Numerical study of the ablative Richtmyer-Meshkov instability of laser-irradiated deuterium and deuterium-tritium targets

The Richtmyer-Meshkov instability (RMI) at the ablation front of laser-irradiated planar targets is investigated by two-dimensional numerical hydrodynamics simulations The linear evolution of perturbations seeded either by surface roughness or target inhomogeneity is studied for perturbation wavelengths in the range 10 <= lambda <= 400 mu m and laser intensity 4 X 10(12) <= I <= 4 X 10(14) W/cm(2) (with laser wavelength lambda(laser) = 0 35 mu m) Thin and thick cryogenic deuterium or deuterium-tritium (DT) planar targets are considered For targets irradiated at constant intensity, it is found that perturbations with wavelength below a given threshold perform damped oscillations, while perturbations above such a threshold are unstable and oscillate with growing amplitude This is qualitatively in agreement with theoretical predictions by Goncharov et al [Phys Plasmas 13, 012702 (2006)], according to which ablation related processes stabilize perturbations with kD(c) >> 1, where D(c) is the distance between the ablation front and critical density for laser propagation For kD(c) < 1 a weakly growing Landau-Darrieus instability (LDI) is instead excited The stability threshold increases substantially with laser intensity, given the dependence of D(c) on laser intensity I (roughly D(c) proportional to I, according to the present simulations) Direct-drive laser fusion targets are irradiated by time-shaped pulses, with a low intensity initial foot In this case, perturbations with wavelengths below some threshold (about 10 mu m, for typical ignition-class all-DT targets) are damped after an initial growth In a thin target, initial perturbations, either damped or amplified by RMI and LDI, seed the subsequent Rayleigh-Taylor instability Finally, it is shown that RMI growth of fusion targets can be reduced by using laser pulses including an initial adiabat-shaping picket (originally proposed to reduce the growth of Rayleigh-Taylor instability) (C) 2010 American Institute of Physics [doi:10.1063/1.3505112]