  The radiation pressure acceleration (RPA) of charged particles has been
considered a challenging task in laser particle acceleration. Laser-driven
proton/ion acceleration has attracted considerable interests due to its
underlying physics and potential for applications such as high-energy density
physics, ultrafast radiography, and cancer therapy. Among critical issues to
overcome the biggest challenge is to produce energetic protons using an
efficient acceleration mechanism. The proton acceleration by radiation pressure
is considerably more efficient than the conventional target normal sheath
acceleration driven by expanding hot electrons. Here we report the generation
of 93-MeV proton beams achieved by applying 30-fs circularly polarized laser
pulses with an intensity of 6.1 x 1020 W/cm2 to ultrathin targets. The
radiation pressure acceleration was confirmed from the obtained optimal target
thickness, quadratic energy scaling, polarization dependence, and 3D-PIC
simulations. We expect this fast energy scaling to facilitate the realization
of laser-driven proton/ion sources delivering stable and short particle beams
for practical applications.
