"'
(1)4 ~ ~ t ' + Z ments i n photon d e t e c t i o n and p u l s e manipulation
In t h e above R and Z a r e t h e r a d i a l and l o n g t i - during t h e i n t e r v e n i n g years have made t h e method
1
a v i a b l e and indeed an a c c u r a t e technique. tudonal half-widths r e s p e c t i v e l y a t t h e / e h e i g h t of t h e d i s t r i b u t i o n a t t h e a r b i t r a r y zero p o s i t i o n , D e t a i l s of t h e p r e s e n t experimental method.
have been published (Blevin e t a 1 1976 a , b , c , 1975 t' = t
-
to, z' = z-
zO.Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797391
For the measurement of W and D y = 0 in L
equation 1 i.e. on axis distributions are measured.
The experimental technique is then to measure distributions at zo and at about six axial posit- ions further down the drift tube. W and DL are then obtained by curve fitting equation 1 with y = o to the experimental distribution.
Transverse diffusion coefficients can be de- termined by scanning the collimator across the swarm at one value of z. In this case, (Blevin et a2 1978a).
The present technique also lends itself to the measurement of ionization coefficients. (Blevin et a2 1978a).
HYDROGEN
Data on the values of W, D D and a/N as a L t
function of E/M in the range 45 S E/N
<
180Td inhydrogen along with the Monte Carlo simulations of the discharge have now been published (Blevin et a2 1976 a,b,c, 1978 a,b.c : Hunter, 1977) and are included here for completeness.
NITROGEN
The electron drift velocity, the diffusion coefficients and the ionization coefficients have all been measured in spectroscopically pure nitrog- en. These data are currently being analyzed and will be presented.
HELIUM
The present technique is currently being applied to pure helium and while only preliminary results are available at present it is evident that secondary electron emission plays an important role in the helium discharge. The proRress in helium will be reported.
SECONDARY ELECTRON EMISSION
The present technique also enables a direct
study to be made of secondary electron emission from the cathode. Flost previous methods have been indirect and have involved approximations in the theoretical analysis of the experimental results.
In hydrogen only two secondary mechanisms are possible, ultra-violet nhoton bombardment of the cathode and ion impact on the cathode. Since those secondary electrons produced by photon bombardment will occur very soon after the primary pulse
(<1.0 us) while those produced by ion impact will be released approximately one ion transit time
(-20~s) later the contribution of each to the total secondary emission can be detected.
The results for hydrogen so obtained will be presented. A. clear "threshold" for secondary ionization by positive ion impact is observed at E/N -200 Td.
It is believed that this is due to the nroduction within the discharge of H+ ions at E/N 5200 Td and that these atomic ions are much more efficient at producing secondary electrons than the H ~ + ions which are the only species present for E/N <200 Td. Further experiments to verify this postulate are now in orogress.
IIEFE?WICES
Blevin H.4. et a2 1976 a,b,c J . Phys 3
2
465; Ibid9' 1471, Ibid 9, 1671.
-
-
Blevin N.A. e t a2 1978 a,b,c J . Phys. D
2,
2295;Ibid
11
1653; Austr. J. Phys. 31 299 - Hunter S.A. 1977 Austr. J. Phys.30
53.Lowke J.J. 1962 Austr. J. Phys. 15 33.
Lucas J 1966 Int. J. Flectron 6 535. - Skullerud H . R . 1974 Austr J. Ohys.
11
195.Huxley 1972 a,5. Austr J. Phys.
