HARMONIC-GENERATION IN A PLASMA WITH LASER-HEATED ELECTRON VELOCITY DISTRIBUTIONS

The theory of high-order harmonic generation taking into account plasm a electron heating due to inverse bremsstrahlung absorption of an inte nse, high-frequency electromagnetic wave is developed. We consider phy sical situations in which the electron quiver energy is generally much larger than the electron thermal velocity. High-order harmonic spectr a are calculated for two cases: that in which the heating mechanism fo rms an isotropic electron distribution function, and that in which, in stead, an anisotropic electron distribution function is formed having a prolate shape, i.e., elongated perpendicular to the radiation electr ic-field direction. This kind of anisotropy is found to inhibit consid erably the generation of high-order harmonics, because the energy from the radiation field is preferentially transferred, at least in the fi rst stages of interaction, to the electron degrees of freedom perpendi cular to the electric-field direction. efficiency of high-order harmon ic generation and the kind of anisotropy of the electron distribution is discussed. (C) 1997 Optical Society of America.