Laser plasma proton acceleration experiments using foam-covered and grating targets

Experimental results are reported for two different configurations of laser driven ion acceleration using solid foils with a structured layer on the irradiated side, aiming to improve the laser-target coupling by exploiting engineered surfaces. Two experimental campaigns have been performed exploiting a 100TW 25fs Ti:Sa laser capable of maximum intensity of 4·10¹⁹ W/cm². "Grating" targets have been manufactured by engraving thin mylar foils (0.9, 20 and 40 μm) with a regular modulation having 1.6 μm period and 0.5 μm depth. The periodicity of the grating corresponds to a resonant incident angle of 30° for the excitation of surface waves. Considering a target of 20 μm and changing the angle of incidence from 10° to 45°, a broad maximum in the proton energy cut-off was observed around the resonant angle (about 5 MeV) which was more than a factor two higher than the case of planar target. "Foam" targets have been manufactured by depositing a porous 10 μm nanostructured carbon film with an average density of 1-5 mg/cm³ on a 1 μm thick aluminium foil. At maximum focalization the foam targets gave a maximum proton energy similar to the case of bare aluminium target (about 6 MeV), while educing the intensity the presence of the foam enhanced the maximum proton energy, obtaining about 1.5MeV vs. 500KeV at an intensity of 5·10¹⁶ W/cm ². 2D Particle-In-Cell simulations have been used to support the intepretation of the experimental results.