Anatomy of the PF magnet failure in Alcator C-MOD

The Alcator C-MOD tokamak began operation in late March, 1992. The terminal of one of the poloidal field magnets, located inside the TF magnet, suffered a structural failure in routine service on April 10, 1992. The protective systems functioned as designed and there was virtually no associated damage. Post mortem analysis showed that the terminal might well have failed at or near the design current limit, but a satisfactory explanation for the failure at 17% of design stress has not been produced. The terminal details of the failed magnet were present in 9 other PF magnets. A new design was developed and applied to all affected magnets. The replacement of the magnet terminals and restart of the experimental facility took 13 months. The terminal assembly that failed was on a relatively simple magnet with only moderately high applied loads. The toroidal field magnet and ohmic heating solenoid are both much more complex and subjected to higher loads. The TF magnet, for example, utilizes an innovative sliding joint construction method to carry loads to a massive external superstructure. Yet the TF magnet continues to perform well and is now operating routinely at over 50% of design current. This presentation will examine the factors and decisions that led to the original PF terminal design and subsequent failure. The new design will be presented, followed by a discussion of the lessons learned from this experience.