Implementation of Prototype Permanent Microundulators

We report the implementation of two types of helical microundulators with a 6 mm period, built from axially magnetized helices fabricated from a single piece of rare-earth magnet. The first device, composed of two oppositely magnetized longitudinal helices, produces a transverse on-axis field exceeding 0.93 T. The second, a hybrid design with two oppositely longitudinally pre-magnetized rare-earth helices alternating with two unmagnetized steel helices, reaches about 1.5 T on axis. Both devices have a 1 mm bore, which required dedicated field-measurement techniques. When used in compact FELs from terahertz to X-ray, such microundulators can provide higher electron oscillation amplitude and radiated power than planar microundulators of similar period. As an illustrative case, we present an ultra-compact SASE XFEL concept in which the studied hybrid helical microundulator can provide approximately 48 GW of radiation power at a wavelength of about 5 angstrom.