0428同步年報-2021-全

098 ACTIVITY REPORT 2021 PeFeB-based CPMU (CU15) The development of CU15, which was the first CPMU at the TPS and has a period length of 15 mm, began in 2016. Several technologies, including field measurement systems, field correction methods, and assembly works of vacuum/ cryogenic components have been developed at the TPS laboratory. CU15 was installed at the storage ring in 2019 and has been in operation since January 2021. Figure 3 illustrates the milestones in the development of CU15. In the design of CU15, a cryocooler cooling method is employed because the initial setup for a cryocooler system is simple and liquid nitrogen (LN 2 ) is not available at the laboratory. The cryocoolers are connected to the magnet arrays by flexible thermal straps and heat transfer feedthroughs, as indicated in the schematic in Fig. 4 . (see next page). Such conduction cooling design can satisfy the regulations imposed on the TPS ultra- high vacuum system. The heat transfer feedthrough separates the vacuum system into two sections, so the annual maintenance of the cold heads can be performed without evacuating the ring vacuum and affecting UHV components of the CPMU. The first attempt to operate CU15 in the storage ring at a beam current of under 300 mA was successful. However, the cryogenic performance of CU15 was poor when the beam current was increased gradually to 500 mA. The beam-induced heat load, generated by the broadband impedance at high beam currents, Fig. 2 : (left) CU15 installed at the TPS storage ring for the high-resolution powder diffraction TPS 19A beamline; (right) CUT18 installed for the advanced microcrystal chemical crystallography TPS 15A beamline. Fig. 3 : Milestones of the TPS-CU15 project. led to the available cooling power of the two cryocoolers being insufficient. The underlying cause of this failure was the electromagnetic waves becoming trapped inside the cavity-like structure at the end of the transition taper, which led to energy loss of the electron beam and undesirable beam heating. A new type of water-cooled transition taper was therefore developed to mitigate the heat loads. After several attempted modifications, CU15 was successfully tested at 500 mA with a minimum gap of 4.8 mm. Figure 1 shows the spectral performance of CU15-2m is superior than IU22-3m at photon energy larger than 15 keV. To identify this result, the powder diffraction measurements was performed by Dr. Yu-Chun Chuang's group at TPS 19A (CU15) and 09A (IU22). The photon counts were measured through undulators with same MYTHEN detector and a NIST standard material 660c (LaB6). Measuring at the same beam current, the relative photon counts using the