THE UHF DISCHARGE WITH PRELIMINARY LOCALLY IONIZED GASEOUS MEDIUM

(1)

HAL Id: jpa-00219303

https://hal.archives-ouvertes.fr/jpa-00219303

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.

THE UHF DISCHARGE WITH PRELIMINARY LOCALLY IONIZED GASEOUS MEDIUM

N. Armand, S. Rogashkov, E. Shustin

To cite this version:

N. Armand, S. Rogashkov, E. Shustin. THE UHF DISCHARGE WITH PRELIMINARY LOCALLY IONIZED GASEOUS MEDIUM. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-643-C7-644.

�10.1051/jphyscol:19797312�. �jpa-00219303�

(2)

JOURNAL DE PHYSIQUE CoZZoque C7, suppldrnent au n07, Tome 40, J u i Z l e t 1979, page C7- 643

THE UHF DISCHARGE WITH PRELIMINARY LOCALLY IONIZED GASEOUS MEDIUM

N.A. Armand, S.A. Rogashkov and E.G. Shustin.

I n s t i t u t e of Radioengineering and Electronics, Academy of Sciences, Moscow, U.S.S.R.

Xhen s t u d i n g t h e i n t e r a c t i o n of i n t e n - s e ( E - ~ o v / c ~ ) focused beams of W band microwave w i t h a low temperature plasma stream flowing out of a n e l e c t r o a r c plas- matron i n t o a r a r e f i e k d gas [I], i t i s r e vealed t h a t a change of power of both pas sed and,espessially,reflected s i g n a l s h a s a n o s c i l l a t i n g c h a r a c t e r i n t h e c e r t a i n region of t h e parameters of plasma and gas (Pig. 1 )

.

^{A s}a r u l e , o s c i l l a t i o n s begin w i t h some d e l a y in r e l a t i o n t o t h e o r i g i n

of pulse. The value of d e l a y and period of o s c i l l a t i o n a r e decreased w i t h i n a r e - a s i n g b o t h d e n s i t y of plasma and pressure of gas. The b r i g h t l y luminous region r a n g i n g from t h e boundary o f a luminous p a r t of a plasma stream on t h e d i r e c t i o n t o t h e r a d i a t i n g antenna i s v i s u a l l y observed.The more pulse d u r a t i o n of a microwave beam, t h e more d i s t a n c e through which t h e luminoua region passes in t h i s d i r e c - t i o n under o t h e r equal conditions.

The luminous region r e p r e s e n t s the a r e a w i t h more i n c r e a s e d i o n i z a t i o n mov- i n g from t h e stream t o t h e source of microwave energy,ioe.the microwave discharge i n i t i a t e d by previous i o n i z a t i o n in a plasma stream. This assumption i s suppor- t e d by d i r e c t measurements of t h e plasma d e n s i t y d i s t r i b u t i o n i n space between the stream a x i s and t h e antenna. The measurements a r e made by an i n t e r f e r o m e t e r w i t h a d i e l e c t r i c waveguide s e r v i n g a s a mobi- l e s e n s o r ^[2') .On t h e interferograms, in- s i d e t h e luminous r e g i o n a s h a r p spbke corresponding t o t h e i n c r e a s e of plasma d e n s i t y w i t h the f o l l o w i n g slow decrease i s recorded.The d e l a y time of t h e s p i k e in r e l a t i o n t o beginning a microwave pul- s e i n c r e a s e s w i t h the displacement of t h e

waveguide t o a r a d i a t o r of microwave power I n Pig.2 i s represented t h e plasma den- s i t y space-time dependence p a t t e r n recon- s t r u c t e d from t h e oacillograms of the in- t e r f e r o m e t e r s i g n a l f o r one of t h e r e g i - mes in which t h e described phenomenon h a s been d e t e c t e d . I t i s seen t h a t a f t e r s w i t - ching t h e microwave beam on, t h e "hump"

moving f o r some time towards a microwave beam i s appeared on a s l o p e of t h e r a d i a l p r o f i l e of d e n s i t y d i s t r i b u t i o n . Excess of c o n s e n t r a t i o n over t h e undisturbed plasma accounts f o r 13C%(in t h e r e g i o n of t h e microwave discharge o r i g i n a t i o n ) till 450%(at t h e end of a microv~ave pulse).

