Kinetic effects of inverse bremsstrahlung absorption in the low-field limit

Inverse bremsstrahlung absorption in laser-heated plasmas is studied using the Fokker–Planck equation in the low-field limit. Compared with the commonly used fitting formulas of Langdon and Matte et al., our work employs fewer approximations and provides more accurate predictions for the super-Gaussian order β and the heating rate. Simulation results show that the super-Gaussian order is generally lower than the fitting results of Matte et al., which leads to an increase in absorption. However, we find two other factors that reduce absorption: the high-order term of the collision frequency and the effects caused by high laser intensity. Therefore, the final simulated absorption can either be higher or lower, depending on the conditions. These phenomena are theoretically analyzed using the Fokker–Planck equation. Fitting formulas are proposed for the super-Gaussian order and the heating rate, showing a discrepancy within ∼10% of the simulation results. We also compare the simulation results with the experimental results from several recent papers.