Laser ion acceleration from concave targets by subpicosecond pulses

Laser-driven ion acceleration is a promising approach to enable sources of high-charge, ultrashort pulses of ~MeV ions for a broad range of applications including inertial fusion, imaging, and medicine. Tight focusing of proton beams is especially beneficial in the Proton Fast Ignition (PFI) scheme of inertial fusion. The current approach towards focusing ~MeV protons onto fusion fuel in PFI is using the concave rear side of the proton-accelerating target. Experimental studies attempting to optimize the target design, understand the underlying physics of proton focusing, and implement diagnostic tools are underway, with a focus on scaled-down (~10 J, ~100 fs) and intermediate (~1 kJ, ~5 ps) laser parameters compared to those needed for PFI (~100 kJ, ~10 ps). To help with experimental design, data interpretation, and to benchmark modeling tools, we conduct a comprehensive study of laser-driven proton acceleration from hemisphere targets by subpicosecond pulses. Using Particle-in-Cell (PIC) codes EPOCH and Smilei, we investigate the effect of laser and target parameters on proton acceleration and focusing. Specifically, we address the role of target curvature and opening angle, and analyze the role of experimental imperfections, e.g., finite laser contrast and laser pointing stability. Finally, we assess the feasibility of using analytical models of plasma focusing to predict the long-term evolution of focused proton beams, and discuss the scalability of our results to picosecond laser durations.