Design of a Short-Period Helical Permanent Magnet Undulator

The design of a helical permanent magnet undulator with a period length of 24 mm for the production of circularly and linearly polarized radiation will be discussed. This undulator consists of four helically shaped magnet arrays, which is expected to produce adjustable helical fields in a cylindrical gap. The helical fields were calculated with the 3-D OPERA computer code producing vertical and horizontal peak fields of 1.29 T for a 24-mm-period length within a bore diameter of 5 mm. The sizes and location of the end poles were optimized to minimize the first and second field integrals. Furthermore, on-axis horizontal or vertical magnetic fields can be generated independently by shifting the relative positions of diagonal pairs of magnet arrays in this device. It leads to the generation of circular and linear polarized radiation in one undulator. The undulator phase shifter can be controlled by mechanical changes to adjust field strength and to tune the helical and linear photon energy. This paper describes magnetic field simulations and mechanical design of the short-period helical permanent magnet undulator.