INTERACTION OF A TEA-CO2 LASER PULSE WITH A DENSE HYDROGEN PLASMA

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Submitted on 1 Jan 1979

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INTERACTION OF A TEA-CO2 LASER PULSE WITH A DENSE HYDROGEN PLASMA

C. Richard Neufeld

To cite this version:

C. Richard Neufeld. INTERACTION OF A TEA-CO2 LASER PULSE WITH A DENSE HYDROGEN PLASMA. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-727-C7-728.

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JOURIVAL DE PHYSIQUE CoZZoque C7, suppZdm@nt au n07, Tome 40, JuiZZet 2979, page C7- 727

INTERACTION OF A TEA-C02 LASER WLSE W I T H A DENSE HYDROGEN PLASMA

C. Richard Neufeld.

Direction ,Sciences de base, I n s t i t u t de Recherche de Z 'Hydro-Qugbec, Vmennes, &e'bec, Canada JOL 2PO.

The p h y s i c a l mechanisms i n f l u e n c i n g t h e cou- ⁷ ., ^-

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p l i n g of energy from i n t e n s e l a s e r p u l s e s t o dense plasmas a r e of c o n s i d e r a b l e i n t e r e s t f o r t h e prob- lem of l a s e r - i n i t i a t e d c o n t r o l l e d thermonuclear fu- s i o n . S t u d i e s of t h e s e phenomena i n f u l l - s c a l e ex- periments i n v o l v e high-speed d i a g n o s t i c techniques a p p l i e d t o r a p i d l y changing plasma c o n d i t i o n s . In- t e r e s t h a s consequently been s h o w r ~ l - ~ i n experi- ments of a modest s c a l e w i t h somewhat r e l a x e d phys-

i c a l c o n s t r a i n t s i n o r d e r t o s i m u l a t e c e r t a i n as- p e c t s of t h e energy coupling problem. Here we des- c r i b e such a s i m u l a t i o n s t u d y , i n which a dense plasma t a r g e t , c r e a t e d by a f a s t 2-pinch d i s c h a r g e 5 ,

i s i r r a d i a t e d by a pulsed C02 l a s e r . 1. The Plasma TBrget

The pinch d i s c h a r g e i s produced between cop- p e r e l e c t r o d e s spaced 25 cm a p a r t i n a Pyrex vacuum v e s s e l of 22.2 cm i n t e r n a l diameter. The c u r r e n t i s s u p p l i e d by a low-inductance energy s t o r a g e bank charged t o 23 kV, t h e o s c i l l a t i n g d i s c h a r g e c u r r e n t reaching a maximum amplitude of approximately 500 kA. No p r e i o n i z a t i o n was used. The pinch was op- e r a t e d i n hydrogen a t a n i n i t i a l p r e s s u r e of 3 Torr, maintained by continuously f l u s h i n g g a s from a pres- s u r i z e d c y l i n d e r through t h e d i s c h a r g e v e s s e l w i t h t h e a i d of a mechanical vacuum pump. High-speed photographs i n d i c a t e a r e p r o d u c i b l e f i r s t plasma compression (column diameter approximately 25 mm) l a s t i n g a b o u t 0.5 u s . Time-resolved s p e c t r o s c o p i c measurements of t h e d i s c h a r g e were made, a s shown on Fig. 1.

Ihschww spectrum

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Fig. 1 Plasma emission spectrum. The d o t t e d l i n e i n d i c a t e s t h e assumed continuum i n t e n s i t y . The 5 r a d i a t i o n i s c r u d e l y approximated by t h e t r i a n g u l a r r e g i o n superposed on t h e con- tinuum.

The spectrum i s dominated by % r a d i a t i o n ( i n d i c a t e d c r u d e l y by t h e t r i a n g u l a r r e g i o n on t h e f i g u r e ) , t h e l i n e b e i n g s t r o n g l y Stark-broadened.

The width of t h i s l i n e i n d i c a t e s a n e l e c t r o n densi- t y of ( 1 x 10'' A 201) ~ m - ~ , s u i t a b l e f o r C02 l a s e r - s o l i d t a r g e t s i m u l a t i o n experiments.

