The beam final focus superconducting magnet system for SuperKEKB consists of 55 superconducting magnets, and it is a key system for increasing the electron and positron beam collision frequency. In particular, high quality of the superconducting quadrupole magnet close to the interaction point for each beam was requested in the excitation and magnetic field performance. KEK developed the prototype quadrupole magnets in house. This paper reports on the many conditions experienced during development of the prototype magnets.

The hands-on training courses on superconducting magnet were held for more than 10 years for young engineers and scientists. Many small size 7 T superconducting solenoid magnets were created and cold tested, enabling valuable knowledge and experience and equipment for fabrication and test of superconducting magnets to be obtained. These were utilized for the development of many kinds of superconducting magnets for practical use.

Force-balanced coil （FBC） can balance electromagnetic forces by the effect of a helically wound configuration and minimize the required mass of structures for energy storage. The author developed superconducting model coils based on the FBC design using NbTi strands. This article summarizes the design conditions of the FBC, the hand-made winding techniques and training histories of the model coils. From the excitation test results of the model coils using liquid helium cooling, the model coils maintained the training phenomena even after the coils were heated to room temperature. However, the FBC requires repetitive excitation tests to improve its quenching properties. The author also discusses the development status of a small-sized prototype of a helical winding machine for REBCO-coated conductors and the test winding results.

The superconducting quantum annealing circuit is constructed with four Nb layers and Josephson junctions with a critical current density of 1 μA/μm². Three key points exist to perform the quantum annealing in experiments. First, the shape of the energy potential of the qubit should be accurately designed. Second, the current pulses for modulation of the shape of the energy potential must propagate to the qubit accurately. Third is the suppression of noise in the environment where the qubit is placed. Furthermore, calibration of the measurement data is necessary with consideration of the experimental environment. In this paper, I will present a mechanism for realizing quantum annealing from two perspectives: a theoretical model and an experiment. Experimental findings on quantum annealing will contribute to study of the surrounding circuit of a gate-type quantum computer.

Conditions for obtaining high-quality joined bulk superconductors using the local melt-growth method with sintered Er-Ba-Cu-O as the joining material for Gd-Ba-Cu-O melt-grown bulk superconductors and the effect of applying pulse field magnetization （PFM） to the joined superconductor were investigated. The effects of important melting conditions of the local melt-growth method on microstructure and superconducting properties were clarified, and it was confirmed that a lower maximum temperature, shorter holding time at maximum temperature, and faster cooling rate can obtain a good quality microstructure and improve superconducting properties. Next, measurements of magnetic field behaviors when PFM was applied revealed that the joined part of the joined bulk superconductor is the preferential field penetration path. A single peak with a maximum trapped field of 0.52 T was obtained at above 4 mm from the sample magnetized at an applied field of 5.0 T at 50 K by PFM. Finally, the PFM was simulated by FEM analysis to confirm the details of the thermal and magnetic field behavior, which confirmed that the joined part is the preferential field penetration path as in the actual experiment. This could contribute to the development of superconducting joining technology, which is an important key to the technology in the larger bulk superconductors, and could make pulse magnetization an efficient method of magnetization.

A prototype high-temperature superconducting （HTS） sextupole magnet for the SuperKEKB interaction region has been designed, fabricated and tested. The magnet consists of two types of sextupole coils, normal and skew coil, wound with a REBCO coated conductor 4 mm in width. The design field gradients of the normal and skew coil are 200 T/m² and 20 T/m² at 30 K, respectively. The assembled magnet has an aperture of 128 mm, and a length of 332 mm. After fabrication, the test of the magnet was performed in LN₂ and in LHe, and the properties of the magnet including field quality were studied. The design, fabrication and performance of the magnet are presented.

We developed a simple level meter for a small cryostat, using the thermocouples （TCs）. It comprises one bamboo bar axis （BBA）, four K-type TCs, and one T-type TC connected to the BBA at the same distance. It is an inexpensive calibration-free level sensor. We estimated and measured the heat leaks of a small stainless-steel cryostat and the TC level meter during the design phase and the experiments. We used the integration of thermal conductivity from the Handbook and estimated the heat leaks for Alumel, Chromel, Constantan, and bamboo from similar materials in the Handbook. The heat leak of the small S/S cryostat is 1 ~ 2 W, but the level meterʼs heat leak was ~ 1/1000 that of the other cryostats. Therefore, we can disregard its heat leak. Since the new level meter is simple and cheap, it is easy to use.