Efficient multi-keV X-ray sources from laser-exploded metallic thin foils

A set of materials--titanium, copper, and germanium--has been experimented with at the OMEGA laser facility [Boehly, Opt. Commun. 133, 495 (1997)] by irradiating thin foils with a prepulse prior to a main pulse with variable delay, in order to design efficient x-ray laser-sources for backlighting, material testing, and code validation. This concept led to increasing factors from 2 to 4 comparing to cases without prepulse, in the experimental conditions. As a result, high multi-keV x-ray conversion rates have been obtained: 9% for titanium around 4 keV, 1% for copper around 8 keV, and 2.5 to 3% for germanium around 10 keV, which places these pre-exploded metallic targets close to the gas with respect to their performance, with wider energy range. A good agreement with hydroradiative code FCI2[Schurtz, Phys. Plasmas 7, 4238 (2000)] calculations is found for titanium and copper on all diagnostics, with nonlocal-thermal-equilibrium atomic physics and, either nonlocal thermal conduction taking self-generated B-fields into account, or limited thermal conduction with intensity-dependent factor f. The results for germanium indicate that dielectronic processes could play a more significant role when higher irradiation intensity on higher Z material.