Spectral modification of shock accelerated ions using hydrodynamically shaped gas target

TL;DR

This study demonstrates how hydrodynamically shaped gas targets influence shock acceleration of ions, enabling control over ion energy spectra, with experimental results supported by 2D PIC simulations.

Contribution

It introduces a method to modify ion energy spectra via hydrodynamic shaping of gas targets, advancing control over shock acceleration processes.

Findings

Long density gradients produce broadband ion beams.

Short density gradients enable quasi-monoenergetic proton acceleration.

Ion energy distribution depends on plasma density profile.

Abstract

We report on reproducible shock acceleration from irradiation of a $\lambda = 10$ $\mu$m CO$_2$ laser on optically shaped H$_2$ and He gas targets. A low energy laser prepulse ($I\lesssim10^{14}\, {\rm Wcm^{-2}}$) was used to drive a blast wave inside the gas target, creating a steepened, variable density gradient. This was followed, after 25 ns, by a high intensity laser pulse ($I>10^{16}\, {\rm Wcm^{-2}}$) that produces an electrostatic collisionless shock. Upstream ions were accelerated for a narrow range of prepulse energies ($> 110$ mJ & $< 220$mJ). For long density gradients ($\gtrsim 40 \mu$m), broadband beams of He$^+$ and H$^+$ were routinely produced, whilst for shorter gradients ($\lesssim 20 \mu$m), quasimonoenergetic acceleration of proton was observed. These measurements indicate that the properties of the accelerating shock and the resultant ion energy distribution, in particular the production of narrow energy spread beams, is highly dependent on the plasma density profile. These findings are corroborated by 2D PIC simulations.