Ion acceleration by an ultrashort laser pulse interacting with a near-critical-density gas jet

We demonstrate laser-driven Helium ion acceleration with cut-off energies above 25 MeV and peaked ion number above ${10}^8$ /MeV for 22(2) MeV projectiles from near-critical density gas jet targets. We employed shock gas jet nozzles at the high-repetition-rate (HRR) VEGA-2 laser system with 3 J in pulses of 30 fs focused down to intensities in the range between $9\times {10}^{19}$ W/cm${}^2$ and $1.2\times {10}^{20}$ W/cm${}^2$. We demonstrate acceleration spectra with minor shot-to-shot changes for small variations in the target gas density profile. Difference in gas profiles arise due to nozzles being exposed to a experimental environment, partially ablating and melting.