  We report the experimental generation of highly energetic carbon ions up to
48 MeV per nucleon by shooting double-layer targets composed of well-controlled
slightly underdense plasma (SUP) and ultrathin foils with ultra-intense
femtosecond laser pulses. Particle-in-cell simulations reveal that carbon ions
residing in the ultrathin foils undergo radiation pressure acceleration and
long-time sheath field acceleration in sequence due to the existence of the SUP
in front of the foils. Such an acceleration scheme is especially suited for
heavy ion acceleration with femtosecond laser pulses. The breakthrough of heavy
ion energy up to multi-tens of MeV/u at high-repetition-rate would be able to
trigger significant advances in nuclear physics, high energy density physics,
and medical physics.
