Inverse bremsstrahlung Coulomb logarithm for non-Maxwellian electron distribution functions

We provide analytic expressions for the effective Coulomb logarithm for inverse bremsstrahlung absorption which predict an increase in the absorption by as much as ≈30% over previous estimates. The calculation of the collisional absorption rate of laser energy in a plasma by the inverse bremsstrahlung mechanism usually makes the approximation of a constant Coulomb logarithm. We dispense with this approximation and instead take into account the velocity-dependence of the Coulomb logarithm, leading to a more accurate analytic approximation for the absorption rate valid in both classical and quantum conditions. In contrast to previous work, the laser intensity enters into the Coulomb logarithm. For a Maxwellian electron distribution the absorption rate is enhanced by ≈1-4% above previous estimates. In most laser-plasma interactions the electron distribution function is super-Gaussian [A.B. Langdon, Phys. Rev. Lett. 44, 575 (1980)], and here we find corrections to Langdon’s prediction for the reduction in absorption by factors of ≈2.