Advantages and disadvantages
According to the method of creating a magnetic field, undulators can be divided into electromagnetic (including superconducting) and permanent magnets. Electromagnetic undulators differ from other types of undulators: the ability to change the field strength by simply changing the current in the windings; high field uniformity (which is especially important for long undulators). The disadvantages of electromagnetic undulators include the following: relatively complex designs; large heat losses; the complexity/impossibility of fine tuning the magnetic field, which is practically necessary for the passage of the electron beam through the undulator.
Ordinary (warm) electromagnetic undulators, widely used in accelerator technology, can no longer satisfy the requirements for the field strength, for at least two reasons. First, saturation effects will noticeably affect the steel core with an increase in the field, since the magnetic field strength at the pole edge is 1.5–2 times higher than the field on the undulator axis. Secondly, the space between adjacent poles decreases with a period and the required current density in the electromagnetic coils reaches values of the order of 2 kA/cm or more, exceeding the permissible limit for warm electromagnets. In superconducting undulators, there is also a critical current density of the order of 2.7 kA/mm2, but this is two orders of magnitude higher than the limit for conventional warm electromagnets. Relative simplicity of design; lack of additional equipment (powerful current sources, cooling systems, etc.); possibility of fine adjustment of magnetic fields also should be mentioned [10]. Rare earth magnets significantly outperform other permanent magnets in a number of characteristics and especially in terms of remanence.
