Harmonic generation in a plasma with laser-heated electron velocity distributions

The theory of high-order harmonic generation taking into account plasma electron heating due to inverse bremsstrahlung absorption of an intense, high-frequency electromagnetic wave is developed. We consider physical situations in which the electron quiver energy is generally much larger than the electron thermal velocity. High-order harmonic spectra are calculated for two cases: that in which the heating mechanism forms an isotropic electron distribution function, and that in which, instead, an anisotropic electron distribution function is formed having a prolate shape, i.e., elongated perpendicular to the radiation electric-field direction. This kind of anisotropy is found to inhibit considerably the generation of high-order harmonics, because the energy from the radiation field is preferentially transferred, at least in the first stages of interaction, to the electron degrees of freedom perpendicular to the electric-field direction. efficiency of high-order harmonic generation and the kind of anisotropy of the electron distribution is discussed. (C) 1997 Optical Society of America.