Mitigating laser imprint with a foam overcoating

TL;DR

This study uses radiation hydrodynamic simulations to explore how foam overcoatings can effectively reduce laser imprint by modulating density distribution and shock front oscillations, proposing optimal foam parameters for mitigation.

Contribution

It introduces a detailed physical mechanism for laser imprint mitigation using foam and identifies optimal foam parameters through dimensional analysis.

Findings

Foam reduces laser imprint by modulating density and shock oscillations.

Optimal foam thickness is 2-3 times the perturbation wavelength.

Optimal foam density is 1/2 to 3/2 times the critical density.

Abstract

Foam has been suggested to reduce laser imprint because of its low density. In this paper, the two-dimensional radiation hydrodynamic code FLASH is applied to investigate and characterize the strength of laser imprint through analyzing areal density perturbation. There are two important factors for the mitigation of laser imprint besides the thermal smoothing of the conduction region (between the ablation front and the critical density surface) and the mass ablation of the ablation front. First, radiation ablation dynamically modulates density distribution not only to increase the frequency of the perturbed ablation front oscillation but also to decrease the amplitude of oscillation. Second, a larger length of the shocked compression region reduces the amplitude of the perturbed shock front oscillation. The smaller the perturbation of both ablation front and shock front, the smaller the areal density perturbation. Based on the above physical mechanisms, the optimal way of mitigating laser imprint with foam is that the dynamically modulated density distribution further reduces the amplitude of perturbation reaching the solid CH when the areal density perturbation of foam oscillates to the first minimum value. The optimal ranges of foam parameters to mitigate laser imprint are proposed with the aid of dimensional analysis: the foam thickness is about 2~3 times the perturbation wavelength, and the foam density is about 1/2~3/2 times the mass density corresponding to the critical density.