Development of Magnetic Field Mapping System for the MuSEUM Experiment With High Precision Using CW-NMR Probes

The MuSEUM experiment is planned at J-PARC to measure the hyperfine structure of a muonium at 1.7 T. The aim is to test the standard theory by precisely measuring the hyperfine structure of a muonium in the ground state. An important parameter is the magnetic field uniformity, which is required to be smaller than 0.2 ppm (peak-to-peak) in a rotating ellipsoid region with a diameter of 20 cm and a major axis length of 30 cm. To perform magnetic field shimming and verify the uniformity of the magnetic field, We are developing a magnetic field distribution measurement system using a CW (Continuous Wave)-NMR probe with a resolution of smaller than 10 ppb. In the present design, twenty-four CW-NMR probes are arranged in a semicircular circumference for quick measurement. Because each probe is located at a short distance, the effect of a crosstalk among probes should be minimised. Parameters of each probe has been determined and operation with multiple channels has been tested. Moreover, an influence of iron shims has been measured accurately with a eight-channel system.