Development of a 12 mm period cryogenic permanent magnet undulator for time-resolved X-ray beamline on HEPS

In order to obtain hard x-rays with the fundamental harmonic energy over 20 keV, a Pr2Fe14B based cryogenic permanent magnet undulator (CPMU) with the period length of 12 mm has been developed on High Energy Photon Source (HEPS) for time-resolved beamline. Several technical difficulties arise with the further shortening of the CPMU period length such as magnets and poles clamping, constitutionally stable pole height shimming, multipoles shimming within the small horizontal area according to the dynamic apertures of ultra-low emittance storage ring. Some specific treatments are adopted in response to these difficulties and proved to be effective. Details of development process including design, assembling, methods of optimization, measurement and shimming are presented.