NEW APPROXIMATION FOR CALCULATING FREE-FREE ABSORPTION IN HOT DENSE-PLASMAS

We propose a model for calculating free-free absorption (inverse brems strahlung) in hot dense plasmas. This model writes the total Gaunt fac tor as the product of a static factor and a dynamic factor. The treatm ent of the static part is based on a relation between the absorption c ross section and the elastic scattering cross section, which is exact for very low frequencies and becomes asymptotically correct when the B orn approximation is valid. Generalizing this relation provides an exp ression of the absorption cross section Q(k, k'), which depends on the initial and final wave vectors k and k', as an integral of a unique f unction S(k). The calculation of nondiagonal matrix elements (k not e qual k') is thus avoided. The analytic summation of the high angular m omenta in the partial wave expansion of the cross section makes possib le to apply the model in the limit of weak electron screening. The col lective effects are accounted for in a dynamic Gaunt factor and in an index of refraction different from unity. Numerical results for the Ga unt factor in cesium are presented and discussed. An application to th e mean opacities of carbon is also shown.