Multi-dimensional PIC-simulations of parametric instabilities for shock-ignition conditions

Abstract

Laser-plasma interaction is investigated for conditions relevant for the shock-ignition (SI) scheme of inertial confinement fusion using two-dimensional particle-in-cell (PIC) simulations of an intense laser beam propagating in a hot, large-scale, non-uniform plasma. The temporal evolution and interdependence of Raman- (SRS), and Brillouin- (SBS), side/backscattering as well as Two-Plasmon-Decay (TPD) are studied. TPD is developing in concomitance with SRS creating a broad spectrum of plasma waves near the quarter-critical density. They are rapidly saturated due to plasma cavitation within a few picoseconds. The hot electron spectrum created by SRS and TPD is relatively soft, limited to energies below one hundred keV. 1. SHOCK-IGNITION AND THE IMPORTANCE OF LASER-PLASMA INTERACTION The shock-ignition scheme (1) is based on an energy redistribution of the available energy: the laser pulse inducing compression of the pellet has lower intensity than in the original direct-drive scheme, followed by a second, intense laser pulse (∼ 100ps) which creates an intense short shock that combined with the rebound of the first shock allows the creation of a hot spot. The intensity of the second pulse is up to 10 times higher than the standard ones, and the laser will propagate in a hot (few keV), long, inhomogeneous plasma. For laser plasma-interaction kinetic effects will be dominant. A first estimate on the importance of laser plasma interaction comes from evaluating the local growth rate for parametric instabilities. As a consequence, even though the proposed intensities for the shock pulse are high, Io 2 = 10 15...16 Wm 2 /cm 2 at 3 o, Stimulated Raman Scattering (SRS) was not expected to be a great danger since the pulse propagates in a hot plasma of 3...5keV and the excited electron plasma waves by SRS are in a strongly damped regime. Some concern could come from the fact that a regime of inflationary SRS (2) was predicted, but the main worry was expected to originate from strong Stimulated Brillouin Scattering (SBS) developing along the whole density profile. 1D-simulation investigating this ansatz showed unexpected results (3): after an initial phase SBS is suppressed by absolute SRS and cavitation at nc/4 and subsequent cascade at nc/16. As a result laser absorption is by collective effects and cavitation appears as a dominant mechanism. 2D-simulations confirm some of these results, but show a richer physics (4, 5). Plasma parameters and profiles are motivated by CHIC hydro-simulations of the HiPER target as considered in the 1D-simulations (3). The simulation set up is as follows: the size of the plasma is 160 m × 103 m; Te = 5keV,Ti = 1keV; the density varies from n = 0.04nc to n = 0.4nc, it has exponential profile with scale length Ln = 186 o ≈ 60 m, followed