Dielectric function of an electron-ion plasma in the optical and X-ray regime

The dielectric function of an electron-ion plasma is studied in quasistatic approximation by use of linear response theory and the technique of cluster expansion. The coherent part of the electromagnetic field is treated classically. A general expression for the susceptibility kernel is derived which is exact in the framework of the quasistatic approximation. The approximation implies that the spatial configuration of ions, or more generally the statistical distribution of their positions and velocities, is assumed known, and unaffected by the oscillating electromagnetic field. The approximation is valid in the optical and X-ray regime.