Measurement of residual magnetic field by superconducting magnets of the LHD

After irregular excitations of superconducting magnets of the Large Helical Device, the residual magnetic field prevents uniform glow discharging in a plasma vacuum vessel. In order to measure the changes of residual field, fifteen Hall probes were installed at five periodic positions in three directions on the mid-plane of the inner cylinder of the cryostat. After the first excitation of the magnets in the cooling cycle, the residual field was changed by 0.1 mT at all positions. The direction was the same as the field during the excitation. After the excitation without helical coils, the residual field was clearly changed. During warm-up of the superconducting magnets, the additional residual field disappeared at the same time of the transition to a normal state. The additional residual field can be induced by magnetization of the superconducting magnets. The magnetic field by magnetization of NbTi filaments in the magnets has been calculated and compared with the measured data.