Ballistic Focusing of Polyenergetic Protons Driven by Petawatt Laser Pulses

We report on a scheme to focus polyenergetic proton beams produced by petawatt laser-solid interactions by using curved targets. Due to the laminar nature of the beam, the protons emitted from a cylindrically curved target surface follow ballistic trajectories and focus at a distance that depends on their energy. The focus position can be inferred from the radius of curvature and the divergence of the beam. For a cylindrically curved target with radius of curvature of a few millimeters, micrometer-scale focal spots are obtained with a peak fluence enhancement of over 1000.