A new undulator scheme providing various polarization states with low on-axis power density

A new undulator scheme is proposed to enable flexible utilization of polarization properties of synchrotron radiation, which is based on the composite-period undulator originally proposed to improve the wavelength tunability of free electron lasers. It has an advantage over the conventional undulators that the on-axis power density is kept low even when a high K value is applied to lower the photon energy. The basic principle and availability of various polarization states are presented together with a practical example.