Pushing proton energy frontiers with pre-expanded, actively controlled, near critical density targets

The performance of laser based ion acceleration and the scaling of the laser energy to
achieve increased ion energies strongly depend on the laser temporal contrast and its
effect on the target plasma scale length. Plasma mirror setups have proven to be a
valuable tool to significantly improve the temporal contrast by reducing pre-pulse
intensity and steepening the rising edge of the main laser pulse. With such contrast
enhancement techniques including novel diagnostic schemes, laser proton acceleration
using ultra-thin foil targets as well as renewable debris-free hydrogen jets were
investigated in a series of experiments within the near-critical density regime. An
important implication of this is the demonstration of a credible path toward high
repetition rate laser-based ion acceleration applications.