Yasushi Arimoto

The Main Linac of the International Linear Collider (ILC) includes one superconducting quadrupole (SCQ) package within each of the Type-B cryomodules. This combined-function, superconducting magnet focuses and steers the electrons and positrons. Due to magnet assembly constraints, it must be splittable, and therefore cryogen-free, that is, conductively cooled via thermal links. Additionally, a large pulsed heat deposition is expected in the superconducting coils due to beam-induced dark currents. As an alternative to the Nb-Ti magnet developed by KEK, CIEMAT explores a proposal based on bronze-route Nb₃Sn that withstands the dark current heat loads by profiting from the higher thermal margin of the superconductor. This paper reports the thermal analysis on the magnet design to maximize the temperature margin provided by the Nb₃Sn technology. The selected wire was measured to define its critical surface at the atypical low field and low current operation region of the SCQ of ILC. The operation temperature profile of the coil was examined with analytical and finite element models, including a thorough examination on the anisotropic behaviour of the impregnated Nb₃Sn quadrupole coils. A highly conductive casing for the coil was designed and high purity aluminum sheets were integrated to significantly improve the thermal management via dry cooling.

To improve the beam collision performance of SuperKEKB, installation of superconducting sextupole magnet in being considered. The main superconducting sextupole coils and three types of superconducting correction coils will be incorporated in this magnet. We are developing the correction coils using ultra-fine strand Nb₃Al superconducting cables with strand diameters less than 50 µm fabricated by the React & Wind method. In this paper, we report the measurement results of the superconducting performance of Nb₃Al cables of 0.6 to 0.8 m in length in the solenoid coils with radius of curvature from 15 mm to 25 mm.

The International Linear Collider (ILC) Main Linac consists of a series of cryomodules, one third of which are composed by eight 9-cell superconducting RF (SRF) cavities and a superconducting quadrupole (SCQ) package. The electrons and positrons are focused by the quadrupole and steered by the two dipoles included in the SCQ magnet. Large expected dark currents are originated at the SRF cavities and deposit energy at the SCQ. The superconductor in the coils should not surpass its critical temperature before that heat deposition is evacuated.This paper depicts the SCQ electromagnetic design proposed by CIEMAT. Several superconductors were analysed and compared. Nb₃Sn wire was selected for the quadrupole coils relying on the large temperature margin for the absorption in the superconducting coils of the heat deposition from the dark currents. The magnet cross-section has been carefully optimized for the HE type of the SCQ magnet to achieve a good field quality and superconductor efficiency in spite of the expected iron saturation. The dipole coils were protected from the heat deposition, enabling the election of Nb-Ti wire. The longitudinal space has been defined with an electromagnetic 3D model.

In the straight beam lines of the SuperKEKB Tsukuba area, 16 conventional sextupole magnets have been installed at intervals of 20–35 m. To achieve high luminosity, higher precision beam tuning with sextupole magnets is required. In KEK, a study on a superconducting sextupole magnet system involving three types of corrector magnets is ongoing. Considering the operating temperature of the system, an A15 compound superconductor, Nb₃Al, is being studied for use as the cable material, and the development of the reaction and winding coil production with Nb₃Al cable has been attempted. This study developed an Nb₃Al ultra-fine strand superconducting cable with a strand of φ50 μm for the corrector magnets, and the critical currents of the cable were measured as functions of the bending radius and the temperature. This paper reports the temperature dependence of the critical currents of the bent Nb₃Al cables after heat treatment.

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.

The International Linear Collider (ILC) main linac comprises a series of 12 m-long cryomodules. The cryomodule contains eight 9-cell superconducting (SC) RF (SRF) cavities and an SC quadrupole magnet combined with dipole correctors to focus and steer electron and positron beams. The magnets are installed between the SRF cavity string and at the longitudinal center of a common cryomodule/cryostat. These magnets are conductively cooled by pure aluminum channels thermally connected to a 2 K two-phase helium pipe for cooling the SRF cavities. A recent study shows that field-emitted electrons, so-called “dark current” initiated in the SRF cavities, are transmitted through the SRF cavity string and reach the SC magnet. The energy is inevitably absorbed in the SC coil due to the magnetic field, resulting in risks of a quench caused by the coil heating. We are investigating alternate magnet designs by using Nb $_{3}$ Sn or $\text{MgB}_{2}$ SC and by adding the dark current absorber surrounding the beam-pipe to realize sustainable magnet operation under the dark current heating. We report the design study of the magnet system and interfaces to the cryomodule accommodating it with the SRF cavities.

Along the beam lines of the SuperKEKB main rings, 218 conventional sextupole magnets are installed, and they are used for tuning the chromaticity of the accelerator rings. Especially, 16 sextupole magnets in the beam straight lines in the Tsukuba area are quite essential for the Crab Waist beam colliding. To make the beam tuning process finer, we are studying the superconducting sextupole magnet system with three types of the corrector magnets. From the operation temperature of the system, A15 compound superconductor is chosen as the cable material, and the development of the react and wind coil production has been tried with the Nb ₃ Al cable. The Nb ₃ Al super fine strand superconducting cable for the corrector magnets was manufactured, and its superconducting characteristics are being studied. In this paper, as the first test result of the development, the critical current characteristics of the straight cable as variables of temperature and magnetic field are reported. As the results, the critical current of the Nb ₃ Al cable was higher than the design values for the corrector magnets, and this level of cable performance also indicates the potential for the cable being used for the main sextupole magnet.

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.

The 55 SuperKEKB final focusing superconducting magnets are assembled into two cryostats, located at the left side and the right side of the accelerator interaction point, respectively. The two cryostats are cooled by sub-cooled liquid helium at 0.16 MPa with two independent refrigerators of about 250 W cooling capacity. From August 2016, the two cryostats were installed into the accelerator beam line and were connected with their own sub-coolers and refrigerators. As cryogenic systems, commissioning was carried out to cool down the cryostats, to excite superconducting magnets, and to test system interlock protections against some emergencies. This paper introduces the cryostats and cryogenic systems, and the commissioning processes. The cryostat heat loads were measured and the results are presented in this paper.