  The dynamics of radiation pressure acceleration in the relativistic light
sail regime are analysed by means of large scale, three-dimensional (3D)
particle-in-cell simulations. Differently to other mechanisms, the 3D dynamics
leads to faster and higher energy gain than in 1D or 2D geometry. This effect
is caused by the local decrease of the target density due to transverse
expansion leading to a "lighter sail". However, the rarefaction of the target
leads to an earlier transition to transparency limiting the energy gain. A
transverse instability leads to a structured and inhomogeneous ion
distribution.
