Laser-proton acceleration with cryogenic hydrogen jets

Demanding applications like radiation therapy of cancer have pushed the development of laser proton accelerators and defined necessary proton beam properties as well as levels of control and stability. The presentation will give an overview of the recent experiments for laser driven proton acceleration employing µm-sized cylindrical and sheet-like cryogenic jet targets to produce high energy proton beams without causing any debris. The low plasma density (30 nc) hydrogen jet was irradiated with the Petawatt laser source DRACO at the HZDR. Substantial improvements of the target system stability led to a proton acceleration performance comparable with that obtained with foil targets in optimized TNSA conditions. Furthermore, correlations between laser temporal profile and proton beam performance was investigated by using a synchronized off-harmonic optical probe beam for measuring the temporal evolution of the target expansion prior to the main pulse. Additional tailoring of the jet’s density profile by using artificial pre-pulses allowed for triggering the regime of relativistically induced transparency, yielding proton energies of up to about 80 MeV. Moreover, structured proton beam profiles were used to study the influence of the millimetres scale vacuum environment surrounding the target, where residual gas molecules are ionized by the remnant laser light that is not absorbed into the plasma but reflected or transmitted. This effect leads to the counter-intuitive observation of laser near-field feature imprints in the detected proton beam profiles.