Small-gap undulator research at the NSLS: concepts and results

We report the results of experiments carried out in the X13 straight section of the NSLS X-ray Ring, which explore the limits of operation of small-gap undulators. Two major projects are described: the Prototype Small-Gap Undulator (PSGU) and the in-vacuum undulator (IVUN). The PSGU utilized a variable-aperture vacuum chamber and a separate drive for the 16-mm-period undulator magnet arrays, which were situated in air outside the vacuum vessel. For IVUN, the 11-mm-period undulator arrays were placed directly in the accelerator vacuum. For both devices, stored beam lifetimes in the X-ray Ring were unaffected until electron beam apertures were decreased below 5 mm. A 3 mm aperture resulted in only a slight reduction of beam lifetime, which is acceptable for normal user operations. Measurements of the photon spectra radiated by the PSGU and IVUN were in excellent agreement with theory. Our work provides strong evidence that small-gap, short-period undulators will play an important role in the future. (orig.)