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Submitted on 1 Jan 1979
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MICROWAVE REFLEXION IN RECTANGULAR WAVEGUIDES BY HIGH PRESSURE R.F. PLASMA
COLUMNS
G. Cicconi, E. Bloyet, P. Leprince, J. Marec
To cite this version:
G. Cicconi, E. Bloyet, P. Leprince, J. Marec. MICROWAVE REFLEXION IN RECTANGULAR
WAVEGUIDES BY HIGH PRESSURE R.F. PLASMA COLUMNS. Journal de Physique Colloques,
1979, 40 (C7), pp.C7-827-C7-828. �10.1051/jphyscol:19797399�. �jpa-00219398�
JOURNAL DE PHYSIQUE CoZZoque C7, suppZ6ment au n07, Tome 40, JuiZZet 1979, page C7- 827
MICROWAVE REFLEXION IN RECTANGULAR WAVEGUIDES BY HIGH PRESSURE R.F. PLASMA COLUMNS
G. Cicconi, E. ~ ~ o ~ e t * , P. 1eprincei and J. blareex.
%Department of ElectricaZ Engineering, University of Genoa, I t a l y .
Laborahire de Physique des Guz e t des PZasmas, Universitd dl&say, France.
The aim of this paper is the determination of some parameters of high pressure plasmas.
Principle. High pressure plasma columns can be pro- duced by means of a surface wave generator : the Surfatron
[I].
These plasmas are highly collisional (e.g. v 1 10 10 Hz at 10 torr in argon) and dense (typically n = 10 ~m-~). 14 Consequently we have cho-sen as experimental method for diagnostic, the de- termination of the reflexion coefficient of a mi- crowave (A = 3cm) incident on the plasma column created in very narrow quartz tubes (int. dia. ab- out 1-Zmm), and placed as a cylindrical post in a rectangular waveguide. Then a numerical method [2]
allows us to estimate some plasma parameters as, for instance, density, radial density profile, col- lision frequency and plasma ladius, from the expe- rimental values of modulus and phase of R. The cal- culated values of R are also compared with those obtained from an extension of Marcuvitz variational method [3]
.
Calculation. We assume for the plasma column a con- ventional dielectric description, where the losses are dependent on a global collision frequency v.The radial distribution of the electron density is pa- rabolic, as :
2 n(r)= Neo<l
-
a 31,1
where N is the density on the cylindrical axis, eo
p1 is the column radius and ci a variable parameters
10 12 14 X
of profile with OGaGl. The reflexion coefficient of
Fig.l.Theoretica1 variations of modulus and phase the plasma column in rectangular waveguide, excited
of R versus X=log10Neo(~m~3) for several collision in TE(1,O)mode polarized along the cylinder axis,
rates.
is numerically calculated by the amplitudes of the Bessel-Fourier cylindrical modes of scattering in waveguide. The numerical code [4], for the calcula- tion of this coefficient, solves an infinite system of linetr equations obtained by the boundary condi- tions on the cylinder surface for the tangential components of e.m. fieid modes. This code was deri- ved from an electromagnetic model where the fields
are Fourier exact solutions of Maxwell's equations
[zJ
[5]. In the cylinder, three regions may be con- sidered: I) plasma 06r<pl,II) vacuum shell plGrCp0'
111) dielectric layer po<r<p2.
The calculation results, for the appropriate ge,- ometry and variation range of electron density, are obtained for three values of the plasma radius pl=
0.50,0.65,0.85mm, v/w= 0.0,1.0,2.0 and a=!0-~,1.0.
These calculation values are practically coincident (Ge1X)with those obtained with the harcuvitz varia- tional method(a=O) [3] .In Fig. l are shown the re- sults for the case a=1.0.
Measurements. The experimental value of the refle- xion coefficient modulus and phase is obtained by
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797399
VSWR measurements in standard waveguide(h=3cm).The discharge tube is pierced across the waveguide and centered in the transversal section through two ho- les having a diameter(6m)twice with respect to the tube diameter(3m),made on the waveguide large wal- 1s.The tube is centered in the holes with a preci- sion'of about 0.5mm.The systematic errors of measu- rement on the modulus and phase of R are respecti- vely E =+6% and c8=f2%.The experimental results(ar-
R
-
gon,frequency IOGHz) are shown in Fig.2 versus RF absorbed power by the surfatron plasma(2450MHz),and for the pressure values of 10,50,15Oand 760 torr.
Fig.2.Variations of modulus and'phase of R in Argon for several pressures versus absorbed power in plasma.
Result interpretation.The values of plasma parame- ters,estimated through a comparison between experi- mental and calculation results, are plotted in Fig3 versus pressure.The fitting is obtained within a maximum deviation of 10% for the modulus and 40%for the phase of R, without taking into account the measurement errors.The maximum value of deviation on 6/11 derives essentially from the presence of the holes for tube support on the waveguide walls. If the fitting is performed by using the calculation results obtained for a = ] o - ~ , the enhancement of the calculation-experimental deviations are of the or- der of 15 to 20 %.
Conclusions.This study gives us the three following
information on the behaviour of surfatron plasmas : the density profile is approximately parabolic ex- cept for very high pressure(e.g.100torr in argon and the atmospheric pressure in neon)where the pro- file tends to become flat,this conclusion is also valid for RF absorbed power values higher than 100 watts;that is at the same pressure the plasma does not fill the quartz tube,the losses, expressed by the v/w term,are not proportional to the pressure.
In conclusion this reflexion method can be conside- red as a powerful tool to estimate the plasma den- sity and other plasma parameters.Moreover the va- riational Marcuvitz method is easier than the me- thod here used but gives only an approximate avera- ged density value.The more important information here obtained is the estimation of the density gra- dient in high pressure plasmas.
Fig.3.Comparison of Argon and Neon parameters in surfatron discharges.
References. 1) Leprince P. et al. 1975. IEEE Trans.
Plasma Sci., PS-3, pp. 55-59.
2) Cicconi G.,Molinari V.,and Rosatelli,l973, Jap.
Journ.App1. Phys.
2,
pp.721-733.3) Marec J.et al.l978,J.Phys.D, g,pp.1021-1027.
4) Benedetti A.P.and Poli P.1975,CNEN RT/FIMA(75)3.
5) Cicconi G.and Rosatelli C.1977,IEEE Trans.Micro- wave Theory and Tech. MTT-25,11, pp. 885-892.
