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Submitted on 1 Jan 1979
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INVESTIGATION OF PLASMA FORMATION DYNAMICS AND PROPERTIES OF
UNDEREXPANDED SUPERSONIC EROSIVE LASER PLASMA FLARES
G. Bakanovich, L. Min’Ko, A. Chumakov
To cite this version:
G. Bakanovich, L. Min’Ko, A. Chumakov. INVESTIGATION OF PLASMA FORMA- TION DYNAMICS AND PROPERTIES OF UNDEREXPANDED SUPERSONIC EROSIVE LASER PLASMA FLARES. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-761-C7-762.
�10.1051/jphyscol:19797368�. �jpa-00219364�
JOUBiWlL DE PHYSIQUE CozZoque C7, suppZ6ment au n07, Tome 40, JuiZZet 1979, Page C7- 761
INVESTWATION OF PLASMA FORMATION DYNAMtCS AN6 PROPERTIES OF UNDEREXPANDED SUPERSONIC EROSlVE LASER PLASMA FLARES
G.I. ~akanovidh, L.Ya. Min'ko and A.N. Chumakov.
I n s t i t u t e o f Physics, ByeZorussian Academy o f Sciences, Knsk, U.S.S.R.
In view of a wide l a s e r a p p l i c a t i o n f o r obtaining movirlg plasmas by the e f f e c t of a free-running mode l a s e r r a d i a t i o n of moderate i n t e n s i t i e s on d i f f e r e n t absorp- t i v e materials it i s of g r e a t importance t o examine plasma formation dynamics and heating the underexpanded supersonic ero- s i v e l a s e r plasma f l a r e s under formation.
Spectroscopic s t u d i e s of the e f f e c t o f free-running mode l a s e r r a d i a t i o n might give us a required information only in case me use the methods providing f o r both space- and r a t h e r high time resolu- t i o n . In t h e present paper t h i s i s a t t a i - nable by high-speed frame-wise spectrum recording which enables t h e diagnostics of plasma c l u s t e r s corresponding t o sepa- r a t e l a s e r spikes t o be performed.
In t h e experiments a Nd g l a s s l a s e r with pulse energy a ~ o u t 500j a t a half-ampli- tude l e v e l duration of 500 p s e c was used.
Laser r a d i a t i o n was focused i n t o the s p o t of ~ 0 . 2 c m diameter on t h e t a r g e t surface.
The d e n s i t y of l a s e r r a d i a t i o n flux was about 2.10~\Vt/cm 2
,
and was s u f f i c i e n t l y enough f o r i n t e n s i v e evaporation and pla- sma formation under e f f e c t of every indi- vidual spike. Aluminium, brass and l e a d were used as plasma formation materials.a t o m and ions of t a r g e t m a t e r i d elements may be used f o r measurement of the main plasma parameters. For t h i s purpose t h e c a l c u l a t i o n of plasma composition and t h e emission and absorption c o e f f i c i e n t s over a wide s p e c t r a l range have been made fox
a wide range of e l e c t r o n d e n s i t i g s a t dif- f e r e n t temperatures with t h e Minsk-32 computer. The continuum c o e f f i c i e n t s have been c a l c u l a t e d from experimentally measured space-time resolved s p e c t r a l brightnesses of plasma. The e l e c t r o n den- s i t i e s have been found both by ' t h e calcu- l a t e d graphs
LA= f
( Ne ) f o r d i f f e r e n t temperatures and by t h e s p e c t r a l l i n e broadening due t o the quadratic S t a r k e f f e c t .The study of frames of spectrograms an'd s t r e a k s showed t h a t f o r every plasma clu- s t e r corresponding t o a s e p a r a t e l a s e r spike the glow brightness amounts t o maxb mum a t some distance from t h e t a r g e t sur- f a c e ( f o r Al 0.4-0.6mm, b r a s s 0.5-0.8mm, Pb 0-2-0,3mm). The glow i n t e n s i t y d i s t r i - bution along t h e plasma c l u s t e r a t d i f f e - r e n t periods of time i s represented i n Fig.1 ("1" i s t h s glow o r i g i n , " Z " O . I ~ s e c ,
1 9 11
3 0.3psec and 1141t o.,$sec; the case o f t h e b r a s s ) .
