Superconducting magnet system for the interaction region of SuperKEKB. Magnetic measurement of quadrupole magnets with harmonic coils

After a phase 1 operation (Feb. ∼ Jun. 2016) of SuperKEKB, a superconducting-final-focus-quadrupole system (QCS) has been installed into an interaction region (IR) of SuperKEKB for the following operation period, phase 2. The QCS has eight-superconducting-quadrupole magnets and squeezes electron and positron beam at an interaction point to about 50 nm in vertical direction. To confirm magnetic qualities of the quadrupole magnets at operation conditions where solenoid fields of Belle II detector and compensation solenoids of QCS are energized, we performed magnetic measurements at IR. Higher order harmonics of the quadrupole magnets were measured by harmonic coils. The amplitudes for vertical focus-quadrupole magnets were below level of 1 unit. One of the horizontal-focusing-quadrupole magnets for an electron beam exhibits a large amplitude of 20 units in sextupole components. The large sextupole components are diminished when the all solenoids are turned off. It was found that the error fields are generated at ends of an iron-support-block surrounding the quadrupole magnet. (author)