Design study of power supplies for LHD superconducting magnets

The Large Helical Device (LHD) is designed to have six pairs of superconducting coils. In the first phase of the project, these will be energized by six steady-power supplies. These power supplies are thyristor types, which generate positive and negative voltages and large currents (up to 30 kA). Each of them has a smoothing filter which reduces the voltage ripple to less than 0.2 V, and a protection circuit to transfer rectifier current to a dump resistor and drop it rapidly during coil quenches. These six steady power supplies are powered from the commercial line. In the second phase of the project, three more power supplies powered by a motor generator (120 MVA) or superconducting energy storage will be added to the poloidal coil circuits. These will be used in experiments which require rapid changes in the poloidal coil currents during a plasma discharge of 5 s. A protection circuit is now under development, composed of disconnectors, a vacuum circuit breaker, a power fuse and diodes. It has worked well up to 21 kA, and will be tested up to 30 kA.