Identifying the cause of the NSTX TF coil bundle failure

The NSTX TF coil assembly experienced a failure in the TF center bundle during pulse number 143912 on July 20, 2011. A short circuit between turns 14 and 30 spread to turn 13 and resulted in melting of approximately 2 kg of copper and ejection of copper and solder from the cooling channels at the lower end of the TF bundle. An autopsy of the coil was immediately begun by PPPL personnel that included cutting numerous sections from the affected region of the coil and examining the chemical makeup of the insulation. In addition, careful machining to isolate various areas of turn to turn insulation uncovered suspicious spots that exhibited low resistance. This paper will present the methods used to isolate the location and identify the causes of the failure in the coil bundle and the design changes made to the NSTX Upgrade TF bundle and fabrication processes to prevent a recurrence of the fault.