LASER GAS-DISCHARGE PLASMA DIAGNOSTICS BASED ON FARADAY ROTATION

(1)

HAL Id: jpa-00219419

https://hal.archives-ouvertes.fr/jpa-00219419

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 published 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, est destinée 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.

LASER GAS-DISCHARGE PLASMA DIAGNOSTICS BASED ON FARADAY ROTATION

V. Gudelev, A. Shkadarevich, V. Jasinskii

To cite this version:

V. Gudelev, A. Shkadarevich, V. Jasinskii. LASER GAS-DISCHARGE PLASMA DIAGNOSTICS BASED ON FARADAY ROTATION. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-865-C7- 866. �10.1051/jphyscol:19797418�. �jpa-00219419�

(2)

JOURNAL DE PHYSIQUE ColZoque C7, suppZe'rnent au n07, Tome 40, JuiZZet 1979, page C7- 865

LASER GAS-DISCHARGE PLASMA MAGNOSTICS BASED ON SARADAY ROTATION

V.G. Gudelev, A.P. Shkadarevich and V.M. Jasinskii.

I n s t i t u t e o f Physics, ByeZorussian Academy o f Sciences, Minsk, U.S. S. R.

Introduction. Faraday r o t a t i o n application t o plasma i n v e s t i g a t i o n has been known f o r a long time. However, new p o s s i b i l i t i e s f o r i t s usage appeared with the develop- ment o f l a s e r s providing ,tunable r a d i a t i o n of high s p e c t r a l density. The i n s e r t i o n of t h e medium under study i n t o l a s e r resona- t o r ensures often an additional increase i n s e n s i t ; i v i t y of Faraday r o t a t i o n measurement. Here several d e f i n i t e examples of the r i n g l a s e r application t o determine a s e r i e s of parameters of a discharge plasma placed in a longitudinal magnetic f i e l d a r e described.

Method. Paraday r o t a t i o n angle is re- l a t e d t o electron density He and magnetia f i e l d i n t e n s i t y H as followfj /'I/

- l ? 2

Q = 2 , 6 3 10 x

^O

J H N ~ ~ ~

⁽²⁾

where t h e angle in radians,

1

the wave- l e n g t h of probe r a d i a t i o n in cm, Ng in cm, 3

f

t h e plasma length, A in Oe. Faraday ro- t a t i o n , i n i t s t u r n , causes counterrunning wave of a r i n g l a s e r t o be d i f f e r d

a ( L 4

3 9 ) (2)

here. t h e perimeter of t h e r i n g l a s e r ,

C

the l i g h t v e l o c i t y , the

$8

term i s due t o t h e active medium dispersion. The scaling f a c t o r of 10 6 in

3-

-dependence of

A$

i n d i c a t e s the p o s s i b i l i t y of a very

s m a 1 r o t a t i o n measurement by this i n t r a - c a v i t y technique.

Paraday r o t a t i o n can be used a l s o t o der termine spectroscopic parameters o f the gas-discharge plasma having absorption o r amplification l i n e s , As was shown In /2/, f o r this purpose it i s q u i t e promissing t o use Faraday r o t a t i o n of ' l i n e a r l y pola- r i z e d probe r a d i a t i o n i t s frequency being in coincidence with the c e n t e r of absorp- t i o n l i n e of gas-discharge plasma.

Faraday r o t a t i o n usage t o t h e Gurposes mentioned provides some advantages ,over

conventional methods. I n particular, the l o c a l plasma analysis i s available with t h e magnetic f i e l d applied t o d e f i n i t e plasma regions. In some cases by means o f t h e magnetic f i e l d one can avoid absorp- t i o n band e f f e c t of nodeeman molecules.

F i n a l l y , magnetic f i e l d permits t h e moau- l a t i o n of t h e measured parameter, bein@;

the important and u s e m f a c t o r enabling an experimental s e n s i t i v i t y t o be incre- ased. Naturally, an inverse problem cah be solved by methods involved,, i.8. using the above parameters found independently the magnetic f i e l d can be estimated.

Emerimental. Experimental study was car-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797418

(3)

r i e d out using a He-Ne r i n g l a s e r with the wave l e n g t h

')r

= 1 ,I 5 ,um and perimeter

L=

l'/Ucm, Block sfiagran of the set-up i s 'shown in Big.1.

!I

-

^{mirror? 2}

-

piesoceramics; 3

-

^system

of automatic frequency control; 4

-

^sole-

m i d ;

5 -

gas-discharge plasma unaer stu- dy; 6

-

a c t i v e medium; 7

- y

^-plate;

8

-

p a r t i a l p o l a r i z e r ; 9

-

^Faraday^rota-

Gor; 1 0

-

magnetic f i e l d supply; Il-scan- ,ning interferometer; 1 2

-

photodetector;

13

-

oscilloscope; 14

-

frequency-voltage converter; 1 5

-

lock-in amplifier; 16-di- g i t a l voltmeter; 1 7

-

x-y-recorder.

'The application of moaulation technique permits us t o acquire t h e s e n s i t i v i t y o f t h e set-up used of 0,Ol Hz f o r beating frequency measurement.

Results. Figm2 sh o w s t h e g a i n distribu- t i o n along t h e a c t i v e medium f o r gas-discharge k b e obtained by l o c a l magnetic f i e l d application, The gain i s found t o disimibute nonuniformly along the posi- t i v e column of gas-ascharge tube.

Curve 1 corresponds t o the case when t h e aluminivm hollow cylinder is used a s a

cathode and g e t t e r body

-

âsânânode.

For curve 2 t h e cylinder and g e t t e r body is exchanged t h e i r function. In both ca- s e s the gain increase has been obtained near the ca6hode. It demonstrates a non- uniformity of the p o s i t i v e column of plasma. The additional IIE e x c i t a t i o n makes t h e gain d i s t r i b u t i o n more smooth (see curve 3). Such behaviour of g a b s e e m t o be aue t o t h e discharge s t r a t i f i c a t i o n . The electron density measurement has been c a r r i e d ou* using discharge tube with l e n g t h 20cm f i l l e d with helium and loca- t e d i n t o solenoid. The evaluated experi- ment shows t h a t using t h e technique involved permits t o r e g i s t r a t e d the electron density a s low a s 10"-10 11 om-3. The app- l i c a t i o n of probe r a d i a t i o n with relaeive- l y small wavelength enable t h e transverse gradients of plasma r e f r a c t i v e index t o be decreased s i g n i f i c a n t l y . Also, an opti-

c a l density a s low a s

l o m 9

has been regi- s t r a t e d .

References

.

/~/.A.A.Dougal ,et.al. Phys .Rev.Lett. ,IJ, 156,1964.

/2/.AmP.Voitovich,et.al. DoMaay of Aca- d e w of Sciences of t h e BSSR ,2,801 ,I 977.