Overcurrent protection for the TFTR neutral beam sources during spark down

The accelerating grid of a neutral beam source (NBS) of the Tokamak Fusion Test Reactor (TFTR) operates at 120 kV and 65 A. The capacitance to ground between the switch tube (ST) and the NBS is C1 approx. 5 nF (approx. 36 J). The arc and filament power supplies for the NBS float at 120 kV and have a capacitance to ground of C2 approx. 2 nF (approx. 14 J). When the NBS sparks to ground, C2 begins to discharge immediately. The ST impedance limits the fault current from the high voltage (HV) power supply to approx. 100 A until it disconnects the power source <2 μs after spark down. In case the ST fails, a crowbar removes the power source approx. 4 μs after spark down and C1 begins to discharge. During spark down, fault currents are limited with a saturated time-delay transformer (STDT) connected between the ST and the NBS and with a snubber, which is in the arc and filament power leads, in connection with a spark gap. Alternatively, STDT's can be used for the HV and for the arc and filament power leads. This paper presents design details and experimental results of the overcurrent protection circuits