Helical undulators of magnetized ring sectors

A periodic helical array of magnetized ring rare-earth sectors (quasi-helices) creates a helical field on its axis, close in structure and magnitude to the field of a solid helix. In some cases, such permanent undulators, made of a relatively small number of readily available magnets, are easier to manufacture and assemble than a system containing magnetized helices made of one piece. The field magnitude of the quasi-helices depends only weakly on the number of sectors per undulator period. According to calculations and measurements, the prototype Halbach-type helical undulator assembled from longitudinally and “radially” magnetized quasi-helices consisting of NdFeB ring sectors with a period of 2 cm and a relatively large inner diameter of 8 mm, provides an on-axis field of about 0.6T. By reducing the inner diameter to 5 mm, one can achieve twice as strong a field. When assembling such an undulator, to weaken the mutual effect of strongly interacting magnetized elements, it turned out to be structurally more convenient, while maintaining the position of all ring sectors, to use the division of the undulator not into quasi-helices, but into cylindrical sectors shifted along the axis and rotated relative to each other. Permanent undulators made from ring sectors can provide high oscillation velocity of electrons and seem promising for increasing the efficiency of Free Electron Lasers in various frequency ranges.