Observation of spatially modulated laser-driven proton beams from micrometer thick targets

The advent of a new generation of high repetition rate Petawatt (PW) laser systems in combination with recent experimentally achieved proton energies of up to 45 MeV from ultra-short pulse (~ 50 fs) facilities is expected to strongly advance the application of laser-plasma based accelerators for ions, e.g. in medicine. In our presentation, we report on the experimental observation of spatially modulated proton beams emitted from micrometer thick targets which were irradiated with ultrashort (30 fs) laser pulses of a peak intensity of 5•1020W/cm2. The net-like proton beam modulations were recorded using stacks of radio-chromic films and the investigation of different target systems for a laser energy range of 0.9 to 2.9 J revealed a clear dependence on laser energy and target thickness for the onset and strength of the modulations. Numerical simulations suggest that intensity-dependent instabilities in the laser-produced plasma at the target front side lead to electron beam break-up or filamentation, then serving as the source of the observed proton beam modulations. We propose that these results on laser intensity dependent plasma instabilities may have implications for the scaling of present acceleration mechanisms, such as target normal sheath acceleration, to higher proton energies and hence higher laser powers. Furthermore a brief overview of the recent laser and target area upgrade for laser-driven ion acceleration experiments at the HZDR will be given.