Effect of SHe Temperature on Cool-Down Speed in JT-60SA CS Module

The JT-60SA central solenoid (CS), which consists of four stacked modules, is cooled to an operating temperature of 4.5 K by supercritical helium (SHe). During cool-down from room temperature to 4.5 K, in order to avoid a damage on the coil, it is required to reduce thermal stress due to temperature difference in the coil. In the cool-down operation, the CS needs to control the maximum temperature difference in the longitudinal direction of the cable-in-conduit (CIC) conductor below 40 K. However, since that could damage the coil, correctly estimating the temperature distribution in the conductor is important for ensuring the cool-down operation is performed safely. In the present study, the JT-60SA CS module was prepared and the inlet and outlet temperatures, as well as the SHe mass flow rate, were measured until the CS module was cooled to operating temperature, after which a CS module simulation was performed in order to determine the effect of the inlet temperature on the cool-down speed. From the simulation results, it was concluded that the average cool-down speed was 0.73 K/h and the CS module reached 80 K in approximately 8.3 days while maintaining the maximum temperature difference within 40 K. Furthermore, the temperature difference between the inlet and outlet was kept within 25 K in order to limit the thermal stress. Taken together, the results of our analyses provide fundamental data that can be used to evaluate the safe cool-down operation of the coil and thus protect the coil systems.