Control of proton energy in ultra-high intensity laser-matter interaction

Recent breakthroughs in short pulse laser technology resulted in (i) generation of ultrahigh intensity (2x10(22) W/cm(2)) and (ii) ultra-high contrast (10(-11)) short pulses at the Hercules facility of the University of Michigan, which has created the possibility of exploring a new regime of ion acceleration - the regime of Directed Coulomb Explosion (DCE). In this regime of sufficiently high laser intensities and target thicknesses approaching the relativistic plasma skin depth it is possible to expel electrons from the target focal volume by the laser's ponderomotive force allowing for direct laser ion acceleration combined with a Coulomb explosion. That results in greater than 100 MeV protons with a quasi-monoenergetic energy spectrum. The utilization of beam shaping, namely, the use of flat-top beams, leads to more efficient proton acceleration due to the increase of the longitudinal field. According to the results of 2D PIC simulations a 500 TW laser pulse with a super-Gaussian beam profile interacting with 0.1 micron aluminium-hydrogen foil is able to produce monoenergetic protons with the energy up to 240 MeV.