Laser driven proton accelerators with solid hydrogen jets for application

Laser plasma-based particle accelerators attract great interest in fields where conventional accelerators reach limits based on size, cost or beam parameters. However, despite the fact that first principles particle in cell simulations have predicted several advantageous ion acceleration schemes,
laser accelerators have not yet reached their full potential in producing simultaneous high-radiation
doses at high particle energies. The most stringent limitation is the lack of a suitable high-repetition
rate target that also provides a high degree of control of the plasma conditions which is required
to access these advanced regimes. Here, we demonstrate that the interaction of petawatt-class laser
pulses with a pre-formed micrometer-sized cryogenic hydrogen jet plasma overcomes these limitations. Controlled pre-expansion of the initially solid target by low intensity pre-pulses allowed for tailored density scans from the solid to the underdense regime. Our experiment demonstrates that
the near-critical plasma density profile produces proton energies of 80 MeV. This energy presents
more than a factor of two increase compared to the solid hydrogen target. Our three-dimensional
particle in cell simulations show the transition between different acceleration mechanisms and suggest enhanced proton acceleration at the relativistic transparency front for the optimal case.