Failure analysis and design improvement of the Alcator C-Mod toroidal field coil sliding joint

In March of 1998 a short occurred in the TF system. Small amounts of copper and carbon tracking were found in the A-right TF. A crack in a finger in the central column was also found. Subsequent disassembly revealed arcing damage to the outer-upper horizontal leg finger joint region, at the lead connection leg where the voltage difference is large. Material was also removed from the neighboring torque shell. Significant degradation of the felt metal in the inner fingers at the central column was also found. This initiated re-analysis of the finger design, testing of the felt metal, and a decision to replace all the felt metal in the machine. Design improvements have been made but some uncertainty as to the cause of the failure remains.