Efficient backward x-ray emission in a finite-length plasma irradiated by a laser pulse of ps duration

TL;DR

This study uses 2D particle-in-cell simulations to demonstrate that finite-length plasmas irradiated by picosecond laser pulses can efficiently emit hard x-rays in both forward and backward directions, with backward emission being particularly effective.

Contribution

It reveals the mechanism of backward x-ray emission due to laser exit and plasma electric fields, highlighting the importance of laser duration for spatial overlap and efficient backward photon generation.

Findings

Backward emission occurs when the laser exits the plasma.

Backward photon energy conversion efficiency is three times higher than forward.

Backward source size is three times smaller than forward source.

Abstract

Motivated by experiments employing ps-long, kilojoule laser pulses, we examined x-ray emission in a finite-length underdense plasma irradiated by such a pulse using two dimensional particle-in-cell simulations. We found that, in addition to the expected forward emission, the plasma also efficiently emits in the backward direction. Our simulations reveal that the backward emission occurs when the laser exits the plasma. The longitudinal plasma electric field generated by the laser at the density down-ramp turns around some of the laser-accelerated electrons and re-accelerates them in the backward direction. As the electrons collide with the laser, they emit hard x-rays. The energy conversion efficiency is comparable to that for the forward emission, but the effective source size is smaller. We show that the ps laser duration is required for achieving a spatial overlap between the laser and the backward energetic electrons. At peak laser intensity of $1.4\times 10^{20}~\rm{W/cm^2}$, backward emitted photons (energies above 100~keV and $10^{\circ}$ divergence angle) account for $2 \times 10^{-5}$ of the incident laser energy. This conversion efficiency is three times higher than that for similarly selected forward emitted photons. The source size of the backward photons ($5~\rm{\mu m}$) is three times smaller than the source size of the forward photons.