Laser-Driven Proton Acceleration from Pre-Expanded Converging Water Jets*

Intense bursts of MeV-energy protons generated by short pulse lasers have generated significant interest due to their potential in tumor therapy, isotope production, material science, and radiography. While target normal sheath acceleration has been the most studied proton acceleration mechanism, its resulting proton beam properties, such as peak energy and divergence, are often inadequate for practical applications. Creating a low-density preplasma in front of the target can result in higher peak proton energies by enhancing the density and energy of the hot electrons that create the sheath field accelerating the protons. To investigate the effect of target density shaping on proton beam properties a high repetition rate target platform has been developed and implemented in an experiment conducted using the 20 TW, 10 Hz high-contrast NePTUN laser at Tel Aviv University. The target consists of a microfluidic nozzle that produces water sheets varying in thickness from 250 nanometers to 1 micron. The water is transported out of the chamber with a heated in-vacuum catcher system and recirculated through a pump to provide a continuously replenishing target. A comprehensive set of density profiles were generated by pre-expanding the water jet with a low energy pre-heating beam with delays ranging from 1 ps to 10 ns. The protons accelerated by the interaction were characterized by a Thomson parabola and a scintillator-based ion imager as scans over target densities and thicknesses were performed.