2020同步年報

Progress on Elliptically Polarized Undulators at the NSRRC Facility Status 091 Fig. 7 : Downtime distribution of TPS accelerator in 2020. (Downtime 68 hours) PS, 6.24, 9% Cryogenic, 0.00, 0% BD, 2.37, 3% RF, 6.70, 10% I&C, 8.25, 12% Operation, 0.47, 1% Magnet, 0.00, 0% Vacuum, 25.02, 37% Utility, 1.96, 3% Safety, 0.69, 1% Linac, 2.28, 3% Other, 12.63, 19% Beamline, 1.36, 2% Introduction Because demand for circularly polarized radiation is strong, insertion devices (ID) have been used to provide elliptical fields since the late 1980s. The trend is to increase both the degree of circular polarization and the brightness of the radiation. Among many ID designs, an elliptically polarized undulator (EPU) provides the greatest merit flux and has thus become the workhorse in several facilities. NSRRC has also been developing its own EPU since the 1990s. EPU56 (length 3.9 m) with period length 56 mm was the longest EPU in the world at the time, as seen in Fig. 1(a) . It was installed at Taiwan Light Source (TLS, 1.5-GeV storage ring) in service to date. At Taiwan Photon Source (TPS, 3-GeV storage ring), the EPU continues to play an important role in fulfil the needs of EUV of the soft X-ray user community. Up to phase II, in total five EPUs have been in- stalled, as seen in Table 1 . It is worth noting that, for full utilization of the TPS straight section, the EPU installed this year has length 4.36 m; a new device, flat wire, is installed, which is expected to correct the dynamic multipoles induced by the EPU. Table 1 : History of EPU development at NSRRC. Year Name Facility Contents Remarks 1999 EPU56 TLS λ u a = 56 mm, G min b = 20 mm, L c = 3.9 m longest EPU in the world, at that time 2015 EPU46 TPS (phase I) λ u = 46 mm, G min = 14 mm, L = 3.8 m refurbished by NSRRC 2015 EPU48A λ u = 48 mm, G min = 13 mm, L = 3.4 m double EPUs for a long straight section 2015 EPU48B 2020 EPU66 TPS (phase II) λ u = 66 mm, G min = 16 mm, L = 4.36 m flat wire installed within a gap 2020 EPU168 λ u = 168 mm, G min = 27.2 mm, L = 4.36 m for EUV beamline, flat wire installed Notes: a period length; b minimum gap; c physical length of EPU. Description of the Mechanical Structure The EPU is composed of four Halbach arrays that regulate the amplitude and phase of the vertical and horizontal magnetic fields by the relative motion of the magnet arrays. On driving the diagonal arrays, the EPU can provide various polarizations, such as horizontal linear, circular and vertical linear (VL) polarization. As with general ID operation, the EPU varies the mag- netic fields to fulfil the spectral tuning by opening and closing a gap of magnet arrays. In terms of the mechanical structure, the EPU has two driving systems to move magnet arrays: vertical movement to tune photon energy, longitudinal movement to control polarization. The vertical movement is achieved on fixing the magnet arrays on the backing beam, which opens