  A single laser pulse with spot size smaller than half its wavelength ($w_0 <
\lambda/2$) can provide a net energy gain to ultra-relativistic particles. In
this paper, we discuss the properties of an optical cell consisting of $N$
sub-cycle pulses that propagate in the direction perpendicular to the electron
motion. We show that the energy gain produced by the cell is proportional to
$N$ and it is sizable even for $\mathcal{O}(1\mathrm{~TW})$ pulses.
