High resolution bremsstrahlung and fast electron characterization in ultrafast intense laser–solid interactions

Abstract

The scaling of the intensity, angular and material dependence of bremsstrahlung radiation from an intense ( I > 10 ^18 W cm ^−2 ) laser–solid interaction has been characterized at energies between 100 keV and 1 MeV. These are the first high resolution ( E /Δ E > 200) measurements of bremsstrahlung photons from a relativistic laser–plasma interaction. The measurement was performed using a high purity germanium detector at the high-repetition rate (500 Hz) λ ^3 laser facility. The bremsstrahlung spectra were observed to have a two effective temperature energy distribution which ranged between 80 (± 10) and 550 (± 60) keV depending on laser intensity and observation angle. The two temperatures were determined to result from separate populations of accelerated electrons. One population was isotropic and produced the lower effective bremsstrahlung temperature. The higher bremsstrahlung temperature was produced by an energetic electron beam directed out of the front of the target in the direction of the specular laser reflection, which was also the direction the bremsstrahlung effective temperature peaked. Both effective bremsstrahlung temperatures scaled consistently with a previously measured experimental electron temperature scaling on λ ^3 . The electron populations and bremsstrahlung temperatures were modeled in the particle-in-cell code OSIRIS and the Monte Carlo code MCNPX and were in good agreement with the experimental results. The observed directionality and intensity scaling suggest a significant difference between picosecond and femtosecond duration pulse interactions.