Investigation of efficient shock acceleration of ions using high energy lasers propagating in low density targets

It was recently shown that a promising way to accelerate ions forward to high energies and in a collimated beam is to use under-dense or near-critical density targets instead of solid ones [1, 2]. Simulations have revealed that the acceleration process depends strongly on the density gradient. Indeed, under certain condition, the most energetic protons are predicted to be accelerated by a collisionless shock mechanism that significantly increases their energy [3]. We present a recent experiment performed on the 100 TW LULI laser facility (France). We obtained proton energies up to 7 MeV when irradiating a 500 nm plastic foil that has been pre-ionized by a long pulse in order for it to be lower-than-critical density, with significant gradients. This is comparable, in the same experiment, with what was obtained under similar laser conditions, but with solid targets. We show as well that varying the plasma density gradient of the target modifies the measured proton spectra.