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Submitted on 1 Jan 1979
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INTERACTION OF THE INTENSE MICROWAVES WITH A FLOW OF LOW TEMPERATURE PLASMA
(I. The experiment)
N. Armand, S. Rogashkov, E. Shustin
To cite this version:
N. Armand, S. Rogashkov, E. Shustin. INTERACTION OF THE INTENSE MICROWAVES WITH A FLOW OF LOW TEMPERATURE PLASMA (I. The experiment). Journal de Physique Colloques, 1979, 40 (C7), pp.C7-565-C7-566. �10.1051/jphyscol:19797273�. �jpa-00219260�
JOURNAL DE PHYSIQUE CoZZoque C7, suppldment au n07, Tome 40, JuiZZet 1979, page C7- 565
INTERACTION OF THE INTENSE MICROWAVES WITH A FLOW OF LOW TEMPERATURE PLASMA (1. The experiment)
N.A. Armand, S.A. Rogashkov and E.G. Shustin.
I n s t i t u t e o f Radioengineering and EZectronics, Academy o f Sciences, Moscow, U.S.S.R.
The experimental study of e f f e c t s a r i s i n g i n a course of passing a focused beam of TI band microwaves a c r o s s the a r - gon plasma stream flowing out of an a r c plasmatron i n t o r a r e f i e d gas through a supersonic nozzle have been c a r r i e d out.
The s p e c i f i c parameter determining a l e - v e l of the n o n l i n e a r i t y of plasma E/E / I / under the experimental conditions re- P ached the value 5+10. The parameters of undisturbed plasma i n the region of irra- d i a t i o n were: concentration of e l e c t r o n s n - 1 0 ' ~ + 1 0 ~ ~ c r n - ~ , e l e c t r o n temperature
Te=O, P- 1+0,3e-V, n e u t r a l gas d e n s i t y ,=N
I O ' ~ + I O ~ ~ C ~ - ~ , v e l o c i t y of flow V0=(I+2)- 10 cm.sec-I. 4
The d i a g n o s t i c s of plamna was c a r r i e d out by means of a microwave interferorne- t e r with enhanced r e s o l u t i o n /2/. The space-time d i s t r i b u t i o n of a microwave f i e l d behind a plasma l a y e r was studied with a mobile r e c e i v i n g antenna t o d e t e r - mine the e f f e c t s of self-action of the wave passed through plasma.
The pulse of the microwave s i g n a l of lmseo duration a t frequency f l was mixing with the t e s t s i g n a l of 2msec duration a t
the displaced frequency f2 ( ~ f = 0 , 2 g c p s , P2/PI in a waveguide l i n e loaded t o the horn-lens antenna. The t e s t pulse passed ahead the powerful pulse over 0,5 msec. Ln plasma region the microwave f i e l d represents the almost p a r a l l e l beam w i t h half-width 3cm and i n t e n s i t y up t o
GOV/cm. The i n t e n s i t y measurements of the t e s t s i g n a l behind a plasma l a y e r have made easy discrimination of the nonlinear e f f e c t s a g a i n s t a background of the ef- f e c t s r e l a t e d t o the nonhomogeneity and n o n s t a t i o n a r i t y of a plasma stream and
made i t possible t o a p p r e c i a t e a charac- t e r of the r e l a x a t i o n of the plasma d i - sturbance caused a powerful signal.
The time-dependencies of the p l a s m d e n s i t y on the a x i s t r a v e r s e of the micro wave beam and plasma stream and the ampli tude of the passed (on the frequency fg) wave a s a function of gas expenditure by
plasmatron and on gas pressure in a vacu- um v e s s e l , the space-time d i s t r i b u t i o n s of the plasma d e n s i t y and wave amplitude behind plasma flow a t various power l e - v e l s of the microwave beam were studied.
The oscillograms (Fig, 1 ) show a time dependence of the amplitude of the pass- ing s i g n a l and e l e c t r o n concentration in a c e n t e r of the microwave beam on the plasma a x i s i n regimes i n which the non- l i n e a r i n t e r a c t i o n of a tvave with plasma i s observed. The regimes w i t h nonlinear growth of the i n t e n s i t y of a s i g n a l on the a x i s of a beam ( P i g a l a ) a r e observed i n the range n ,0,2.10 12 c c 3 and 0 , 1 2 < p <
0,25 t o r r . S p e c i f i c time of e s t a b l i s h i n g P the s t a t i o n a r y s t a t e i s varied from lmsec t o 0,1~0,2msec with t h e increasing p and n Over t h i s period of time the l e v e l of
the t e s t beam passed through plasma beco- P'
mes 2+8 times a s high a s the undisturbed one. A s seen from the oscillograrns the in fluence of a powerful microwave beam on Aasma i n these regimes l e a d s t o e a s e n t i - a 1 decreasing the d e n s i t y of plasma on the a x i s of a beam; t h i s decreasing rea- ches 50-8@ from the undisturbed d e n s i t y and seems t o be the determining f a c t o r in
the process of s e l f - a c t i o n of a powerful wave, Wit@ the i n c r e a s i n g n and p the
growth of i n t e n s i t y during a time of a P powerful pulse i s changed by i t s decreas-
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797273
i n g and f u r t h e r by nonmonotonic time de- pendence. A s a r u l e , t h e i n i t i a l f a l l i s changed by t h e more weak i n c r e a s i n g (Fig.
l b ) .
