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Submitted on 1 Jan 1979
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IONIZATION GROWTH IN ARGON
R. Abdulla, J. Dutton, A. Williams
To cite this version:
R. Abdulla, J. Dutton, A. Williams. IONIZATION GROWTH IN ARGON. Journal de Physique
Colloques, 1979, 40 (C7), pp.C7-73-C7-74. �10.1051/jphyscol:1979736�. �jpa-00219355�
JOURNAL DE PHYSIQUE CoZZoque C7, suppze'ment au n07, Tome 40, JuiZZet 1979, page C7- 73
IONIZATION GROWTH IN
ARGONR.R. Abdulla, J. Outton and A.W. Williams
Department o f Physics, University College o f Swl
Introduction.
The growth of ionization currents in argon have been studied experimentally by a number of authors (1)s
(a,
(3).The experiments of Kruithof and penning") and of ~ e ~ l e n ' ~ ) used the standard Townsend method of measuring I a s a function of d a t constant E/N and gave values of the primary ionization coefficients
a/N which w e r e in agreement f o r purified gases. I t was a l s o shown(3) that unpurified gas gave, a s expected, higher values of a/N. Tilcre is however interest in investigating the pressure dependence of a/N to elucidate whether p r e s s u r e dependent processes such a s associative i o n i ~ a t i o n ' ~ ) a r e significant in ionization growth and breakdown in this gas. The only other high p r e s s u r e study(2) gave no evidence on this question because it was found that I, d curves could not be analysed to give values of a/N s o that I, p curves were obtained which gave values of a/N dependent on the value Of d used.
The present paper r e p o r t s the initial measurements of a n investigation to examine further the question of the values of the primary and secondary ionization coefficients in high p r e s s u r e argon.
Apparatus.
In o r d e r to achieve the high g a s purity necessary f o r ionization growth measurements on the r a r e g a s e s the ionization chamber used in the present investigation could be evacuated t o ultra high vacuum. The chamber has been described by Dutton and
ow ell(^)
and consists of a cylindrical stainless s t e e l chamber, 50 cm in diameter and having a volume of about 40 litres. The chamber was pumped by a combination of sorption and ion pumps and could be baked a t a temperature of 250°C by means of a furnace. All the taps and manifolds exterior to the furnace w e r e degassed using heating tapes. The ultimate p r e s s u r e of the system was-
9-
62 x 10 t o r r w i t h a l e a k r a t e o f 0 . 9 ~ 10 l.ps-l.
The electrodes w e r e 15 cm diameter gold plated copper, machined to a ~ r u c e ' ~ ) profile and gave a n
m s e a , Swansea, U.K.
uniform electric field for electrode separations up to 5 cm. In the centre of the anode were drilled 37 holes 1 mm in diameter which allowed the passage of ultra- violet light on to the cathode t o provide a n initial current of photoelectrons.
The voltage supply was a 6 kV Fluke model 408B with a stability of 1 in 10 5
.
The voltage w a s measured using a 4 M Q potentiometer and ionization c u r r e n t s w e r e measured by means of E. I. L. 33c vibrating reed electrometer.In the present experiment g a s e s of two different purities were used. With standard g r a d e gas of 99.99%
purity the ionization chamber was filled directly from the bottle while for high purity gas the bottle g a s was further purified by passing i t through a B. 0. C. R a r e Gas Purifier. The impurities in the second sample should be less than one part per million. The gas p r e s s u r e s were measured by means of oil and mercury manometers isolated from the chamber by sensitive bellows. All measurements were carried out a t 20 C 0
in a t h e r m o s ' t a t i c a l ~ ~ controlled room.
Experimental Procedure and Analysis of Experimental Data.
In the present experiment ionization coefficients w e r e obtained by measuring the growth of pre-breakdown ionization currents, I
,
a s a function of inter-electrode spacing, d,
for different values of E/N (electric field/g a s number density). In o r d e r t o overcome possible fluctuations in the initial photoelectric current. I.
,
c u r r e n t s I, = cIo w e r e measured a t values of (E/N) = ( E m ) , a t which no ionization took place.
Values of IDc w e r e then obtained for different values of d.
The current growth curves w e r e analyzed in t e r m s of the Townsend current growth equation in the form
where a i s the ionization coefficient and w / a a generalized secondary coefficient. The analysis
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979736
followed the method given by Crompton e t a d 7 ) and Jones and Llewellyn-Jones (8)
.
Results.
Current growth curves have been obtained with standard grade argon (99.99% purity) a t N = 3 8 . 5 ~ 1 0 17
and for high purity argon a t N = 41.8 x 10 17 cm-3
t
and a r e shown in Figure 1.
li-.&. . . . , . , , . . . L - i
1 2 3
d (Crn.) Fig. 1. High Purity A r on.
E/N (volts CT$, 3.04 x 1 0 ~ : ~ . r 3.65~10-l6
.
4.26 x 10-
.
A 4.86 x 10,
5. 47x10-16.The values obtained for the ionization coefficients a r e shown in Figure 2 and in Tabie i, together with Kruithof and Penning's r e s u l t s f o r comparison. It can be seen that there is good agreement with Kruithof and Penning's r e s u l t s for the standard g r a d e argon but that the coefficients a r e consistently about 15% lower f o r the high purity argon. This difference is well outside the uncertainty of about
+
5% in the determination of a/p.This trend is to be expected a s any increase in the purity of the argon would decrease the ionization coefficient.
-! E/N~IU" 5 6
10 L-- .-7- r--------r-
TABLE I.
This work is being continued with argon a t higher p r e s s u r e s and with a m a s s spectrographic analysis of the gas samples.
References.
Kruithof &
Penning
0.069 0.18 0.39 0.73
1.17 1.68 2.3
1. Kruithof, A. A . and Penning, F. M., 1936, Physica 3, 515-533.
Standard Grade
0.055 0.191 0.39
1.18
2.4 High Purity
2. Golden, D. E. and Fisher, L. H., 1961, Phys. Rev. 123, 1079-1086.
2.43 3.04 3.65 4.26 4.56 4.86 5.47 6.08
3. Heylen, A.E.D., 1968, Brit. J. Appl. Phys., Ser. 2, Vol. 1, 179-188.
-
0.148 0.35 0.74
-
1.06 1.53
4. Lozansky, E .C. and Robouch, B. V., 1978, Proc. 5th Int. Conf. on Gas Discharges, Liverpool, September 1978.
5. Dutton, J. and Powell, J.M., 1972,
J. Phys. B. Atom. Molec. Phys. 5, 1236-1240.
6. Bruce, F. M., 1947, J. Inst. Elec. Eng., 94, 138-149.
7. Crompton, R. W., Dutton, J. and Haydon, S. C., 1956. Proc. Phys. Soc. B, 69, 2-13.
8. Jones, Eifionydd and L.lewellyn-Jones. F., 1958, Proc. Phys. Soc., 62, 363-368.
Fig. 2. a/N (cmL) a s a function of E/N (volts cmL).
---
rn- - - -
Present experiment High Purity Argon,(N
= 41.8 x l 0 l 7 'A Present experiment Standard Grade Argon, (N = 38.5 x 1017
4- Kruithof and Penning (0.229 x 1017 cm3<hk 53.3 x 1017 c m - 3 ).
