Experimental qualification of the Hybrid Circuit Breaker developed for JT-60SA Quench Protection Cir

This paper presents the experimental qualification of the Hybrid Circuit Breaker developed for the full scale prototypes of the Quench Protection Circuit (QPC) used for Poloidal Field Coils (PFC) and Toroidal Field Coils (TFC) of JT-60SA within a contract placed to Ansaldo Sistemi Industriali S.p.A.. JT-60SA will be built in Naka, Japan, as a satellite tokamak of ITER in the framework of the Broader Approach Agreement; it is a complementary facility which accelerates the development of magnetic confinement fusion reactors. It is designed to perform long lasting plasma pulses up to 5.5 MA, with a flat-top duration of 100 seconds thanks to additional heating systems. In order to limit the power dissipation it will be equipped with both toroidal and poloidal magnets manufactured with low temperature superconductors and protected against quench by means of QPCs, which are capable of interrupting and diverting the current into a dump resistor avoiding major damages to the coils. The heart of the QPC is the Hybrid mechanical-static Circuit Breaker (HCB) an innovative concept which has no similar applications either industrialor laboratory at such power values. HCB is a device which combines a mechanical bypass switch (BPS) and a static circuit breaker (SCB): It takes advantage of the low power losses during normal operation while the BPS is closed, and the fast, reliable, arch-less current interruption of the SCB when it is operated. The SCB does not need a counterpulse network to turn-OFF the SCBthanks to the selection of static devices (IGCTs) which can be controlled both at turn-ON andturn-OFF. The HCB has been developed by Consorzio RFX team since 2007, starting from the conceptual design and the feasibility studies [1] which allowed to explore and solve open issues such as the reliable turn-ON of single and paralleled static switches with a very low voltage. The 10 kA unidirectional HCB prototype [2] has been a major step through the adoption of this solution for QPC, because it highlighted the suitability and reliability of the arc voltage to divert the current into the static devices in few ms. The final step has been the development and construction of the full scale prototypes for toroidal and poloidal HCBs which match JT-60SA requirements in terms of current polarity, current value, reapplied voltage and intervention time. In particular HCB for PFC is rated for a bidirectional current up to 21 kA and a maximum voltage of 5 kV, while the HCB for TFC is rated for a unidirectional current up to 25.7 kA and a maximum voltage of 2.8 kV; the operational time for both HCBs is 350 ms. Since the QPC is a safety component a full qualification process, based on a complete set of tests, has been designed and performed during 2012. It allowed to highlight the commutation between BPS and SCB which is one key aspect of the HCB and also the robustness of solutions and technologies employed for the HCB prototypes. The performances of the prototypes during the tests proved the compliance with the requirements and verified also the goodness of the electrical and thermal models developed during the design phase. [1] Novello et al. "Feasibility Study of a Hybrid Mechanical-Static DC Circuit Breaker for Superconducting Magnet Protection" IEEE Transactions on applied superconductivity, vol. 19, no. 2, April 2009, pages 76-83 [2] Novello et al. "Development and Testing of a 10-kA Hybrid Mechanical-Static DC Circuit Breaker" IEEE Transactions on applied superconductivity, vol. 21, no. 6, December 2011, pages 3621-3627