  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).
