Plasma physics with intense VUV radiation

Complete test of publication follows. New intense VUV-FEL beams, presently becoming operational, as well as existing intense high harmonics sources make it possible to heat solid foils with photons of 10 - 100 eV energy and to study specifics of the interaction with hot dense matter. First transmission experiments with 38 eV photons performed in autumn 2005 at the VUV-FEL facility 'FLASH' at DESY/Hamburg have reached beam intensities up to 1014 W/cm2 heating metal foils to temperatures of 1-2 eV. Depending on focusing, intensities up to 1018 W/cm2 may become possible generating solid-density plasmas with temperatures up to 1 keV and multi-Gbar pressures. This would open unprecedented possibilities for high-precision dense-plasma studies and would be of great importance for the physics of inertial fusion. The major problem presently to achieve this ambitious goal is an experimental one: improved focusing techniques. Here we are concerned with theoretical modelling of VUV absorption in hot dense matter. In contrast to visible laser light, VUV radiation with frequencies beyond the plasma frequency ω-barp can propagate in the metal foils, though increasing ionization may abruptly block the propagation mode. At higher energies (> 100 eV), energy deposition in metals is dominated by bound-free photo-ionization processes. In the regime 10-100 eV, however, considered here, collisional absorption mediated by inverse bremsstrahlung of conduction electrons still plays a major role. It turns out that VUV collisional absorption is quite different from the well-known optical laser absorption, mainly because the photon energy exceeds the Fermi energy of metals, and target electrons are to be considered as 'slow' electrons in radiative Coulomb collisions has been considered recently by Krainov. Here we use his results to model the VUV absorption coefficients. Results are compared with the recent DESY experiments.