Efficient backward x-ray emission in a plasma irradiated by a ps laser pulse

Motivated by experiments employing ps-long laser pulses, we examined x-ray emission in a plasma irradiated by $10^{20}$ W $ m{cm^{-2}}$ ps-long pulse using particle-in-cell simulations. We found that, in addition to the expected forward emission, the plasma also efficiently emits in the backward direction. The backward emission occurs when the laser exits the plasma. The longitudinal plasma electric field that is generated by the laser at the density down-ramp turns around some of the electrons and accelerates them in the backward direction. As the electrons collide with the laser, they emit hundred keV photons. The ps-duration is required for an overlap between the laser and backward energetic electrons. The conversion efficiency is comparable to the forward emission, but the effective source size is much smaller.