High energy X-ray production in laser-solid interactions

The radiation produced in laser-solid interactions has been characterised at intensities relevant to petawatt-class short-pulse lasers (1e20 to 1e22 W/cm²). Particular attention is paid to photons of energy over 1 MeV which emerge from synchrotron radiation in the laser focal spot, and from bremsstrahlung radiation as hot electrons traverse the solid. Bremsstrahlung modelling was performed in 3D simulations using a novel hybrid-PIC code, where it was found that the emission lasted significantly longer than the laser pulse. For a driving pulse of 1 μm wavelength and 40 fs duration, the bremsstrahlung emission was found to last on the order of 10-100 ps. The efficiency of laser energy to bremsstrahlung radiation was found to be significantly higher than that reported by earlier simulations using full-PIC codes, with a peak efficiency of (7.4 ± 1.0)% for a 1e22 W/cm² shot on a cubic gold target of side length 100 μm. A simple analytic model is provided to estimate these bremsstrahlung efficiencies.

The hybrid-PIC code has been benchmarked against experiments on the Vulcan petawatt laser, and an empirical model has been used to emulate the complex electron refluxing behaviour on the boundaries of the simulation window. The free parameters of this model were set using both 1D and 2D full-PIC simulations. The role of self-generated magnetic fields on the X-ray efficiency was also investigated.

Despite the high bremsstrahlung efficiencies, 2D full-PIC and 3D hybrid-PIC codes were run together to demonstrate that synchrotron radiation could still dominate bremsstrahlung in some set-ups with petawatt-class lasers. In a simulation where a laser of intensity 1e22 W/cm² and wavelength 1 μm shot a plastic target, the efficiency of laser energy to synchrotron radiation was found to be 0.84%, while the bremsstrahlung efficiency measured only 0.083%.