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Cl-204 JOURNAL DE PHYSIQUE

The axial emission is imaged by a grazing-incidence mi rr or onto theentrance slit of a multichannel soft X-ray spectrometer. A second spectrometerviews the plasma transversely. A tra nsv erse scan of the axial spectrometergives information on the relative divergence of the s t~ mu la te d emissioncompared to spontaneous emission lines. In lithium-like ions the 4d-3p and4p-3s tran sitions have gA values significantly less than the 4f-3d transition. (gis the statistical weight of the upper level and A is the transition probability).In th e prescence of a n = 4 to m = 3 population inversion, gain is expected to befirs t apparent in the transition wi th th e highest gA value. A convenientmeasure of thi s increase is th e 4f-3d inten sity rela tive to the 4d-3p and 4p-3stransitions. The measurements indicate a maximum gain length ( g ~ ) f gL =3-4 a t 15.4 nm and gl, = 1-2 at 12.9 nm ( ~i -l ik eS ~ X I I ) . ~

An impo rtan t new avenue of research a t Princeton is the application ofth e 18.2 nm soft X-ray laser to high resolution contact microscopy. The softX-ray laser beam is collimated by a rudimentary toroidal mirror onto abiological specimen chamber in a contact microscope. The specimen is isolatedfro m the laser va cuum system by a silicon nitride window, 1000 thick and,during th e exposure, th e shadow of the specimen is recorded in PMMA-MAAcopolymer photoresist. The experiment is in an early stage but experimentsare planned for the near future using live cells in which the resist will beviewed a t high resolution using an electron microscope. Higher qua lity opticsand more advanced microscopes a re being designed and constructed

A s par t of a larger system which is being constructed for the developmentof sho rt wavelength X-ray las ers and which includes a C02 laser (1 kJ energy,10-50 nsec pulse duration), a powerful picosecond KrF* laser has been developedand used in in itia l t arge t inte raction e ~ ~ e r i m e n t s . ~ > ~he master oscillator ofthe PP-laser system w as a cavity-dumped dye laser tuned to a wavelength of,648 nm . The dye laser used a hybrid mode-locking scheme and produced 1 psecpul ses . The dye laser w a s pumped by the frequency-doubled output of amode-locked YAG laser. The output of t he dye laser wa s injected into athree-stage dye amplifier that w a s pumped by amplified, frequency-doubledpulses from the YAG laser. The amplified 648 nm pulses we refrequency-doubled and, then, mixed wit h 1064 nm YAG pulses. The resulting248 nm pulses were injected into two KrF* amplifiers. The amplified pulsestypically had an energy of 20 to 25 mJ and a duration of 1 to 1.2 psec. Thepulses we re focused by an f/5 spherical lens to an in tensity of approximately1016 ~ / c m 2 ,

The laser pulses were incident on a rotating cylindrical target eithercomposed or else coated with the material under study. The spectra wererecorded on Kodak 101 photographic plates, and the spectra produced by up to7000 laser shots, were integrated onto each exposure.The spectrum from a solid aluminum target is shown in Fig. la.Transitions (not shown) occurring in the low-density expansion plasma, haveline width s consistent with th e 30 mA ins tru menta l broadening (resulting fromthe 10 pm en trance slit) and Doppler source broadening. The much la rge rwid th s of th e Li-like n = 2 - 4 and n = 2 - 5 transitions shown in Fig. 1 areconsistent with quasi-static ion Stark broadening at an electron density In therange 1021- ~ m - . ~ .he same transitions in Li-like fluorine from a teflontarget a re shown in Fig. lb. These line profiles ar e asymmetric and a re up to

2 A in width.It is well known th at the microfield due to the plasma can cause shif ts ofthe energy levels of emitting ions.9 In dense plasmas, when the shi fts becomecomparable to the energy level splittings, the wave func tions of neighboringstat es become mixed, and this permits transitions tha t a r e not allowed in th eabsence of the microfield. The ra tio of the intensities of the forbidden and

allowed transitions increases with plasma density.

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1 1 . 1 1 1 1 1 / 1 1 1 , I I l I I , ,

35 40 5 50 55 60WAVELENGTH (I)

- -90 1 0 0 110 12 0 130 1 4 0 1 %

WAVELENGTH a)Figure I : High resolution XUV spectra. Figure Ial i s a r e s u l t from an a2uminz.mta rg et showing Li -l ik e AIXI and Be-like AIX. Figure Ibl i s a result from at ef l o n ta rg et showing L i-- lik e FVII and Be-ZikeFVI.

