MEASUREMENT OF ELECTRIC FIELD IN TURBULENT PLASMA BY THE METHOD OF SATELLITES OF FORBIDDEN TRANSITIONS IN HELIUM

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MEASUREMENT OF ELECTRIC FIELD IN TURBULENT PLASMA BY THE METHOD OF SATELLITES OF FORBIDDEN TRANSITIONS IN

HELIUM

M. Brizhinev, S. Egorov, B. Eremin, A. Kostrov, A. Stepanushkin

To cite this version:

M. Brizhinev, S. Egorov, B. Eremin, A. Kostrov, A. Stepanushkin. MEASUREMENT OF ELEC- TRIC FIELD IN TURBULENT PLASMA BY THE METHOD OF SATELLITES OF FORBID- DEN TRANSITIONS IN HELIUM. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-867-C7-868.

�10.1051/jphyscol:19797419�. �jpa-00219420�

JOURNAL DE PHYSIQUE CoZZoque C7, suppZ6ment a u n 0 7 , Tome 40, J u i Z Z e t 1973, puge C7- 867

MEASUREMENT OF ELECTRIC FIELD IN TTURBULENT PLASMA BY THE METHOD OF SATELLITES OF FORBIDDEN TRANSITIONS IN HELIUM

M.P. Brizhinev, S.V. Egorov, B.G. Erernin, A.V. Kostrov and A.D. Stepanushkin.

I n s t i t u t e o f A p p l i e d P h y s i c s , U.S.S.R. A c a d e q o f S c i e n c e s , Gorky, U.S.S.R.

The measurement of electric fields of pla~ma waves is one of the important prcb- lem in experimental investigation of in- teraction of intense electromagnetic wave with plaama,which can give rise to a strong Leulgnn\ir turbulence In our communication we present the tl]

.

results of measurements of field in plaa- ma, baaed on the forbidden transitions satellites method suggested by Baranger and Yoaer [27 We used the quasioptical beam of elec-

.

tromagnetic waves in our experiments. A plasma column with transverse dimensions L-10 cm was placed in a c natant magne- tic field ( WL 3 2.109sec-9). A helium plasma was produced at preesure 10-2torr with the initial electron and ion tenpe- ratures Te-10 ev a n d T ^N 1 ev reapecti- vely. A n electromagnet'ic wave beam (

A,--,

8 mm) formed with a system of quasiop- tical converters was focused at surface of a plasma column (K H El. The maximun el ctric field without plasma achieved

4

(&/& = 0.15). The optical emiaaion

spectra were analysed by a monochromator DFS 2 with the instrumental halfwidth 0,2

-1

sufficient for satellite resoluti- on G),,- 2*l0l leec-1, Forbidden tran itions He I fo three lines ( = 4026

1,

^{A =}

= 4922

fi, ^A

^{= 4472}

⁸

⁾ were investige ted. A n intense line 4922

61

was most aui- table for measurements.

The line spectrum was obtained during many operation cycles of the set up.

The operating conditions of a plasma source were chosen so that durin the pump m v e pulse ( 5 = 200

/isec f

^{the pl.s}

ma density slowly increased passed tbro- ugh the critical value ( &P" 2.101

~CBI-3>,

The typical emission oecillograms of the allowed line and of satellite near the critical plawa density a r e displayed by Pig.1, A sharp increase of the satellite emission near the critical density consi- derably greater than that of the allowed line points to growth of hf electric fi- elds in plaema. In a transparent plasma

(He 4 Hecr) the satellite strength m a defined by the electric field of the pump wave

.

Fig.2 shows three a ectra of lasma eniaaion near the line

4

^{= 4922}

!

^of

He I. Curve X represents the emiasion of a "coldw plasma (without pump wave) cur- ves 11 and I11 represents the emission from plasma under the action of pump wave for ugdercritical (Ne

<

ETecr) and near critical density respectively.

