The cooling system for the helical coils of the Large Helical Device (LHD) was upgraded in 2006 in order to improve the stability of the LHD helical coils. The cooling system that has 20 parallel paths to provide subcooled He to the helical coils fulfills the following design requirements: inlet temperature of 3.2 K and mass flow rate for the coils of 50 g/s. Although the system meets these requirements, the characteristics of the system are not understood in practice. In this study, the influences of the mass flow rate and cooling temperature on the cooling system were investigated to determine what is required for stable system operation. The results show that the temperatures at each position changed linearly with the temperature of the supplied subcooled He; and that an unstable state in the parallel paths occurred when the mass flow rate decreased. A mass flow rate of at least 30 g/s is needed to prevent an unstable state in the cooling system when subcooled He of 3.2 K is supplied to the helical coils. In addition, the temperature of the helical coil winding was evaluated using a numerical calculation, in which the numerical model simplifying the helical coil configuration was utilized. This was done because the helical coil winding is not equipped with thermometers in terms of electrical insulation. Using the numerical model, the temperatures at the coil outlet and coil case can be predicted not only under a steady heat load, but also under coil excitation. The coil winding temperature is close to the coil outlet temperature undercoil excitation because of AC loss.
