2D hydrodynamic simulations of generation of high-current proton beams by relativistic skin-layer laser-plasma interaction

The properties of fast proton beam generation by skin-layer ponderomotive acceleration (SLPA) induced by a short (< 1 ps) laser pulse are studied using 2D two-fluid relativistic code. It is found that both for subrelativistic and relativistic laser intensities, the crucial parameter determining the spatial structure and angular divergence (theta(b)) of the proton beam is the ratio d(L)/L-n where d(L) is the laser beam diameter and L. is the density gradient, scale length of preplasma in front of the target. When this ratio is relatively small (< 10), the proton beam generated at relativistic laser intensities has a bubble-like structure and theta(b) is large. When dL is at least tens times greater than L-n, the transverse distribution of the proton density and the proton current density in the beam follow the spatial shape of the laser beam and theta(b) is small. In the last case, highly collimated high-current (MA) proton beams can be produced by SLPA at relativistic laser intensities.