Early-Time Harmonic Generation from a Single-Mode Perturbation Driven by X-Ray Ablation

X-ray ablation dynamics of the planar foil with preimposed sinusoidal ripples is investigated at the SG 100 kJ Laser Facility. A significant fraction of the second harmonics is observed and identified at the beginning of the ablative drive when the amplitude of the perturbation is within the linear regime. With radiation-hydrodynamic simulations and a developed simple model, we can reveal that such a novel phenomenon is due to the fact that a sustained deformation of the ablation front is initiated since the ablation pressure is directed to the normal direction of the perturbation surface. We find that this deformation dominates the early-time perturbation evolution and results in a specific stage in addition to the traditional ablative Richtmyer-Meshkov instability phase. Our results can be applicable to various regions such as implosions in inertial confinement fusion and the dynamics of molecular clouds in astrophysics.