Characterization of sub-picosecond laser-produced fast electrons via modeling of bremsstrahlung with 3-D hybrid PIC simulations for hard x-ray radiography

Broadband hard x-rays from an intense laser-solid interaction are essential for radiographs of high areal density objects and Inertial Confinement Fusion implosion cores. To accurately simulate angular- and time-dependent bremsstrahlung, it is critical to benchmark a numerical code with well-defined fast electron characteristics: an electron energy distribution, divergence angle, and laser-to-electron conversion efficiency. Here, we show validation of a 3-D hybrid particle-in-cell LSP code using angularly resolved bremsstrahlung. By irradiating a 100 µm thick Cu foil with or without a large CH backing (Cu-CH) with a 50 TW Leopard laser (15J, 0.35 ps, 2×10¹⁹ W/cm²), we measured escaped electrons and angularly resolved bremsstrahlung from the refluxing and non-refluxing targets. The measured bremsstrahlung at two angular positions was simultaneously fit to determine a divergence angle and energy of an injected electron beam, while a single slope temperature was inferred from the electron measurement. A fitting result for the Cu-CH target shows that the divergence angle and conversion efficiency are estimated to be 52° ± 8° and 10.8 ± 1.4% when the slope temperature of 1.15 MeV is used. A parameter study for the Cu target shows a similar conversion efficiency, but simulations with any divergence angle match the measurement, indicating that strong electron recirculation in the Cu foil makes the divergence angle indistinguishable. The benchmarked code is further used to simulate angular- and time-dependent bremsstrahlung. Details of the simulated x-ray source for radiography applications will be discussed.