The observed spectrum i s n o t c l a s s i c a l , and i t s a n a l y s i s p r e s e n t s some d i f f i c u l t i e s . However, a s e l f - c o n s i s t e n t i n t e r p r e t a t i o n of t h e measure- ments, i n c l u d i n g t h e s t r o n g a b s o r p t i o n of t h e Ha l i n e (Ao = 6563 A) and t h e absence of r e c o g n i z a b l e H and Ifi r a d i a t i o n , can b e made6 by r e c o g n i z i n g

Y

t h a t s u b s t a n t i a l d e p a r t u r e s from l o c a l thermodynamic e q u i l i b r i u m (LTE) can e x i s t i n a plasma undergoing a r a p i d l y - r i s i n g d e g r e e of i o n i z a t i o n 6 ' '. This l a c k of LTE i s n o t c r u c i a l t o t h e e l e c t r o n d e n s i t y d e t e r -

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797351

mination, but clearly precludes spectroscopic tem- perature measurements of the pinch plasma. On the basis of previous work by other authors8" a kinetic temperature of approximately 10 eV was assumed, a value consistent with elementary pressure balance considerations.

2. The Laser-Plasma Interaction

The plasma target was irradiated transversely by a TEA-CO laser pulse, focussed to give a laser

2

power density (measured in vacuum) of approximately 2 x 1011 W Visible perturbations of the plas- ma emission can be observed using high-speed streak photography 10 . In addition, quantitative measure- ments were made of the laser radiation backscatter- ed from the plasma column.

A spectrum of the backscattered radiation is shown on Fig. 2. Both a broadening and a shift to longer wavelengths are evident, with essentially no signal present at the incident laser wavelength

(AX = 0). In addition, the temporal behaviour of the backscattered radiation can be very complex, and often bears no apparent relation to the incident laser pulse. The results indicate that stimulated Brillouin scattering dominates the backscatter pro-

cess. This interpretation leads to a plasma kinetic temperature in excess of 200 eV during the laser- plasma interaction. At these relatively modest la- ser power densities only a few per cent of the inci- dent radiation is backscattered. For the higher la- ser power densities proposed for laser fusion, how- ever, the backscattered fraction may be considerably greater, especially if relatively long (> 1 ns) la- ser pulses are used1'. The plasma target described here can provide a means for checking such a hypoth- esis experimentally.

References

1. D.G. Steel, P.D. Rockett, D.R. Bach, and P.L.

Colestock, Rev. Sci. Instr. 49, 456 (1978).

2. A.A. Offenberger, A. Ng, and M.R. Cervenan, Can.

J. Phys. 56, 381 (1978).

3. D.R. Gray, J.D. Kilkenny, and M.S. White, Phys.

Rev. Lett. 39, 1270 (1977).

4. K.H. Finken, G. Bertschinger, S. Maurmann, and H.-J. Kunze, J. Quant. Spectrosc. Radiat. Trans- fer 20, 467 (1978).

5. J. Mercier and C.R. Neufeld, Journ. Appl. Phys.

48, 4498 (1977).

-

6. C.R. Neufeld, Journ. Appl. Phys. (Accepted for publication).

7. W. Engelhardt, W. ~ H ~ ~ e n d u r f e r , and J. Sommer, Z. Physik 246, 29 (1971).

8. L.A. Artsimovich, Controlled Thermonuclear Reac- tions (Gordon and Breach, New York, 1964)p. 129.

9. H. Zwicker and U. Schumacher, Z. Phys. 183, ⁴⁵³

- ~ 1 1 j , 1 _*I 10. C.R. Neufeld, P. NOPI, and A. Robert, Bull. Am.

Phys. SOC. 2 l , 1139 (1976).

0

-20 0 20 40 60 80 100 140 160 220 260

AA ( A ) 11. B.H. Ripin, F.C. Young, J.A. Stamper, C.M.

Fig. 2 Spectrum of backscattered CO laser radia-

2 Armstrong, R. Decoste, E.A. Mc Lean, and S.E.

tion shown in terms of the displacement, AA,

Bodner, Phys. Rev. Lett. 39, 611 (1977).

from the incident laser wavelength (A -