Z-PINCH IMPLOSION DRIVEN X-RAY LASER RESEARCH

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Z-PINCH IMPLOSION DRIVEN X-RAY LASER RESEARCH

M. Matzen, R. Dukart, B. Hammel, D. Hanson, W. Hsing, T. Hussey, E.

Mcguire, M. Palmer, R. Spielman

To cite this version:

M. Matzen, R. Dukart, B. Hammel, D. Hanson, W. Hsing, et al.. Z-PINCH IMPLOSION DRIVEN X-RAY LASER RESEARCH. Journal de Physique Colloques, 1986, 47 (C6), pp.C6-135-C6-139.

�10.1051/jphyscol:1986618�. �jpa-00225861�

2-PINCH IMPLOSION DRIVEN X-RAY LASER RESEARCH

M.K. MATZEN, R.J. DUKART, 3.A. HAMMEL, D.L. HANSON, W.W. HSING, T.W. HUSSEY, E.J. McGUIRE, M.A. PALMER and R.B. SPIELMAN

Sandia National Laboratories, Albuquerque, N M 87185, U.S.A.

A b s t r a c t - I n experiments performed d u r i n g t h e p a s t two y e a r s on Proto I1 ( a 10-TW pulsed-power a c c e l e r a t o r ) , we imploded annular plasmas onto thin-walled annular x- r a y l a s e r t a r g e t s i n o r d e r t o c r e a t e a r a d i a t i o n pump s o u r c e f o r x-ray l a s e r physics s t u d i e s . This 2-pinch must be a x i a l l y uniform and must e f f i c i e n t l y produce t h e pump r a d i a t i o n without d e s t r o y i n g t h e l a s e r medium on t h e c y l i n d r i c a l a x i s of symmetry. To c h a r a c t e r i z e t h e pump source x-rays and l a s a n t c o n d i t i o n s , we r e g u l a r l y f i e l d a l a r g e number of x-ray d i a g n o s t i c s . I n r e c e n t experiments, we produced over 15 kJ of 21-keV pump r a d i a t i o n w i t h an' imploding neon gas-puff l o a d . We a r e considering both recombination and resonance-pumped x-ray l a s e r schemes.

Discussion

I n our previous experimental .work1 ' 2 we emphasized development of an x-ray 77flashlampn -- an x-ray source t h a t can be used t o pump an x-ray l a s e r medium.

Using Proto I1 and a diode developed f o r f o i l implosion s t u d i e s , we demonstrated t h a t gas-puff implosions can be an e f f i c i e n t s o u r c e of x-rays with e n e r g i e s

21 keV,' and t h a t t h e i n t e n s i t y of t h e s e high energy x-rays does not degrade when a

-

t a r g e t i s placed on t h e c y l i n d r i c a l a x i s of symmetry." In our more r e c e n t experiments, we have developed diodes t h a t a r e optimized f o r gas-puff implosion s t u d i e s , increased t h e k i l o e l e c t r o n v o l t (keV) pump source r a d i a t i o n t o 215 kJ, and begun preliminary x-ray l a s e r t a r g e t experiments. This paper d e s c r i b e s our r e c e n t experimental progress on pulsed-power-driven x-ray l a s e r t a r g e t s .

A l l Z-pinch plasma implosions a r e known t o e x h i b i t magnetic Rayleigh-Taylor

9

i n s t a b i l i t i e s , which make i t d i f f i c u l t t o c r e a t e a homogeneous l a s a n t plasma .' ^To

minimize t h e s e problems., w e developed a concept f o r f i e l d i n g an x-ray l a s e r i n a pulsed-power e n ~ i r o n m e n t ~ ' ~ i n which a n annular s t a g n a t i o n s h e l l surrounds an x-ray l a s e r rod. This t a r g e t c o n f i g u r a t i o n i s mounted on t h e c y l i n d r i c a l a x i s of

symmetry of a 2-pinch implosion, a s shown i n Fig. 1 . The o u t e r c y l i n d r i c a l s h e l l (Fig. 4 of Ref. 4) s e r v e s both t o s t a g n a t e t h e imploding plasma, converting plasma k i n e t i c energy i n t o r a d i a t i o n , and t o hydrodynamically i s o l a t e t h e l a s e r rod from t h e imploding plasma. With t h i s geometry t h e l a s a n t can remain homogeneous f o r a time before being destroyed by t h e plasma implosion.- In p r i n c i p l e , t h e low d e n s i t y l a s e r medium could be e i t h e r a g a s - f i l l e d soda s t r a w ( f i l l e d s t a t i c a l l y o r

dynamically) o r a h i g h l y collimated gas flow (high Mach-number gas f l o w ) . I n p r a c t i c e , both schemes a r e d i f f i c u l t t o f i e l d i n a pulsed-power environment.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986618

