Density profile of the ablating plasma produced by soft x-ray irradiation

Indirect/direct-hybrid irradiation has been proposed for suppressing the in itial imprint of laser-irradiation nonuniformities. Indirect soft x-ray pre irradiation of the target generates a plasma expansion layer on the target surface. When the drive laser beam comes later, the target has a substantia l stand-off distance between the ablation front and the beam absorption reg ion, in which the thermal smoothing effect is expected to take place, and t hen, the initial imprint of the laser irradiation nonuniformities can be si gnificantly reduced. Stand-off distance is of great importance in the impri nt mitigation. We measured the preformed plasma profile by using the x-ray side-on backlighting method. The preformed plasma was backlit by x rays and imaged onto an x-ray streak camera. The scale length of the preformed plas ma was found to increase with duration of the soft x-ray irradiation. The e xpansion velocity was found to be 6 x 10(5) cm/s at an x-ray irradiation of 1 x 10(11) W/cm(2). (C) 2001 American Institute of Physics.