Recent trends of insertion-device technology for X-ray sources

As a consequence of the developments of in-vacuum undulators, where permanent magnet arrays are located in an ultrahigh vacuum, the in-vacuum mini-gap undulator with very short period has been successful in the X-ray ring of NSLS in collaboration with SPring-8, producing a new concept for synchrotron radiation facilities. The key feature of the concept is the combination of in-vacuum mini-gap undulators and a low-emittance ring with moderate beam energy. In other words there are possibilities for the design of a moderate-cost medium-sized synchrotron radiation facility, the performance of which may be comparable with that of the existing large-scale facilities. Here the performance of in-vacuum undulators is described, particularly of the mini-gap type. Also described is the essence of the new concept for synchrotron radiation facilities, the so-called new third-generation light source.