A. Sgattoni

The interaction of laser pulses with thin grating targets, having a periodic groove at the irradiated surface, is experimentally investigated. Ultrahigh contrast (~10(12)) pulses allow us to demonstrate an enhanced laser-target coupling for the first time in the relativistic regime of ultrahigh intensity >10(19) W/cm(2). A maximum increase by a factor of 2.5 of the cutoff energy of protons produced by target normal sheath acceleration is observed with respect to plane targets, around the incidence angle expected for the resonant excitation of surface waves. A significant enhancement is also observed for small angles of incidence, out of resonance.

We investigate by particle-in-cell simulations in two and three dimensions the laser-plasma interaction and the proton acceleration in multilayer targets where a low-density ("near-critical") layer of a few-micron thickness is added on the illuminated side of a thin, high-density layer. This target design can be obtained by depositing a "foam" layer on a thin metallic foil. The presence of the near-critical plasma strongly increases both the conversion efficiency and the energy of electrons and leads to enhanced acceleration of protons from a rear side layer via the target normal sheath acceleration mechanism. The electrons of the foam are strongly accelerated in the forward direction and propagate on the rear side of the target, building up a high electric field with a relatively flat longitudinal profile. In these conditions the maximum proton energy is up to three times higher than in the case of the bare solid target.

The acceleration of dense targets driven by the radiation pressure of high-intensity lasers leads to a Rayleigh-Taylor instability (RTI) with rippling of the interaction surface. Using a simple model it is shown that the self-consistent modulation of the radiation pressure caused by a sinusoidal rippling affects substantially the wave vector spectrum of the RTI, depending on the laser polarization. The plasmonic enhancement of the local field when the rippling period is close to a laser wavelength sets the dominant RTI scale. The nonlinear evolution is investigated by three-dimensional simulations, which show the formation of stable structures with "wallpaper" symmetry.

In this paper, we present acceleration by laser and dynamics of charged particles (ALaDyn), a particle-in-cell code, to investigate the interaction of a laser pulse with a preformed plasma and/or an externally injected beam. The code, fully parallelized, works in 1D, 2D, and 3D Cartesian geometry, and it is based on compact high-order finite-difference schemes ensuring higher spectral accuracy. We discuss the features, the performances, and the validation tests of the code. We finally present a preliminary application on a physically relevant case based on the PLASMON-X experiment of the CNR-INFN.

We report on the development of foam-based double-layer targets (DLTs) for laser-driven ion acceleration. Foam layers with a density of a few mg cm-3 and controlled thickness in the 8-36 μm range were grown on μm-thick Al foils by pulsed laser deposition (PLD). The DLTs were experimentally investigated by varying the pulse intensity, laser polarisation and target properties. Comparing DLTs with simple Al foils, we observed a systematic enhancement of the maximum and average energies and number of accelerated ions. Maximum energies up to 30 MeV for protons and 130 MeV for C6+ ions were detected. Dedicated three-dimensional particle-in-cell (3D-PIC) simulations were performed considering both uniform and cluster-assembled foams to interpret the effect of the foam nanostructure on the acceleration process.