Rare-earth magnets made of NdFeB alloy have very high remanent magnetization

Rare-earth magnets made of NdFeB alloy have very high remanent magnetization (up to 1.47 T), but a small maximum operating temperature, while magnets from the Sm2Coi7 alloy retain their characteristics (Br up to 1.2 T) at significantly higher temperatures (up to 250ºC). When choosing rare-earth magnets for the designed undulator, one should be guided, first of all, by the temperature conditions in which the device will operate, and only secondarily by their magnetic properties. One of the main drawbacks of the permanent magnet undulator design is a large spread in the magnetization force and direction in different magnetic blocks. To reduce the spread, one has to make a careful selection of magnets, combinatorially group them into pairs, fours, etc. By their design, permanent magnet undulators (PM) are subdivided into pure rare-earth (iron-free) and hybrid [10]. Undulators of this type are the simplest to design and manufacture, but require a computer calculation of the location of preliminary carefully measured magnetic prisms.

The fields adjustment is very difficult (it is practically impossible) and the amplitudes of the magnetic fields of such undulators are small. Substantially stronger magnetic fields can be achieved with the so-called hybrid configuration. Since the pole pieces in the hybrid circuit are usually in saturation, changes in the magnetic field caused by inhomogeneities in the magnetization of the rare earth magnets are reduced. It becomes an option to fine-tune the field of an individual pole using shunts, which makes it possible to achieve high field uniformity.