THE SANDIA X-RAY LASER PROGRAM

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Submitted on 1 Jan 1986

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THE SANDIA X-RAY LASER PROGRAM

E. Mcguire, K. Matzen, R. Spielman, M. Palmer, B. Hammel, D. Hansen, T.

Hussey, W. Hsing, R. Dukart

To cite this version:

E. Mcguire, K. Matzen, R. Spielman, M. Palmer, B. Hammel, et al.. THE SANDIA X- RAY LASER PROGRAM. Journal de Physique Colloques, 1986, 47 (C6), pp.C6-81-C6-88.

�10.1051/jphyscol:1986611�. �jpa-00225854�

THE SANDIA X-RAY LASER PROGRAM

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

Sandia National Laboratories, Albuquerque, NM 87185, U.S.A.

R6sum6 - Le laser au rayon X en train d'stre dBvelopp6 aux Laboratoires Sandia utilise un rayonnement keV intense, produit par la compression cylindrique ("Z-pinch") d'une bouff6e de gaz, pour obtenir les ions de la sBrie isoBlec- tronique F par la photoionisation des ions Ne. Les populations des niveaux de la configuration (2p)5(3p) et de la configuration (2p)5(3s) sont inverties par des processus de recombinaison des ions. Une enveloppe annulaire de stagnation maintient la sgparation entre la source du rayonnement et le plasma Btant lass.

Une couche mince d'aluminiumou sodium sert pour changer la longueur d'onde du rayonnement. La mBthode pour arriver aux dimensions et h la densit6 du laser au rayon X est d6crite.

Abstract - The Sandia X-ray Laser Program is based on the use of intense keV radiation produced by gas puff, Z-pinch implosions to photoionize Ne-like ions to F-like ions. A 3p-3s population inversion is generated via recombination processes. An annular stagnation shell is used to separate the imploding pump source from the lasant. We are also developing a converter technology for exa- mining the Na-Ne line matching scheme. Design considerations and some computa- tional results are presented.

Discussion

The Sandia x-ray laser program is based on the availability of large pulsed-power drivers developed for our light ion beam ICF program. When thin cylindrical foils arp imploded in a pulsed-power diode, the implosions

a r e observed to be unstable. Calculations have indicated that annuli of

comparable mass/length, but greatly increased thickness (reduced density) would have improved stability properties 1 . But the same calculations have indicated anomalously intense photon emission at 1 keV and above. This emission, nominally 10 kJ in 10 ns from a cylindrical stagnation region of radius 0.1 cm and length 2 cm, produced an intensity at the center line of the cylinder of about 2 TWIcm of 1 keV X-rays. 2 If this radiation pulse were to rise to its maximum in 5 ns, the rate of increase of pump power would be 0.4 TW/cm -ns. In the first publication on x-ray lasers, Duguay 2 and ~entzepis' estimated a requirement of 0.004 TI/cm -ns for 3s-2p lasing 2 at 372 8 in ~ a l * , and 0.25 x 1013 TW/cm -nr for a Cu 2 KO laser at 1.54 8 .

Clearly, we have a respectable pump for a soft x-ray laser, but are orders of magnitude below that required for a hard x-ray laser. (The estimates in Ref.(2) suggest pump power scaling as inverse laser wavelength to the sixth power.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986611

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To s i m u l a t e low d e n s i t y , t h i c k - s h e l l e d " f o i l s " we u s e a n n u l a r g a s p u f f s p r o d u c e d by high (4-8) Mach number n o z z l e s . The g a s i s p r e i o n i z e d and i m p l o d e d . F i g s . ( l ) and (2) show t i m e i n t e g r a t e d s p e c t r a from Re a n d Xe g a s p u f f i m p l o s i o n s u s i n g t h e P r o t o I1 a c c e i e r a t o r . B e c a u s e t h e n o z z l e d o e s n o t p r o d u c e a t r u e a n n u l u s , i n t e n s e p h o t o n e m i s s i o n i s o b s e r v e d f i r s t a t t h e n o z z l e end o f t h e d i o d e , a d v a n c i n g a l o n g t h e a z i m u t h a l a x i s o f symmetry w i t s i n c r e a s i n g t i m e .

v a r i e t y of schemes a r e b e i n g examined t o r e d u c e t h i s

" z i p p e r i n g " phenomenon.

1) S p e c t r a l i n t e n s i t y f o r a Ne g a s p u f f i m p l o d i n g o n i t s e l f .

0.0

800.0 1000.0 1200.0 1400.0 1600.0

P h o t o n e n e r g y (eV)

2) S p e c t r a l i n t e n s i t y f o r a Xe gas p u f f i m p l o d i n g o n i t s e l f .