The line profiles were modeled using a formalism developed for thecalculation of spe ctral line profiles of multie lectron ions in dense plasmas usingthe quasi-static ion approximation.1 A quantum mechanical relaxation theoryw a s used to calculate th e piasma electron broadening of the spectral lines. Thecalculated profile of the F6+ 2p-3d spectral fea tu re showed th at a t electrondensities greater than 5 x ~ m - ~ ,he 2p-3p forbidden component contributessignificantly to the red wing of the spectral featu re. This is consistent w iththe enhanced red win g of the observed 2p-3d spectral fea tur e shown in Fig. 2b.Similarl y, t he red wing of the observed 2p-4d fe at ur e is enhanced by the 2p-4pforbidden component.

From numerical computations the line wid th a nd as ym me tr y of theobserved F6+ 2p-3d spectral f eat ur e ar e consistent wit h a n electron density ofapproximately ~ m - ~ .owever, not all features show such agreement andcannot be explained in te rm s of densi ty alone. It is possible tha t the electricfield associated with the intense laser pulse may also be influencing thechara cte r of the spect ral emission Nevertheless, it is of int erest to note th atthe dens ity implied from calculations is more comparable to the criticale lec tron densi ty (2 x ~ m - ~ )or 248 nm l ase r radiation than th e targetdensity (4 x ~ m - ~ ) . ince the plasma expansion during the picosecondlase r pulse i s negligible, it seem s likely that the observed emission occu rs in theexpanding plasma immediately afte r the laser pulse. Fur ther mor e, the timefor collisional ionization to F6' is approximately 1 psec, and the time for n = 2-3collisional excitation is less th an a picosecond. The times for the rad iat ivetransitions 3p-2s and 3d-2p a r e 20 psec and 6 psec, respectively, and ar e muchlarger than the laser pulse duration . Thus it seems reasonable that theexcited F6+ ions ar e formed dur ing the picosecond lase r pulse, and the bulkof t he emission occurs af ter the laser pulse. The F5+ ionization ener gy is 160 eVand the F ~ + = 2-3 excitation energy is 100 eV, an d it is possible tha tmultiphoton (nonresonance) processes, req uir ing a large number of 248 n m (5eV) photons, con tribute to the format ion of excited F6+ durin g the laser pulse.

Finally we note th at w it h the iron target t he presence of Na-like el"lines, representing the most highly charged ion roduced in plasmas created byPpicosecond la se r. The He-like ls2 - ls2p P i resonance line of a lum inumat 7.757 A (1.6 keV excitation energy) w a s identified in the spectra fr om t healum inum targets (more details in Ref. 8).

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CI-206 JOURNAL DE PHYSIQUE

C ON C LU SI ON S AND F U T U R E P L A N SWe have demonstrated gain at 15.4 nm a nd 12.9 nm in a recombining

X-ray laser scheme. We hope to achieve still shorter wavelength output byusing higher Z elements (e.g. sulphur). Another area in which an intensiveeffort is being directed is the development of an X-ray laser cav ity using softX-ray mirrors with -, 0% reflectivity .

We ha ve also obtained high resolution XUV spec tra from the in teraction ofthe powerful picosecond KrF laser system with several solid targets.Extremely broad and asymmetric line profiles were observed along with highlevels of ionization. Experiments to stud y the effect of a prepulse on theinteraction and wi th new layered targets a r e beginning. The development of afinal stage amplifier For the PP-laser, wh ich will provide much higher powerlevels, is proceeding.

REFERENCES1. S. Suckewer, C.A. Skinner, D. Kim, D. Voorhees, and A . Wouters, Phys.Rev. Lett. 3, 004 (1986).2. D. Matthews, M. Rosen, S. Brown, N. Ceglio, D. Eder, A. Hawryluk, C.Keane, R. London, B. MacGowan, S. Maxon, D. Wilson, J . Scofield, and J .Trebes, J . Opt. Soc. Am. B & 575-587 (1987).3. S. Suckewer, C.H. Skinner, H. Milchberg. C. Keane, and D. Voorhees,Phys. Rev. Lett. 1753-1756 (1985).4. C.H. Nam, E. Valeo, S. Suckewer, and U. Feldman, J . Opt. Soc. Am. B 3,1199 (1986).5. C.H. Skinn er and C. Keane, Appl. Phys. Lett.&, 1334 (1986).6. C.H. Skinner, D. Kim, A. Wouters, D. Voorhees, and S. Suckewer, Proc.of SPIE 31st In t. Tech, Sym ., 831 36 (1987).7 . W.G. ighe, C .H . Nam, S. Suckewer , and J . Robinson, to be published.8. C.H. Nam, W.G. Tighe, S. Suckewer, J.F. Seely, U. Feldman, and L.A.Woltz, Phys. Rev. Lett. 2427 (1987).9. H.R. Griem, Academic Press, New York, (1974).10. L.A. Woltz and C.P. Hooper, to be published in Phys. Rev. A. (1987).