Radiation Pressure Acceleration: the factors limiting maximum attainable ion energy

TL;DR

This paper investigates the fundamental and practical factors limiting the maximum ion energy in radiation pressure acceleration, emphasizing the roles of laser group velocity, target expansion, and incidence angle.

Contribution

It identifies and analyzes the combined effects of group velocity, target transverse expansion, and laser incidence angle as key limits in RPA, providing insights for optimizing ion acceleration.

Findings

Group velocity less than light speed limits ion energy.

Target expansion reduces areal density, terminating acceleration.

Off-normal laser incidence further constrains maximum ion energy.

Abstract

Radiation pressure acceleration (RPA) is a highly efficient mechanism of laser-driven ion acceleration, with with near complete transfer of the laser energy to the ions in the relativistic regime. However, there is a fundamental limit on the maximum attainable ion energy, which is determined by the group velocity of the laser. The tightly focused laser pulses have group velocities smaller than the vacuum light speed, and, since they offer the high intensity needed for the RPA regime, it is plausible that group velocity effects would manifest themselves in the experiments involving tightly focused pulses and thin foils. However, in this case, finite spot size effects are important, and another limiting factor, the transverse expansion of the target, may dominate over the group velocity effect. As the laser pulse diffracts after passing the focus, the target expands accordingly due to the transverse intensity profile of the laser. Due to this expansion, the areal density of the target decreases, making it transparent for radiation and effectively terminating the acceleration. The off-normal incidence of the laser on the target, due either to the experimental setup, or to the deformation of the target, will also lead to establishing a limit on maximum ion energy.