0.0

800.0 1000.0 1200.0 1400.0

P h o t o n E n e r g y (eV)

laser medium that might show a population inversion on a ns or longer time

5 5

scale. The (2p) (3p)-(2p) (3s' transition in Ne-like ions is an appealing candidate. The population inversion is produced by photoionizing Ne-like ions with our imploding gas puff source, and developing the population inversion via recombination of F-like ions and the disparity of radiative lifetimes of the upper and lower laser levels. Fig.(3) shows the ionization energy of Ne-like ions as a function of nuclear Z, and the range of high energy photon emission observed for various gases. The intersection identifies suitable Z for laser media. (Pump power requirements are not included in these calculations.)

3) Graphical determination of laser medium to match pump spectrum

1

-

-

- -

-

105s ^-

-

Fig.(4) shows an idealized target design, with a 1 mm radius gas puff stagnation layer surrounding a laser medium. The stagnation layer is introduced to delay shock wave disruption of the laser medium until lasing has occurred. Experiments have shown that a stagnation layer made of CH

foils or low density foam does not significantly change the high energy

output from the stagnating gas puff. Given the geometry, and assuming that

the laser medium is composed of ground state Ne-like ions, the pumping

requirement is the high energy output (J) required for an ionization time

constant of 0.1 ns, 1.0 ns, and 10 ns. The required energies are called

JlO0, JlO, and J1, respectively. If we assume that the high energy output

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4) Idealized laser target

X-RAY LASER CONFIGURATION

ANNULAR STAGNATION

IMPLODING SHELL

LASER ROD

is a delta function in energy located at the threshold for (2p) 6 photoionization, we calculate the dashed curves in Fig.(5). If we use realistic photoionization cross sections and broad spectral

distributions (e.g. Fig.2) for the various gases, we calculate the hooked shaped curves in Fig.(5) (i.e. there is an optimum Z for each gas puff spectrum). A J, criterion is likely to determine a lower energy limit,

I

while with JIOO and a broad pump, one will rapidly photoionize F-like ions to 0-like ions, etc. Thus J1-J10 establishes a reasonable criterion for pump energy. The results in Figs. (3) and (5) suggest a pump power scaling with inverse wavelength to the fourth power, neglecting the energy required to produce the Ne-like plasma. Also shown in Fig. (5) are outputs from various modifications of the Proto I1 accelerator, and a projected future driver. We are currently using a diode designed by PSI to optimize our gas puff photon source; we have measured > ²⁰ KJ at or above 1 keV with

imploding He gas puffs. Thus

can examine laser media near Z = 30.

In modeling these systems with broacioanb pumps such as shown in Fig.(2), one finds that a JI0 criterion produces a maximum F-like Ion popu:ation In 2-3 ns. after whlch it decreases as the 0-like and N-like pn~uidt~ons Increase because the intense broadband pump can photoionize F-

lin:

and 0-like ions. To avoid this waste of both photons and F-like ion population we are developing low Z-converters to produce output spectra simiiar to Fig.(l), i.e. emission on the Ka lines from the A-like and Ae- like ions of the low Z-converter. This is done by coating the outer surface of the stagnation layer with the low-Z material. The laser medium is produced by thermally exploding a solid coating on the inner surface of the stagnation layer. Schematics of some current targets are shown in Fig.(6).

With Ka radiation from A-like Al, one can photoionize the (2p)6 shell of the Ne-like ion of both Cu and Ni. But while this pump wavelength can

photoionize the (2p)5 shell of F-like Ni, it cannot photoionize the (2p) 5

shell of F-like Cu. Thus, with an A1 converter and a Cu target one can

prevent the photoionization of F-like ions, and by varying the target from

Cu to Ni, one can test the hypothesis experimentally. By varying the

and Al converters show about 25% of the output above 1 KeV in A1 H- and He- like K O lines. Fig.(7) shows a time integrated spectrum from a thin foil stagnation layer coated with Al on the outside and C u on the inside.

