Investigation of fast-electron-induced K α x rays in laser-produced blow-off plasma.

Refluxing of fast electrons generated by high-intensity, short-pulse lasers was investigated by measuring electron-induced Ka x rays from a buried tracer layer. Using planar foils of Au/Cu/CH, the 150-J, 0.7-ps TITAN short-pulse laser was focused on the gold foil to generate fast electrons and the 3-ns, 300-J long pulse beam irradiated on the CH side to create expanding plasma as a conducting medium. By delaying the short-pulse beam timing from the long pulse laser irradiation, the plasma size was varied to change electron refluxing in the target rear. The total yields and two-dimensional images of 8.05-keV Cu-Ka x ray were recorded with an x-ray spectrometer and two monochromatic crystal imagers. The measurements show that the integrated yields decrease by a factor of 10 from refluxing to the nonrefluxing limit. Similar radial profiles of the Ka images in the rear were observed at all delays. Hybrid-particle-in-cell simulations using plasma profiles calculated by a radiation-hydrodynamic code HYDRA agree well with the measured Ka yields. The simulations suggest that conducting plasma with the size of ~300 µm in the laser direction and ~600 µm in the lateral direction at the density of 2 × 1020 1/cm³ is sufficiently large to prevent electrons from refluxing in the target. The parameters found in this study can be useful in designing experiments utilizing a Ka x-ray source in refluxing regime or a tracer layer in nonrefluxing regime. [ABSTRACT FROM AUTHOR]