UCRL-JD-123779

XUV Radiography Measurements of Direct Drive Imprint in Thin Aluminum Foils Using a GE X-ray Laser on Valcun

D. H. Kalantar, A. Demir, M. H. Key, N. S. Kim, C. L. S. Lewis, J. Lin, D. Neely, A. McPhee, B. A. Remington, R. Smith, G. J. Tallents,

J. S. Wark, J. Warwick, S. V. Weber, E. Wolfrum, and J. Zhang

March 29,1996

4 This is an informal report intended primarily for internal or limited external distribution. The opinions and conclusions stated are those of the author and may or may not be those of the Laboratory.

~ Work performed under the auspices of the US. Department of Energy by the Lawrence Livermore National Laboratory under Contract W-7405-Eng-48.

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XUV RADIOGRAPHY MEASUREMENTS OF DIRECT DRIVE IMpRINT IN THIN ALUMINUM FOILS USING AGE X-RAY ER ON WJLCAN ?‘f+

D. H. Kdantar”, A. &mi?’, M. H. Key:] N S. B. A. Remington’’, R. SmithzJ, G. J. Tallents’~, J:S.

Lewis4J, J. Lin5’, D. NozIY”, A. McPhee4! S. V. Weber’’, E. Wolfrum-”, J. B a n

’JLawrence Livermore Nationat Laboratory, USA LJEssex University, UK

”Rutherford Appleton Laboratory, UK ‘’Queen’s Univeristy, UK ”Oxford University, UK

INTRODUCTION

One key aspect for high gain direct drive inertial confinement fusion is the imprint of perturbations in the outer surface of a capsule due to nonuniformities in the direct laser illumination of the capsule. D i m drive implosions are achieved by uniformly irradiating the outside surface of a hollow spherical capsule that contains a layer of fusionable D-T o n its inner surface. The intensity of laser irradiation is down with a low intensity ‘foot‘ at 10” W/cm2 for several nanoseconds before it builds up to more tha IOg5 W/cm2 during the main drive portion of the pulse. Laser ablation of the capsule surface produces a high pressure that accelerates the capsue1 shell radially inward in a spherical implosion. During this acceleration, perturbations due to surface roughriess and due lo imprint from spatial non-uniformities in the laser irradiation undergo Rayleigh-Taylor growth, potentially severely degrading performance.

Our interest is in studying the imprint process and subsequent Kayleigh-Taylor growth of perturbations in a foil target that is irradiated by a low intensity laser speckle pattern. Previous experiments havc been done to study laser imprint with an x-ray laser backlighter at the Nova laser using 0.3s pin laser irradiation of at 3 pin Si In thesc cxpcriinents wc irradiated a 2 pin thick AI foil with 0.53 pm lascr light at 2-8x IO’* W/crn2 using the Vulcan laser. Wc used a Ge x-ray laser4 as an X U V hacklighter to nicasure the rnodulalion in optical depth of the foil on a CCD during the initial imprint phasc and aftcr Kayleigh-Taylor growth with differcnt lascr smoothing schemes.

We used a single Vulcan laser bcarn with a static random phase plate speckle pattern, smoothing by spcctral dispcnion, and smoothing by IS1 We compared tllc results with results from a multiple bcam overlap of static speckle patterns and SSD smoothed speckle patterns. We also measured the growth of a single wavelengrh modulation that was imprinted by a single mode optical intensity modulation onto the target. We used AI foil targets since AI has the lowest opacity for the Ne-like Ge x-ray

XUV sensitive CCD . XY pm

I I

laser wavelength of 19.6 nm. The AI is still highly attenuating, which limits the expwiment to chin foils. It also means, however, that the technique is sensitive to sm11 modulations in the thickncss of the foil. At 19.6 nm. the product of opacity time density for AI is 2.24 pm I . With this high an opacity, a thickness variation of only 50 nm results in a 10% change in signal intensity.

EXPERIMENT

We uscd six beams of the Vulcan laser to generate a Ge X-I-ay laser. Three of the six bcariis of 1 .Xi pm laser light were focused with an overlapping line focus onto each of two 100 p n wide strips of Ge deposited on glass slides. For these experiments we uscd two 18 mm targets that had a separation of 200 pin, .as illustrated in Figure I . We used 100 ps pulses with a 10% pre- pulse 2 ns before the main pulsc. Under these conditions. the x- my laser bcam had a divergence of about 30 mrad i ~ ] the plane of the x-ray laser target surface. and 10 nirad normal to the plane.

