High-quality proton bunch from laser interaction with a gas-filled cone target

Generation of high-energy proton bunch from interaction of an intense short circularly polarized(CP) laser pulse with a gas-filled cone target(GCT) is investigated using two-dimensional particle-in-cell simulation. The GCT target consists of a hollow cone filled with near-critical gas-plasma and a thin foil attached to the tip of the cone. It is observed that as the laser pulse propagates in the gas-plasma, the nonlinear focusing will result in an enhancement of the laser pulse intensity. It is shown that a large number of energetic electrons are generated from the gas-plasma and accelerated by the self-focused laser pulse. The energetic electrons then transports through the foil, forming a backside sheath field which is stronger than that produced by a simple planar target. A quasi-monoenergetic proton beam with maximum energy of 181 MeV is produced from this GCT target irradiated by a CP laser pulse at an intensity of $2.6\times10^{20}W/cm^2$, which is nearly three times higher compared to simple planar target(67MeV).