Characterizing Fast Electron Energy Distribution and Divergence in Ultra-Intense Laser-Matter Interactions by Modeling of Angularly Resolved Bremsstrahlung Measurements

Abstract

The interaction of an ultrahigh intensity, short-pulse laser with solids generates a large number of energetic electrons with well above relativistic energies. Characterization of the fast electrons is important for advancing fundamental knowledge of high-energy-density physics and for many applications such as charged particle accelerations, generation of bright x-ray and gamma-ray sources and Fast Ignition laser fusion. This thesis reports experimental and numerical investigation of fast electron characterization using angularly resolved bremsstrahlung measurements and hybrid-particle-in-cell modeling. In an ultra-intense laser experiment, a 100-μm thick copper foil was irradiated by a 50 TW Leopard laser at the Nevada Terawatt Facility. High-energy bremsstrahlung produced by the transport of fast electrons was recorded with two differential filter-stack bremsstrahlung spectrometers in the photon energy range between 10 and 800 keV. A computation study using a 2D hybrid Particle-In-Cell code with self-consistent fields was performed to model the angularly resolved bremsstrahlung spectra. It was found that a comparison to the experimental results show that the energy distribution and divergence can be constrained by multiple angular bremsstrahlung positions. The two-spectrometer signals simultaneously fit by varying single slope temperatures (Thot) and divergence angles (θ) enable for determining Thot and θ to be 1.1±0.3MeV and 15° ± 8°, respectively. Using the same experimental setup, the dependence of the fast electron energy distribution and divergence on the atomic number Z of the target was studied by varying the material (Al, Cu and Ag). Modeling using the 2D hybrid-PIC code Large Scale Plasmas (LSP) shows that Thot remains unaltered with varying atomic number. The inferred fast electron divergence, however, decreases as the atomic number Z was increased.