Effect of density gradients on the generation of a highly energetic and strongly collimated proton beam from a laser-irradiated Gaussian-shaped hydrogen microsphere

An investigation is made on the influence of the sharpness of the density gradients on the generation of energetic protons in a radially Gaussian density profile of a spherical hydrogen plasma. It is possible to create such density gradients by impinging a solid density target with a secondary lower intensity pulse, which ionizes the target and explodes it to create an expanded plasma target of lower effective density for the high-intensity main pulse to hit on. The density gradients are scanned in the near-critical regime, and separate regimes of proton motion are identified based on the density sharpness. An intermediate-density gradient [ n peak ≈ ( 1.5 – 2.5 ) γ n c] favors the generation of high energetic protons with narrow energy spectra that are emitted with better collimation from the target rear surface. Protons with energies exceeding 100 MeVs could be achieved using such modified plasma targets with circularly polarized lasers of peak intensities I 0 ∼ 10 20 W cm − 2 and peak energy ∼ 10 J.