  We report on the experimental studies of laser driven ion acceleration from
double-layer target where a near-critical density target with a few-micron
thickness is coated in front of a nanometer thin diamond-like carbon foil. A
significant enhancement of proton maximum energies from 12 to ~30 MeV is
observed when relativistic laser pulse impinge on the double-layer target under
linear polarization. We attributed the enhanced acceleration to
superponderomotive electrons that were simultaneously measured in the
experiments with energies far beyond the free-electron ponderomotive limit. Our
interpretation is supported by two-dimensional simulation results.
