Masami Iio

COMET experiment is planned to be conducted at the Hadron facility of J-PARC. This experiment searches for the neutrino-less transition of a muon to an electron in a nuclear field known as “$\mu$–e conversion,” which is one of the Charged Lepton Flavor Violation processes. A detector solenoid (DS), which is located at the downstream end of a muon beam line for COMET, is the world’s largest cryogen-free superconducting magnet, which is directly cooled by cryocoolers. The DS was fabricated, and the commissioning tests were performed in 2024. The DS has successfully been cooled down to $4.2 \,{\mathrm{K}}$ and excited to the rated current without training quenches. In addition, the preliminary magnetic field measurement using a 3-axis Hall probe is carried out, and these results have proven the integrity of the magnet.

The High Energy Accelerator Research Organization (KEK) has been conducting neutron irradiation studies on Rare-earth Barium Copper Oxide (REBCO) coated conductors for application in muon source solenoids. A proton beam is extracted from the accelerator and directed onto a secondary-particle production target to generate pions, which subsequently decay into muons. To efficiently capture the produced pions, a superconducting solenoid is positioned close to the production target, where the radiation environment is extremely intense. To study the degradation of superconducting properties under thermal and fast-neutron irradiation, four sample types containing different rare-earth elements were irradiated at the Belgian Reactor 2 (BR2) and the Japan Research Reactor No. 3 (JRR-3). GdYBCO was irradiated at BR2 with fast-neutron fluences in the range of 0.8–7.9 × 10²² n/m² and thermal-neutron fluences in the range of 0.2–1.4 × 10²² n/m². GdBCO, EuBCO, and YBCO were irradiated at JRR-3 with a fast-neutron fluence of 2.4 × 10²¹ n/m² and a thermal-neutron fluence of 7.9 × 10²² n/m². In the JRR-3 irradiation, the samples were loaded into capsules both with and without Cd shielding, which suppresses the effects of thermal neutrons, in order to investigate the specific impact of thermal-neutron irradiation. In the absence of Cd shielding, the critical temperature (T_C) decreased most in GdBCO, followed by EuBCO and YBCO, reflecting their respective neutron-absorption cross sections. The use of Cd shielding effectively suppressed this reduction in T_C. These results are consistent with, and corroborated by, previous studies (Sickafus et al., 1992). The critical current (I_C) degradation of GdYBCO samples agreed with (Unterrainer et al., 2024), while EuBCO, despite a smaller ΔT_C, exhibited a larger I_C degradation than GdYBCO.

Nuclear fusion is an enticing alternative to current sources of energy, with multilayered Rare-Earth Barium Copper Oxide (REBCO) coated conductors deemed pivotal in the race toward fully realized, commercially viable, and magnetic confinement fusion reactors. In this study, we simulated the ion spectrum expected to evolve from REBCO's nickel-based Hastelloy C-276 substrate and copper stabilizer in an affordable robust compact-like reactor. We then emulated this gas production through helium implantation to investigate changes in materials and superconducting properties. Our results revealed that the substrate and stabilizer are capable of producing protons energetic enough to recoil throughout the tape thickness in appreciable doses, and alphas energetic enough to deposit 7.54 × 1014 ions/cm2 or 50.1 helium appm in the superconducting layer over a 30-year reactor lifetime. The superconducting layer of SuperPower® tapes exhibited at least double the swelling rate of the other major layers, and both SuperPower and Fujikura Ltd. tapes displayed microstructural changes in the REBCO layer not observed in isotropic metals. For the estimated lifetime fluence, the Fujikura tapes showed a ∼1 K reduction in critical temperature and a 32% degradation in critical current for compact reactor-relevant conditions (16 T, 20 K). Nuclear transmutation, low-temperature solder implantations, gas-ion evolution, the influence of gas production on vortex dynamics, and other related considerations are also discussed.

To explore the feasibility of using high-temperature superconducting (HTS) REBCO coated conductors in future accelerator magnets, two REBCO flat racetrack coils were fabricated using 4-mm wide EuBCO tapes at the High Energy Accelerator Research Organization (KEK). These coils were tested as an insert inside a Nb$_{3}$Sn common-coil dipole magnet, which provides a background field of up to $\sim$ 9.5T, at the Brookhaven National Laboratory (BNL). REBCO tapes offer exceptionally high critical current density under strong magnetic fields; however, they also exhibit significant magnetization due to screening currents, leading to magnetic field errors. This study presents a 2D finite element model of screening current-induced fields (SCIF) in REBCO coils using the T-A formulation, along with the results obtained. Simulations were then performed for two KEK test cases: one where the REBCO conductors were oriented with the HTS tapes parallel to the background field, and another where the tapes were perpendicular to it. Since screening currents also influence the stress distribution and increase the peak stress in the coils, the mechanical effects of these currents were analyzed. The implications of these simulation and test results for the design of Nb$_{3}$Sn/REBCO superconducting accelerator magnets are discussed.

