  By using multi-dimensional particle-in-cell simulation, we present a new
regime of stable proton beam acceleration which takes place when a two-specie
shaped foil is illuminated by a circularly polarized laser pulse. It is
observed that the lighter protons are nearly-instantaneously separated from the
heavier carbon ions due to the charge-to-mass ratio difference. The heavy-ions
layer extensively expands in space and acts to buffer the proton layer from the
Rayleigh-Taylor-like (RT) instability that would have otherwise degraded the
proton beam acceleration. A simple three-interface model is formulated to
qualitatively explain the stabilization of the light-ions acceleration. Due to
the absence of the RT-like instability, the produced high quality
mono-energetic proton bunch can be well maintained even after the laser-foil
interaction concludes.
