  The interaction of ultraintense laser pulses with solids is largely affected
by the plasma gradient at the vacuum-solid interface, which modifies the
absorption and ultimately, controls the energy distribution function of heated
electrons. A micrometer scale-length plasma has been predicted to yield a
significant enhancement of the energy and weight of the fast electron
population and to play a major role in laser-driven proton acceleration with
thin foils. We report on recent experimental results on proton acceleration
from laser interaction with foil targets at ultra-relativistic intensities. We
show a three-fold increase of the proton cut-off energy when a micrometer
scale-length pre-plasma is introduced by irradiation with a low energy
femtosecond pre-pulse. Our realistic numerical simulations agree with the
observed gain of the proton cut-off energy and confirm the role of stochastic
heating of fast electrons in the enhancement of the accelerating sheath field.
