Enhanced laser-driven proton acceleration with gas-foil targets

TL;DR

This study demonstrates through simulations that combining a near-critical-density gas with a solid foil significantly enhances proton acceleration efficiency by increasing maximum proton energy by about four times, due to improved laser absorption and electron dynamics.

Contribution

The paper introduces a novel gas-foil target configuration that markedly improves laser-driven proton acceleration, highlighting the role of relativistic electrons and multidimensional effects.

Findings

Maximum proton energy increased by ~4 times with gas-foil targets.

Relativistic thermal bulk electrons dominate the acceleration field.

Multidimensional effects influence proton layer curvature.

Abstract

We study numerically the mechanisms of proton acceleration in gas-foil targets driven by an ultraintense femtosecond laser pulse. The target consists of a near-critical-density hydrogen gas layer of a few tens of microns attached to a solid carbon foil with a contaminant thin proton layer at its back side. Two-dimensional particle-in-cell simulations show that, at optimal gas density, the maximum energy of the contaminant protons is increased by a factor of $\sim 4$ compared to a single foil target. This improvement originates from the near-complete laser absorption into relativistic electrons in the gas. Several energetic electron populations are identified, and their respective effect on the proton acceleration is quantified by computing the electrostatic fields that they generate at the protons' positions. While each of those electron groups is found to contribute substantially to the overall accelerating field, the dominant one is the relativistic thermal bulk that results from the nonlinear wakefield excited in the gas, as analyzed recently by Debayle et al. [New J. Phys. 19, 123013 (2017)]. Our analysis also reveals the important role of the neighboring ions in the acceleration of the fastest protons, and the onset of multidimensional effects caused by the time-increasing curvature of the proton layer.