San Diego, California, 17-19 June 2002

Laser Imprint Problem and its Treatment
 
 

M. KÁLAL and J. LIMPOUCH

Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Brehová 7, 115 19, Praha 1, Czech Republic

kalaltroja.fjfi.cvut.cz

E. KROUSKY, K. MASEK, M. PFEIFER, K. ROHLENA,

J. SKÁLA, P. STRAKA and J. ULLSCHMIED

Institute of Physics of the Academy of Sciences of the Czech Republic, Na Slovance 2, 180 40 Praha 8, Czech Republic

A. KASPERCZUK and T. PISARCZYK

Institute of Plasma Physics and Laser Microfusion,

23 Hery St., 00-908 Warsaw 49, Poland

S. Yu. GUSKOV and V. B. ROZANOV

P. N. Lebedev Physical Institute of RAS,

Leninski Ave., 53, 119991, Moscow, Russia

V. V. GAVRILOV, V. N. KONDRASHOV and N. G. KOVALSKIY

Troitsk Institute for Innovation and Fusion Research,

 142190 Troitsk, Moscow Reg., Russia

Residual structures imprinted by laser beam non-uniformities on initially cold target surface are difficult to avoid during the early-time stage of the laser target interaction. This phenomenon - laser imprint - may seed disturbances in the developing plasma with detrimental consequences on the compression uniformity, so critical e.g. in the direct drive fusion schemes. In this paper results from our analytical, numerical and experimental studies of two approaches which could provide some solution to the laser imprint problem are reported.

In the first method a prepulse was employed to generate an absorption zone near the target surface with non-zero thermal conduction before arrival of the heating pulse. The critical surface position and the size of the absorption zone of the heating pulse were controlled by a relative time delay of both laser pulses and by ratio of their intensities. The aim was to assess conditions under which an optimal smoothing effect would occur.

In the second method the smoothing process due to the fast energy transfer in low density porous matter of light elements (with average densities of 10^-3 - 5.10^-2 g/cm³) was studied. A theory of laser light absorption and energy transfer mechanisms in porous matter with supercritical density was developed. 2-D numerical simulations of laser beam interaction with the media emulating the properties of foam were performed. Acceleration of 5 m m Al foils under the pressure of laser-produced plasma from porous matter was also investigated. The fast smoothing process with transverse energy transfer speed of 10⁷ cm/s and higher, pressure of laser-produced plasma of porous matter of several Mbars and Al foil velocities of the order of 10⁶ cm/s were observed. Experiments were performed on the Prague Asterix Laser System using the laser beam parameters: E_L @ 10¹⁴ W/cm², l  = 0.438 m m, t_L@ 400 ps. To visualize the laser-produced plasma at different stages of its development the 3-channel interferometry with various delays as well as the soft X-ray streak camera were employed.