Mitigation of initial imprinting with diamond ablator

In direct drive inertial confinement fusion, where laser light directly irradiates the target, surface perturbations on the target are seeded by initial imprint due to laser irradiation nonuniformity. It is the initial imprint that become the seed of the hydrodynamic instability, and decisive solutions for the mitigation of initial imprinting is required. We focused on material stiffness of ablator as an idea that was effective for mitigation of imprinting and adopted the diamond with low compressibility as an ablator material. In the imprint experiments, the diamond foils were irradiated with a foot pulse at an intensity of $\sim$ 4.0 $\times$ 10$^{12}$W/cm$^{2}$ with 1.3 ns width, on which a stationary spatial nonuniformity with sinusoidal shape of 100$\mu$m wavelength was imposed by implementing a grid mask. The foils were subsequently accelerated by a uniform main laser pulse of $\sim$ 1.0 $\times$ 10$^{14}$ W/cm$^{2}$ and imprinted perturbation were observed to be amplified by Rayleigh-Taylor instability through face-on x-ray backlight measurements. We deduced the equivalent initial surface roughness for the imprinted foil. We verified the mitigation of initial imprinting with diamond from the quantitative evaluation.