Tests of proton laser-acceleration using circular laser polarization, foams and half gas-bag targets

We have analyzed laser-accelerated protons, generated at the rear surface of a solid target irradiated by an ultra-intense (I ∼ 10¹⁸ to 5 × 10¹⁹ Wcm∼²) short laser pulse in different conditions: (1) using linear and circular polarization; (2) using 'foam' targets, i.e. targets where a density gradient has been artificially generated by adding a foam onto a solid and (3) using 'half gas-bag' targets, where the density gradient has been artificially generated by having a gas assembly in front of the solid. In all these varied conditions, and for our laser and target parameters, no enhancement of proton acceleration could be found, compared with the standard set-up using a linearly polarized laser irradiating a flat metal foil. Potential issues and solutions are discussed.