Enhanced gamma-ray emission in structured targets irradiated by counter-propagating laser pulses

Previous research has shown that a structured over-dense target irradiated by a high-intensity laser pulse ($I\approx5\times10^{22}$W/cm$^2$) is an efficient source for collimated beams of multi-MeV gamma-rays [Stark et al. PRL 116, 185003 (2016)]. The newly-constructed laser facilities, such as ELI Beamlines, will enable experiments with multiple laser pulses instead of just one. Motivated by this experimental capability, we consider a setup where a structured target with an embedded relativistically transparent channel is irradiated by two counter-propagating laser pulses. Using 2D particle-in-cell simulations, we show that the laser energy conversion rate into multi-MeV photons is significantly increased due to head-on collision between laser-accelerated electrons and a counter-propagating laser. The main advantage of the setup is that the target channel provides automatic alignment between the electrons and the beam.