  Using fully three-dimensional particle-in-cell simulations, we show that
readily available femtosecond laser systems can stably generate proton beams
with hundred MeV energy and low spread at $\sim1\%$ level by parallel
irradiation of a tens of micrometers long plasma plate. As the laser pulse
sweeps along the plate, it drags out a huge charge ($\sim$100 nC) of collimated
energetic electrons and accelerates them along the plate surface to
superponderomotive energies. When this dense electron current arrives at the
rear end of the plate, it induces a strong electrostatic field. Due to the
excessive space charge of electrons, the longitudinal field becomes bunching
while the transverse field is focusing. Together, this leads to a highly
monoenergetic energy spectrum and much higher proton energy as compared to
simulation results from typical target normal sheath acceleration and radiation
pressure acceleration at the same laser parameters.
