Vulnerability of Feedthroughs Operated at a Low-Impedance Point on DIII-D

Two vacuum feedthrough ceramics failed in separate incidents on DIII-D when operating at high power with the feedthrough near a low-voltage point in the transmission line. In each case a sustained arc in the feedthrough was not detected by the arc protection circuit because the impedance seen by directional couplers upstream was nearly the same as that observed with no arc. A different arc detection methodology is needed when operating with the vacuum feedthrough at a low-voltage point than the more usual situation with the vacuum feedthrough at high-voltage