Characterization of Fast Electron Divergence and Energy Spectrum from Modeling of Angularly Resolved Bremsstrahlung Measurements

Characteristics of fast electrons generated in an intense laser-solid target interaction are studied by modeling angularly resolved bremsstrahlung measurements with a hybrid Particle-In-Cell code, Large Scale Plasmas. The experiment was performed using the 50 TW Leopard laser at the Nevada Terawatt Facility. A 100 μm thick Cu foil was irradiated by the 15 J, 0.35 ps laser at a peak laser intensity of 2 × 1019 W/cm2. Bremsstrahlung produced by transport and recirculation of the fast electrons in the foil was measured with two differential filter-stack x-ray spectrometers at 22° and 40° from the laser axis. 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 ± 0.3 MeV and 15° ± 8°, respectively. The Thot inferred from the bremsstrahlung signals agrees with that from an escaped electron measurement, suggesting that that the bremsstrahlung is predominantly produced by the transport of the high energy fast electrons in the first pass.