DOPPLER SHIFT IN X-RAY SPECTRUM FOR LASER-IMPLODED SPHERICAL MICROSHELLS

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DOPPLER SHIFT IN X-RAY SPECTRUM FOR LASER-IMPLODED SPHERICAL MICROSHELLS

H. Fiedorowicz, S. Denus, K. Jeziak, M. Kolanowski, P. Parys, W. Pawlowicz, J. Wolowski

To cite this version:

H. Fiedorowicz, S. Denus, K. Jeziak, M. Kolanowski, P. Parys, et al.. DOPPLER SHIFT IN X-

RAY SPECTRUM FOR LASER-IMPLODED SPHERICAL MICROSHELLS. Journal de Physique

Colloques, 1988, 49 (C1), pp.C1-369-C1-389. �10.1051/jphyscol:1988180�. �jpa-00227594�

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JOURNAL DE PHYSIQUE

Colloque Cl, Suppl6ment au n03, Tome 49, Mars 1988

DOPPLER SHIFT I N X-RAY SPECTRUM FOR LASER-IMPLODED SPHERICAL MICROSHELLS

H. FIEDOROWICZ, S. DENUS, K. JEZIAK, M. KOLANOWSKI, P. PARYS, W. PAWLOWICZ and J. WOLOWSKI

Institute of Plasma Physics and Laser Microfusion, PO Box 4 9 , PL-00-908 Warsaw 4 9 , Poland

Doppler shifts due to plasma motion have been observed in the S ce resolved X-ray spectra for the laser-imploded spherical microshells. The anal sis of t r Y-ray spectrum makes it possible to study the ablation process from the microsiel surface.

Space-resolved X-ray spectroscop is a powerful diagnostic technique for the study of phsmas produced by laser-driven impLsion of spherical microtar ets. It gives information on parameters of the plasma expanding from the microtarget s d c e and of the compressed plasma due to the spherical implosion Cl].

Electron densities of the plasma can be obtained by line brwdenin analysis and line related intensit measurements. Electron temperature of the plasma is usuily ?! determined from the slope of t L recombination continuum and from line ratios.

As a consequence of the spherical expansion of the lasma from the microtarget surface, shifts and broadening of the spectral lim connected

wit1

motional Doppler effects should be observed. Typical expansion velocities Vexk have (at laser intensity about

M1=

W/cm21 values of the order of a few times 107 c /S C21 and for X-ra ener range the motional D o e r shifts of the order of a few m are expected. ~etaifed anqsis of the Doppter shifted X-ra l i s allows to study the laser implosion process C3, 41 and to determine the energy specrrum of fast ions C51.

2. Experiment

The laser im losion experiment was carried out with the use of the four beam Nd-glass laser system. glass microshells of 100-150 W in diameter and 1

-

1.5 W wall thickness, filled with deuterium-neon gas mixture at a pressure of 10 atm, were irradiated in a tetrahedral ometry. Some shells were coated with polystyrene and metal la ers. The laser energy was a g u t 40 J in 1.5 ns pulse

-

it gives approximately 10'' W/cm

J

irradiance on the microtarget surface.

The X-ray space-resolved S tra ggistered b means of the miniature spctrograph with ADP flat crystaF2d = usin a $0

m

slit oriented along the d~spersion direction. The 10 Be filter and RAR 2490 !-ray film were used. The slit was made in the 10 Ni foil which covered one half of the entrance window.

3. Results and discussion

The t pical X-ray spectrum obtained in the implosion experiment for the lass microshells is presen!ed in F

.

1. The X-ray spectral image of the irradiated microshel? formed in the resonance line of ?he He-like Si ions is characteristicaNy crescent-shaped ( Ib). The shape

"%

can be ex lained as a "blue" shift of the central part of the ima caused y Doppler effect in the spRericm expanding plasma from the irradiated microsher The central t of the image is formed by the radiation from the plasma expandin in the crystal d k K (velocity component in the observer direction has the maximum value!. The lateral arts of the image are formed b the X-ray emission from the lasma expanding perpendicul!rly to the c stal

7

direction (in Xis case velocit component in tRe observer direction has the minimum vaue) The value of the Do p t shid is determined as a difference AA,, =

4 -

^Ac ^where

4

and 1, correspond to tl!e wavelength of lateral and central parts of the X-ray spectral Image, respectively.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988180

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C 1-370 JOURNAL DE PHYSIQUE

From the shift, the ex ansion velocity of t e emitting plasma can be estimated. In the case resented in F

.

1 the s i f t dlD = 6.0 t 0.5 m

1

giies the velocity Vex = (2.7 0.2).107 cmls.

&e electron k i t y and the electron tem rature of the plasmif in the emitting r ion can be estimated from the intensit ratios o r t h e intercombination and dielectronic sazliie lines to the resonance line of the &-like Si ions C6. A. For the case shown in Fig. I the electron density n, = 5-1020 and temperature Te

=

500 eV were obtained. In this region plasma expands with the rate of mass loss rh

=

p V

-

4.5.10* g . ~ m - ~ - s - ' where

p

-

IUma density, thus giving a reaction force per unit areavP; h.Vex = 1.2 Mbar.

R*

X-ray spectrum obtained for the lass microshells coated Cith 1.0 thick polystyrene layer and covered externally with 0% pn copper layer is presented in 2. In this case the c u m of the X-ray s ctral image of the irradiated microsheY is rtegfiffe. It means that the ex ansion velocity o r t h e emitting S ions is much less than 10' cmjs. The reduction of the

&

ions expansion velocity can be caused by the external polystyrene and metal layer (inhibitor).

The measured quantities can be compared with the results of the hydrodynamic modelling of the laser implosion using the computer code BALON IM.Kolanowski

-

unpublishedl. The calculated profiles of the asma (electron densit , electron temperature and plasma velocit in the time moment to

Pk

the laser im l o d d microshells are presented in Fig. 3. ll!e implosions of the glass microshell (Fig. 3aP and of the glass microshell coated with I pm thick po styrene layer (Fq. 3b) are caused by absorption of 50 J laser radiation in 1.5 ns pulse.

k

e diameters and wall thicknesses of the lass microshells are the same. For the time moment t o the electron temperature of the heafed glass has the maximum value.

The temperatures of the plasma in the critical region (n, = , n, =

lo2'

~ m - ~ ) calculated numeri- cally are higher compri to the measured values. It IS connected with higher laser energy absorbed by the m i c r o s d in the modelling. The calculated ex nsion v e h i t y in the subcritical region i n = 5.1020 c ~ n - ~ ) for the imploded glass microshe~equaJs Vex = 2-5-16 m / s . For the &ss microshell coated with the polystyrene layer the ekpansion v&)ocity of the glass plasma heated to 'he maximum temperature equals V = 6.106 cm/s.

CXP

Doppler shifts in the space-resolved X-ray spectra connected with s herical expansion of plasma in laser implosion experiments can be observed. Detailed study o

I'

the X-ray spectral Images gives information about the mass abiation rate, ablation pressure, plasma density and temperature. The inhibition of the Si ion expansion from the irradiated microshell surface caused by external polystyrene layer has been observed.

Fig.1. S ace integrated (a) and space- resolved (b) spectra of the He-like Si ions o l t a i n 2 in the laser implosion experiment for the glass microshell.

Fig.2. S ace integrated (a) and space

-

resolved (b) s ctra of the He-like Si ions ogtained in the laser implosion experiment for t E glass microshell coated with external polystyrene and metal layers.

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Rg.3. The calculated profiles of the plasma for the imploded glass microshell (a) and the glass microshell coated with polystyrene layer (b).

-

Cll

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