Properties of laser-driven hard x-ray sources over a wide range of laser intensities

We present measurements of the hard x-ray emission from targets irradiated at relativistic laser intensities, with the objective of comprehensively characterizing source properties relevant to x-ray radiography backlighting in high energy density experiments. Thin gold foil and tungsten wire targets were irradiated at peak laser intensities varying between 10¹⁸ and 10²¹ W/cm², with laser pulse energies >100 J. We have measured the absolute x-ray yield in the spectral range between 20 and 200 keV, angularly resolved over a large range of emission angles with respect to the incident laser. In addition, we have determined the x-ray source sizes for the two target types in the direction both along and across the target. The results are compared with the predictions of a simple model for the hot electron propagation, x-ray generation, collisional stopping, and expansion cooling. Based on this model, our measurements allow extraction of the laser to hot electron conversion over the wide range of intensities covered by our experiment.