Nakamura Kazuya

The JT-60SA Poloidal Field (PF) coils system comprises four Central Solenoid (CS) modules and six Equilibrium Field (EF) coils, and the cryostat, vacuum vessel (VV), and stabilizing plate (SP) are installed around the PF coil. Evaluating the voltage between conductors in the PF coils is one of the most critical factors in the energized coil operation and the insulation design. The power supply voltage of the PF coils has some frequency components caused by the voltage control system. The resonance phenomena caused by the voltage fluctuations of the power supply induce a non-uniform voltage distribution in the coil. The locally concentrated voltage may exceed the withstand voltage of the insulation, affecting the operation of the JT-60SA. In this study, a circuital model was assembled including four CS modules and six EF coils, to estimate the resonance frequency effect into the coil voltage distribution. In addition, the effects of the cryostat, VV, and SP on the voltage between conductors in the PF coils were evaluated.

The JT-60SA central solenoid (CS) module is cooled to the operating temperature by the gas and supercritical helium. The cool-down tests of center solenoid model coil (CSMC), manufactured to validate the CS module manufacturing process for JT-60SA, showed that the maximum temperature in the CSMC occurs at the innermost turn of the coil. In the JT-60SA cool-down operation, the temperature can only be measured at the flow pipes connected to the outlet and inlet. Therefore, the maximum temperature in the CS module, which has a flow path similar to that of the CSMC, cannot be measured directly. In this study, we developed an analytical model to estimate the temperature distribution for the helium, strands, and jacket in the conductor from the measurement of the JT-60SA CSMC cool-down test. When the inlet temperature of the CS module in the JT-60SA cool-down test in 2023 was used for the inlet temperature in the analytical model, the coil was safely cooled to the operating temperature. Furthermore, the maximum temperature difference in the CS module and the cool-down speed were evaluated when the inlet was adjusted to keep the temperature difference between the inlet and outlet constant.

We have studied a magnetic levitation system including an HTS bulk as magnetic shield. This system consists of a ferromagnetic rail, HTS bulks, and HTS coil. In this paper, the vertical attractive force was discussed in two types of the analytical models. First, the model with the racetrack coil was proposed. We analyzed the vertical attractive force depending on the length of the coil's straight section. As a results, the maximum vertical attractive force became large with increasing a straight section, and the force reached approximately 20 kg when the straight section was 1 m. Next, we proposed a scaled-up model which was a 10-fold scaled-up of the HTS racetrack coil, HTS bulk, and ferromagnetic rail assuming an actual train size. Here we considered that one vehicle body, whose weight was 25 t, was levitated by four racetrack coils. Hence the vertical attractive force of each racetrack coil needed to be about 6.25 t. According to the analysis, the maximum vertical attractive force was about 16 t. The force was generated by one racetrack coil. Therefore, the vertical attractive force was enough larger than the weight of the actual vehicle body.

The authors previously proposed a quench protection method for a high-temperature superconducting (HTS) coil that uses Cu tape co-wound with HTS tape (Cu-CW method). In this method, when a quench occurs in the HTS coil, part of the current in the HTS coil is quickly transferred to the co-wound Cu tape coil by shorting the Cu coil with a resistor due to the tight magnetic coupling of both coils, and the hot-spot temperature of the HTS coil is reduced due to the quick reduction of the HTS coil current. The previous work showed by a numerical simulation that the method is effective to improve the quench protection performance compared with a commonly used detect and dump method (D&D method). In this work, the effectiveness of the method was studied by an experiment using small-scale test coils wound with yttrium barium copper oxide (YBCO) coated tapes to experimentally simulate quench behaviours of HTS coils of significant scale.

The JT-60SA central solenoid (CS) module is cooled to 4.5 K using liquid helium. This cool-down operation of the coil is planned to take about one month. During cool-down, the maximum temperature difference in the coil (dT(C-M)) must be kept below 40 K to avoid excessive thermal stress. Thermometers are attached only at the inlet and outlet on the outer perimeter of the coil. However, because the maximum temperature inside the coil occurs near its inner edge, dT(C-M) cannot be measured directly. Hence, estimating dT(C-M) is important to allow the cool-down process to be completed within one month without excessive thermal stress on the CS. In the present study, based on the results of the JT-60SA cool-down test performed in 2020, a temperature simulation model of the CS module, including the strands, jacket, and He in the bundle and hole regions was created using MATLAB, and dT(C-M) was estimated during the cool-down test. This model was then used to evaluate the effect of the mass flow rate and inlet pressure on the cool-down speed.