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LASER - DOPPLER VELOCIMETRY IN IONIZED GASES : A REVIEW PAPER
G. Gouesbet
To cite this version:
G. Gouesbet. LASER - DOPPLER VELOCIMETRY IN IONIZED GASES : A REVIEW PAPER.
Journal de Physique Colloques, 1979, 40 (C7), pp.C7-789-C7-790. �10.1051/jphyscol:19797380�. �jpa-
00219378�
J O U R N A ~ DE PHYSIQUE CoZZoque C7, suppZ6ment au n07, Tome 40, JuiZZet 2979, pape C7- 789
LASER
-
DOPPLER VELOCIMETRY IN I O N I W GASES : A REVIEW PAPER.G.
Gouesbet.
Laboratoire de I"nermodynamique, associg au C.N.R.S., N0230, Facult6 des Sciences e t des Techniques de Rouen, 76130, Mont Saint Aignan, France.
AbstPact
A
review of plasmas v e l o c i t i e s measurements by means of Laser-Doppler velocimetry i s presented.
Specific d i f f i c u l t i e s a r e pointed o u t . A1 though such experiments a r e u n t i l now very scarce, there i s no doubt t h a t the future will see a tremendous increase of LDV-appl i c a t i o n s t o plasmas owing t o i t s , g r e a t p o t e n t i a l .
The reader i s assumed t o be we1 1 aware of the LDV general theory and appiications. Basic books a r e available (1, 2,
3 ) .LDV-measurements have been successively made i n l i q u i d s
(4), cold gases, then combustion devices.Plasmas investigations s t a r t e d more l a t e l y , owing t o the expected a i f f i c u l t i e s
: high temperaturesgiving r i s e t o noise problems due t o plasma and seeding p a r t i c l e s radiations
;electromagnetic sa- t u r a t i o n of the e l e c t r o n i c devices ( r . f . torches) or e l e c t r i c a l interference from the large current supplied t o arcs
;f a s t evaporation of the s c a t t e r centers, coupled with d i f f i c u l t i e s t o be s u r e t h a t they a r e small enough t o follow the f l u i d when they reach the optical probe
;thermophoresis phenome- non producing a very slow r a t e of data acquisition when some c r i t i c a l zones are studied
;s e n s i t i v i t y of the plasma s t a t e t o the presence of possible a r t i f i c i a l seeding p a r t i c l e s , e t c . Nevertheless, workers attempted t o solve these problems i n order f o r instance t o provide a b e t t e r understanding of the heat and mass t r a n s f e r processes between a plasma flow and suspended p a r t i c l e s , with emphasis on plasma chemistry.
S.A.SELF s t a r t e d experiments i n a
MHDboun- dary layer (5, 6 , 7) w i t h additional problems due t o r e s t r i c t e d optical access, high v e l o c i t i e s
(>
500 m/s) , high s p a t i a l resolution requirements, r e f r a c t i v e index inhomogenei t y , s c a t t e r from the wall . On the other hand, temperatures were r a t h e r
low
(* 2700 K ) , and the f l u i d was more a flame than s t r i c t l y speaking a plasma. Furthermore the detection used an optical Fabry-Perot system, so
without heterodyning. I t was then spectroscopic mea- surements r a t h e r than c l a s s i c a l LDV-ones.
Al a s e r l i -
0
ne f i l t e r i s used (10
Abandwidth) t o remove thermal radiation and ambient l i g h t . Experiments have been then carried out with a differential-forward s c a t - t e r method f o r sidewall measurements
(8, 9 ) .Pro- cessing was achieved through a high-speed counter linked t o an P-2100 computer. Mean v e l o c i t i e s , turbulence i n t e n s i t i e s and spectrum of velocity fluctuations have been measured.
M.R.BARRAULT e t a1 reported about
LDV-experiments i n a t r a n s i e n t (12 ms) a r c c i k u i t brea- k e r ( l 0 ) . Specific problems arose plus the extra pro- blems of t r a n s i e n t nature of the a r c , high veloci- t i e s (up t o
3000 ms-l ) and d r a s t i c seeding pro- blems. The real f r i n g e mode optical set-up was used and forward s c a t t e r i n g from an ~ r ' , 1.5
W ,l a s e r . The p a r a s i t i c radiation was removed w i t h a
10A interference f i 1 te r and associated p o l a r i s e r , plus
0
a Fabry-Perot f i l t e r of 0.5
Abandwidth. Signal processing was achieved through an osci 11 ographic r a s t e r display. Then improvements were made J n or- der t o cover the e n t i r e arcing period (11). B u t problems of i n t e r p r e t a t i o n remained due t o the un- known s i z e of s c a t t e r centers which did not always follow the f l u i d , An approximate c o r r e l a t i o n was used between signal frequencies ( p a r t i c l e veloci- t i e s ) and signal amplitudes ( p a r t i c l e s i z e s ) i n order t o deduce the plasma velocity from the p a r t i - c l e one. Such a correlation is not very r e l i a b l e
:as a matter of f a c t , big p a r t i c l e s passing through the edges of the control volume give r i s e t o the same signal amplitudes than small ones passing through i t s center. f f e c t i v e l y , the authors consis der t h a t the accuracy i s not b e t t e r than about 40
%.Then, IRIE and BARRAULT carried out measurements using the Doppler s h i f t of a Q-switched ruby l a s e r radiation without foreign elements. Again, there was no heterodyne detection, so t h a t i t was spec- troscopy measurements r a t h e r than
LDVones (12).
