A design study of a staggered array undulator for high longitudinal uniformity of undulator peak fields by use of a 2-D code

A staggered array undulator was studied using a 2-D computer code, aiming at high longitudinal uniformity of undulator peak fields. Firstly, insertion of ferromagnetic pieces at both ends of the undulator is found to give higher uniformity of the undulator peak fields, while there are still undesirable leakages of axial magnetic fields in the vicinity of both ends of the undulator. Secondly, to further reduce these leakages, an additional external closed magnetic circuit was designed and found to successfully reduce the leakages as well as giving a much higher uniformity of the undulator peak fields. As a result, it is found in our simulation that, compared with the undulator without the closed magnetic circuit, the transmittance of electrons through the undulator increases by 15%, and consequently the spontaneous emission intensity also increases by 50%.