Effect of the surface finish on the stability of targets imploded by thermal radiation

Abstract

The effect of surface finish on the evolution of ICF targets has been studied. As driver for the implosion has been used a temporally profiled thermal radiation pulse (25 ns in duration) as that expected for the case of a high-gain (cm sized) capsule enclosed in a cavity. 2-D numerical simulations have been performed on a planar analog to investigate the stability to perturbations having wavelengths in the interval 2.5–40 μm and initial amplitudes of 0.03 and 1 μm. The simulations show perturbation growing due to hydrodynamic instabilities. The distortion of the ablation front reaches such a level to strongly enhance the ablation rate because of the increase in the exposed area and in the power absorbed per unit area. Two different phenomena have been identified: one, preferentially occurring for small wavelengths, in which the motion behind the first shock wave becomes turbulent and the symmetry is lost; the other, appearing for long wavelengths, in which a thermal radiation wave penetrates into the fuel. This preheating phenomenon is related to the formation of holes in the thermal shield due to the enhanced ablation and to matter transverse piling up. All these effects may render useless the 1-D-based design of a high-gain capsule exposed to a given thermal bath. However, it has been also observed that, for the longer wavelength (40 μm) and a smaller initial perturbation amplitude (e.g. 0.01 μm), during the temperature pulse the preheating does not occur and most of the fuel can be accelerated as a whole to speeds of the order of 2 × 107 cm/s or greater without a substantial onset of transverse velocities, even in the presence of the ablator piling up.