Thus, suppression of laser imprinting due to irradiation nonuniformity is important for direct-drive ICF targets. When a target is irradiated with intense laser light, a core plasma is formed and expands over the target surface. The target is imploded by the pressure of plasma formed on the surface. Since stiffer and denser materials reduce surface perturbations due to spatially nonuniform laser irradiation, diamond is a top candidate as an ablator material for direct-drive ICF targets.

In this study, the researchers fabricated diamond capsules to investigate the influence of material strength on the generation of perturbation on a diamond surface by changing laser irradiation nonuniformity (laser intensity variation) at the GeKKO XII during the process of verifying the suppression effects in reducing surface perturbations.

They measured areal-density perturbations using x-ray shadowgraphy. While with irradiation non-uniformity ~10%, typical sinusoidal-like perturbations were seen, under large irradiation non-uniformity of ~40%, a non-sinusoidal perturbation with a “sharp” structure was observed.

They also conducted two-dimensional radiation hydrodynamic simulations in order to clarify the mechanisms of the perturbation with the “sharp” structure. From previous shock compression experiments, the Hugoniot elastic limit HEL (or elastic yield strength) on diamond applied by laser irradiation was measured around 80 GPa.

When the irradiation nonuniformity was large (40%), two regions (elastic and plastic states) appeared on the diamond surface. Most of the compressed area was over the HEL and crystal was collapsed and transformed into a plastic state. From this, the mechanical local fracture due to the elastic-plastic transition is the most probable interpretation for the generation of the crack-like structure.

This crack-like structure is not good for laser-induced fusion using diamond capsules. Thus, this group used diamond with a thin high atomic number (high-Z) coating on its surface (Cu coating (0.1 μm^t) foils) so as to mitigate surface perturbations due to nonuniform irradiation, through the effects of laser-induced plasmas. As a result, due to mitigation effects by laser-induced plasmas, the crack-like structure did not occur.

The 2D simulations also showed that surface perturbations were reduced due to mitigation of the pressure perturbation through the effects of laser-induced plasmas. This means a local fracture on the surface was suppressed by relaxation of the pressure perturbation.

This group’s achievements will lead to more efficient fuel compression in ICF. Laser nuclear fusion research is important as basic research on plasmas at high temperature and density. The knowledge from this study, which is deeply related to the elemental process of laser processing technologies, will be also applied to industry.

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