GOUESBET reported LDV-mean v e l o c i t i e s mea- surements i n a 4 MHz, r f torch, by means of a 5 mW
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797380
He-Ne forward r e a l f r i n g e system w i t h o s c i l l o g r a - p h i c d i s p l a y o f t h e s i g n a l s (13, 14). But the exact f l u i d v e l o c i t y was n o t measured due t o t h e t o o l a r - ge s i z e s o f the used p a r t i c l e s , although t h e d i s a - greement was f u r t h e r found t o be l e s s than t y p i c a l - l y 20 %. F u r t h e r experiments were c a r r i e d o u t w i t h a more powerful l a s e r (Krypton, 800 mW on the 647,l nm l i n e ) and an automatic data a c q u i s i t i o n and pro- cessing system using a s i n g l e counter (15, 16, 17, 18)
.
The p a r a s i t i c e m i t t e d 1 i g h t was removed by0
means o f a monochromator ( 2 A bandwidth). Seeding was achieved by means o f a c o u n t e r - c u r r e n t system s u p p l y i n g t h e plasma i n .L 5 urn, A1 203-particles, a t a r a t e s u f f i c i e n t l y small f o r n o t d i s t u r b i n g t h e f l o w . The beam was expanded b e f o r e f o c u s i n g t o im- prove t h e S/N r a t i o and the s p a t i a l r e s o l u t i o n , The s p a t i a l r e s o l u t i o n was a g a i n increased by c o l 1 ec- t i n g the s c a t t e r e d l i g h t o f f - a x i s . The e f f e c t o f t h e p a r a s i t i c rf r a d i a t i o n was decreased by means o f a Faraday cage and f i l t e r i n g t h e s i g n a l through r e j e c t o r s . Mean v e l o c i t i e s and ' f l u c t u a t i o n s ' have been s u c c e s s f u l l y c a r r i e d o u t . Furthermore, t h e atom temperatures were measured by c o u p l i n g LDV- and dynamic pressures measurements.
Mme THI HIEN HO attempted measurements i n a 4 MHz rf t o r c h w i t h a 5 mW He-Ne l a s e r , using a r e a l f r i n g e mode set-up (19). Processing was achie- ved through a frequency t r a c k e r and a frequency analyser. Measurements were n o t r e a l l y successful.
I t i s suggested t h a t a frequency t r a c k e r c o u l d be d i f f i c u l t t o work p r o p e r l y owing t o t h e small r a t e of p a r t i c l e s a r r i v a l . The frequency analyser proba- b l y picked up the r a d i o frequency o f t h e plasma generator. Furthermore, GOUESBET's experiments showed t h a t a
5
mW power i s t o o c r i t i c a l f o r 'com- f o r t a b l e ' mkasurements i n such a s i t u a t i o n . The photon c o r r e l a t i o n technique c o u l d be p o s s i b l y suc- c e s s f u l l y used.LDV-measurements a r e r e p o r t e d from t h e Im- p e r i a l College (London) i n a DC t r a n s f e r r e d a r c heater a t t h e National p h y s i c a l Laboratory (20,21).
Real f r i n g e systems were t e s t e d i n forward and backward s c a t t e r i n g s . A spectrum analyser, a f r e - quency t r a c k e r and a frequency counter have been t e s t e d . The seeding p a r t i c l e s were Ni,ckel ( ~ 5 0 p m ) and alumina ones (% 1 wm). V e l o c i t y histograms were recorded. Alumina p a r t i c l e s e x h i b i t much
h i g h e r v e l o c i t i e s than n i c k e l ones, as i t c o u l d be expected from drag arguments.
The author i s aware o f experinents c a r r i e d o u t i n Limoges b u t no r e p o r t has been r e c e i v e d a t time t o discuss about them. L e t us a l s o mention T i l l e r ' s paper (22). F i n a l l y , p r e l i m i n a r y plasma experiments have been made a t t h e ONERA i n 1 975. A plasma work i s a c t u a l l y now planned ( B o u t i e r , ONERA, C h a t i l l o n / Bbgneux)
.
CONCLUSSON
The previous r e p o r t e d experiments show t h a t LDV can be s u c c e s s f u l l y used f o r plasma d i a g n o s t i c s . The d i f f i c u l t i e s remain important, b u t t h e r e i s now no doubt t h a t such techniques w i l l become a very popular t o o l f o r such experiments owing t o t h e t r e - mendous v e r s a t i l i t y and p o t e n t i a l o f LDV-systems.
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