Design and implementation of four 20 kA, 5 kV hybrid switching networks for plasma ignition in the international tokamak JT-60SA

This paper describes the design and implementation of the Switching Network Unit (SNU) for the superconducting Central Solenoid coils of the international nuclear fusion experiment JT-60SA to be built in Naka, Japan. Fusion experiments require an overvoltage in the poloidal coils inducing an overvoltage in the plasma chamber in order to produce the plasma breakdown. In the modern fusion devices, as JT-60SA, this is achieved by a SNU. The designed SNU can interrupt a DC current up to 20 kA in less than 1 ms to create a voltage up to 5 kV. It is realized with a hybrid switch integrating an electro-mechanical device and a solid state static circuit breaker, parallel connected. The SNU resistance can be prearranged and dynamically reduced by a solid state making switch to comply with the experimental requests. Preliminary test results confirmed the current balance of the multiple parallel branches constituting the solid state switch and proper behavior of the devices, confirming simulations results. The developed technical solutions may be employed in many other fields where is required a DC high current interruption, as medium and high voltage DC networks (HVDC systems).