Proton acceleration from relativistically transparent solid targets at 400 nm

Relativistically induced transparency (RIT) enables a laser pulse to penetrate a classically opaque plasma and interact directly with bulk electrons. RIT-enhanced proton acceleration has applications in hadron cancer therapy, high-energy density radiography, and ion fast-ignition schemes, among other applications. We performed laser solid interaction experiments at the ALEPH Laser Facility using the frequency doubled 400 nm mode of operation to provide sufficient contrast. With intensities exceeding 10^21 Wcm^-2, we measured proton spectra as a function of direction with solid density foils ranging in target thickness. We performed Particle-In-Cell modeling of the interaction, which showed a strong correlation with experimental results and suggests that the ions were accelerated in the relativistically induced transparency regime.