Tuning of the magnetic measurement system for the SuperKEKB damping ring

The magnetic measurement system for the positron damping ring of the SuperKEKB was designed and manufactured in 2010. From 2011, the magnetic measurements began. The rotating harmonic coil system for the quadrupole measurements has problem of the low accuracy. In 2012, the shorter harmonic coil of 60cm length was manufactured and the system was reconstructed to fit the shorter coil. Although the accuracy improved with the shorter coil, it did not reach the required value. With the reconstruction of the coil fixing mechanism of the girder, finally the accuracy satisfied the requirements. In this proceedings, we show the struggle to improve the measurement accuracy. (author)