  Target normal sheath acceleration (TNSA) is a method employed in
laser--matter interaction experiments to accelerate light ions (usually
protons). Laser setups with durations of a few 10 fs and relatively low
intensity contrasts observe plateau regions in their ion energy spectra when
shooting on thin foil targets with thicknesses of order 10 $\mathrm{\mu}$m. In
this paper we identify a mechanism which explains this phenomenon using one
dimensional particle-in-cell simulations. Fast electrons generated from the
laser interaction recirculate back and forth through the target, giving rise to
time-oscillating charge and current densities at the target backside. Periodic
decreases in the electron density lead to transient disruptions of the TNSA
sheath field: peaks in the ion spectra form as a result, which are then spread
in energy from a modified potential driven by further electron recirculation.
The ratio between the laser pulse duration and the recirculation period
(dependent on the target thickness, including the portion of the pre-plasma
which is denser than the critical density) determines if a plateau forms in the
energy spectra.