C6- 136 JOURNAL DE PHYSIQUE

Therefore, t o form t h e l a s e r medium, t h e l a s e r t a r g e t s f o r our r e c e n t and f u t u r e experiments r e l y on t h e expansion of a t h i n l a y e r of m a t e r i a l on t h e i n s i d e of t h e s t a g n a t i o n s h e l l . A sample t a r g e t i s shown schematically i n Fig. 2. These t a r g e t s have t h e advantage of f l e x i b i l i t y i n t h e choice of l a s a n t m a t e r i a l ; t h e i r

disadvantage i s nonuniformities a s t h e l a s a n t plasma assembles on-axis. In t h i s r e s e a r c h we have emphasized a photon-pumped, Ne-like recombination l a s e r scheme. 4 I n t h i s scheme, keV r a d i a t i o n from t h e pump i s used t o photoionize Ne-like l a s a n t ions t o F-like i o n s . Subsequent recombination can l e a d t o an i n v e r s i o n of t h e 3p and 3s l e v e l s , a l d s o f t x-ray (-40 eV) l a s i n g . Our r e c e n t experiments used t h e K-shell r a d i a t i o n from Ne t o pump V and from A 1 t o pump N i and Cu. We have a l s o s t u d i e d t h e production of resonance r a d i a t i o n from He-like Na f o r a p o t e n t i a l Na/Ne l i n e coincidence l a s e r scheme.

CURREN RETURN POST

.FOAM T AAGET

Fig. 1 Schematic of gas n o z z l e / t a r g e t assembly showing t h e r e l a t i v e p o s i t i o n s of t h e t a r g e t , nozzle, t h e c u r r e n t r e t u r n p o s t s , and t h e e l e c t r i c a l feed.

X-RAY CONM#TER LAVER LOW-Z SODA STRAW OR LOW DENSrrY FOAM

Fig. 2 Schematic of a pulsed-power-driven x-ray l a s e r t a r g e t .

We used t h e Proto-I1 a c c e l e r a t o r f o r a l l of t h e s e pulsed-power-driven x-ray

l a s e r experiments. In i t s present c o n f i g u r a t i o n , t h i s multimodule water-line

machine produces a s 1.2 MV p u l s e with a 4 5 n s f u l l width a t h a l f maximum (FWHM) a t

0.12532. The t o t a l e l e c t r i c a l energy going forward was approximately 320 kJ. We

d e l i v e r e d i n excess of 6 MA t o a g a s puff load t h a t has a t o t a l inductance of

approximately 6.5 nH. The maximum k i n e t i c energy d e l i v e r e d t o t h e imploding gas

puff i s approximately 120 k J .

emphasis was t o minimize t h e a x i a l inhomogeneities of t h e plasma implosion and t o measure neon-like i o n i z a t i o n s t a t e s i n t h e l a s e r t a r g e t . We continue t o measure t h e pump source energy w i t h t h i n - f i l m bolometers, x-ray diodes (XRDs), and c r y s t a l spectrographs. We measured t h e a x i a l nonuniformity with a framing x-ray pinhole camera, and t r i e d t o minimize t h e s e nonuniformities through more d e t a i l e d nozzle design and c h a r a c t e r i z a t i o n . We measured t h e presence of Ne-like l a s a n t i o n s i n r e a l i s t i c x-ray l a s e r t a r g e t s with c r y s t a l spectrographs, and monitored t h e time-dependent pump photons with a s t r e a k e d e l l i p t i c a l spectrograph and a framing c r y s t a l spectrograph. F i n a l l y , we f i e l d e d a framing g r a z i n g incidence spectrograph t o r e c o r d s p e c t r a i n t h e r e g i o n of t h e s o f t x-ray l a s e r t r a n s i t i o n s .

