Development of AC Magnetic Field Imaging Technique Using Polarized Pulsed Neutrons at J-PARC

We have been developing a quantitative magnetic field imaging technique at J-PARC. As was previously reported [1], we successfully quantified strength and direction of a static magnetic field by analyzing the wavelength dependence of polarization position by position for images, which were obtained using a time-of-flight (TOF) method of pulsed neutrons. Applying this method to observe a magnetic field in industrial products, such as voltage converters and motors, it is necessary to extend this technique to the AC magnetic field driving at a frequency of commercial power supply (50~60Hz). In this study, we attempted to measure an AC magnetic field quantitatively with the TOF method. Magnetic field imaging experiments were performed at the beam line of BL10 “NOBORU” in the Materials and Life science experimental Facility (MLF) of J-PARC. The experimental setup was the same as the previous experiment [1]. An AC magnetic field was produced by applying an AC electric current to a small solenoid coil with the diameter of 5 mm and length of about 50 mm. The frequency of applied field was set to 50.5 Hz, which is slightly higher than that of a two-fold repetition of the pulsed neutrons of J-PARC. Polarization images were obtained under applying the AC field in the coil and wavelength dependence of polarization in a selected area was analyzed, in which polarization changes due to the neutron spin rotation were observed. By fitting the results with a model assuming that only the magnetic field inside the coil contributed to the neutron spin rotation, the amplitude of applied AC field was estimated to be 3.22±0.14×10 A/m, which was corresponded to the designed value of 3.3×10 A/m. This work was supported by Photon and Quantum Basic Research Coordinated Development Program from the Ministry of Education, Culture, Sports, Science and Technology, Japan.