Study Of Free-free Absorption In Plasmas Using Distorted Wave Approximation

Free-free absorptions play important roles in inertial confinement fusion, nuclear physics, astrophysics and X ray laser, etc. As the advancement of spectrum detection, more accurate measurement can be made for free-free absorptions. The classical Kramers formula and hydrogenic approximation are not enough to obtain accurate atomic data for the free-free processes. With the development of experimental technology, hot dense plasmas can be generated in laboratory, where strong coupling appears between ions and ions, electrons and electrons, ions and electrons. More accurate theoretical methods should be developed to meet this challenge.Dipole approximation calculation of free-free absorption cross section was carried out based on the distorted wave approximation. The formulae of free-free absorption cross section were deduced according to the perturbation theory of quantum mechanics and then the formulae were simplified to calculate the reduced matrix elements. The integration on phase method was apllied to calculate the free-free integrals. The standard of dividing points chosen was discussed. The reliability and correctness of the present procedure is verified by comparing the thermal averaged gaunt factor with the results of classical and close-coupling methods. It is found that our program can provide correct result except for energy of the photon absorbed is small. The partial wave summation converged very slowly at low photon energy and therefore the cut off of partial wave summation will result in inaccuracy.The calculation for specific cases showed that for hydrogenic system, the present theoretical method agrees well with accurate quantum treatment except at low photon energy. For non-hydrogenic system, calculations show that the cross section deviate evidently from the hydrogenic approximation, and thus the hydrogenic and Kramers formula is inappropriate.