Description
We present experimental results obtained with PW laser class on LFEX facility in Osaka in 2025. Several types of targets (foams of different densities, foils, and foams with foils) and laser parameters have been scanned. We show in some sets of parameters that the number of MeV protons sharply increases (>10) when the laser is focused on a foam target instead of a foil target (3.1014 to 3.1015 protons/sr). Despite the maximum energy cut-off being higher using foil targets than foams (13MeV to 22MeV).
Several proton diagnostics have been used and compared to get a good estimation of the absolute numbers of protons and find out the laser energy conversion to proton energy. We used RCF stacks, which give a very good spatial distribution of the proton spectrum but with a low energy resolution. Thomson Parabola gives a very high energy resolution in a very small solid angle (<sr). Very fast Time of Flight detectors give the low-energy spectrum. CR39 track detectors also allow information on the proton spectrum and, in some case alpha particles using a Boron target. Nuclear activation sample allows information about protons above the energy threshold of nuclear reaction, such as 11B(p,n)11C and 10B(p,α)7Be. We also explore the production of alpha particles in the topic of the proton boron fusion. By combining the experimental data obtained from all these diagnostics, it was possible to reconstruct the entire proton spectrum and to confirm our results in terms of absolute quantities.
Simulations allowed us to confirm our experimental results and highlight the physical processes involved. For thin foils, TNSA (Target Normal Sheath Acceleration) is the dominant process, unsurprisingly. This corresponds to a surface acceleration of impurities on the back side of the target by a strong electric field perpendicular to the rear side. This field is induced by the MeV electrons accelerated by the interaction of the PW laser with the target front side. This explains the low number of accelerated protons but very high energies. For foam targets, proton acceleration occurs within the foam's volume, either through a multitude of micro-TNSA on the microstructure of the foam and/or by the strong laser radiation pressure. This explains the higher number of accelerated protons but lower energies. Therefore, potential applications must be considered to determine which target to use.