!Phe emission of a forbidden transiti- on 2 3 ~

-

^4% in a "coldn plasma is asso- ciated with a quasi-static electric fields existing in plasma L4I.

It should be noted that its is diffi- cult to make abaolute measurements of the electric field in plasma by the satellite to the allowed line intensity ratio under experimental conditions. This is due to the fact that satellites emission escapes from a small region located in a strong electric field, while the contribution to the allowed line emisaion was made by the whole plasma layer. Therefore the obtained experimental values for the electric field in plasma are too low.

Fig, 3 shows the electric field in plasma measured by satellites of fo bidden transitions of th6 line h = 4922

%

^{as a}

function of the electric field strength in the pump wave. In calculations we have used th linear suggested by Baranger and

~ o z e r [2f.

In a transparent plasma the electric field is linearly dependent on strength of the electric pump field. This fact sup- ported the correctness of the field calcu- lations made using the Baranger and Nozer linear theory. Measurements in a transpa- rent plasma enabled us to determine the formfactor due to, as was mentioned above, the nonlocal character of optical measure- ments. The formfactor was found to be 3.

Hear the plasma resonance ( E = 0) the increase in the electric field was ob- served only with the pump electric field exceeding the threshold value (E-'500

2)

and reached saturation for the elect- Cm ric pump field The polarization of the electric

-

^{8 - 10'21, - -} 2000 v/cm (

LL /

^{L ~ T ~}

field in plawa may be determined by that of satellites of the forbidden transition L 5 1

Polarization measurements in a trans- parent plasma in the resence of the focu- aed (

L E

^,- 10 nun? beam of the pump wa- ve indicated that the beam field is aniso- tropic with the ratio E?/ E: = 4 2 1 where the direction of the field components is determined with respect to the wave vector

K . !his result rather well correspondens to the electrodynamic estimate of the field components at the focus of the Gausaian be- am.

The dependence of polarization degree on the electric field in the pump wave is shown in Fig.4. For fields Eo <.I000 v/cm polarization measurements are dxfficult to be made due to the influence of unpolarized quasi-static fields. An essential a n i a o t w

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797419

was observed for the pump fields 1000v/cm with dominating electric field along the wave vector K

.

^{With 8} further growth of the beam field the anisotropy in the electr pic field distribution m a not observed*

Thus, in the experiments it irs diaco- vexed Ehat when the plaama is affected by an intense electromagnetic wave as a re- sult of nonlinear interaction there ap- peared plasma waves with the amplitude of the electric field exceeding that of the pump wave field possibly up to the charac- teristic plaama fields. When the threshold field was exceeted (Eo

>

500 v/cm) the eleatric field component along the wave vector of the pump wa

3

e was increas ed. Ttp:

maximum increase of E,, reas ed 25 compared to the incident radiation. For the elect- ric field component perpendicular to the wave vector, the enhancement of ~2 amounted to 4. m e increase inthe longituginal

(with rerspect to the wave vector K 1 com- ponent of the electric field in plasma is apparently due to nonlinear deformation of the plasma denrsity profile and trana- formation of an electromagnetic wave into plasma one directed along the denaity gra- dient.

Reference

1. V.P.Silin. Parametric high power in- fluence on plasma. Xzd,"Naykan, 1973.

2. Boranger N., Mozer B., Bye-Rev.

123,

25 (1961).

3. Hicka W.W., Hess R.A., Cooper 1.S.

Phgs* Rev*,

2,

490 (1972)c 4. G-Grim. Spectrolgcopy of plaama,

Atomizdat, M., 1964.

5. Cooper HV.S,, Ringler H., Phya. Rev.,

m,

226 (1969).

Pig. 2.

I . , . , . , J

Pig.1 O~lcillogrsms: I-Pump signal;

11-Intensity of allowed transition emis- sion; 111

-

Satellite intensity;

IP-"Faat" electron current (We

>

100 ev).