Measurement of NdFeB permanent magnets demagnetization induced by high energy electron radiation

Demagnetization of NdFeB permanent magnets has been measured as function of radiation dose induced by high energy electrons. The magnet samples were of different intrinsic coercive forces, ≃12≃12 and ≃20KOe, dimensions and direction of magnetization. 5 GeV electron beam from 12 GeV Cornell Synchrotron was used as a radiation source. A calorimetric technique was employed for radiation dose measurement. Results indicated that depending on the sample intrinsic coercive force, shape and direction of magnetization the radiation dose causing 1% of demagnetization of the sample varies from 0.0765±0.005Mrad to 11.3±3.0Mrad, i.e., by more than a factor of 100.Experimental data analysis revealed that demagnetization of the given sample induced by radiation is strongly correlated with the sample demagnetizing temperature. This correlation was approximated by an exponential function with two parameters obtained from the data fitting. The function can be used to predict the critical radiation dose for permanent magnet assemblies like undulator magnets based on its demagnetizing temperature. The latter (demagnetization temperature) can be determined at the design stage from 3-D magnetic modeling and permanent magnet material properties.