The t o t a l r a d i a t i o n f l u x e s were measured w i t h c a l i b r a t e d t h i n - f i l m bolometers t h a t were f i l t e r e d t o r e c o r d t o t a l r a d i a t i o n f l u x and f l u x above 800 eV with nanosecond time r e s o l u t i o n . We observed a s much a s 15 kJ of K-shell r a d i a t i o n and a s much a s 70 kJ t o t a l r a d i a t i o n from Ne gas puff implosions. With K r o r Xe a s t h e imploding g a s , t h e t o t a l y i e l d increased but t h e y i e l d near 1 keV decreased. Thus we used Ne i n most of our t a r g e t experiments. We used a r r a y s of f i l t e r e d X R D s t o o b t a i n time-resolution with crude s p e c t r a l c u t s . The time-integrated c r y s t a l spectrographs ( s e e Figs. 1 and 7 of Ref. 4) confirmed t h e bolometer measurements of t o t a l pump energy, and implied t h a t approximately 1 / 4 of t h e Ne K-shell energy can be converted i n t o A 1 K-shell energy i n o u r converter t a r g e t s .

An important i s s u e i n t h e development of an x-ray l a s e r i s t h e s i m u l t a n e i t y of t h e x-ray pump along t h e a x i s of t h e l a s e r rod. The expansion of t h e g a s j e t a s i t i s i n j e c t e d i n t o vacuum l e a d s t o a time-dependent s t a g n a t i o n of t h e gas puff along t h e x-ray l a s e r t a r g e t . 6 With a framing x-ray pinhole camera, we measured t h i s

"zipperingn e f f e c t a s t h e x-ray pump s o u r c e emission proceeds from t h e cathode ( n o z z l e ) t o t h e anode ( r e t u r n c u r r e n t s c r e e n ) . With our p r e s e n t gas nozzles we t y p i c a l l y f i n d t h a t t h e imploding g a s s t a g n a t e s a t t h e anode more than 1 2 n s a f t e r i t s t a g n a t e s a t t h e cathode. This long s t a g n a t i o n time i s unacceptable f o r our x-ray l a s e r t a r g e t designs. Thus we developed an e l e c t r o n beam s c a t t e r i n g f a c i l i t y t o measure t h e time- and space-dependent n e u t r a l d e n s i t y produced by t h e s e

supersonic nozzles. I n Fig. 3 we show t h e Ne n e u t r a l d e n s i t y p r o f i l e s produced from a nozzle t h a t was designed t o g e n e r a t e a Mach 4 gas p r o f i l e . A t a d i s t a n c e of 22 mm from t h e nozzle e x i t plane, t h e r e i s s i g n i f i c a n t spreading of t h e g a s

compared t o t h e d e n s i t y p r o f i l e a t 2 mm from t h e e x i t plane. We have c a l c u l a t e d 6 t h e e f f e c t of t h i s spread on t h e implosion dynamics and have suggested s e v e r a l methods t o minimize i t s e f f e c t . Experiments t o minimize t h e a x i a l nonuniformities a r e c u r r e n t l y underway.

In our most r e c e n t experiments we emphasized time-resolved x-ray (4-20 A ) and s o f t x-ray ( 175-550 A) measurements of t h e r a d i a t i o n from thin-walled annular x-ray l a s e r t a r g e t s . The measurements i n t h e x-ray r e g i o n , where t h e Ne-like l a s a n t resonance l i n e s occur, a r e necessary t o v e r i f y t h a t we could i o n i z e t h e l a s a n t m a t e r i a l t o t h e Ne-like s t a t e . I n a d d i t i o n t h i s s p e c t r a l region allows us t o s t u d y t h e pump r a d i a t i o n f o r both Ne-like recombination and l i n e coincidence schemes.

The x-ray l a s e r t a r g e t s c o n s i s t e d of a parylene annulus ( 2 t o 4 pm) coated on t h e o u t s i d e with 2000 A of A 1 and on t h e i n s i d e with from 250 t o 1000 A of Cu o r N i . A time-integrated c r y s t a l spectrum of an Al/CH/Cu(1000 A) t a r g e t taken a t 60° t o t h e c y l i n d r i c a l a x i s of symmetry i s shown i n Fig. 4. I n a d d i t i o n t o t h e K-shell resonance l i n e s from Ne, t h e resonance l i n e s from Ne-like Cu a r e a l s o present. I n an attempt t o measure t h e time-dependent i o n i z a t i o n balance of t h e l a s a n t r e l a t i v e t o t h e pump source photons, we a l s o f i e l d e d a s t r e a k e d e l l i p t i c a l spectrograph and a framing c r y s t a l spectrograph a t 60° t o t h e c y l i n d r i c a l a x i s of symmetry. Thus f a r we have not obtained time-resolved information on t h e resonance l i n e s from t h e Ne-like l a s a n t due t o t h e i r low i n t e n s i t y r e l a t i v e t o t h e pump x-rays. In f u t u r e experiments we w i l l i n c r e a s e t h e s e n s i t i v i t y of t h e time-resolved c r y s t a l