We placed a thin (2 pin) AI foil about 3 cm from the output of the Ge x-ray laser. We then used two multilayer mirrors to image tlie AI foil in thc x-ray la.scr wavelength ~ n t o an XUV sensitive CCD (Figurc I ) . A spl~crical mirror with a 1 m radius of curvature was placed S3 frn from the AI foil, providing a 16X magnified image of the foil on the CCD at near normal incidence (<Ode). The CCD was filtered with an additional 0.8 pin AI foil to reduce thermal emission from the foil. and a 45” anglc of incidence planar mirror was uscd t o relay thc iinage onto thc CCD and spcctrally isolate tlie image from the thcrinal background noise.

The XUV mirror imaging system used near normal incidence reflection from the spherical imaging mirror to minimize spherical aberrations. The resolution of this imaging system was better than 1 pm.

We conducted a series of experiments to study the imprinting of a 0.53 pm law bcam on a thin AI foil by measuring the modulation

100 pm Ge stripe

1.06 pm beams for x-ray laser

Figure 1: Geometry for the x-ray laser target and XUV imaging system to measure the modulation in optical depth of a thin AI foil due to direct drive laser imprint on Vulcan.

of the foil a? a function of time wilh various laser smoothing schemes. This modulation was imprinted by variation in optical intensity and enhanced by Rayleigh-Taylor growth at late time.

We used up to two beams of the Vulcan laxer as drive beams to directly irradiate the AI foil with the following series of configurations:

I . Multiple iiiode lasci intencity iiioduiation: a) single beam irradiation

static speckle pattern 1-11 SSD smoothed speckle pnttcrn IS1 smoothed speckle patfern

stalic speckle pattci-n 1-11 SSI) smoothrd s 1 ~ c k l c puttcrii

h) two heam overlap

2 Single riiotlc I:i\ci- intciisity iiiodul;ilioii a ) I 5 girl wavclcllptll 11) 70}lIll \ v ~ l v < ~ l ' ~ l ~ ~ t l l

lindnun gating, providing a dispersion of about 0.17 rnrad. W e used an RPP with IS1 smoothing to &encrate the smoother irradiation pattern shown in Figure 2c.

We irradiated the 2 pn thick AI foils directly by an intensity of 3- 8 ~ 1 0 ' ~ W/cm2 of 0.53 pni laser light using the different spccklc patterns shown above in Figui-e 2. We recorded the modulation i n optical depth in fhc foil duc to Iascr iinpnnl and subseqeiit Kaylcigh-Taylor growth using the Gc x-ray 1:isdr backlighter. We show several X U V radiographs as modulation in optical depth rccordcd at 0.2 ns into the laser pulse in Figur-e 3 for the three cascs of imprint due to a static random phase plate (RPP) spccklr p:ittcni. an spechle pa[tcni snioothcd by spclc~ral dispcrsion (SSI)). and an spc.ckfe pattern wth induced spatial incoherence ( IS1 smoothing). Power spectra foi- tlx iriipnntcd inodulation iiicasured in these iniages aic sliowii i n Figure 4 Thcsc arc plottcd as pwvcr prr iiiotlc. such that the square root of the iritcgrnl untlcr the curves is lhc roof iiic:iii squ:irc (KMS) riicdulatioii in tlic r:idlograpii IIII:I~C Not<. tli:it we also show flic power \I)c'ctruiii olmiiicd liii ai1 IIII~IIIVTII tiiigct 1 0 1 coiiil):iii\oii i i i till\ ulplL.

0 2.5 so 75 IO 12s 0 25 so 7s I(X) 12.5 0 25 so 7s loo 12.5 h) SSD sruoolhcd speckle pai\em c ) IS1 sinwthcd speckle pattrrn

Figure 2 : I~iiivalcnr taigct plane iiiiagcs of thc lascr focal spot recorded with a) ;i static KPI' specklc pattern. h) a I - diilic'nsional SSI) siii~withcd qwcl.lc pattern. and c) an IS1 srnoothcd speckle pattern. l h c sc;ilc is i l l micron\ :it the t;irgcf

a) Static Ut'l' speckle pmcm

0 2s SO 75 103 125 0 25 so 75 loo 12s 0 25 50 75 I 0 0 125 b) SSD smoothed speckle pattern c) IS1 smoothed speckle pattern

Figure 3 : Modulation in optical depth 0 1 an AI foil irradiated by a 0.53 pm direct drive laser beam sinoothed with a) a static KI'P specklc pattern. h) a Idimensional SSD smoothed speckle pattern. and c) an IS1 smoothed speckle pattern. These iiilages were recorded at 200 ps into the drive pulse using the Ce x-ray laser backlighter. The scale is in microns a( the rarge(,'and they are plotted on the same grayscale from - 1.2 to + 1.2 in optical depth.

a) Sialic RPP speckle pattern

.-

e 5 - * slatlcRPP a - I( SSDsn-dhed % 0.8 - A I S l m ~ -

IO“

10“

Zj L 0.

