  Using a kilojoule class laser, we demonstrate for the first time that
high-contrast picosecond pulses are advantageous for ion acceleration. We show
that a laser pulse with optimum duration and a large focal spot accelerates
electrons beyond the ponderomotive energy. This anomalous electron heating
enables efficient ion acceleration reaching 52 MeV at an intensity of 1.2X10^19
Wcm^-2. The proton energy observed agrees quantitatively with a one-dimensional
plasma expansion model newly developed by taking the anomalous heating effect
into account. The heating process is confirmed by both measurements with an
electron spectrometer and a one-dimensional particle-in-cell simulation. By
extending the pulse duration to 6 ps, 5% energy conversion efficiency to
protons (50 J out of 1 kJ laser energy) is achieved with an intensity of
10^18-Wcm^-2. The present results are quite encouraging for realizing
ion-driven fast ignition and novel ion beamlines.
