Buffered spectrally-peaked proton beams in the relativistic-transparency regime

TL;DR

This paper reports the generation of spectrally-peaked proton beams with narrow divergence from ultrathin foils irradiated by intense lasers, demonstrating buffered acceleration due to relativistic transparency effects.

Contribution

It introduces the observation of spectrally-peaked proton beams from ultrathin foils and explains their formation via buffered acceleration in the relativistic transparency regime.

Findings

Proton beams with ~8 MeV energy and ~4 MeV spread were reproducibly generated.

Beam divergence narrows to about 8 degrees with decreasing foil thickness.

Simulations confirm buffered acceleration as the mechanism for spectral peaking.

Abstract

Spectrally-peaked proton beams ($E_{p}\approx 8$ MeV, $\Delta E\approx 4$ MeV) have been observed from the interaction of an intense laser ($> 10^{19 }$ Wcm$^{-2}$) with ultrathin CH foils, as measured by spectrally-resolved full beam profiles. These beams are reproducibly generated for foil thicknesses (5-100 nm), and exhibit narrowing divergence with decreasing target thickness down to $\approx 8^\circ$ for 5 nm. Simulations demonstrate that the narrow energy spread feature is a result of buffered acceleration of protons. Due to their higher charge-to-mass ratio, the protons outrun a carbon plasma driven in the relativistic transparency regime.