HAL Id: jpa-00219167
https://hal.archives-ouvertes.fr/jpa-00219167
Submitted on 1 Jan 1979
HAL
is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire
HAL, estdestinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
MEASUREMENT OF THE ELECTRON ENERGY DISTRIBUTION IN A CO2 LASER PLASMA
S. Bourquard, J. Mayor, P. Kocian
To cite this version:
S. Bourquard, J. Mayor, P. Kocian. MEASUREMENT OF THE ELECTRON ENERGY DISTRIBU- TION IN A CO2 LASER PLASMA. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-385-C7-386.
�10.1051/jphyscol:19797189�. �jpa-00219167�
JOURNAL DE PHYSIQUE ColZoque C7, suppZ6ment au n07, Tome 40, J u i l Z e t 1979, page C7- 385
MEASUREMENT OF THE ELECTRON ENERGY DlSTFUBUnON IN A C02 LASER PLASMA
S. Bourquard, J.M. Mayor and P. Kocian.
Laboratory o f Applied Physics Federal I n s t i t u t e o f Teehndlogy, h u s a n n e , Switzerland.
Introduction
Much theoretical work has been carried out on GO2 laser plasmas in order to understand the excitation mechanisms and to optimize the laser efficiency. The electron energy distribution (EED) is obtained by numerical solution of the Boltvnann equation.The different excitation rates are then obtained by a convolution of the cal- culated EED and the excitation cross sections.
/1,2/. Direct measurement of the EED would be difficult in high pressure, high ;hurrent laser discharges. However we can obtain valuable info- rmation if we perform the measurement in a D.C.
glow discharge at low pressure. It turns out that the essential parameters
,
the reduced fieldthe
reduced current density j / ~,
the ratioof the electronic density to the neutraldensity are the same as in'high pressme discharges.
Experiment
All measurements were conducted in a 2/1/1 He-N2-C02 mix at 0.35 Torr, in a 80 cm long
,
65 mm int
.
diam. Pyrex tube. The axial electric field was measured between two probes 15 cm apart.The plane Langmuir probe is situated between the two probes and is perpendicular to the axis.
A thermocouple'gives the temperature of the neutral gas. All probes are located 10 mm off the center of the tube.The discharge was stable for currents ranging from 80 to 500 mA. We have noticed six striations between the anode and the cathode. The last striation next to the cathode was clearer indicating decomposition of CO into
2
CO. The electronic density could be obtained f r a the Langmuir probe saturation current
,
the+dues agreed within 30
%
with the integral of the distribution function. This one was determine3 from the second derivative of the probe current, obtained with the second harmonic.
The moddation frequency was 450 Hz.
Results
Two series of experiments were done, one at constant flow, the other one at constant current, The essential data and measurements of the temperature, electronic density and reduced field are reported in Table 1. The EED for each series are depicted in Fig. 1 and2. We see that varying the current has a relative little effect on the field
.
On the other hand, the EED is considerably modified. This effect was also observed by Pol& et al. /3/ in pure C02 at 2 Torr, however the low-energy part of the EED decreased with increasing current while in our case the reverse is observed. We suggest the following hypothesis : at higher current more mo1ecu;les are decomposed and the resulting products then slow down the electrons. This hypothesis is confirmed when we investigate the effect of the flow for a given cupllenk.The concentration of slow electrons increases when the flow is reduced or stopped, whereas the EED becomes wider with an inportant flow.
Ih the fast flow regime
,
the effect can still be reinforced by decreasing the current as depicted in Fig. 3.Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797189
For these conditions the electronic density is reduced by a factor of 2 and the discharge tends to vanish
TABLE I.
Experiment No I I1 I11 IV V VI flow rate
(cc/min STP) 1 1 1 0 3 5
current, mA 100 150 250 250 250 250
E/N 2.0 2.3 2.5 2.4 2.5 2.6
electronic
density 2.8 5.0 5.4 4.8 3.1 2.6 109 /CC
Conclusion
Our measurements have shown the non-maxwellian characteristic of the EED with an important con- centration of low energy electrons
,
as predicted by the computations. The current and the flow rate do have an important effect on the measured distributions.Fig. 1. Measured EED with the current (indicated in m.4) as variable parameter . Flow 1 cc/min stp
.
Fig. 2* Measured EED with the flow rate (indicated in cc/min stp) as variable
parameter. Current 250 mA.
Fig. 3 (below). Measured EED at high flow (5cc/min) for 250 and 150 mA.
References
/I/ W.L .Nighan
,
R.H.Bullis,
M.C .Fowler, W.J.Wiegand,
A I A A Journal lO,4,1972,407 and references cited therein.
/2/ C.J.Eliott
,
O.P. Judd,
A.M. Lockett,
S.D.Rockwood
,
Los Alamos Report 5562-MS.
/j/ L.S Pol&
