  A new method for the generation of a train of pulses from a single
high-energy, ultra short pulse is presented, suited for Resonant Multi-Pulse
Ionization injection. The method is based on different transverse portion of
the pulse being delayed by a "mask" sectioned in concentric zones with
different thicknesses, in order to deliver multiple laser pulses. The mask is
placed right before the last focusing parabola. A hole in the middle of the
mask lets part of the original pulse to pass through to drive electron
injection. In this paper a full numerical modelling of this scheme is
presented. In particular we discuss the spatial and temporal profile of the
pulses emerging from the mask and how they are related to the radius and
thickness of each section.
