Proton jet formation with focused symmetric spatially chirped beams

Three-dimensional particle-in-cell simulations are used to demonstrate the generation of highly collimated, dense proton jets from planar hydrogen targets using shaped, petawatt-class, laser pulses formed via simultaneous spatial and temporal focusing (SSTF). Simulations assume a 30 J, 30 fs laser pulse that is engineered with a radial spatial chirp and then focused into a conical space-time structure. An idealized conical structure that might be produced using a pair of circular diffraction gratings is first modeled and then compared to an approximate implementation using a multibeam combination of four 7.5 J pulses that are produced using conventional single-pass grating pairs. Tightly collimated, 5 µm diameter proton jets with peak energies up to 60 MeV are produced in both instances. Unlike target-shaping schemes that require precise manufacturing and alignment, this shaped-pulse approach can enable highrepetition rate, collimated proton beams using existing thin hydrogen sheet technology.