Phase combination for self-cancellation of magnetic force in undulators

A simple arrangement of undulator magnets having nearly zero magnetic force between the upper and lower magnet arrays is proposed, which significantly simplifies the mechanical structure of undulators. In this arrangement, each magnet array is divided into a number of sections, half of which are phase-shifted according to a particular rule without breaking the periodic condition required for the undulator field. Calculation results show that, when the gap over period is in practical range, more than or equal to 1 /18 , the magnetic force can be reduced down to less than 1 /50 of that in the Halbach undulator by applying the proposed scheme.