Conceptual design of the quench protection circuits for the JT-60SA superconducting magnets

In JT-60SA, the new satellite tokamak to be built in Naka, Japan, the magnetic field for plasma generation and confinement is produced by a number of superconducting coils whose rated current ranges from 20 kA to 25.7 kA, with a nominal voltage up to 5 kV. This paper deals with the conceptual design of the Quench Protection Circuits (QPC) of JT-60SA, which must provide a fast removal of the energy stored in the superconducting coils in case of quench. The core of the QPC units is based on a dc current breaker, which diverts the coil current into a discharge resistor. Different concepts of dc circuit breaker for QPC were considered, including vacuum and static circuit breakers; then, a hybrid solution, composed of a mechanical bypass switch paralleled to a static switch based on Integrated Gate Commutated Thyristor (IGCT) technology, has been worked out. A pyrobreaker is added in series to the hybrid switch as a backup protection. The resulting design allows benefiting from the fast breaking and very low maintenance requirement of the static switches, besides maintaining the advantage of the much lower power losses of the mechanical bypass in normal operation. After a brief presentation of the preliminary studies, the selected design solution for JT-60SA is presented in detail.