Experimental characterization of fast electron stopping power in dense media ranging from solid to warm and dense matter

The intrinsic high gain of the fast ignition scheme for inertial confinement fusion makes it an ideal candidate for energy production at the industrial level. It demands a high-power laser system delivering an energy in the level of 100 kJ, generating a high current density fast electron beam which must depose a minimum of ~20 kJ over 20 ps into a 20 µm radius lateral hot spot in the compressed fuel core. The fast electron transport through dense deutered plasma with steeply increasing density and temperature determines the efficiency of the energy coupling. The stopping power in warm dense matter is not yet well known. Resistive effects associated to higher density currents in regions closer to the fast electron source can enhance energy deposition far from the core.

The experimental research presented here, carried out in sub-ignition energy facilities, describes and quantifies the fast electrons energy loss mechanisms, namely the transition from the resistive to collisional-loss regimes as a function of the electron current density. In Al foil targets, we detect the increase of both energy loss mechanisms with planar compression and target heating to temperatures close to the Fermi temperature. In the temperature and density conditions achieved by cylindrical compression of CH targets, the increase of the collisional energy losses with density is compensated by the decreasing resistive energy losses due to the passage of the material resistivity to the high temperature Spitzer regime