Proton acceleration via high-power laser interactions with near-critical-density foam targets

Ultraintense laser interactions with near-critical density (NCD) targets have emerged as a promising approach to enhance proton energy and flux. In this study, we used three-dimensional particle-in-cell simulations to investigate proton acceleration by a linearly polarized 100 TW, 40-fs laser beam interacting with near-critical-density targets. Four target types were compared: log-pile foam structure, spongelike foam structure, uniform plasma, and solid foil. The results indicate that log-pile foam targets enhance proton yield by up to a factor of 9 and increase total energy by up to a factor of 10, compared to the other target types. This improvement is attributed to the efficient laser absorption (more than twice the absorption of other targets) into the log-pile structure and the multitarget normal sheath acceleration mechanism, which occurs between the surfaces of the log-pile structure within the target. Additionally, the self-generated radial magnetic field, arising from the low-density plasma, collimates the electrons, while the strong quasistatic transverse electric field focuses the proton beams along the laser axis. These improvements in proton generation, coupled with the practically feasible additive fabrication, make the log-pile foam target more advantageous than other NCD target types.