High temperature superconductors (HTS) are being considered for use in future accelerator projects where they will be subjected to high levels of radiation. Development of coated conductor HTS magnets has been undertaken at the High Energy Accelerator Research Organization (KEK). This paper presents the fabrication and testing of seamless double pancake racetrack-type HTS coils wound with EuBCO tape coated with a ceramic insulation material which has a higher radiation tolerance than organic materials. The coils are instrumented with voltage taps and Hall sensors, and an excitation test at 77 K in self-field was completed. The preparation for excitation tests in the background field (∼ 10 T) are currently in progress at Brookhaven National Laboratory (BNL).

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 $\pm$ 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.

A new muon beamline is under construction for the COMET Phase-I experiment at J-PARC. The experiment requires a negative muon beam with the world-highest intensity to search for muon-electron conversion processes. The muon beamline consists of a series of superconducting solenoid magnets to generate muons by 8 GeV pulsed proton beam from MR of J-PARC, to transport muons to the detectors. A radiation-tolerant superconducting solenoid, the Pion Capture Solenoid, is developed to generate a high magnetic field on a production target to trap pions and muons efficiently. A long curved solenoid, the Muon Transport Solenoid, with correction dipole coils is utilized not only to transport muons but also to select low-momentum muons against unnecessary particles. A large detector solenoid is planned to connect to the muon beamline with a gradually reduced solenoidal field tuned by additional small solenoids. This paper reports the status of the construction and commissioning of the superconducting magnet system.

Construction of a second target station of the Materials and Life Science Experimental Facility is one of the future plans of the Japan Proton Accelerator Research Complex. A superconducting magnet called a pion capture solenoid, which requires 100 MGy class high radiation resistance, will be installed for the muon beamline in the second target station. Therefore, we are researching and developing radiation-resistant magnets based on high temperature superconductors. As one of the research subjects, we have been investigating the effect of irradiation on rare-earth barium copper oxide (REBCO) conductors. Results of irradiation using the ⁶⁰ Co gamma-ray facility indicated no significant degradation in superconductivity up to 27.4 MGy. However, decreases in the superconducting transition temperature and critical current were confirmed with a neutron irradiation fluence of 8.23 × 10 ²¹ n/m ² .

The COMET experiment, which is being prepared at the Japan Proton Accelerator Research Complex (J-PARC), aims to explore the rare decay phenomenon of muons. This phenomenon is not allowed in the Standard Model of elementary particles but is expected to occur due to new physics beyond the Standard Model. In the COMET experiment, superconducting magnets are used throughout the muon beamline. The Pion Capture Solenoid concentrates the pions generated by the injection of proton beams into the target. Since this magnet surrounds the target, it is designed to operate in a high radiation environment. The Muon Transport Solenoid guides the muons generated by the decay of pions. This magnet maximizes the muon yield and reduces the other background particles. The Detector Solenoid acts as a spectrometer of the electrons generated by the decay of muons. These low-temperature superconducting magnets are now being developed and constructed. The current lead box is also constructed to supply high currents to the magnets. It provides a thermal gradient from the room-temperature part of the power supply to the low-temperature part of the coil. It is designed to provide a large thermal gradient over a short distance using high-temperature superconductors. Using the current lead box, all magnets, including the high-temperature superconducting leads, are cooled by conduction cooling. In this paper, the construction status of the COMET superconducting magnet system is reported.

Research and development of a mineral insulated superconducting magnet based on REBCO coated conductor has been performed at the High Energy Accelerator Research Organization (KEK) to establish technology for a future radiation resistant high field magnet. A goal of the research is an experimental proof of a small-scale demonstration magnet composed of mineral insulated REBCO coils. Application research on ceramic coating technology to REBCO coated conductor is in progress. A short sample study confirms the adaptability of ceramic coating for electrical insulation of REBCO tape. In addition, an electrical insulation film is formed on the surface of a long tape of about 14 m in length by applying the ceramic coating. A small prototype double pancake coil was wound with the ceramic coated REBCO tape with a ceramic adhesive.