Effect of front surface engineering on high energy electron, X-ray and heavy ion generation from Relativistic laser interaction with thick high-Z targets

TL;DR

This study investigates how front surface engineering of high-Z targets with various coatings affects the generation of high-energy electrons, X-rays, and heavy ions during relativistic laser interactions, revealing that bare targets produce the most energetic electrons and X-rays, while coated targets enhance heavy ion acceleration.

Contribution

It demonstrates that coating thickness and material influence energy coupling and particle emission, with plastic coatings improving electron and X-ray generation compared to bare targets.

Findings

Bare targets produce 30 MeV X-rays and high-energy electrons.

Thicker coatings reduce laser absorption and particle emission.

Foam and nanowire coatings enhance heavy ion acceleration.

Abstract

Relativistic lasers on solid targets generate hot electrons, and other secondary particles. These particles can be used for radiography, cancer therapy, or isochoric heating. A lower density or structured coating on high-Z targets can improve laser-target energy coupling and subsequently enhance overall particle emission. In this work performed at the Scarlet Facility, a $10^{21}$ W/cm$^2$ intense pulse was incident on front surface coatings on 1 mm thick Ta. These coatings include a 12 $\mu$m plastic coating, a 50 $\mu$m thick foam coating, and a Au nanowire (NW) coating. Post-damage craters are correlated with reflected light on a MACOR screen, illustrating that less absorption in a target is directly tied to smaller craters. Additionally, more absorption in a target also leads to more MeV electrons and X-rays. Bare targets performed the best for electron and MeV X-ray generation, with X-rays of 30 MeV detected, as coatings tested were too thick and thus experienced lower intensities. Due to this larger spot size, foam and NW-coated targets generated the greatest heavy ion acceleration. Particle-in-cell simulations tested on bare and plastic-coated targets illustrate that $\sim \mu$m thick plastic coatings perform better than bare Ta. These results underline the importance of density and thickness control of coatings on high-Z materials. In the future, post-damage crater analysis could provide an easy way to benchmark absorption in a sample, and could later be compared against absorption estimates from particle-in-cell simulations.