Image tuning techniques for enhancing the performance of pure permanent magnet undulators with small gap/period ratios
The on-axis field of a small-gap undulator constructed out of pure permanent magnet (PM) blocks arranged in an alternating-dipole (i.e., 2 dipoles/period) array can be substantially varied by positioning monolithic permeable plates above and below the undulator jaws. This simple technique, which can be used to control the fundamental photon energy in conventional synchrotron radiation (SR) or Free Electron Laser (FEL) applications requiring sub-octave tuning, can also be shown to suppress magnetic inhomogeneities that can contribute to the undulator's on-axis field errors. If a standard (4 vector rotations/period) Halbach undulator, composed of PM blocks with square cross sections, is rearranged into an alternating-dipole array with the same period, the peak field that can be generated with superimposed image plates can, for a certain range of magnet dimensions, exceed that of the pure-PM Halbach array. This design technique, which can be viewed as intermediate between the "pure-PM" and standard "hybrid/PM" configurations, provides a potentially cost-effective method of enhancing the performance of small-gap, pure-PM insertion devices. In this paper we report on the analysis and recent characterization of pure-PM undulator with superimposed image plates, and discuss possible applications to FEL research.