  In the ion acceleration by radiation pressure a transverse inhomogeneity of
the electromagnetic pulse results in the displacement of the irradiated target
in the off-axis direction limiting achievable ion energy. This effect is
described analytically within the framework of the thin foil target model and
with the particle-in-cell simulations showing that the maximum energy of
accelerated ions decreases while the displacement from the axis of the target
initial position increases. The results obtained can be applied for
optimization of the ion acceleration by the laser radiation pressure with the
mass limited targets.
