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