Direct Diagnostic of Multi-Temperate Fast Electrons Beams on UHI Laser-Solid Interactions by Optical Transition Radiation Diagnostic

Within the context of fast electron transport in dense matter, we report on results from high repetition rate-type ultra-intense laser (up to 9 10''19 W/cm''2) interaction with Aluminium foil targets. In such extreme interaction conditions, an intense and collimated supra-thermal electron current is injected form the plasma created on the laser focal spot into the cold interior of the target. A detailed characterisation of the accelerated hot electrons is fundamental for the global understanding of the highly complex physics involves in its generation and transport through dense matter, ruled by both collisions and self-induced field effects, and of crucial importance for several applications like the laser-produced proton and heavy ion beam sources and the inertial fusion fast ignitor. The geometry and dynamics of the fast electron propagation in the conducting targets has been directly investigated by means of the Optical Transition Radiation (OTR) emitted when the fast electrons cross the target-into-vacuum boundary. We estimate a fast-electron population with a temperature of 1 MeV and a total kinetic energy of about 50% of the on target laser energy. A simultaneous coherent and incoherent character of the emitted radiation allowed to conclude on both the micro-bunching and the small radial spreading of the relativistic component on the electron distribution is pointed out. We discuss on if it corresponds to a higher temperate (between 5 and 10 MeV) or just to the tail of the 1 MeV bulk population. The OTR-based diagnostics reveal to be the only capable to characterize in detail the form of the electron distribution at high energies. (Author)