Multibeam laser-plasma interaction at the Gekko XII laser facility in conditions relevant for direct-drive inertial confinement fusion

Laser–plasma interaction and hot electrons have been characterized in detail in laser irradiation conditions relevant for direct-drive inertial confinement fusion.The experiment was carried out at the Gekko XII laser facility in multibeam planar target geometry at an intensity of approximately 3×10^(15)W/cm^(2).Experimental data suggest that high-energy electrons,with temperatures of 20–50 keV and conversion efficiencies ofη<1%,were mainly produced by the damping of electron plasma waves driven by two-plasmon decay(TPD).Stimulated Raman scattering(SRS)is observed in a near-threshold growth regime,producing a reflectivity of approximately 0.01%,and is well described by an analytical model accounting for the convective growth in independent speckles.The experiment reveals that both TPD and SRS are collectively driven by multiple beams,resulting in a more vigorous growth than that driven by single-beam laser intensity.

Time evolution of stimulated Raman scattering and two-plasmon decay at laser intensities relevant for shock ignition in a hot plasma

Laser–plasma interaction(LPI)at intensities 1015–1016 W·cm^-2 is dominated by parametric instabilities which can be responsible for a significant amount of non-collisional absorption and generate large fluxes of high-energy nonthermal electrons.Such a regime is of paramount importance for inertial confinement fusion(ICF)and in particular for the shock ignition scheme.In this paper we report on an experiment carried out at the Prague Asterix Laser System(PALS)facility to investigate the extent and time history of stimulated Raman scattering(SRS)and two-plasmon decay(TPD)instabilities,driven by the interaction of an infrared laser pulse at an intensity^1.2×1016 W·cm^-2 with a^100μm scalelength plasma produced from irradiation of a flat plastic target.The laser pulse duration(300 ps)and the high value of plasma temperature(~4 ke V)expected from hydrodynamic simulations make these results interesting for a deeper understanding of LPI in shock ignition conditions.Experimental results show that absolute TPD/SRS,driven at a quarter of the critical density,and convective SRS,driven at lower plasma densities,are well separated in time,with absolute instabilities driven at early times of interaction and convective backward SRS emerging at the laser peak and persisting all over the tail of the pulse.Side-scattering SRS,driven at low plasma densities,is also clearly observed.Experimental results are compared to fully kinetic large-scale,two-dimensional simulations.Particle-in-cell results,beyond reproducing the framework delineated by the experimental measurements,reveal the importance of filamentation instability in ruling the onset of SRS and stimulated Brillouin scattering instabilities and confirm the crucial role of collisionless absorption in the LPI energy balance.

Collective absorption of laser radiation in plasma at sub-relativistic intensities

Processes of laser energy absorption and electron heating in an expanding plasma in the range of irradiances Iλ^2=1015–1016 W·μm^2/cm^2 are studied with the aid of kinetic simulations.The results show a strong reflection due to stimulated Brillouin scattering and a significant collisionless absorption related to stimulated Raman scattering near and below the quarter critical density.Also presented are parametric decay instability and resonant excitation of plasma waves near the critical density.All these processes result in the excitation of high-amplitude electron plasma waves and electron acceleration.The spectrum of scattered radiation is significantly modified by secondary parametric processes,which provide information on the spatial localization of nonlinear absorption and hot electron characteristics.The considered domain of laser and plasma parameters is relevant for the shock ignition scheme of inertial confinement fusion.