Generation of high-order harmonics in plasmas of multicharged atomic ions produced by an intense laser pulse.

The yield of high-order harmonics has been derived for relativistic plasmas of multicharged atomic ions produced by an intense, linearly polarized laser pulse. Harmonics of the laser field are excited at the elastic electron-ion collisions in plasmas. In the nonrelativistic case only odd harmonics can be excited. The ultrarelativistic limit is considered when both even and odd harmonics are excited with the comparable efficiencies. Drift electron velocities are assumed to be small compared to the quiver electron velocities. It is shown that the harmonic yields do not depend on the instantaneous energy distribution of electrons produced at the multiple ionization on the leading edge of the laser pulse. The harmonic conductivity is a tensor with odd components along the laser polarization and with even components along the direction of propagation of the laser pulse. This conductivity tensor has been derived analytically in the ultrarelativistic limit. Possible observation of even harmonics is discussed with respect to the Weibel effect in laser plasmas.