Efficient backward x-ray emission in a finite-length plasma irradiated by a laser pulse of ps duration
arxiv(2024)
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×
10^20 W/cm^2, backward emitted photons (energies above 100 keV and
10^∘ divergence angle) account for 2 × 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 μ m) is three times smaller than the source size of the
forward photons.
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