  Radiation Pressure Acceleration relies on high intensity laser pulse
interacting with solid target to obtain high maximum energy, quasimonoenergetic
ion beams. Either extremely high power laser pulses or tight focusing of laser
radiation is required. The latter would lead to the appearance of the maximum
attainable ion energy, which is determined by the laser group velocity and is
highly influenced by the transverse expansion of the target. Ion acceleration
is only possible with target velocities less than the group velocity of the
laser. The transverse expansion of the target makes it transparent for
radiation, thus reducing the effectiveness of acceleration. Utilization of an
external guiding structure for the accelerating laser pulse may provide a way
of compensating for the group velocity and transverse expansion effects.
