Linear response of a variational average atom in plasma: Semi-classical model

The frequency-dependent linear response of a plasma is studied in the finite-temperature Thomas–Fermi approximation, with electron dynamics described using Bloch hydrodynamics. The variational framework of average-atoms in a plasma is used. Extinction cross-sections are calculated for several plasma conditions. Comparisons with a previously studied Thomas–Fermi Impurity in Jellium model are presented. An Ehrenfest-type sum rule, originally proposed in a full quantum approach is derived in the present formalism and checked numerically. This sum rule is used to define Bremsstrahlung and collective contributions to the extinction cross-section. It is shown that none of these is negligible. Each can constitute the main contribution to the cross-section, depending on the frequency region and plasma conditions. This result obtained in the Thomas–Fermi–Bloch case stresses the importance of the self-consistent approach to the linear response in general. Some of the methods used in this study can be extended to the linear response in the quantum case.