The property of universality for pure permanent magnet wigglers

This paper deals with electromagnetic radiation generated by relativistic particles in arbitrary planar magnetic field (in a wiggler or undulator for example). Magnetic system producing this field is assumed to be planar and to consist of permanent magnets. It is shown that there is a special rotations for vectors of magnetization in such magnetic system: while magnetic field is varying but the spectrum of spontaneous radiation generated by relativistic particles remains the same. This property of radiation can be used in design of the new undulators and wigglers.