Ultra-short pulse laser-driven acceleration of protons to 80 MeV from density tailored cryogenic hydrogen jets

Laser plasma based particle accelerators have attracted great interest in fields where conventional accelerators reach limits based on size, cost or beam parameters. Designing future laser ion accelerators requires a high degree of control of the advanced acceleration schemes involved and predictive modelling capabilities. Here, we investigate the interaction of petawatt class laser pulses with a micrometer sized cryogenic hydrogen jet target. Controlled pre-expansion of the target by low intensity pre-pulses allowed for tailored density scans over more than two orders of magnitude. On-shot target characterization using two-color optical probing provided precise starting conditions for numerical simulations. For the optimal target density profile in the near critical regime proton energies of up to 80 MeV were observed which represents a two-fold increase with respect to the initial solid target case. Three-dimensional particle in cell simulations confirmed the transition between different acceleration mechanisms involved and suggested the magnetic-vortex regime to be responsible for the highest proton energies achieved.