Plasma heating by a strong laser field with statistically distributed parameters

Abstract

The energy absorption rate by a classical homogeneous plasma irradiated by a strong fluctuating laser field via inverse bremsstrahlung is considered. A chaotic-field model is used and comparison is made with the fundamental model of a purely coherent field. In the present analysis, the emphasis is put on the interplay between the laser field statistics and the plasma electron energy distribution. Numerical calculations are concerned with the dependence of the energy absorption rates on laser intensity and frequency. Laser intensity values up to 4.6\1015 W/cm2 are considered. The multiphoton structure of the energy absorption is analysed as well. Concerning the joint influence of the radiation and particle statistics on the absorption rate, the basic result may be stated as follows. For situations where the particle thermal velocityv T is larger than the oscillatory velocityv 0 imparted by the field (v T ≥v 0, relatively weak field), the absorption rate is only weakly dependent on the field statistics. For situations, instead, whenv 0≫v T , which occurs for very high intensities, the reverse becomes true: now the initial particle velocity distribution plays the modest role of a velocity spread of an electron beam oscillating atv 0. In general, for very high intensities (v 0≫v T ), the energy absorption via bremsstrahlung becomes less effective because the high oscillatory velocityv 0 reduces the time available to electrons for the interaction with the ions, the third body which makes possible the exchange for energy between electrons and a radiation field. We report also, for the first time, results on the Marcuse effect for the case of a chaotic laser field, along with calculations of the absorption rate for a directed electron beam.