Radiation-induced magnetization reversal causing a large flux loss in undulator permanent magnets

Abstract

We report an unexpectedly large flux loss observed in permanent magnets in one of the undulators operated in SACLA, the x-ray free electron laser facility in Japan. Characterizations of individual magnets extracted from the relevant undulator have revealed that the flux loss was caused by a homogeneous magnetization reversal extending over a wide area, but not by demagnetization of individual magnets damaged by radiation. We show that the estimated flux-loss rate is much higher than what is reported in previous papers, and its distribution is much more localized to the upstream side. Results of numerical and experimental studies carried out to validate the magnetization reversal and quantify the flux loss are presented, together with possible countermeasures against rapid degradation of the undulator performance.

dc septum magnet based on permanent magnet for next-generation light sources

“…Nd-Fe-B has a higher B r and H c than Sm 2 Co 17 , and the total number of PMs can be reduced. However, we chose one of the highest grades of commercially available Sm 2 Co 17 (LM-32SH, TOKIN Corporation [14]) considering its toughness against demagnetization in an irradiated environment [15]. The specification of Sm 2 Co 17 adopted for the septum magnet is summarized in Table II.…”

Challenges of in-vacuum and cryogenic permanent magnet undulator technologies

“…However, the above treatment was found to be noneffective for IVUs installed in the linac-based XFEL facilitiy, SACLA. Some of the PMs in the IVU generated a reverse field [15]. It should be noted that the radiation damage in IVUs installed in a linac-based-XFEL is more noticeable, compared to IVUs in a storage ring, because the electron halo contains many more particles than the outer tails in a Gaussian distribution while the loss of electrons cannot be detected easily.…”

“…In addition, the pressure for IVUs can be higher than 10 −6 Pa, so the vacuum components need not necessarily be UHV compatible and have high thermal resistance against UHV bakeout. On the other hand, radiation damage issues in linacs become more serious [14,15] compared to storage rings because the electron halo surrounding the particle beam can result in radiation damage of permanent magnet materials. Electron losses from the halo are not always detectable.…”

Permanent magnet based dipole magnets for next generation light sources

“…A permanent magnet is demagnetized when radiation hits the magnet. Demagnetization of undulators has been observed because of the high radiation doses in accelerators, and the effects have been discussed at several accelerator facilities [14,16,18,37,38]. However, it should be emphasized that the conditions for the previous observations for undulators are different than those for dipole magnets in several aspects [39].…”

“…Second, the magnetic field generated by a permanent magnet is known to be temperature dependent [14,15], which affects the electron energy and lattice functions when the ambient temperature changes. Third, demagnetization of permanent magnets in undulators have been observed at several light sources [16][17][18][19][20], which should be avoided for the main magnets. Further, it is necessary to discuss the feasibility of producing the newly proposed bending magnets with field gradients as permanent magnet based main magnets for light sources.…”