Fast-electron transport and induced heating in aluminum foils

Beams of fast electrons have been generated from the ultra-intense laser interaction (6 × 10¹⁹ W cm^-2, 40 fs) with aluminum foil targets. The dynamics of fast-electron propagation as well as the level of induced in-depth heating have been investigated using the optical emission from the foil's rear side. The dependence of the emitted signals spectrum and size on the target thickness allowed the identification of the coherent (coherent transition radiation) and incoherent (thermal radiation) mechanisms of the optical emission. We demonstrate a two-temperature energy distribution for the laser-generated fast-electron population: a divergent bulk component (θ^bulk=35°±5°) with ≈35% of the laser focal spot energy and a 400-600 keV temperature, plus a relativistic tail highly collimated (θ^tail=7°±3°), with a 10 MeV temperature and a periodic modulation in microbunches, representing less than. 1 % of the laser energy. Important yields of thermal emission, observed for targets thinner than 50 μm, are consequence of a hot plasma near the front surface. The important heating at shallow depth (< 15 μm) results from collective mechanisms associated to the fast-electron transport, in particular from, a resistive heating upon the neutralizing return current of background electrons. For deeper layers, because of the bulk component divergence, the fast-electron energy losses are dominated by collisions.