Monitoring, Modeling, and Protecting Against Insulation Failures in the NSTX-U TF Outer Legs

The purpose of the effort described here is to model and monitor the insulation shear bonds between the three conductors of the toroidal field (TF) outer legs. Mechanical failure of the insulation could be a precursor to an electrical failure that could damage the TF inner leg central column, which is more difficult to repair. The shear stress in these bonded layers is proportional to the TF outer-leg out-of-plane (OOP) bending. This is a part of a larger program at National Spherical Tokamak Upgrade to instrument and monitor the structural performance of the machine. Components of the instrumentation plan are shown in this study. Bending of the outer leg due to out-of-plane loads is supported partly by shear in the bond between the three conductors that are bonded together to form the outer leg. Bending stress in the outer conductors provides an indication of the integrity of the shear bond. As a part of the orderly planned increase in operational parameters to 0.8 T (as of Feb 2016 it was at 0.61 T). Ten new FISO gauges were purchased and installed in March 2016. They yielded useful data prior to the forced shut-down due to the failure of PF1aUpper coil. The analytic process used to split out the thermal, in-plane and out-of-plane bending strains is described and compared with the measured results. The measured results provide reasonable benchmarks for the analysis. Ultimately, the main purpose of the instrumentation is to compare coil-to-coil behavior, to watch for consistency. If a coil starts to deviate from the others, the coil can be inspected for possible debonding between the three conductors that make up the outer leg. To properly monitor the full TF system, more channels are needed than Fabry-Pérot systems can provide. The planned fiber-Bragg grating (FBG) system is introduced in this paper. Monitoring and evaluation of the TF outer leg strains are also related to computed quantities that are monitored and protected by the digital coil protection system. Currently, the bending strain is expected to be adequately represented by upper-outer leg global moment sums. This relationship is discussed in this paper.