Undulator Development Towards Very Short Period Lengths

Abstract

Synchrotron light sources have made progress through the third generation to the fourth generation. Realization of linac-based free electron lasers is a representative example of recent remarkable achievements in fourth-generation light sources. In this progress, there has been a demand to reach shorter wave lengths to expand research possibilities. The on-axis wavelength λk of the k-th harmonic of the undulator radiation is given byHere, K is a deflection parameter proportional to the period length λu of the undulator magnetic field and the strength B0 of a periodic field of the undulator,At the High Energy Accelerator Research Organization of the Photon Factory (KEK-PF), research and development for in-vacuum undulators have focused on obtaining shorter wavelengths and higher-energy photons. Here, the undulator magnets are contained in the vacuum of the light source accelerator. This method allows one to utilize the shortest period length of the undulator field which can be produced by novel magnet materials provided with the newest magnet technology. The first breakthrough was a successful in-vacuum undulator with a period length of 4 cm, which was installed in the 6.5 GeV Photon Factory-Accumulation Ring (PF-AR) [1, 2] for use on the Moessbouer beamline. This was followed by several in-vacuum undulators with the same period length which were installed in the PF-AR, and by several in-vacuum short gap undulators (SGU) installed in the 2.5 GeV PF ring. The latter have period lengths of 1–2 cm and are capable of producing hard X-rays with the third or fifth (or higher) harmonics [3, 4].