Efficient generation of high-energy ion bunches via laser-induced cavity radiation pressure acceleration

Ion beams driven by ultra-intense lasers have many applications, such as the proton radiography, hadrontherapy and the fast ignition of inertial confinement fusion targets. An important issue in many of the applications is the problem of laser-to-ion beam energy conversion efficiency, which is generally below 10%. In this contribution we discuss one possible method for improving this parameter by using targets with a “keyhole” structure, where the laser beam is introduced into a cavity through an aperture, which allows for partial “recycling” of the incident laser beam energy by redirecting part of the reflected beam back to the target¹. This mechanism, which was dubbed the laser-induced cavity pressure acceleration (LICPA), has already been successfully applied at sub-relativistic laser intensities², where it was utilized for an efficient generation of dense plasma macroprojectiles, mostly by the thermal plasma pressure generated in the cavity. In this note we discuss the application of the LICPA mechanism in the ultra-intense radiation pressure regime, which would soon become available within the Extreme Light Infrastructure project. We report results of a series of particle-in-cell simulations, mostly in 1D, in which the acceleration of thin foils was investigated for a circularly and linearly polarized super-Gaussian laser pulse with 130 fs FWHM and 10²² W/cm² peak intensity (a slightly defocused 1.3 kJ pulse of a 10 PW ELI laser). It was found that LICPA mechanism is very effective in this regime, allowing for an increse in energies of accelerated ions by as much as a factor of 7 in the case of a 2 µm carbon target. The enhancement factor is smaller for thinner targets, but the energies achieved are higher. Predictions are made also for a 100 PW ELI laser: the enhancement factor due to cavity reflections is smaller, but energies as high as 220 MeV per nucleon were found for thin carbon targets. The results of kinetic simulations are compared with predictions obtained from the cavity-enhanced laser sail acceleration model³, and - interestingly — a good agreement is found. This observation allows us to quickly predict, how the cavity enhancement effect would scale with target parameters and the laser intensity.