I n Fig.2 a r e shown the space p r o f i l e s of t h e passed wave i n t e n s i t y . The p r o f i - l e s a r e p l o t t e d f o r v a r i o u s p e r i o d s of time a f t e r switching t h e powerful p u l s e on, The p r o f i l e s of Fig.2a correspond t o t h e regime in which t h e oscillograms ( i n Pig. l a ) a r e recorded. I n t h i s regime nar- rowing t h e b a s i c lobe of t h e d i r e c t i v i t y diagram of t h e antenna i n t h e n o n l i n e a r regime ( s e l f f o c u s i n g ) a s w e l l a s t h e in- c r e a s i n g the t o t a l power passed through plasma ("transparency enhancement" of plasma) t a k e s place. The e f f e c t i n t h i s regime h a s pronounced t h r e s h o l d c h a r a c t e r : t h e decrease of power 2 times completely t a k e s off the observed e f f e c t .
The p r o f i l e s shown in Pig.2b c o r r e s - pond t o t h e c o n d i t i o n s when on t h e a x i s of a beam one can observe t h e f a s t decre- a s e of t h e s i g n a l i n t e n s i t y in time of ac- t i o n of the powerful p u l s e o r e l s e s e e t h e o s c i l l a t i o n s of t h i s i n t e n s i t y (Fig.
Ib). A s seen fram t h e p l o t t e d p r o f i l e s t h e displacement of t h e maximum i n t e n s i t y r e g i o n t a k e s place,i.e. t h e t u r n of a mic- rowave beam i n t h e d i r e c t i o n of moving t h e plasma, i.e. t h e n o n l i n e a r r e f r a c t i o n . The nonmonotonic dependence of t h e i n t e n - s i t y on the a x i s of a microwave beam on time,as seen from t h i s F i g . i s caused by l a m i n a t i o n of a microwave beam. The dec- r e a s e of power of t h e b a s i c beam l e a d s t o d e c r e a s i n g the v e l o c i t y of t h e beam t u r n and d e c r e a s i n g t h e displacement of t h e r e - g i o n w i t h maximum i n t e n s i t y from t h e un- d i s t u r b e d s t a t e . The mechanisms o f t h e ob-
served phenomena of the microwave-beam ac- t i o n on t h e c h a r a c t e r i s t i c s of t h e t e s t beam passed plasma may be s a t i s f i e d from t h e a n a l y s i s of t h e space d i s t r i b u t i o n of plasma d e n s i t y d i s t u r b e d by a powerful pulse. A s seen from t h e i n t e r f e r o g r a m s ta- ken r e a d i n g s in d i f f e r e n t r e g i o n s of p l a s - ma flow n e a r t h e a x i s of a microwave beam under i n f l u e n c e of a powerful wave, t h e d e n s i t y d i s t r i b u t i o n "dip" i s formed.
This "dip1* i s smoothly lowered down on a stream arid turned more o r l e s s s h a r p l y i n t o a "hump" on moving t h e s e n s o r of an i n t e r f e r o m e t e r up on a stream t o t h e p l a s ma i n j e c t o r . So t h e asymmetric nonunifor- mity of the plasma d e n s i t y d i s t r i b u t i o n a c r o s s t h e d i r e c t i o n of the microwave be- am propagation i s developed. J u s t a s t h e depth of d i s t u r b a n c e s o Lts a s p i e t r y de- pend on t h e r e l a t i o n s be:;veen the g a s p r e s s u r e , d e n s i t y of undi-turbed plasma and stream v e l o c i t y . The self-focusing of wave o r t h e n o n l i n e a r r e f r a c t i o n i n t h e wave caused n o n u n i f o m i t y a r e observed on dependence on the d e p t h of a "dip" and s t e e p n e s s of i n c l i n e of t h e plasma d e n s i - t y d i s t r i b u t i o n .
FC3FERENCES :
l.A.V.Gurevich,ii.B.Shvartsburg.The non- l i n e a r theory of radiowave propagation i n ionosphere,M.,1973.
2.~.~.~hustin,k.~.~isitskaya.~he micro- wave i n t e r f e r o m e t e r w i t h h i g h r e s o l u -
t i o n f o r t h e study of plasma i n a micro wave f ield.Rep.on All-Union conf .on low
temperature plasma physics,Kiev, 1979.
