Characterization of a cryogenic hydrogen jet target for Laser Proton acceleration

With the rise of a new generation of high intensity, high repetition rate laser systems, new fields of applications emerge, like e.g. hadron therapy using laser-accelerated protons. This trend imposes new demands on target systems for laser ion acceleration to offer high repetition rate capability as well as good acceleration performance and precise control of all target parameters. In recent experiments, a cryogenically cooled, solid hydrogen jet target has been implemented into the Draco PW Laser system at HZDR. This debris-free, renewable target allows for the acceleration of pure proton bunches while offering superb acceleration performance due to its low plasma density. Using an artificial pre-pulse to expand the target before the main Laser interaction, a further increase in maximum proton energy has been demonstrated. In this talk, we present a new study, which employs ultra-high-resolution, synchronized off-harmonic optical probing to characterize the expansion dynamics of the hydrogen jet target after interaction with a pre-pulse at intensities ranging from 1017 W/cm² down to the ionization threshold of hydrogen.