Proton acceleration by a pair of successive ultraintense femtosecond laser pulses

TL;DR

This study explores how two successive ultraintense femtosecond laser pulses can enhance proton acceleration from thin aluminum targets, revealing a time-delay-dependent boost in proton energy and charge through experimental and simulation analyses.

Contribution

It demonstrates the optimal timing for a second laser pulse to enhance proton acceleration, combining experimental results with particle-in-cell simulations to understand the underlying mechanisms.

Findings

Second pulse boosts proton energy and charge for delays below ~0.6-1 ps.

Target surface expansion enhances hot-electron generation by the second pulse.

There is a maximum delay beyond which proton energy enhancement does not occur.

Abstract

We investigate the target normal sheath acceleration of protons in thin aluminum targets irradiated at relativistic intensity by two time-separated ultrashort (35 fs) laser pulses. For identical laser pulses and target thicknesses of 3 and 6 $\mu$m, we observe experimentally that the second pulse boosts the maximum energy and charge of the proton beam produced by the first pulse for time delays below $\sim0.6-1$ ps. By using two-dimensional particle-in-cell simulations we examine the variation of the proton energy spectra with respect to the time-delay between the two pulses. We demonstrate that the expansion of the target front surface caused by the first pulse significantly enhances the hot-electron generation by the second pulse arriving after a few hundreds of fs time delay. This enhancement, however, does not suffice to further accelerate the fastest protons driven by the first pulse once three-dimensional quenching effects have set in. This implies a limit to the maximum time delay that leads to proton energy enhancement, which we theoretically determine.