Origins of plateau formation in ion energy spectra under target normal sheath acceleration

TL;DR

This paper explains the formation of plateaus in ion energy spectra during target normal sheath acceleration by analyzing electron recirculation effects through particle-in-cell simulations, revealing the influence of laser pulse duration and target thickness.

Contribution

It introduces a new mechanism linking electron recirculation to plateau formation in ion spectra, supported by one-dimensional PIC simulations.

Findings

Electron recirculation causes oscillations in charge density at the target rear.

Transient sheath disruptions lead to peaks in ion energy spectra.

The ratio of laser pulse duration to recirculation period determines plateau formation.

Abstract

Target normal sheath acceleration (TNSA) is a method employed in laser--matter interaction experiments to accelerate light ions (usually protons). Laser setups with durations of a few 10 fs and relatively low intensity contrasts observe plateau regions in their ion energy spectra when shooting on thin foil targets with thicknesses of order 10 $\mathrm{\mu}$m. In this paper we identify a mechanism which explains this phenomenon using one dimensional particle-in-cell simulations. Fast electrons generated from the laser interaction recirculate back and forth through the target, giving rise to time-oscillating charge and current densities at the target backside. Periodic decreases in the electron density lead to transient disruptions of the TNSA sheath field: peaks in the ion spectra form as a result, which are then spread in energy from a modified potential driven by further electron recirculation. The ratio between the laser pulse duration and the recirculation period (dependent on the target thickness, including the portion of the pre-plasma which is denser than the critical density) determines if a plateau forms in the energy spectra.