Emission from Re-like C u is observed. Gse of a Na converter would allow experiments on the Na-Ne line matching laser scheme. 3

5) Graphical determination of laser medium to match pump energy

6) Schematic of current targets

1.5 mm CH SHELL mm

LAYER

E

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7 ) E m i s s i o n s p e c t r u m from Al/CH/Cu s o d a s t r a w t a r g e t

0.3

5.0 7.0 9.0 11.0 13.0

WAVELENGTH (angstroms)

The s i z e o f t h e l a s e r t a r g e t c a n be e s t i m a t e d f r o m r a d i a t i o n t r a p p i n g c o n s i d e r a t i o n s . The b a c k g r o u n d b l a c k b o d y r a d i a t i o n c a n d e s t r o y a ( 2 p ) ( 3 p ) - 5

5 5 5

( 2 p ) ( 3 s ) p o p u l a t i o n i n v e r s i o n by pumping t h e (2p) ( 3 p ) - (2p) (3d) t r a n s i t i o n w i t h t h e s u b s e q u e n t r a p i d r a d i a t i v e d e c a y of t h e (2p) ( 3 d ) 5 t o t h e ( 2 p ) ⁶ g r o u n d s t a t e . A c r i t i c a l blackbody f l u x c a n be d e f l n e d a s

w h e r e A .

i s t h e E i n s : e i n c o e f f i c i e n t , kT i s t h e r a d i a t i o n t e m p e r a t u r ? i n

1 , J R

Ry and f Z g (AE) i s t h e 3p-3d o s c i l l a t o r s t r e n g t h ( e n e r g y d i f f e r e n c e i n Ry).

-.

The c r i t i c a l b l a c k b o d ? f l u x i s shown i n F i g . (8) as a functi%_of r a . d i a t i o n t e m p e r a t u r e f o r a v a r i e t y of Z . A l s o shown a r e c r i t i c a l b l a c k b o d y f l u x e s a s s o c i a t e d w i t h o u r g e o m e t r y , p u l s e w i d t h , and J v a l u e s . F o r 80 k3 and a 4(

eV r a d i a t i o n t e m p e r a t u r e , F i g . ( 8 ) shows t h a t t h e b l a c k b o d y background c o u l d d e s t r o y a p o p u l a t i o n i n v e r s i o n

Z=23, w h i l e f o r Z=29 and 34 t h e same b l a c k b o b y b a c k g r o u n d i s below c r i t i c a l by a f a c t o r of 3 - 5 . However, b a c k g r o u n d b l a c k b o d y r a d i a t i o n i s n o t a s e r i o u s p r o b l e m u n l e s s t h e

( 2 p > ( 3 d ) - ( 2 p ) 6 r a d i a t i o n c a n e s c a p e from t h e s y s t e m . U s i n g a c r u d e 5 r a d i a t i o n t r a p p i n g c r i t e r i o n N [ ( 2 p ) 6 ] R a > I , t h e n a c r o s s s e c t i o n and Ne-

2 ~ ,3d

l i k e i o n d e n s i t y d e f i n e a minimum t a r g e t r a d i u s . However, s i n c e o n e d o e s

n o t w a n t t h e ( 2 p ) ( 3 ~ ) - ( 2 ~ ) ~ 5 l i n e t r a p p e d , a s i m i l a r r a d i a t i o n t r a p p i n g

c r i t e r i o n d e t e r m i n e s a maximum t a r g e t r a d i u s . I n F i g . ( 9 ) minimum a n d

maximum N [ ( 2 p ) ]R a r e shown 6 as a f u n c t i o n o f Z , f o r a 2 0 eV D o p p l e r w i d t h .

l d e L

'

'

' , ^'

. ,

-

A

N

-

'E - ^-

a

I 0

10

: -

9) Critical radius-ion density product determined by radiation trapping.

13

10

18

26

NUCLEAR

Z

C6-88 JOURNAL DE PHYSIQUE

The maximum electron density and, hence, the maximum Ne-like ion density is determined as the critical electron density for destruction of a 3p-3s population inversion due to the excess of superelastic 3p-3s over inelastic 3s-3p collisions. For a system pumped by recombination a critical electron density is given by

where C. is a velocity weighted electron excitation cross section. In

1. ,"

i

Fig.(lO) nec is shown as a function of Z for various electron temperatures (kT ) . Peak gain occurs for n about n 12. With kTR=40 eV and kTe= 125

ec

eV, Figs.(3) and (8)-(10) suggest targets of V pumped directly with Ne gas puff radiation, Ni or Cu pumped with radiation from an A1 converter, and possibly Se pumped with radiation from a S converter. For such laser media created from solid coatings on the inside of the stagnation layer the coating thickness should be 100 or less.

The approach followed in the Sandia X-ray laser program is traditional in that we separate the pump source from the laser medium. Much of our effort so far has been on source development; laser experiments are planned for the coming year.

10) Critical electron density determined by superelastic/

inelastic electron collisions.

References

1) T. W. Hussey, Bull. Am. Phys. Soc. 22, 1065 (1982)

2) M. A. Duguay and P. M. Rentzepis, Appl. Phys. Lett. 1_0, 350 (1967)

3) J. P . A~ruzese and J. Davis, Phys. Rev. A g , 2976 (1985)