Helical Undulators of Magnetized Helices and Ring Sectors

The periodic system of a spiral array of magnetized ring sectors (quasi-helix) creates a helical field, which is close in structure and magnitude to the field in the system of helical magnets. Such a system of a relatively small number of readily accessible magnets can be easier to manufacture and assemble than a system containing mag-netized helices made from single pieces. In this paper, we study the dependence of the helical field on the number of sectors per undulator period. Short prototypes consisting of longitudinally and radially magnetized sectors were experimentally studied. The maximum value of the helical field on the axis of a quasi-helix of longitudinally and radially magnetized ring NdFeB sectors with a period of 2 cm and a relatively large inner diameter of 8 mm is 0.28 T and 0.34 T, respectively. This corresponds to a field of 0.74 T in the case of four alternately longitudinally and radially magnetized Halbach-like quasi-helices of pre-magnetized sectors and a hybrid system assembled from longitudinally pre-magnetized sectors and preliminarily non-magnetized steel sectors. Such undulators can provide a high oscillatory electron velocity and seem promising for increasing the efficiency of FELs and inverse FELs in various frequency ranges.