0

2 IO‘( a

1 o6 0 10 20 30 40 50 60 70 80

Mode number (250 pm square region) Figure 4: Power per mode caluclated from the XUV optical depth

modulation measured from the radiograph images shown in Figure 3.

-

speckle pattern and two beams with an SSD smoothed speckle pattern.

C c - 2 0 4 1 3 -0 -

cn

0 0 z

Figure 6 shows XU\‘ radiographs recorded at about 0.2 ns for the.% two cases with an intensity of about XX. The equivalent focal plane image for the overlapped static speckle patterns appears 10 show snlaller scale structure than for a single static KPP speckle pattern in Figure 33. The image for the overlapped SSD smoothed beams shows streaks in two dircctions hecause the dispenion direction on the two beams was orthogonal. The KMS modulation in optical depth with two overlappjng static speckle patterns was 0.30. It was 0.17 for the two overlapped SSI) smoothed bcanis.

0

- rn *

a 4 4 A ‘ I I ” l ’ l i 1 1 * ” ’ 1 1 1 ’ 1 , 1 1 1

We also conducted preliminary expcrinicnts to compare the imprint and KT growth of single mode vs multiniodc niodulatioris in a thin AI foil. We placed a two-slit aperture in the 0.53 prn I:iser drive beam that provided an Airy pattern to illunrinatc thc carget at ahouf 2 x IO” W/cni’. The slits wen: designed to provide and interference pattern with a single doininan! wavelength at IS p n and 30 p i . We recorded a series of images at different times to measure the growth of the niodulaiion. Preliminary results from these incasureincnts arc prescntcd separately in this proceedings.

SUMMAKY

These rxpenments showed that we could make nieasurnicnls of the modulation imprinted by direct drive on a thin foil using thc Gc x-ray laser a# the Vulcan Iaxr facility. We made ipasurenlcnts

of the imprinted modulation and suhsoquent Kayleigh-Taylor growth as a function of time with various laser smoothing schemes. We also made measurements of the inodulation imprinted by a single mode optical perturbation.

Full analysis of the iniprint and suhsquent Kayleigh-Taylor growth measurenlents is in progress, and will be reporied in full detail in future publications. This will includc comparisons of the imprinted modulation with previous experinlents iwdc on t he Nova lascr that uscd a 0.35 p i 1 laser imprint wavelength, as well a< with simulations.

KEFEK ENCES

’ M. H . Key. T. W . H&e. Jr.. L. H . IhSilva, S . G. Glendinning. 1). H. Kalantar, S. 1. Kosc. and S. V . Weber. ‘New plasma diagnostic possibilities from radiography with x-ray lasers’. J. L)uaiir Sprcln>sc. RuIi[il. 7 i n r i . ~ j < v 54. 22 I ( 1995). 2 1). 14. Kalantar, 1‘. W . 13arbcs.. Jr. , L. H. IlaSilva, S . C. Clendinning, M. 1-1. Key. J. P. Knauer. F. Weber. and S. V. Wcher. *X-ray laser radiography of erturhations due to imprint of laqcr speckle in 0 35 niicron laser irradiation of a thin Si foil’. Kri!. Sci. Iiixlrurti. (in press).

I). H. Kalantar, M. 14. Key, L. 13. DaSiIva. S. G. Glendinning, J. 1’. Knauer. H. A. Kenlington. F. Webcr. and S. V . Wcbcr. “Measureinent of 0.3.5 micron laser imprint in a thin Si foil using an x-ray laser hacklighter*, Phys Krv. Irfrc,r.s ( in press).

3 . Zhang about the Ge x-ray laser

0 25 50 75 100 125 0 25 $ 75 i& 125 a) Overlapped static RPP speckle patterns b) Overlapped SSD smoothed speckle patterns

Figure 6: modulation in optical depth of an A1 foil irradiated by two overlapping 0.53 pm laser beams with a) static RPP speckle patterns, and b) SSD smoothed speckle patterns.