Characterising x-ray emission from laser-solid interactions and QED plasmas

With the commissioning of new multi-PW laser facilities, laser intensities beyond 10²² Wcm^-2 are now achievable. These lasers can accelerate ions and electrons in solid targets to ultra-relativistic energies, generating a QED plasma where both relativistic and quantum effects must be considered [1]. QED plasmas are predicted to produce intense bursts of hard x-rays through nonlinear Compton scattering (NCS). Indeed, previous simulation work has shown that a bright NCS x-ray flash is a key observable when we transition into the QED plasma regime. However, at current achievable laser intensities of 10²⁰ -10²² Wcm^-2, the additional process of inverse bremsstrahlung emission is also present and generates x-rays that confound the signal from NCS x-rays.

Simulations can be conducted to investigate optimal parameters that simultaneously enhance NCS emission and minimise bremsstrahlung emission including laser intensity, target shape, and target density. The work presented directly compares NCS and bremsstrahlung x-ray emission using results from simulations conducted using EPOCH2D and the hybrid-PIC EPOCH3D extension [2]. The results presented include a comparison of the angular distribution of x-ray energy and laser to photon conversion efficiency for a novel low-Z target in the intensity range 10²⁰ -10²³ Wcm^-2.

[1] C.P. Ridgers et al 2012 PhysRevLett. 108, 165006

[2] S. Morris et al 2021 Phys. Plasmas 28, 103304