Experiments of multi-keV x-ray production from pre-pulsed germanium foils

Previous work with laser pre-exploded thin foils of titanium (He alpha at 4.7 keV) and copper (He alpha at 8.3 keV) showed high multi-keV x-ray conversion efficiencies. They are increased by a factor of more than 2 in comparison with solid materials and are close to gas targets. Experiments with a thin foil irradiated with 2 laser pulses (one delayed in time) lead to hot and underdense plasmas, which are efficient to produce multi-keV K-shell emission. Exploded thin foil experiments have been performed on the OMEGA laser facility at LLE (University of Rochester) to quantify the multi-keV x-ray output from germanium targets. X-ray power was measured by filtered diodes (DMX broadband spectrometer), which was fit to the germanium K-shell emission around 10.3 keV. Within the spectral bandwidth of 10 $<$ h$\nu$ $<$ 13 keV, a conversion efficiency enhancement by a factor of 2.2 is measured relative to the case without pre-pulse.