Current control of magneticlly coupled superconducting coils for large helical device using H-infinity control scheme

Precise control is required for the current control of the superconducting coils in the Large Helical Device (LHD) in NIFS, Japan. There are strong mutual couplings between coils, and the effect of structure materials surrounding materials of the coils and plasma, which act like shorted coils seen from the power source, must be considered for the design of the current controller. The authors designed the current controller for the LHD coil, which is composed of an H-infinity controller and a feedforward controller. Features of the controller are as follows. At first, it is possible to design the controller by considering frequency domain characteristics of the closed loop system and stability of the designed controller is guaranteed according to the theory of the H-infinity control. Secondly, the response characteristics to current reference values and DC-coupled characteristics of current control can be determined by the design of the feedforward controller. The designed controller showed excellent characteristics in both simulation and experimental results