Insertion devices for third-generation light sources (invited)

By the introduction of the concepts of short-period undulators or minipole undulators, we can raise the upper limit of the fundamental photon energy of undulator radiation up to 300 keV region without any unreasonable heat load, so that the utilization of synchrotron radiation without any wiggler may be possible. Furthermore, we can bring down the lower limit of the fundamental without either unreasonable heat load or unnecessary higher harmonics by adopting helical undulators.