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Submitted on 1 Jan 1979
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THE INFLUENCE OF THE ELECTRON-NEUTRAL COLLISIONS ON THE IONIZATION WAVES IN A
HELIUM PLASMA
G. Popa
To cite this version:
G. Popa. THE INFLUENCE OF THE ELECTRON-NEUTRAL COLLISIONS ON THE IONIZA- TION WAVES IN A HELIUM PLASMA. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-189- C7-190. �10.1051/jphyscol:1979793�. �jpa-00219499�
JOURNAL DE PHYSIQUE CoZZoque C7, suppZ6ment au n07, Tome 40, JuiZZet 1979, page C7- 189
THE lNFLUENCE OF THE ELECfRON-NEUTRAL COLLISIONS ON THE IONIZATION WAVES IN A HELIUM PLASMA
G. Popa.
Faculty of Physics ' 2 2 . 1 . Cuza" University, Iasi, Romania.
The g e n e r a l b e h a v i o r o f t h e i o n i z a t i o n waves i n a d.c. p o z i t i v s column hsve been w e l l e x p l a i n e d i n a hydrodynamic mode l/l/.
It shows a good sgreement w i t h e x p e r i m e n t s a t h i g h d i s c h a r g e c u r r e n t s / 2 / .
A more e l a b o r a t e d t h e o r y is a k i n e t i c one /3/.It t a k e 8 a c c o u n t of t h e ~ p a t i a l e l e c - t r o n g a s r e s o n a n c e s which e x i s t a t low p r e s s u r e s and c u r r e n t s where e l e c t r o n - n e u - t r a l c o l l i s i o n s c o u l d Dot always e s t a b l i e h a l o c a l Maxwellian d i s t r i b u t ion.The model e x p l a i n s remarkable a s p e c t s o f t h e i o n i z a - t i o n waves a s h i g h r a t i o o f t h e g r o u p t o phsse v e l o c i t y and MovaP'e law nnd more wave v a r i e t i e s /4/.Thus t h e wave p o t e n t i a l
i.e. t h e p r o d u c t o f t h e l o n g i t u d i n a l e l e c - t r i c f i e l d B t i m e s t h e optimum w a v e l e n g t h
is g i v e n by :
Ua/BA = g(1-#) f o r q = 1 o r 2 ( 1 ) U, f a t h e f i r s t e x c i t a t i o n p o t e n t i a l and
d is t h e r a t i o of t h e e l a a t i c o v e r i n e l a s - t i e , e l e c t r o n e n e r g y l o a a e s i n u n d i s t u r b e d column.
The aim o f t h i s paper is t o p r e s e n t aome e x p e r i m e n t a l e v i d e n c e on r e s o n a n t b e h a v i o r o f t h e i o n i z a t i o n waves end i n f l u e n c e o f t h e e l e c t r o n - n e u t r a l c o l l i s i o n s on t h i s r e s o n a n c e f o r t h e h e l i u m plasma, I n a h e l i u m p o z i t i v e column t h e small coef i c i e n t
# i a u s i n g /5/ g i v e n by: cc = c o n s t . (p/B \ r ~ ) ~ , where: p is t h e g a s p r e s s u r e , B e l e c t r i c f i e l d i n t e n s i t y and U e l e c t r o n energy. T h e r e f o r e t h e e q u a t i o n (1) becomes f o r q = 1 :
1
- t u , / s ~ ) = c o n s t . ( p / ~ fi12
( 2
which c o u l d be e x p e r i m e n t a l y checked. For
t h i s purpose t h e e l e c t r i c f i e l d B , e l e c -
t r o n t e m p e r a t u r e U and optimum w a v e l e n g t h
axe measured f o r d i f f e r e n t h e l i u m p r e s s u r e
p ( f o r h e l i u m Ua = 19.82 V).
