Laser ion acceleration from concave targets: focusing, scaling, and robustness studies*

Proton fast ignition (PFI) is one of the promising approaches for achieving fusion power. PFI employs separate lasers for target compression and heating, potentially enhancing the control over both stages. Achieving this requires precise focusing of energetic proton beams and high laser-to-proton energy conversion efficiency for effective target heating. Additionally, for a future fusion power plant to be economically viable, it must operate at a high repetition rate of at least 10 Hz.

It is widely recognized that laser ion acceleration from concave targets may satisfy the aforementioned requirements for PFI. Despite decades of theoretical and experimental exploration, the parameters of deliverable proton beams and their controllability still need improvement to meet the PFI requirements. To bridge this gap, experimental studies investigating laser-driven proton acceleration and focusing by concave targets are underway at the CSU ALEPH laser facility (20 J, 40 fs), with future experiments planned to cover a wider range of laser and target parameters.

To support the experimental campaign, we perform a numerical study of laser proton acceleration and focusing using hemisphere targets. We use the radiation hydrodynamic code FLASH to estimate the parameters of preplasma and the fully kinetic relativistic Particle-In-Cell code EPOCH to simulate laser-driven proton acceleration. We assess the role of target geometry, characterize the proton focusing characteristics, and evaluate robustness concerning factors such as laser pointing stability and finite laser contrast.