With t h e a b s o l u t e value t h e plasma densi- t y i n the r e g i o n of UHF d i s c h a r g e accou- n t s f o r 0,7-0,9 of t h e c r i t i c a l d e n s i t y n - 1 , 1 . 1 0 ~ ~ r n - ~ . c r - It i s e v i d e n t t h a t t h e o s c i l l a t i o n s of t h e r e f l e c t e d s i g n a l e in t h e regimes of t h e i n i t i a t e d microwave d i s c h a r g e a r e determined by b e a t i n g between t h e r a d i a t i n g and Doppler d i s p l a c e d

( r e f l e c t e d from t h e running f r o n t of ionization)waves.So i t i s easy t o d e f i n e t h e f r o n t propagation v e l o c i t y : v= c3/24

where R

-

t h e b e a t frequency determined from t h e oscillograms. The v e l o c i t y o f propagation of a microwave d i s c h a r g e i s found t o depend on t h e undisturbed ( " p r i - mingu)plasma parameters and t o be

(0,543) .I0 cm/sec. These d a t a a g r e e w i t h 4 fhe r e s u l t s of t h e measurements of t h e f r o n t of i o n i z a t i o n v e l o c i t y according t o t h e d e l a y time of peak on t h e i n t e r f e r o - gram corresponding t o t h e microwave d i - scharge on moving t h e s e n s o r t o t h e an- t enna

.

Following Yu.P.Raiser 131 , l e t u s es- t i m a t e the magnitude of t h e s t a t i o n a r y

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797312

(3)

e l e c t r i c f i e l d f o r a breakdown of n e u t r a l argon under experimental conditions:

~ ~ ( v / c m ) = 5 , 7 . 1 0 -

where d i s i g n a t i o n s correspond t o the ac- cepted ones i n [3]

.

According t o our conditions in the range of pressures of 0,1+0,5torr, t h i s estimation d e f i n e s the value of the i n t e n s i t y of the breakdown f i e l d E=150V/cm,i.er2,5 times more than the i n t e n s i t y r e a l i z e d i n the course of the experiment. The observed phenomenon has analogs in some experiments on the op

t i c a l breakdown of gases and studing the microwave discharges in waveguides [3,4].

A s shown i n [47 , t h e d i f f u s i o n of op- t i c a l resonance r a d i a t i o n i s responsible f o r the formation and propagation of the i n i t i a t e d microwave discharge in a waveguide.In our conditions except t h i s me- chanism, two more physical phenomena seems t o be of concern. F i r s t , t h e change of f a s t f r e e d i f f u s i o n determining the par- t i c l e runaway in conditions of the neu- t r a l gas breakdown by the ambipolar one ( p e c u l i a r t o plasma) f a c i l i t a t e s the pos- s i b i l i t y of t h e avalanche breeding the particles.Second,the increase of the t o n i zing microwave e l e c t r i c f i e l d due t o both wswellingn the f i e l d in the nonuniform plasma near the resonance point and reflcc

t i n g the wave from the gradient of plasm concentration has g r e a t sagnif icance. The determination of a comparative importance of these mechanisms f o r the described phe- .bornenon w i l l be the s u b j e c t t o be inves-

tigated.

REFEREbTCES :

1 .W.A.Armand,S.A.Rogashkov,E.G.Shustin.

I n t e r a c t i o n of the i n t e n s e microwave beam with t h e flow of the low tempera- t u r e plasma.Proceedings of the 14th Con ference on PIG,Grenoble,France,l979.

2.E.A.Shustin,A.A.Lisitskaya.The microwave interferometer with enchanced re-

s o l u t i o n f o r observing the plasma in a microwave f ield.The r e p o r t of the A l l - Union Conference on physics of the low

temperature plasrna,Kiev,l979.

3.Yu.P.Raiser.The l a s e r spark and propaga- t i o n of discharges,M., t1Nauka",1974.

4.G.W.Bettke,A.DeRuess and 0th. Phys.Flu- ids,1963,ve6,p.594; 1966,~.9,~.1430;