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COLLECTIVE SCATTERING OF CO2-LASER LIGHT BY THE HIGHLY IONIZED ARGON PLASMA OF A
HOLLOW CATHODE ARC DISCHARGE
B. Pots, J. Coumans, D. Schram
To cite this version:
B. Pots, J. Coumans, D. Schram. COLLECTIVE SCATTERING OF CO2-LASER LIGHT BY THE
HIGHLY IONIZED ARGON PLASMA OF A HOLLOW CATHODE ARC DISCHARGE. Journal de
Physique Colloques, 1979, 40 (C7), pp.C7-797-C7-798. �10.1051/jphyscol:19797384�. �jpa-00219382�
JOURNAL
DEPHYSIQUE CoZZoque
C7,suppZ6ment au n07, Tom
40,JuiZZet
1979,page C7-* 797
COLLECTIVE SCATTERING OF
C02-LASER
LIGHT BY THE HIGKYIONIZED
ARGON PLASMA OF A HOLLOW CATHODE ARCDISCHARGE
B.F.M. Pots,
J.J.H.
Coumans and D.C. Schram.Physics Department, Eindhoven University o f TechnoZogy, Eindhoven, fie Netherlands.
I n t r o d u c t i o n : C o l l e c t i v e s c a t t e r i n g of CO 2 - l a s e r r a d i a t i o n i s an i m p o r t a n t d i a g n o s t i c f o r t h e s t u d y o f t u r b u l e n c e i n plasmas. T h i s d i a g n o s t i c h a s o b t a i n e d a l o t of i n t e r e s t d u r i n g t h e p a s t few y e a r s . Most of t h e s t u d i e s s o f a r r e f e r t o : a ) d e t e c t i o n of e x t e r n a l l y e x c i t e d s i n g l e frequency waves, b ) s c a t t e r i n g i n high d e n s i t y plasmas ( e l e c t r o n d e n s i t y neb1o2l m-3), o r C )
The (k
,
w ) -window can b e v a r i e d by changing t h e s c a t t e r i n g a n g l e 8,which is p r o p o r t i o n a l t o k f o r s m a l l 8 , and by changing t h e a n g u l a r frequency wf of a v a r i a b l e bandpass f i l t e r a f t e r t h e o p t i c a l mixer. For homodyne d e t e c t i o n one g e t s w = I w l , s o
f t h a t t h e spectrum a f t e r t h e o p t i c a l mixing i s a d i r e c t measure of t h e f l u c t u a t i o n spectrum o f t h e plasma.
experiments w i t h h i g h power ( p u l s e d o r c.w.1 CO 2
-
Experimental equipment: The low power C.W. CO-
l a s e r s [ 1 , 2 , 3 , 4 ] . We p r e s e n t a s f a r a s we know f o r 2
l a s e r i s focussed i n t h e plasma t o a s p o t w i t h a t h e f i r s t time c o l l e c t i v e s c a t t e r i n g measurements
diameter of 2 mm-(see f i g u r e 11. The plasma is of spontaneously e x c i t e d t u r b u l e n c e i n a medium
c r e a t e d i n a s t a t i o n a r y , c u r r e n t d r i v e n (10-300 A ) ,
d e n s i t y plasma w i t h a low power C.W. 2 W C02- -3
low p r e s s u r e (10 t o r r ) hollow cathode a r c l a s e r .
d i s c h a r g e . The plasma i s c y l i n d r i c a l ( r a d i u s 10 mm, C o l l e c t i v e s c a t t e r i n g r e f e r s t o plasma s c a t t e r i n g l e n g t h 0-2.5 m)
,
m a g n e t i c a l l y confined (0-0.5 T) o f e l e c t r o m a g n e t i c r a d i a t i o n f o r which t h e so-and h i g h l y i o n i z e d . T y p i c a l v a l u e s o f t h e plasma c a l l e d s c a t t e r i n g parameter a = ( k ~ ~ ) - ' > l . Here k=
parameters a r e : e l e c t r o n d e n s i t y l ~ ~m-3r ~ - l ~ ~ ~
Ikl=llfs-5ii,
w h e r e 5 and k a r e t h e wavenumbers o fI e l e c t r o n temperature 3-4 eV, i o n temperature 1-2
s c a t t e r e d and i n c i d e n t r a d i a : i o n r e s p e c t i v e l y ; 'D
eV, n e u t r a l d e n s i t y 1018-1019 m-3, i o n plasma i s t h e Debye l e n g t h . I t i s p o s s i b l e t o o b t a i n
frequency 200 MHz and Debye l e n g t h 1.5 p ~ . A i n f o r m a t i o n about t h e e l e c t r o n d e n s i t y f l u c t u a t i o n s
f r a c t i o n o f t h e i n c i d e n t l a s e r beam i s used a s f o r v a r i o u s wavenumbers
5
and a n g u l a r f r e q u e n c i e sl o c a l o s c i l l a t o r (10 mW). The r a d i a t i o n s c a t t e r e d w. Here w=w -w where w and wi a r e t h e a n g u l a r
s i' with a n g l e 8 i s t h e s i g n a l beam, which i s chopped
f r e q u e n c i e s o f t h e s c a t t e r e d and i n c i d e n t r a d i a t i o n f o r phase s e n s i t i v e d e t e c t i o n . Both beams a r e r e s p e c t i v e l y . The s c a t t e r e d power P i s proport-
combined v i a a beam s p l i t t e r and focussed on a i o n a l t o neS (k -k. ,us-ui)
,
where S(k,w)
i s t h e-s -1 l i q u i d helium cooled Ge:Cu d e t e c t o r ( 1
mm
2,
c u r r e n ts p e c t r a l d e n s i t y f u n c t i o n , which r e p r e s e n t s t h e
1 mA)
.
