Description
We present the experimental results of proton acceleration obtained at the LFEX laser facility in Osaka, 1 kJ, 1 ps, I > 10¹⁹ W/cm², with foam targets of 3, 8, and 16 mg/cc and 200 microns thick, as well as with thin foil targets. For the 3 mg/cc foams, the proton distribution, emitted on the back side of the target, exhibits two hot temperatures: one of 1 MeV for energies below 8 MeV, and one of 4,5 MeV for energies up to 20 MeV. The novel result is that the number of protons increases by more than a decade, and significantly increasing the conversion efficiency from laser energy to the proton beam. By increasing the angle of incidence of the laser beam on the target from 0 to 15°, we observe that the proton beam remains perpendicular to the back face of the target. We discuss the competition between the different acceleration processes involved, such as TNSA and RPA. In the case of thin foil (6 microns CH), the formation of energy bunches in the proton spectra which are due to the interplay between the protons and the highly charged carbon ions (mostly C6+) which are also accelerated by the lasers. These finding are coherent with theoretical formulation predictions that the carbon ions act as a “Coulomb piston” on protons.