Thermal demagnetization in in-vacuum undulators and effect of magnetic configuration on choice of magnet grade

Resistance of undulator magnetic arrays composed of neodymium–iron–boron (NdFeB) permanent magnets (PMs) against thermal demagnetization, which is important especially for in-vacuum undulators (IVUs), has been investigated under different conditions of intrinsic coercivity, application of grain boundary diffusion, and magnetic configurations, to explore reasonable choices of NdFeB PMs for IVUs. The experimental results show that the resistance of a special magnetic configuration based on PMs with 45∘-tilted easy axis, which has been proposed before to enhance the resistance against radiation damage, is much higher than other conventional ones. This suggests the potential of the special magnetic configuration to enhance the performances of IVUs.