Tuning of Laser-Driven Proton Beams from Near-Critical Density Targets

Collimation, conversion efficiency, and spectral control remain critical challenges for realizing the full potential of laser-driven proton beams. We perform particle-in-cell simulations of high-intensity laser-plasma interactions with near-critical density targets to first characterize the spectral features and collimation for varying pulse durations – and fixed total energy – with flat targets. Streaming dynamics and electrostatic mode structures are characterized in this relativistically transparent interaction regime. Modifications with concave target shaping are then demonstrated to preferentially populate protons onto the laser axis, and the alterations to the on-axis spectra are discussed in terms of the changes to the flow and mode structures. By identifying the key control parameters in this system, we can potentially tune many properties in the accelerated proton beam.