Beam Based Search for Linear Imperfection Fields in 11 m Long Undulator at NewSUBARU

The NewSUBARU storage ring has an 11 m long undulator. It is a permanent magnet planner undulator, consisting of eight units. We measured and adjusted its magnetic field distribution to minimize the imperfection fields mainly by an in-situ sorting method developed by T. Tanaka of SPring-8. The imperfection fields were minimized for each unit to prevent them from exciting unnecessary betatron resonance in the ring. We measured the gap dependence of linear imperfection fields, dipole and quadrupole field components, using the stored electron beam in the ring. The results confirmed that the field adjustment reduced the imperfection fields. Strengths of the dipole imperfection fields in the undulator were at the same level as the earth field. The normal and skewed quadrupole imperfection fields were about three times of the guideline, but were smaller than the imperfections produced by the ring magnets. We conclude that the linear imperfection fields were reduced to the acceptable level