Bxper iment a 1 d e v i c e and method a r e pre s e n - t e d i n a p r e v i o u s paper /6/.The b o t h i n - t e r f e r o m e t e r end s p a c e time diagram me thode have been ueed.The l o n g i t u d f n a l e l e c t r i c f i e l d B and plasma p a r a m e t e r s were meaaured w i t h c y l i n d r i c a l probee
p l a c e d n e s r anode .The d i s c h a r g e was produ -
c e d i n a pyrex t u b e ( 6 0 cm l o n g and R =
= 0.5 cm i n n e r r a d i u e ) f i l l e d w i t h epeo- t r a l l y pure helium, An e x t e r n a l a i n u s o i d a l s i g n a l which modulates t h e d i s c h a r g e c u r r e n t ( l e e s t h a n 10%) waa used f o r i o n i - z a t i o n wave excit8tiOt1,The waves a r e de- t e c t e d by a p h o t o m u l t i p l i e r c o u n l e d w i t h an f n t e r f e r o m e t ; e r s g a t e m and r e g i s t e r e d on XY r e c o r d e r . S u c h i n t e r f e r o m e t e r t r a c e e a l l o w e d t o measure t h e b o t h w a v e l e n g t h e and apace a m p l i t u d e v a r i a t i o n r a t e e K i
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979793
f o r d i f f e r e n t f r e q u e n c i e s , In f i g . 1 t h e i o n i z a t i o n wsveq c o r r e s ~ o n d i n q t o t h e b o t h d i s p e r s i o n and s p a t i a l a m n l i f i c a t f o n
r a t e cULXv68 3re p r e s e n t e d for a c o n s t a n t d i s c h a r g e c u r r a n t and v a r i o u s g a s p r e s s u - res.The c a r v e s show t h e backward f a s t
wsves o f whfch s p a c e amp1 i f i c a t i o n c o e f f i- o i e n t f e l s x g e r t h e n u n i t y o n l y f o r r e d u - c e d p r e s s u r s above a b o u t 1 Torr.cm.
F i g . 1
Tn s u c k c o n d i t i o n s t h a s p a c e time diagram shows a wsve p r o p a g a t i o n whfch c o r r e s p o n d e o n l y t o t h e ontimum w a v e l e n g t h and i t is a l s o i n F i c a t e d b y maximum o f t h e spsze a m p l i f i c a t i o n r a t e c u r v e ( c i r c l e nof n t on b o t t o m P i s p e r s i o n c u r v e f i g . l ) . T h e maxl- mum o f t h e s p a c e a m p l f f i c a t i o n c u r v e s show i n f ia.1 t h e o ~ t i m u m w a v e l e n g t h 9 f o r va- r 3 o u e pressures and f o r t h e c o n s t s n t r e - duce d d i s c h a r n e c u r r a n t ( 6 0 ma/cm) .The l o n -
~ i t u d i n a l e l e c t r i c f i e l d s n d t h e e l e c t r o n t e m p e r s t u r e have been messured f o r un
-
d i s t u r b e d column.The i r v a l u e s a r e p r e s e n - t e d i n t a b l e I t o g e t h e r w i t h t h e b o t h p h a s e ( v f ) and g r o u p v e l o c i t i e s ( v g ) of t b
o p t ?mum w3ve 1 - e n s t h ~
T a b l e 1
p(T?/m2) R(V/m) U(V) vg(m/s) vi(m/a) vv/vF
-.
85 940 6.0 76.900 1.800 8.88
133 1.000 5.6 12.6C0 1 , 2 7 0 S.92
213 1.040 5.3 3.320 ROO 3.87
399 1.050 4 . 3 540 5 3 0 1.C2
Tn f i g . 2 t h e deper,Aence o f loq(l-1Ja/EA)
a t r a f g h t ! l n e f o r e x p a r i v e n t s l p o i n t a up t o
"ig.2
The above c x p c : ~ i m n t s l r e a u l t s ahown t h a t a t low p r e s s u r e u p t o 1 Torr.cm a r e s o n a n t b e h a v i o r o f t h e i o n i z a t i o n wavse e x i s t s . With j n c r e a s i n g p r e s s u r e f o r c o n s t m t
c u r r e c t t h e e l e c t r o n - n e u t r a l c o l l i e i o n number i n c r e o s o s , t h e p h a s e and g r o u p v e l o - c i t i e s became comparable and d i s p e r s i o n c u r v e s become f l a t . It shows a t r a n s i t i o n t o hydrodynsmia b e h a v i o r and e q . ( 2 ) i a no more v a l i d .
R e f e r e n c e s
l . L . P e k 6 r e k , ~ . F . F . %,(1968)363
2.R.Wojaczek,Be i t r .P1asn,aphye. ,11(1971)335 3.K.Rohlena 6 t . s l . P h y s . l e t t e r s =(1972)e39
4.L.PekBrek e t a l . h . o ~ . 7 ~ ~ ~ ~ 1 ~ , ~ o v i n j 1974 5.V.Perina e t al.Czech. J.Phya.R25(1975)660
6. G.Popa, Rev.Roum.Phya. ( i n nr i n t )'
