Monitoring of superconducting bus-line of LHD using fuzzy theorem

In the LHD (Large-scale Helical Device) system of National Institute far Fusion Science (NIFS), Toki, Gifu, Japan, the experiments using superconducting magnets with total stored energy of roughly 1.6 GJ are carried out. The LHD consists of nine coils those are fed by large-scale DC power supply. The superconducting bus-lines are connecting the power supply and coils with a length of about 50 m and a current capacity of 32 kA (rated current). As this bus-line has thick stabilizing barking metal (high purity Al), a resistive voltage generated at small normal-transitioned region is very small. Meanwhile, the inductive voltage noise induced by the external magnetic field or its own magnetic field is large because the length of the bus-line is large. Besides, the voltage detection leads cannot be attached directly just outside the conductors because of the thick thermal insulation and the care for electrical insulation. Therefore, it is difficult to decide the state of the superconducting bus-line only by the voltage signals, So, the authors proposed to introduce the Fuzzy theorem calculating the "dangerous rate" of the bus-line. In this system, not only the voltage signals of the bus-line but also the transport current and the liquid helium flow-rate in the bus-line are taken into consideration. The experimental result shows that the state of the bus-lines is well expressed by the "dangerous rate", and the bus-line has enough operation margin.