Ballistic focusing of polyenergetic protons driven by petawatt laser pulses.

By using a thick (250  μm) target with 350  μm radius of curvature, the intense proton beam driven by a petawatt laser is focused at a distance of ∼1  mm from the target for all detectable energies up to ∼25  MeV. The thickness of the foil facilitates beam focusing as it suppresses the dynamic evolution of the beam divergence caused by peaked electron flux distribution at the target rear side. In addition, reduction in inherent beam divergence due to the target thickness relaxes the curvature requirement for short-range focusing. Energy resolved mapping of the proton beam trajectories from mesh radiographs infers the focusing and the data agree with a simple geometrical modeling based on ballistic beam propagation.