Experimental observation of transverse modulations in laser-driven proton beams

We report on the experimental observation of transverse modulations in proton beams accelerated from micrometer thick targets which were irradiated with ultra-short (30 fs) laser pulses of a peak intensity of 5·10^20 W/cm^2. The net-like proton beam modulations were recorded using radiochromic film and the data suggest a dependence on laser energy and target thickness for their onset and strength. 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.