MeV quasi‐mono‐energetic proton beam created by a combination of a laser‐plasma ion accelerator and synchronous rf cavity

MeV quasi‐mono‐energetic proton beam is produced by a combination of a synchronous radio frequency (rf) electric field and laser‐plasma ion acceleration. The experiment was carried out at the Kansai Photon Science Institute, JAEA, using the Ti:Sapphire laser system called J‐KAREN. The proton beam is emitted normal to the rear surface of the thin polyimide target of the thickness 7.5 μm irradiated at peak intensity of 4×1018 W/cm2. The energy spread is compressed from 100% to less than 11% at FWHM by the rf field. The focusing and defocusing effect of the transverse direction is also observed. These are also studied by a Monte Carlo simulation. The relation between the transverse focusing and the energy spectrum of the phase‐rotated beam is systematically shown by the simulation.