Optimizing proton acceleration using hollow laser pulses and a double-layer target: effects of focal spot position and polarization

Abstract

We study the influence of hollow laser pulse settings, specifically adjustments to the focal plane and the phase structure and polarization, on the acceleration of high-energy, low-divergence proton beams from a double-layer target using a feasible laser energy (~ 3.7 J). We investigate the laser's relativistic self-focusing dynamics in the near-critical plasma layer of the target and examine hot electron generation and proton acceleration using three-dimensional particle-in-cell (PIC) simulations. First, we vary the focal spot position of a tightly focused twisted driver, leading to distinct laser beam width evolution that impacts hot electron generation in the near-critical plasma, particularly in the transverse direction. The resulting proton beams differ in maximum longitudinal momentum (0.29-0.35 mi c) and transverse profile. The most efficient acceleration, with a cutoff energy of 40 MeV and ultra-low divergence (~25 mrad), occurs when the driver is focused at the center of the target.
Next, we modify the phase structure and polarization of the laser pulse, transitioning from a linearly polarized twisted beam to cylindrical vector beams with radial and azimuthal polarization, all exhibiting hollow, cylindrically symmetric intensity profiles. This change primarily affects the proton beam's maximum energies, influenced by the ultra-relativistic direct laser-accelerated (DLA) electrons, efficiently generated only by the radially polarized and twisted laser pulses. However, the bulk properties of the proton beam remain similar, as the relativistic mid-energy electrons (< 25 MeV), which sustain a strong sheath field, are the main contributors to proton acceleration in this setup.