Magnetic Field Measurement of Superconducting Transport Solenoid for COMET

The COMET experiment, currently under construction at J-PARC, aims to explore the process of muon-to-electron conversion in a nucleus. This phenomenon, known as charged lepton flavor violation, is an elementary process beyond the Standard Model. The muon beam is produced from pion decays generated by bombarding a proton beam on a target and corrected and transported from the production target to a stopping target in a detector with superconducting solenoid magnets. Their momentum is selected by a curved solenoid field (3 T) and dipole field (0.05 T) in the Muon Transport Solenoid (MTS), which has a diameter of 468 mm and a curvature radius of 3 m with a bending angle of 90 degrees. We energized the solenoid up to 105 A and the dipole to +/- 175 A in the summer of 2022. Although the solenoid was operated at a half of its design value (210 A) due to the limitation of the support structure, this commissioning provided a sufficient current for the subsequent engineering beam operation (Phase-alpha) to be conducted successfully. We performed in-situ magnetic measurements of the curved solenoid with Hall probes and compared them to 3D calculations. The paper describes the scheme and results of the magnetic filed measurement in MTS.