Reduction of early-time perturbation growth in ablatively driven laser targets using tailored density profiles

The effects of tailoring the density profile in a laser target in order to decrease imprinting of mass perturbations due to the long-wavelength modes are investigated analytically and numerically. Inverting the acceleration o f the ablation front during the shock transit time could reduce the early-t ime mass perturbation amplitudes developed in the target after the shock tr ansit. This principle was first suggested for mitigating the Rayleigh-Taylo r (RT) instability of imploding Z-pinches [Velikovich , Phys. Rev. Lett. 77 , 853 (1996); Phys. Plasmas 5, 3377 (1998)]. As the shock wave slows down p ropagating into higher density layers, the effective gravity near the ablat ion front has the same direction as the density gradient. This makes the ma ss perturbations near it oscillate at a higher frequency and at a lower amp litude than they normally would due to the "rocket effect" caused by mass a blation [Sanz, Phys. Rev. Lett. 73, 2700 (1994); Piriz , Phys. Plasmas 4, 1 117 (1997)]. So, tailoring density profiles instead of using flat densities is demonstrated to reduce the "seed" mass perturbation amplitude at the on set of the exponential RT growth. (C) 1999 American Institute of Physics. [ S1070-664X(99)00908-8].