instruments, and w i l l f i e l d them t o view t h e t a r g e t a x i a l l y . I n Fig. 5 we show a

time-integrated c r y s t a l spectrograph from a Ne implosion o n t o a parylene annulus

coated w i t h NaF. The r e l a t i v e s t r e n g t h of t h e He-like Na resonance l i n e s u g g e s t s

t h a t t h i s t a r g e t c o n f i g u r a t i o n might be used f o r a Na/Ne l i n e coincidence scheme.

C6-138 JOURNAL DE PHYSIQUE

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

Nozzle Radius (cm)

Fig. 3 Measured n e u t r a l N e d e n s i t y at 2 and 22 mm from t h e e x i t p l a n e of t h e s u p e r s o n i c n o z z l e .

0.0

9.0 10.0 11.0 12.0 13.0 14.0

WAVELENGTH

Fig. 4 Time-integrated KAP c y r s t a l spectrum of Ne s t a g n a t i o n of a Al/CH/Cu(1000 A )

a n n u l a r t a r g e t .

Fig. 5 Time-integrated KAP c r y s t a l spectrum of Ne s t a g n a t i o n o n a NaF coated parylene annulus.

The s o f t x-ray r e g i o n , where t h e p o t e n t i a l l a s i n g t r a n s i t i o n s occur, h a s been s t u d i e d using a framing g r a z i n g incidence spectrograph f i e l d e d on t h e t a r g e t a x i s of symmetry. The observed s p e c t r a , which a r e r i c h i n l i n e s , a r e dominated by r a d i a t i o n from t h e imploding plasma and o t h e r elements i n t h e l a s e r t a r g e t . Using a more c a r e f u l l y c o n t r o l l e d l i n e - o f - s i g h t , we hope t o d i s c r i m i n a t e a g a i n s t t h i s r a d i a t i o n i n our f u t u r e experiments.

I n summary, i n our r e s e a r c h t_o produce a s o f t x-ray l a s e r we have developed an i n t e n s e pulsed-power-driven x-ray source and t h e time-resolved d i a g n o s t i c s

necessary t o study t h e x-ray l a s e r physics. Our p r i o r i t i e s f o r f-uture experiments a r e t o minimize t h e a x i a l nonuniformities, c h a r a c t e r i z e t h e time-dependent

i o n i z a t i o n balance of t h e l a s a n t r e l a t i v e t o t h e pump s o u r c e , benchmark t h e design codes with t h e experiments, and observe Ne-like n=3, An=O l i n e r a d i a t i o n from t h e l a s a n t . F i n a l l y , we plan t o s t u d y a l t e r n a t e l a s e r concepts.

References

1 ) R. B. Spielman, D. L. Hanson, M. A. Palmer, M. K. Matzen, T. W. Hussey, and J.

M. Peek, J. Appl. Phys. 57, 830 (1985); and r e f e r e n c e s t h e r e i n .

2) R. B. Spielman, M. K . Matzen, M. A. Palmer, P. B. Rand, T. W . Hussey, and D. H.

McDaniel, Appl. Phys. L e t t . 9, 229 (1985); and r e f e r e n c e s t h e r e i n .

3) R . Dukart, G. Dahlbacka, R. Stewart, and R. F o r t n e r , "Imploding Plasma X-Ray Lasert1, PITR-1549 (San Leandro, CA:Physics I n t e r n a t i o n a l , November 1982)

4 ) E. J. McGuire, K. Matzen, R. Spielman, M. A. Palmer, B. A. Hammel, D. L. Hanson, T. W. Hussey, W. W - Hsing, and R. J. Dukart, "The Sandia X-Ray Laser Program", i n t h i s i s s u e of t h e Journal de Physique.

5) S. Maxon, P. Hagelstein, K . Reed, and J. S c h o f i e l d , J. Appl. Phys. 57, 971 (1 985).

6) T. W. Hussey, M. K. Matzen, and N. F. Roderick, J. Appl. Phys. 2, 2677 (1986).