Reduction of Early-Time Perturbation Growth in Ablatively Driven Laser Targets Using Tailored Density Profiles

We investigate analytically and numerically 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. Inverting the acceleration of the ablation front during the shock transit time could reduce the early-time mass perturbation amplitudes developed in the target after the shock transit. This principle was first suggested for mitigating the RT instability of imploding Z-pinches [Velikovich et al., Phys. Rev. Lett. 77, 853 (1996); Phys. Plasmas 5, 3377 (1998)]. As the shock wave slows down propagating into higher density layers, the effective gravity near the ablation front has the same direction as the density gradient. This makes the mass perturbations near it oscillate at a higher frequency and at a lower amplitude than they normally would due to the "rocket effect" caused by mass ablation [Sanz, Phys. Rev. Lett. 73, 2700 (1994); Piriz et al., Phys. Plasmas 4, 1117 (1997)]. So, tailoring density profiles instead of using flat densities is demonstrated to reduce the "seed" mass perturbation amplitude at the onset of the exponential RT growth.
This article appeared in Physics of Plasmas, v6, article 3283, 1999.