Ultrahigh-intensity inverse-Bremsstrahlung absorption

We calculate the rate of collisional absorption for ultrahigh-intensity (UHI) laser-plasma interaction when the plasma temperature is nonrelativistic and the electron quiver velocity is fully relativistic. This regime is relevant to most of the UHI experiments. Both linear and circular polarization cases are considered. It is shown that, the dipole approximation may break down in the nonrelativistic regime.