Fast Free–Free Absorption-Coefficient Calculations Including Plasma Screening

Inverse bremsstrahlung (free–free) absorption is the key absorption mechanism in inertial confinement fusion plasma conditions. Current models for free–free absorption often neglect plasma screening, which becomes relevant at high densities. We present a fast and robust code that utilizes quantum mechanical calculations of the Gaunt factor using a screened potential. Energy-dependent Gaunt factors are calculated using an efficient semiclassical method. Partial-wave summation is accelerated for high energies by a Born approximation-based method, resulting in faster convergence. Through Gaunt factor tables of wide-ranging electron energies and plasma conditions, a parametrized analytical fit was created. This can serve as a low-cost model for the effect of plasma screening that can be implemented in radiation-hydrodynamics codes. Energy-dependent Gaunt factors can be averaged based on a distribution of energies. This material is based upon work supported by the Department of Energy [National Nuclear Security Administration] University of Rochester “National Inertial Confinement Fusion Program” under Award Number DE-NA0004144.