The d e t e c t o r i s coupled v i a a source f o l l o w e r e l e c t r o n d e n s i t y f l u c t u a t i o n s [5]. Mostly o p t i c a l(600 Q-50 0) and a h i g h p a s s frequency f i l t e r homodyne o r heterodyne d e t e c t i o n i s used. (1 MHz) t o broad-band low n o i s e a m p l i f i e r s (0-500
HIGH H.F. L
.
F. PHASEPASS BANDPASS BANDPASS SENSITIVE
--..-- ..
rnDETECTOR
r:w.
WAIST: 2 w 4 m m 2 m m (PLASMA) C02 10,6p1 mm (DETECTOR) L.0. POWER 10 mW
WE-DEWAR, DETECTOR OE:CU 1 mm2
DETECTOR
\
PLASMA VESSEL
'-.-
- -
-
CO LASER 2Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797384
C7- 798
MHz, g a i n 40 dB). Home-made band p a s s f i l t e r s ( c e n t e r frequency vf 1-50 MHz, bandwidth 0.1-0.3 of v f , 24 dB/octave) a r e used f o r s p e c t r a l a n a l y s i s . We can v a r y t h e s c a t t e r i n g a n g l e 0 from l o t o 7'
(0.02<kX < 0 . 1 ) . We emphasised optimkzation of t h e D
s i g n a l t o n o i s e r a t i o and d e t e r m i n a t i o n o f t h e a b s o l u t e s e n s i t i v i t y of t h e d i a g n o s t i c a s a whole.
Both a r e c r u c i a l p o i n t s . I n o p t i c a l mixing experiments it i s t h e ambition t o keep t h e s h o t n o i s e o f t h e l o c a l o s c i l l a t o r dominant w i t h r e s p e o t t o o t h e r n o i s e s o u r c e s . We r e a l i z e d t h i s by t h e use of an impedance t r a n s f o r m e r and u l t r a low n o i s e a c t i v e components; I n t h i s way a l s o t h e l o c a l o s c i l l a t o r power i s k e p t a t a low l e v e l . The a b s o l u t e s e n s i t i v i t y was determined by performing c o h e r e n t and i n c o h e r e n t measurements with a b l a c k body and s c a t t e r i n g f r o ? h i g h l e v e l a c o u s t i c waves i n 1 atm. argon. We a r e a b l e t o d e t e c t a s c a t t e r e d s i g n a l c l o s e t o t h e quantum l i m i t .
Experimental r e s u l t s : We used two s c a t t e r i n g geometries: - k / / g and
LAB.
We found t h a t f o r a l l plasma c o n d i t i o n s considered t h e f l u c t u a t i o n l e v e l with&//g
was c l o s e t o thermal. A s i n t h i s c a s e t h e s i g n a l s a r e very s m a l l we merely measured t h e t o t a l c o n t e n t s o f t h e s p e c t r a . However, p r e l i m i n a r y masurements i n d i c a t e a maximum a t t h e frequency- e q u i v a l e n t of t h e i o n a c o u s t i c v e l o c i t y . ForklB
we found f l u c t u a t i o n l e v e l s which could reach 3 o r d e r s of magnitude above thermal. I n f i g . 2 we show f o r a s p e c i f i c plasma c o n d i t i o n S(LJl,o) f o r 8=2"for v a r i o u s r a d i a l p o s i t i o n s i n t h e plasma. In f i g . 3 we show S (&,&I) a s f u n c t i o n s o f kL and o f o r a s p e c i f i c plasma c o n d i t i o n f o r t h e c e n t e r of t h e plasma. The d i s p e r s i o n i n d i c a t e s t h e presence o f i o n a c o u s t i c - l i k e waves.
Discussion: One of t h e most e v i d e n t r e s u l t i s t h e s t r o h g a n i s o t r o p y i n &-space, which i n d i c a t e s a l s o t h e l a r g e s e l e c t i v i t y i n &-space of t h e c o l l e c t i v e s c a t t e r i n g method. For - k / / g t h e l e v e l i S c l o s e t o thermal; f o r t h e l e v e l i s h i g h l y non-thermal.
This was c o n t r a r y t o o u r e x p e c t a t i o n s f o r a plasma with t h e e l e c t r o n d r i f t below b u t c l o s e t o t h e c r i t i c a l d r i f t f o r e x c k t i o n o f l o n g i t u d i n a l i o n a c o u s t i c waves. P e r p e n d i c u l a r l y p r o p a g a t i n g d i s t - urbances a r e p r e f e r e d , moving w i t h i o n a c o u s t i c p h a s s v e l o c i t i e s , w i t h an accumulation a t long wavelengths. The freauencv Anmain we s t u d i e d s o
f a r i s f a r above t h e i o n c y c l o t r o n f r e q u e n c i e s . We n o t e , t h a t t h e measurements with low frequency
o p t i c a l probing i n d i c a t e a l s o a s t r o n g a n i s o t r o p y i n k s p a c e and mainly p e r p e n d i c u l a r l y p r o p a g a t i n g waves.
References:
11 Surko e t a l , Phys. Rev. L e t t . ,
2
(1972) 81.21 Holzhauer, Phys. L e t t . ,
62A
(1977) 495.33 P e r a t t e t a l , Phys. F l u i d s ,
20
(1977) 1900.41 Massig, Phys. L e t t . , (1978) 207
5 1 S h e f f i e l d , "Plasma s c a t t e r i n g of e l e c t r o m a g n e t i c r a d i a t i o n " , Academic P r e s s , New York, 1975.
