2020同步年報

092 ACTIVITY REPORT 2020 and closes vertically, as seen in Fig. 1(a) . In contrast to a general ID, which suffers only a vertical attractive force, the EPU suffers changes in both attractive and repulsive forces and in three directions. A rigid frame and base are hence required to support the backing beams. A cast structure replaces the screwed-in frame to improve the stiffness of the member, as seen in a filled region (blue) in Fig. 1(b) . Beginning with the Phase-I EPU, a freely sliding end of a backing beam was designed to diminish the mechanical deformation caused by thermal expansion and contraction of heterogeneous materials due to changes in ambient temperature. Strategies to Resist Radiation Damage In addition to a robust mechanical structure, the electron- ics in a driving system must be stable, even in a radiation environment. Radiation damage to and protection for the EPU becomes a worthy item for discussion to obtain a reli- able operation. The experience gained from commission- ing and operation of the Phase-I EPUs at the TPS taught us that EPU driving systems can behave erratically following a beam dump or loss. According to the related research, this effect is attributed to the malfunction of memory elements in an optical encoder. Although the damage is “soft” and recovers after a power cycle, the Phase-II EPUs are designed with various strategies to address this issue. Encoders are enclosed in lead material of thickness more than 10 mm, which is a tenth and a half value layer re- quired for gamma rays of energy 412 to 1,332 keV. The Fig. 1 : Mechanical structure of an EPU. Side view (a) on the backing beam side. From top to bottom: EPU56 (TLS), EPU48 (TPS Phase I) and EPU66 (TPS Phase II). (b) In the direction of the electron beam. Left: EPU56 (TLS). Right: EPU66 (TPS Phase II). The magnet block and the fixed structure are enlarged in the figure. distance between the encoder and the nominal electron beam position is increased; the distance with the Phase-II EPUs is more than twice as great as the least distance of an encoder in Phase I. The radiation dose of neutrons is thus supposed to be decreased by 50 to 75%. In addition, we reserve an installation space for a second encoder in the Phase-II EPUs. Apart from the encoders for TR-electronics, Renishaw’s encoders will be installed and tested. Challenges and Methods for Optimization of the Magnetic Field To achieve the desired properties of radiation and to have no net effect on an electron beam are two main require- ments of an EPU. To realize the former objective, the mag- netic field is desired to be a purely periodic distribution. Radiation emitted from each pole has a correct correlation, resulting in constructive interference. The phase error describes the deviation of the phase between a real device with field error and an ideal one at each pole. The r.m.s. phase error, which has been shown to be well correlated with the spectral intensity, must thus be minimized. The first and second field integrals represent the exit angle and position of the electron beam, respectively. For the latter objective, these field integrals in transverse directions must be minimized, to effect a negligible multipole error. In contrast to a general ID, an EPU must be operated in several polarization modes, each of which must meet the specified requirements. How to arrange thousands of magnet blocks to fulfil the multiple requirements hence becomes the task