Quasi-Monoenergetic, Low-Divergence Proton Beams via Radiation Pressure Acceleration in Laguerre-Gaussian Laser-Driven Disk-shaped CH Target

Abstract High-quality proton beams with low energy spread and divergence are essential for applications such as fast ignition in inertial confinement fusion, isochoric heating of warm dense matter, and ion radiotherapy. Laser-driven ion acceleration via radiation pressure acceleration (RPA) offers promise for generating quasi-monoenergetic beams, yet it is often limited by instabilities and transverse particle expansion. Here, we propose an enhanced RPA scheme employing a mass-limited disk-shaped carbon-hydrogen (CH) target irradiated by an ultra-intense circularly polarized Laguerre-Gaussian (LG) laser pulse. The target's radius is matched to the LG focal spot to confine particles within the laser's ponderomotive potential well, thereby suppressing transverse instabilities. Additionally, the CH composition provides a buffering effect through heavier carbon ions, while the disk geometry minimizes cold electron reflux to sustain strong accelerating fields. Three-dimensional particle-in-cell simulations demonstrate the generation of a quasi-monoenergetic proton beam by employing a laser intensity of $\sim$5$\times$10$^{22}$ W/cm$^{2}$ with cutoff energy near 1 GeV, peak energy of 795.3 MeV, energy spread of 7.39$\%$, and divergence angle of 2.16$^\circ$. Compared to Gaussian laser counterparts, this configuration yields superior beam quality, with higher energy, improved monochromaticity, and reduced divergence. Parametric studies confirm the scheme's robustness across variations in density, thickness, carbon-to-hydrogen ratio, and laser intensity. These results highlight the potential of LG-driven disk CH targets for advancing compact, high-performance ion sources.