0428同步年報-2021-全

Facility Status 097 Status of Cryogenic Permanent-Magnet Undulators at Taiwan Photon Source C ryogenic permanent-magnet undulators (CPMUs) have become a point of interest in the development of short-period undulators. Two 2-m CPMUs have been developed at the Taiwan Photon Source (TPS) by using different magnet materials and cooling methods: a PrFeB- based CPMU with cryocooler cooling and a NdFeB-based CPMU with liquid nitrogen (LN 2 ) tank cooling. Both CPMUs exhibit spectral performance superior to that of 3-m in- vacuum undulators. CPMUs have great potential to become standard undulators at the TPS for beamlines requiring high-brilliance X-rays at high photon energies. Introduction High-brilliance X-rays from undulators are highly desirable for use in third-generation storage rings. The brilliance of synchrotron radiation can be increased by using short-period, high-field undulators with low phase errors. A cryogenic permanent-magnet undulator (CPMU) is a suitable candidate for this purpose because of its strong magnetic field. The other potential advantages of CPMUs are as follows: (1) Outgassing from the permanent magnet component is suppressed at cryogenic temperatures (CTs), and cold magnets work as cryopumps. (2) Permanent magnets at CTs have high resistance to radiation damage due to their high coercivity. (3) Beam-induced-heat loads are low because image current heating on Cu-Ni foils covered the magnet arrays can be reduced by a factor of 2–3 relative to that at room temperature. These advantages are associated with crucial characteristics for advanced undulators, namely ultra-high-vacuum compatibility and high radiation and thermal resistance. In April 2015, eight in-vacuum undulators (IU22-3m) was installed for phase-I beamlines at the TPS storage ring. IU22s are designed for a photon energy range of 5–20 keV. Two CPMUs for phase-II beamlines, CU15 and CUT18, can provide brilliance higher than that provided by IU22s at photon energies above 15 keV ( Fig. 1 ). The parameters of the three undulators are presented in Table 1 . Beam-induced heating poses a challenge in the development of CPMUs. Because the TPS operates at a beam current of 500 mA with a bunch length of ~16 ps, such beam parameters result in high beam heating on vacuum components and undulators of in-vacuum with small gaps. Therefore, special cryogenic system design is required to ensure that the CPMUs exhibit high cryogenic performance. Figure 2 depicts the CU15 and CUT18 installed for the phase-II beamlines at the TPS. Items Unit IU22 CU15 CUT18 Type IVU CPMU CPMU Length m 3 2 2 Magnet material NdFeB (NMX-38EH)+ Dy diffusion PrFeB (NMX-68CU)+ Dy diffusion NdFeB (NMX-U52SH)+ Tb diffusion Remanence B r T 1.24 at 300 K 1.64 at 80 K 1.57 at 170 K Coercivity H cj kA/m 2743 at 300 K 6385 at 80 K 2854 at 170 K Period length λ u mm 22.000 14.945 17.962 Minimum magnet/vacuum gap mm 6.5 / 6.2 5.2 / 5.0 5.6 / 5.4 Effective magnetic field T 0.86 1.01 1.18 Deflection parameter 1.77 1.42 1.98 Number of periods 140 133 111 Magnet temperature K 300 80 170 Cooling method Water Cryo-cooler LN 2 tank Status Operation Operation Commissioning Table 1 : Specifications of the TPS in-vacuum undulators (IU22, CU15, and CUT18). Fig. 1 : Spectral performance of IU22-3m, CU15-2m, and CUT18-2m. Parameters for the calculation were as follows: E GEV = 3 GeV, β x = 5.3 m, β y = 1.7 m, coupling constant = 0.01, emittance = 1.6 nm rad, and energy spread = 10 −3 .