Evidence of resonant surface wave excitation in the relativistic regime through measurements of proton acceleration from grating targets

The interaction of laser pulses with thin grating targets, having a periodic groove at the irradiated surface, has been experimentally investigated. Ultrahigh contrast ($\sim 10^{12}$) pulses allowed to demonstrate an enhanced laser-target coupling for the first time in the relativistic regime of ultra-high intensity >10^{19} \mbox{W/cm}^{2}. A maximum increase by a factor of 2.5 of the cut-off energy of protons produced by Target Normal Sheath Acceleration has been observed with respect to plane targets, around the incidence angle expected for resonant excitation of surface waves. A significant enhancement is also observed for small angles of incidence, out of resonance.Comment: 5 pages, 5 figures, 2nd version implements final correction

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 targets with a structured layer on the irradiated side. Two experimental campaigns have been performed exploiting the 100TW 25fs Ti:Sa UHI-100 laser pulse at CEA Saclay. The use of a double plasma mirror allowed a contrast ratio of > 1012 so that the structures of the front surface withstanded the prepulse. ”Foam” targets have been manufactured by depositing a 10 μm nanostructured carbon foam with an average density of 1-5 mg/cm3 on a 1 μm thick aluminium foil. At maximum focalization, corresponding to an intensity of 1019 W/cm2. the foam targets gave a maximum proton energy similar to the case of bare aluminium target (about 6 MeV). Reducing the intensity by moving the target from the best focus plane, the presence of the foam enhanced the maximum proton energy, obtaining about 1.5MeV vs. 500KeV with a target 500 μm from the best focus, corresponding to an intensity of 5 · 1016 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 degrees for the excitation of surface waves. Considering a target of 20 micron and changing the angle of incidence from 10 to 45 degrees, a broad maximum in the proton energy cut-off was observed around the resonant angle. The proton energy cut-off was up to 5 MeV for a laser intensity of 1019 W/cm2. As suggested by numerical simulation, radiochromic films placed 300 degrees around the target showed a very intense electron signal suggesting the presence of a peak emission tangent to the target only in presence of the grating. The experiments have been supported by the LaserLAB EU access scheme