Characterization of short-pulse laser-produced fast electrons by 3D hybrid particle-in-cell modeling of angularly resolved bremsstrahlung

The interaction of an intense short-pulse laser with a solid target efficiently generates energetic (fast) electrons above the energy of 1 Mega-electronvolt (MeV). Characterization of such high-energy electrons is critical for numerous applications, such as the generation of secondary particle sources, the creation of warm dense matter (WDM), advanced fusion concepts, and intense x-ray radiation for probing complex high areal density objects and inertial confinement fusion (ICF) fusion cores. However, determining laser-driven fast electron characteristics, specifically, electron energy distribution, divergence angle, and laser-to-electron conversion efficiency, has been challenging partly due to complex electron trajectories caused by electric sheath potential, known as electron recirculation. This thesis reports on developing a novel fast electron characterization technique by modeling angularly resolved bremsstrahlung radiations with a three-dimensional (3D) hybrid Particle-in-cell (PIC) code. An experiment using a 50-TW Leopard laser (15 J, 0.35 ps, 2×10^19 W/cm2) was carried out to measure bremsstrahlung radiations at two angular positions and escaped fast electrons along the laser axis for two types of targets: a 100-μm- thick Cu foil and a same Cu target with a CH backing (Cu-CH target). A 3D hybrid-PIC code, Large Scale Plasma (LSP), is extensively used in this work to simulate the electron transport within the solid target, including electron recirculation around the target, and the x-ray generation of absolute photon yields. The measurements were fitted with a series of simulations by varying all three electron parameters. Fitting results based on chi-squared analyses show good agreements for both target types when the electron slope temperature of 0.8 MeV, the divergence angle of 70 degrees, and the electron beam energy of 1.3 J are used. Furthermore, the effects of electron recirculation on bremsstrahlung generation and the enhancement of a short-pulse laser-produced x-ray intensity in various foil thicknesses are numerically studied. These results provide insight into designing and optimizing an x-ray source target for broadband x-ray radiography of a magnetically compressed aluminum rod at the Zebra pulsed power laboratory