Demagnetization of Nd2Fe14B, Pr2Fe14B, and Sm2Co17 Permanent Magnets in Spallation Irradiation Fields

Prompted by the need for radiation-resistant permanent magnets for insertion devices (IDs) of high-brilliance next-generation synchrotrons such as the National Synchrotron Light Source II, the demagnetization of Nd
₂
Fe
₁₄
B and Pr
₂
Fe
₁₄
B was studied after exposure to a mixed irradiating field. Degradation and damage of the permanent magnetic material by components of electromagnetic showers induced in magnets by intense high-energy electron beams will alter the magnetic field structure of the IDs. Plate-like Nd
₂
Fe
₁₄
B magnets were irradiated to 1.8 Grad dose and were evaluated against Pr
₂
Fe
₁₄
B magnets irradiated to a lower dose of 20 Mrad. In addition, annular Sm
₂
Co
₁₇
and Nd
₂
Fe
₁₄
B magnets integrated within a ferrofluidic feedthrough (FFFT) rotary seal were also irradiated to dose levels of 2 Grad for Sm
₂
Co
₁₇
and 20 Mrad for Nd
₂
Fe
₁₄
B. Post-irradiation measurements of the magnetic intensity revealed that severe demagnetization exceeding 85% occurs in Nd
₂
Fe
₁₄
B magnets after only 50 Mrad dose and over 87% for Pr
₂
Fe
₁₄
B after 10 Mrad dose. The annular-shaped Sm
₂
Co
₁₇
magnets of the FFFTs were almost insensitive to irradiation up to a dose of 2 Grad. Annular-shaped Nd
₂
Fe
₁₄
B magnets also showed resistance to demagnetization, a direct consequence of the annular shape which is characterized by the removal of the stronger demagnetizing field present at the center of a disk-like magnet. The sensitivity of boron-based permanent magnets to neutron energy (thermal versus fast) was also assessed via specifically designed experiments and discussed.