Development of a Quasi-Periodic Undulator for the HLS
  China's first quasi-periodic undulator (QPU) has been developed for the Hefei Light Source (HLS). It uses a magnetic configuration with varied thicknesses of NdFeB blocks, based on the QPU of European Synchrotron Radiation Facility (ESRF). Depression of 3rd harmonic radiation is significantly improved over the ESRF QPU, as deduced from the measured magnetic fields. A method of configuring shims of different geometries and sizes, based on a symmetric principle to correct multi-pole field integrals, was demonstrated. 
Yufeng Yang, Huihua Lu, Wan Chen, Qika Jia, Shuchen Sun and Zhiqiang   Li
physics.acc-ph nucl-ex