Permanent helical micro-undulators for x-ray free electron lasers

Abstract

Helical Halbach undulators, formed by four alternately axially and radially magnetized rare-earth helices, produce a stronger field than two planar Halbach undulators with perpendicular polarization. Hybrid undulators of two longitudinally premagnetized rare-earth helices and two initially unmagnetized steel (or vanadium permendur) helices produce an equal or higher field and are easier to implement. We made sure that the required helices with a period of down to 3 mm or even less could be manufactured with high precision using Wire Electric Discharge Machining. According to the simulation results, when using identical electron bunches with moderate initial spreads in energy and angles, the gain in x-ray free electron laser (XFEL) power from replacing planar micro-undulators with the helical micro-undulators under consideration can be significantly greater than predicted by the simplest 1D theory with a cold electron beam. This makes such micro-undulators promising for creating compact XFELs of various types, both based on modern linear accelerators and plasma wakefield accelerators.