  Generation of high intensity attopulses is investigated in cylindrical
geometry by using 3D particle-in-cell plasma simulation code. Due to the
rotation symmetric target, a circularly polarized laser pulse is considered
propagating on the axis of a hollow cone-like target. The large incidence angle
and constant ponderomotive pressure leads to nano-bunching of relativistic
electrons responsible for the laser-driven synchrotron emission. A numerical
method is developed to find the source and direction of the coherent radiation
that is responsible for the existence of attopulses. The intensity modulation
in the harmonic spectrum is well described by the model of coherent synchrotron
emission extended to the regime of higher order \gamma-spikes. The spatial
distribution of the higher harmonics resembles a spiral shape which gets
focused into a small volume behind the target.
