Parabolic undulator and its application to fast switching of helicity

A novel insertion device called a parabolic undulator is proposed. This device can provide various polarization states simultaneously without changing magnetic configuration, or phasing, therefore it is suited for in-vacuum insertion devices (IDs) with variable polarization. Radiation properties such as flux and polarization are calculated to compare with ordinary IDs. An important application to fast switching of helicity is also mentioned to show the usefulness of the parabolic undulator.