Fusion Applications of Large DC-Current Interruption Units

In fusion experiments, interruption units for high DC currents are widely used for generating the loop voltage required at plasma breakdown and current start-up. Likewise, similar systems are employed for the protection of superconductive coils in case of quench (i.e. a loss of superconductivity). In such event, large resistor banks are inserted in the circuits by means of circuit breakers, so as to dissipate the energy stored in the coils.

The ITER experiment, the largest fusion facility ever conceived, is currently under construction in the south of France at Cadarache site and, as in the already existing fusion experiments, it will be provided with DC interruption units for plasma initiation (the Switching Network Units - SNUs) and coil quench protection (the Fast Discharge Units - FDUs).

The paper, after a survey on the interruption units installed in large fusion facilities worldwide, describes the systems designed for the ITER experiment, pointing out their peculiarities and challenging issues. Then, a comparison among different solutions implemented is given, pointing out critical performances required, issues in the design of key components and possible future developments.