Thyristor DC circuit breakers for Alcator C-MOD

The Alcator C-MOD PF (poloidal field) system requires DC circuit breakers in five PF magnets for plasma initiation. A design based on standard 77-mm SCRs and diodes was chosen for several reasons. Bypass switches and other mechanical components were avoided to reduce maintenance costs and achieve high reliability on five separate systems. The circuit breakers will carry the full charging current pulse as well as the reverse swing current. Standard phase control devices offer superior current-carrying capability, low cost, and commonality with the devices, gate drives, and mounting hardware used in the magnet power supplies. Phase control SCRs have a longer turn-off time and require a large counterpulse capacitor band than inverter thyristors. The net cost of the additional capacitance is very small in a single-pulse, low duty-cycle system. Design values and simulations for three switches are presented. Switch sections are rated 2 kV maximum interrupting voltage with bipolar current ratings of 50, 25, and 15 kA. A pulse-forming network provides the required 400 μs turn-off pulse to the SCRs. The design is strongly influenced by the parasitic inductance in bus and circuit components