I n t e n s i v e continuum and s p e c t r a l l i n e s of TO i d e n t i f y t h e "dark space" ( t h e weak
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797368
glow zone, d.etected a t t h e t a r g e t surface) one had t o diagnose t h e nearsurface plas- ma formation f o r a l u m h i c t a r g e t . Only 6-8 frames of spectrograms corresponding t o s e p a r a t e fragments of the f l a r e deve- lopment no l a t e r than
40 Jusec a f t e r t h e on- s e t of l a s e r e f f e c t were processed quan- t i t a t i v e l y a s a f t e r -
wards t h e spectrum Pig.?
n a t u r e changed sharply. Separate frames served f o r determination of s p e c t r a l radi- ances along t h e plasma formation f o r 3 wavelength regions of t h e continuum f a r from i n t e n s e spectrum l i n e s . The "bright- ness temperature" c a l c u l a t e d i s 5,103 OK
i n t h e dark s p i c e region and increases t o t h e height of 0.,6mm. A t t h e distance of more than I m m from the t a r g e t surface tem- perature was measured by r e l a t i v e i n t e n s i - t i e s of s p e c t r a l l i n e s A 1 111 448nm and 451nm; t h e temperature values a r e i n t h e range 16000-23000 OX. The analysis of ra- d i a l p r o f i l e s o f continuum and s p e c t r a l l i n e s i n t e n s i t i e s i n d i f f e r e n t cross-sec- t i o n s of the f l a r e in-
d i c a t e s an existence of pronounced tempe- r a t u r e g r a d i e n t s along t h e plasma f o s mation radius.
Fig.2
Rue t o t h e usage of t h e technique descri- bed we succeeded i n obtaining the e l e c t r o n d i s t r i b u t i o n along the plasma formation.
Pig.2 gives t h e t y p i c a l e l e c t r o n d e n s i t y d i s t r i b u t i o n along t h e plasma c l u s t e r COD-
responding t o a s e p a r a t e l a s e r spike.
The s t u d i e s show t h a t the maximum values of the e l e c t r o n density (2.10~~crn'~) and t h e temperature in an erosive plasma clus- t e r a r e a v a i l a b l e a t some d i s t a n c e from t h e t a r g e t surface. This c l u s t e r area is separated from the t a r g e t surface by a narrow weakly glowing zone w i t h T N ~ . I O ~ OK
and N e ~ 1 0 17cm-3. The d e t e c t e d dark space presents t h e zone of formation of erosive vapor cloud which absorbs l a s e r r a d i a t i o n f o r every pulse duration. On attainment o f c r i t i c a l flow d e n s i t y i t r e s u l t s i n a p l a s ma formation a t some d i s t a n c e above t h e t a r g e t surface. A s the time required t o obtain t h e c r i t i c a l flow d e n s i t y i s deter- mFned by t h e steepness of t h e pulse f r o n t then t h e dark space value depends on t h e pulse shape.
The dark space value I* can be estimated by t h e distance covered by t h e vapor cloud f o r the time from t h e onset of evaporation t o plasma formation ( a t c r i t i c a l flow den- s i t y ) . The estimation' y i e l d s t h e value of l80.3m which s a t i s f a c t o r y agrees with
t h e experimental one. The dark space value depends a l s o on the ambient pressure and cannot exceed the value o f t h e unidimen- s i o n a l expansion region where t h e absorp- t i o n of t h e incident r a d i a t i o n i s t h e most e f f i c i e n t one. Hence, w i t h increase of t h e i r r a a i a t i o n s p o t and w i t h t h e usage of t h e l a s e r pulse of t h e l a r g e f r o n t duration t h e plasma formation w i l l occur a t a con- s i d e r a b l e d i s t a n c e from t h e evaporating
* s u r f ace,
