Highly efficient conversion of laser energy to hard X-rays in high intensity laser-solid simulations

We present simulations which predict significantly higher laser to X-ray efficiencies than those previously found in high intensity (1020-1022 Wcm-2) laser-solid simulations. The bremsstrahlung emission is shown to last for 10-100 ps, which is difficult to model with conventional particle-in-cell (PIC) codes. The importance of collective effects is also demonstrated, showing the limitations of Monte Carlo modelling in these systems. A new, open-source hybrid-PIC code with bremsstrahlung routines has been developed to model this X-ray production. Special boundary conditions are used to emulate complex electron refluxing behaviour, which has been characterised in 2D-PIC simulations. The peak X-ray efficiency was recorded in thick gold targets, with 7.4% conversion of laser energy into X-rays of energy 1 MeV or higher. The target size is shown to play a role in the conversion efficiency and angular distribution of emitted X-rays, and a simple analytic model is presented for estimating these efficiencies.