Optimization of laser-driven ion acceleration from pre-expanded liquid water microjets*

Laser-driven ion sources exhibit a variety of features (such as MeV energies, high peak brightness, and ultra-high acceleration gradients) that make them compelling alternatives to conventional accelerators for applications in medicine, materials science, and fundamental physics. To advance these sources towards maturity for applications, further advances in ion peak energy and laser-target coupling efficiency are needed. Pre-expansion of solid-density targets has been observed to significantly improve both these parameters, but previous studies have been limited to low repetition rates by available laser and target technology.

Here we present results from a high repetition rate experiment studying proton acceleration from liquid water targets that were pre-expanded by an independent ps-duration pulse. This experiment was performed at the 0.5 J, 30 fs NePTUN laser facility at Tel Aviv University, which delivers high-contrast pulses at 10 Hz. The targets were ambient-temperature planar liquid water sheets developed by our group.  We will describe the characteristics of the proton beams and the target pre-expansion conditions for optimized ion beam energies and yields. These results will guide future experiments studying ion acceleration from targets in the near-critical density regime at higher laser intensities to demonstrate ion acceleration to energies beyond those accessible from the conventional TNSA mechanism.