Divergence reduction and enhanced energies of protons from double-layer target irradiated by intense shaped laser

Multi-MeV proton beams find various applications such as "fast ignition" of inertial confinement fusion targets or modification of material parameters. Target normal sheath acceleration (TNSA) driven by short intense laser pulses is, in this context, a well-established proton acceleration model. The generation of multi-MeV proton beams with ultrashort duration (ps) and a high number of protons in a bunch (10¹¹ – 10¹³) has been successfully demonstrated in experiments, but beam properties still need improvement for applications. Improving the divergence of TNSA generated proton bunches, for example, is an important open question for future progress. Here, we demonstrate that the divergence of protons accelerated from double-layer targets irradiated by intense lasers with orbital angular momentum (OAM) can be significantly smaller (up to factor 7) than when considering pure Gaussian modes. We find that such reduction is closely connected with the laser self-focusing and absorption in the near-critical plasma of the double-layer target, which impacts on hot electron generation and subsequent proton acceleration at the rear side. The self-consistent laser—plasma dynamics is investigated analytically and by relying on three-dimensional particle-in-cell simulations in OSIRIS.