Implementation of prototype permanent microundulators

We report on the implementation of two types of helical microundulators with a small period of 6 mm, containing axially magnetized helices fabricated from a single piece of rare-earth magnet. The first microundulator, formed by two oppositely magnetized longitudinal helices, produces a transverse on-axis field of about 0.93 T, while the second, assembled from two oppositely longitudinally magnetized rare-earth helices and two pre-unmagnetized steel helices, produces a field of about 1.5 T. The apertures inside both microundulators are 1 mm, which required special field measurement techniques. When used in compact FELs in the terahertz to X-ray range, such microundulators can provide significantly higher electron oscillation velocity and radiated power than the previously proposed planar microundulators. An attractive example of an ultra-compact XFEL in SASE regime is presented, in which the use of the studied hybrid helical microundulator can provide a radiation power of 48 GW at a wavelength of about 5 Å.