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Synchrotron radiation is generated when a high-energy electron beam passes through an insertion device — an undulator or a wiggler — that provides a periodic magnetic field. Because the electron trajectory is governed by this field, the magnetic properties of the device, such as field strength and period, directly determine the intensity, wavelength, and directionality of the emitted radiation. Insertion devices are built with either electromagnets or permanent magnets, and among permanent magnets NdFeB is the most widely used material. SmCo is also employed in several facilities, including the Linac Coherent Light Source (LCLS), and is planned for use at the Korea-4GSR multi-purpose synchrotron radiation facility.
The permanent magnets installed in the undulators operating at PLS-II and PAL-XFEL of the Pohang Accelerator Laboratory (PAL) are continuously exposed to radiation produced during beam operation, and their magnetic properties may degrade as a result. Verifying the radiation-induced change of the magnetic field is therefore essential for stable and reproducible synchrotron radiation generation. A previous study at PAL evaluated the magnetic field variation of NdFeB magnets irradiated by a 2.5 GeV electron beam [1]. In the present work, an irradiation experiment was carried out at the PAL-XFEL Main Beam Dump (MBD) in order to reach a considerably higher beam energy than in the earlier study, with the objective of characterizing the change of the magnetic field as a function of the electric charge accumulated on the magnet samples. A related evaluation of electron beam irradiation effects on permanent magnets reported by SOLEIL, a large accelerator facility in France, indicates that the damage threshold of SmCo is more than two orders of magnitude higher than that of NdFeB [2]. Accordingly, two types of permanent magnets, NdFeB and SmCo, were adopted as the study objects so that the effect of electron beam irradiation could be assessed for both materials currently used, or planned to be used, in domestic and overseas light source facilities.
The permanent magnet samples were mounted at the beam center position of the PAL-XFEL MBD, stacked vertically so that several samples could be irradiated simultaneously under the same beam condition while being exposed to different radiation levels depending on their vertical position. Following the approach reported in a previous study [3], the samples were not irradiated directly by the electron beam. Instead, a 4 cm thick copper target was placed in front of the sample assembly so that the samples were exposed to secondary radiation under a well-defined and uniform radiation field. The irradiation was performed continuously over a period of several days using the high-energy electron beam delivered to the beam dump during normal machine operation, and the accumulated charge incident on the assembly was recorded throughout the irradiation.
The residual dose rate at the magnet surface was measured with an ion chamber survey meter (Fluke Biomedical 451P) immediately after irradiation in order to confirm the activation of the samples. After a cooling period of more than two weeks, the three-dimensional magnetic field distribution (X, Y, and Z axes) was mapped at 1 mm intervals using a low-noise magnetic field transducer (SENIS, I3C-03C10L-B02T0K5J). The same measurement was performed before and after irradiation at identical positions so that the difference could be evaluated point by point.
The activation of the irradiated samples was confirmed from the residual dose rate measured near the magnet surface after the irradiation period, and the magnetic field distributions obtained before and after irradiation were compared for each sample. The effect of the electron beam irradiation on the magnetic field of the NdFeB and SmCo permanent magnets, together with its dependence on the accumulated charge, will be discussed.
References
[1] R. Qiu, H.-S. Lee, J. Li, T.-Y. Koo, and T. H. Jang, “Radiation damage of Nd2Fe14B permanent magnets at 2.5 GeV electron accelerator,” Nucl. Instrum. Methods Phys. Res. A, vol. 594, no. 2, pp. 111–118, 2008.
[2] J.-B. Pruvost and P. Berkvens, “Challenges in Radiation Safety associated with SOLEIL II Upgrade project,” 12th International Workshop on Radiation Safety at Synchrotron Radiation Sources.
[3] Y. Asano, T. Bizen, and X. Marechal, “Analyses of the factors for the demagnetization of permanent magnets caused by high-energy electron irradiation,” J. Synchrotron Rad., vol. 16, no. 3, pp. 317–324, 2009.
| Topics | T6. Radiation damage to materials |
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