Electrical Testing of NSTX-U Inner Poloidal Field Coils

The National Spherical Torus eXperiment—Upgrade (NSTX-U) experienced a series of technical problems during its initial operating campaign leading to an extended outage and recovery period. The final event that necessitated shutdown was a turn-to-turn fault in one of the inner PF coils. Investigation of the coil failure and development of remedial measures has become a major focus of the recovery. It was decided that all three inner PF coil pairs that employed similar design features and were fabricated by the same manufacturer needed to be replaced. To further reduce risk, a prototyping program was initiated to qualify suppliers. Four prospective suppliers of replacement coils were mobilized to produce an identical prototype coil that was similar in characteristics to the PF1A coil. The prototype technical evaluation included a comprehensive set of electrical tests on each of the four prototype coils and before and after destructive sectioning into two halves. This article reports on the electrical test methods and results from prototype testing, along with plans for application to the production coils.