  A new regime is described for Radiation Pressure Acceleration of a thin foil
by an intense laser beam of above 10^20 W/cm^2. Highly monoenergetic proton
beams extending to GeV energies can be produced with very high efficiency using
circularly polarized light. The proton beams have a very small divergence angle
(less than 4 degrees). This new method allows the construction of ultra-compact
proton and ion accelerators with ultra-short particle bursts.
