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Submitted on 1 Jan 1979
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SELF-TRAPPING OF LOWER HYBRID WAVES AT THE RF BREAKDOWN OF GAS
G. Markov, V. Mironov, A. Sergeev
To cite this version:
G. Markov, V. Mironov, A. Sergeev. SELF-TRAPPING OF LOWER HYBRID WAVES AT THE RF BREAKDOWN OF GAS. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-639-C7-640.
�10.1051/jphyscol:19797310�. �jpa-00219301�
JOURNRL DE PHYSIQUE CoZZoque C7, s u p p z d m e n t au n07, Tome 40, J u i Z Z e t 1979, page C7- 633
SELF-TRAPPING OF LOWER HYBRID WAVES AT THE RF BREAKDOWN OF: GAS
G.A. Markov, V.A. Mironov and A.M. Sergeev.
A p p l i e d . P h y s i c s I n s t i t u t e , Academy of S c i e n c e s o f t h e U.S.S. R., Gorky, U.S. S . R.
The study of the interaction between intense rf radiation and magnetoplasma is stimulated at present by searching for the effective methods of directed and lo- calised field energy transfer to plerama.
In this connection the poesibilities of nresonancen focusing of lower hybrid wa- ves excited in plasma by cylindrical sour- ces (1) are of particular interest. It is evident that the role of nonlinear effects in formation of a strong field region is ef great importance. In this paper the interaction between field and plasma is investigated at rf breakdown of gas near a cylindrical inductor .at the frequencies of uw,<<cLt<<O"o are the cyclotron electron and ion frequencies). A plasma waveguide is found to be essentially iso- lated from walls and to trap lower hybrid waves creating it.
An air (or helium) discharge was exci- ted with a double-coil inductor mounted coaxially with a glass balloon. The in- ductor was the anode load of the two-cy- cle generator and was placed outside the balloon walls to eliminate the inductor teminal influence. The system parameters were chosen aa follows: the balloon dia- meter 2a=20cm, its length lnl20cm, the in- ductor diameter 6cm, its length cm, th operating pressure range p3. 10-3+5 . I O ~ Torr, the longitudinal magnetic field Bn 500 Ga, ita inhomogeneity along the airs
s
65,
the rf field frequency f=50 MHa, the power W=5+150w.To determine the plaslas parameters mo- vable cylindrical and plane probes were umed.!i!he rf potential diatribution in plaanar wae measured by a screened pin ern- Canna, connected with the spectral analyzer.
The sbaolute measurements of the rf field amplitude were made using the oscillo~cope and the dipole antem.
The important peculiarity of the rf dis- charge within the given pressure interval is the presence of a thin plasma filament extended from the inductor region alone
For p
<
?0-*~orr and W 3 low the length of the fllament was limited by that of the taystem. Por comparatively small va- lues of the rf power input W.-
low plas-ma dens3ty ip the filament centre
no,nc
( n c =
mffile
is the critical plasma density) and is approximately the same along the axis far from the inductor.The curves of Fig.2 worn obtained for W=9.6w, p=10-2 Torr at different dis-
tances
L
from the inductor. The rf po- tential distribution over the discharge balloon cross sectioncpunder the same experimental conditions is shown in Fig.3. The measurements of the diffe- rence between field oscillation phases at different points along the system axis indicate that the length of a wave prepagating in the canal, approximately coincides with the double inductor size14 cm. With the growth of the rf power feeding in the discharge plasma density in the filament and its thick- ness increase. A regime is possible when the electron density exceeds the criti- cal value everywhere.
Passing to the interpretation of ex- perimental data we note firat of all that investigated rf potential and plas- ma distributione are in a quasi-static zone of the soprce
,
i. e. k
-<
Xfi Y whereLC= elf({
+%ep
is the whistler wavelength in plasma. Therefore, density and conical angle aperture measurements indicate that the pattern observed is definitely associated with excitation of the potential ionizing lower hybrid waves propagating in the consistent plas- ma distributioneFor a qualitative explanation of the canal structure we ahall consider a simple model* It is based on the appro- ximation of the axially raymmetric plas- m s density and rf potential amplitude distribution being homogeneous d o n g the magnetic field B, i.e.
where r. is the distance from the bal- loon axis,
z
is the longitudinal coor- dinate,
K = 211~/h.
For the rf field the Poisson equation with the longitudinal and 'tranaverse n ~1asma.permittivities-
(A$-
&,=I
& 11'-4
--=nc
{ --
W 2 is satisfied.When writing the material equation we take into account that the electron tem- perature locally depends on the electric field amplitude in the discharge crose section.-~hia permits to represent the frequency of molecule ionization at elec-
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797310
tron impact in the form $i =
$ i ( l ~ @ )
This complicated dependence is usually approximated by the power one:
Ji=dl~y12P (j3
> I )
The diffusion f l uh
on the walls and the electron adhesion to electro-negative molecules of gas are assumed to
So the self-consistent distribution of rf potential of a low-hybrid wave and of plasma density is the solution of non- linear differential equations
d2V
4 dY
+
- ~ ' ( ~ - n / n d v . o
(1)d&+-- ' d n
+ [ d ( n l p ~ ~ ~ - $ ~ ] & = ~d"t2
1.d~
where QL is the coefficient of the ambi- polar diffusion across the magnetic field,
$a is the electron adhesion frequency.
System*.j)
allows the space localized solutions for which near the axisn >n,
,
and in the peripheral regionn < n that determines an exponential
decrease in the field. At; the same time the density decrease is due to the adhe- sion action, Similar solutions correspond
to the possibility of self-sustaining References:
plasm and field distribution without the
ballosn wall influence. 1. R.L.Stenze1 and W.Gekelman, Phys.
Thus, the main reeult of the paper is Fluids
20,
108, (1977).that the localized region of field and
plasma extended as a thin filament along 2. A.G*Litvak, Radiofizika
4,
629, the magnetic field is found under the rfdischarge conditions. This phenomenon is (1966)*
explained by propagation of an ionizing lower hybrid wave in the consistent plas- ma distribution.
*)When describing a discharge in helium, a weak inhomogeneity of the canal along the axis should be taken into account. It leads to the same qualitative results as the adhesion in the air.
