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

TL;DR

This paper introduces a new hybrid-PIC simulation approach that predicts higher laser-to-X-ray conversion efficiencies in high-intensity laser-solid interactions, revealing the importance of collective effects and target geometry.

Contribution

A novel open-source hybrid-PIC code with bremsstrahlung routines and specialized boundary conditions for 3D modeling of X-ray production at high laser intensities.

Findings

Peak X-ray efficiency of 7.4% in gold targets for >1 MeV X-rays

Collective effects significantly influence bremsstrahlung emission

Target size affects conversion efficiency and emission angular distribution

Abstract

We present simulations which predict significantly higher laser to X-ray efficiencies than those previously found in high intensity (1e20-1e22 W/cm2) 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 in 3D. Special boundary conditions are used to emulate complex electron refluxing behaviour, which has been characterised in 2D full-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.