Electron kinetic simulations of solid density AI plasmas produced by intense subpicosecond laser pulses. I. Ionization dynamics in 30 femtosecond pulses

The interaction of a 10¹⁸ W/cm², 30 fs laser pulse with solid AI was simulated with the electron kinetic code "FPI" [J. P. Matte et al., Phys. Rev. Lett. 72, 1208 (1994)] in which an improved average ion module was fully coupled to the electron kinetics. It includes electron impact ionization and excitation and their inverse processes: collisional recombination and de-excitation; as well as radiative decay and pressure ionization. We compare to runs without the inverse processes, and also without atomic physics (with 〈Z〉 set to 11). Atomic physics strongly affects the energy balance and the shape of the distribution function. Line radiation is mostly due to three body recombination into excited states after the peak of the pulse, as the plasma cools down. Despite the atomic processes and the high density, strongly non-Maxwellian distribution functions were obtained due to very steep temperature gradients and strong collisional heating, at the peak of the pulse. However, after the pulse, there is a very rapid thermalization of the electron distribution to which inverse processes strongly contribute.