Attospiral generation upon interaction of circularly polarized intense laser pulses with conelike targets

The generation of high-intensity attopulses has been investigated in cylindrical geometry by using a three-dimensional 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 conelike target. The large incidence angle and constant ponderomotive pressure lead to nanobunching 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 ensures 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 γ spikes. The spatial distribution of the higher harmonics resembles a spiral shape which gets focused into a small volume behind the target.