Planar helical undulator sources of circularly polarized X-rays

The planar helical undulator is a charged particle accelerator insertion device that produces a helical magnetic field using only planar magnet arrays. The helical field causes the particles to execute helical trajectories, and to emit X-rays with a degree of circular polarization. To switch the X-ray helicity, some designs require a mechanical shift of the magnet arrays. Other designs use a chicane, a separate undulator to produce each polarization, and a chopper to switch between them. We present an analysis of the different magnetic designs that have been proposed, with emphasis on a device for the 500-1000 eV X-ray range on the SPEAR storage ring