Abstract
We present simulations, which predict significantly higher laser to x-ray efficiencies than those previously found in high-intensity (1020–1022W cm−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 modeling 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 behavior, which has been characterized in 1D and 2D full-PIC simulations. The peak x-ray efficiency was recorded in thick gold targets, with (7.4±1.0)% 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.
Highlights
When a high-intensity laser pulse strikes a solid target, the illuminated surface is ionised and forms a plasma layer
This plasma is further heated by the laser, injecting a large current of high energy electrons into the solid, with a roughly exponential energy distribution.1. Multipetawatt laser facilities such as ELI2 and Apollon3 are expected to reach intensities between 1022-1023 Wcm−2, creating hot electrons over 100 MeV in energy. Such electrons could lead to efficient X-ray generation through either nonlinear Compton scatter (NCS) in the laser focus, or through bremsstrahlung as the electrons traverse the solid
We have developed a hybrid extension25 to the PIC code EPOCH,18,26,27 including resistive fields and elastic scatter equations,28,29 with additional bremsstrahlung and Moller scatter algorithms adapted from Geant4.22–24 This provides a similar functionality to the hybrid-PIC
Summary
When a high-intensity laser pulse strikes a solid target, the illuminated surface is ionised and forms a plasma layer This plasma is further heated by the laser, injecting a large current of high energy (hot) electrons into the solid, with a roughly exponential energy distribution.. Multipetawatt laser facilities such as ELI2 and Apollon are expected to reach intensities between 1022-1023 Wcm−2, creating hot electrons over 100 MeV in energy Such electrons could lead to efficient X-ray generation through either nonlinear Compton scatter (NCS) in the laser focus, or through bremsstrahlung as the electrons traverse the solid. Several groups have already characterised the Xray emission in laser-solid simulations by adding bremsstrahlung radiation to particle-in-cell (PIC) codes, and treating the solid as a cold, dense plasma.10–17 This approach has the advantage of directly modelling the absorption of laser energy in the pre-plasma, but requires a large number of computational macro-particles to suppress self-heating..
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