Observation of proton rear emission and possible gigagauss scale magnetic fields from ultra-intense laser illuminated plastic target

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Abstract
CR-39 film stacks are used to measure the energy and angular distribution of protons emitted from the rear surface of ultra-intense laser illuminated plastic foils. The experiment suggests that the energetic protons are dragged away from the rear surface, where the hot electron formed a virtual cathode. The two-dimensional particle in cell simulation supports this hypothesis. For 5 (100) μm thick target, 1.8×109 protons have a slope temperature of 3 (2) MeV. The ring structure of proton emission leads us to the hypothesis that a toroidal magnetic field associated with the hot electrons works on the fast ions and deflects them. From the logarithmic slope of the ring diameter versus the ion energy, the product of the magnetic field × the length over which it works on the ions is estimated to be up to 1000 MG⋅μm. The simulation shows that a strong toroidal magnetic field was excited at the target rear side with expansion of plasmas. The proton’s angular distribution from the rear surface has the logarithmic slope similar to the experimental results due to this magnetic field.