Pulsation of laser–plasma interaction explained by density ripple buildup and relaxation for understanding smoothing by random-phase plate, ISI, and broadband

Abstract

In recent years experimental results about the pulsation of reflectivity or 3ω/2 harmonics from laser-irradiated plasma with a period of 10–20 ps initiated a reconsideration of a broad field of phenomena of laser-plasma interaction. We present here numerical results from a very general hydrodynamic computation showing that the pulsation is due to a standing-wave-produced density ripple in the widespread plasma corona causing these after 2 ps at Laue-Bragg reflection at very low density, followed by a hydrodynamic relaxation of the ripple. This cycle has a turnaround time of about 6–10 ps and immediately explains the observation of pulsation. The suppression of pulsation and the achievement of smooth direct drive is then understood by a washing-out process of the ripples when using the random-phase plate (RPP) or broadband irradiation, or by just preventing the buildup of the ripple at the induced spatial incoherence (ISI), where coherence of the laser field is 1 or 2 ps only.