Hard X-ray generation from solids driven by relativistic intensity in the lambda-cubed regime

Interaction of relativistic intensity laser pulses encompassed at focus by a volume of a few wavelengths cubed with solids is examined. Spectroscopy of hard X-rays of several metallic targets, including Cu, Ge, Mo, Ag, and Sn, irradiated in this regime at a high repetition rate (0.4 kHz), has been experimentally studied. The K Delta *a and K Delta *b peaks of all targets were obtained. Averaged electron temperatures of several tens of keV and total X-ray conversion efficiencies up to 0.02% are calculated. The X-ray source size is measured to be ~10 micron with varying elliptical shape.