Radiation-Induced Demagnetization of Nd-Fe-B Permanent Magnets

Introduction The Advanced Photon Source (APS), as well as other third-generation synchrotron light sources, uses permanent magnets in the insertion devices to produce x-rays for scientific research [1,2]. When placed in a high-energy storage ring, these permanent magnets are subjected to irradiation from synchrotron radiation, high-energy bremsstrahlung, and bremsstrahlung-produced neutrons. Previous investigations have exhibited varying degrees of degradation in the intensity of magnetization of these magnets [3] due to The APS specifically uses Nd-Fe-B permanent magnets in the insertion devices [2]. Although no detectable radiation-induced demagnetization has been observed in the APS insertion devices so far [12], partial demagnetization has been observed in at least one insertion device at the European Synchrotron Radiation Facility (ESRF) [4,6], where Nd-Fe-B permanent magnets are also used. A growing concern for the APS insertion devices as well as the insertion devices of next-generation light sources, such as free-electron lasers (FELs) [13,14] where permanent magnets will also be used, resulted from the partial demagnetization observed at the ESRF. Greater potential for irradiation of the Nd-Fe-B permanent magnets used in the APS insertion devices results from the close proximity of the electron beam to the magnets themselves. During normal operation the magnets will be as close as 3-5 mm from the electron beam. This close proximity increases the potential of magnet irradiation from synchrotron radiation and high-energy bremsstrahlung. Typically the APS insertion devices receive a photon dose of approximately 1 Mrad during every APS run, which generally lasts two months with approximately 100 Amp-h of beam current [15]. This results in an absorbed photon dose of approximately 7 Mrad per year, or approximately 130 Mrad after 20 years of operation, which is the desired lifespan of the APS insertion devices. At this point in time no measurements have been done to determine the neutron dose received by the insertion devices during a normal APS run. However the neutron dose is expected to be much lower than the combined photon dose of x-rays and γ-rays because the photoneutron production in an electromagnetic shower is minimal [16]. Calculations performed at the APS of the neutron flux present as a result of the entire 100 mA of stored beam loss on a beamline element revealed an instantaneous neutron pulse of 2x10 8 neutrons. Similar calculations for the neutron flux produced during a continuous injection loss of 84 watts at the design performance goal revealed a flux of 1.9x10 11 …