Beam-Based Design of a Correction Coil for the Stray Field of a Pulse Septum Magnet

A direct-drive pulse septum magnet has been in operation for injection of a 1 GeV electron beam at synchrotron light source ring NewSUBARU. Its nominal field is 0.35 T with waveform of 0.92 ms half-sine. Its stray field was measured using the stored electron beam and found to be different from the original design, probably because of an alignment error of the septum current sheet. In the data analysis, the stray field was separated in two components. One component had the expected normal field symmetry, and the other had an unexpected opposite field symmetry. As reported here, a correction coil system for the stray field was designed on the basis of measurements using the stored electron beam. The magnetic field produced by the correction coil was calculated with the field calculation code OPERA-3d. With this correction system, the stray field with the opposite symmetry was reduced from 213 $\mu{\rm Tm}$ (Tesla meter) maximum to 8 $\mu{\rm Tm}$ maximum.