Inverse bremsstrahlung absorption in laser heated plasmas is studied by Fokker-Planck equation, limited to the low-field limit. Compared to the commonly used fitting formula by Matte et al.[1], our work employs fewer approximations and provides more accurate predictions for the super-Gaussian order  and the heating rate. The simulation results show that the super-Gaussian order is generally lower than the fitting results from Matte, which leading to a increase of absorption. However, for the same the super-Gaussian order, the simulated heating rate is lower, thereby the final simulated absorption can either be higher or lower, depending on the conditions. These phenomena are theoretically analyzed by the Fokker-Planck equation. A new fitting formula for the super-Gaussian order and heating rate are proposed, showing a discrepancy within approximately 10% of the simulation results.

laser-plasma interactions,inverse bremsstrahlung absorption,Fokker-Planck simulation