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Tensile strain mitigation during the NSTX-U Ohmic Heating (OH) coil cooldown

NSTX-U uses an inertially cooled OH coil that is cooled with water between shots. Cooling is fed from the bottom of the coil and a cooling wave propagates up the height of the coil. The finite height of the cooling “wave” causes a thermal gradient in the coil that causes a bending stress in the coil build. The larger radial build of the new NSTX Upgrade OH coil produces a shorter “wave” than the previous coil, and larger bending stress. The OH insulation system uses CTD 425 epoxy with interleaved glass and Kapton. This insulation is intended to provide some accommodation of tensile strains and delamination. Localized tensile strains, and shear stresses beyond recommended allowables have been a characteristic of many coil winding packs. Mechanical and electrical array testing is often used to qualify these winding packs. Mitigation of tensile strains via preloads is also often employed. For NSTX-U, only a small preload is practical. Strain controlled array testing has been performed. This has demonstrated a robustly acceptable electrical behavior after cyclic loading. This allows some relief in the requirement to control the cooling water “thermal shock”. To provide additional protection of the insulation, an active system that introduces cooling water with a more gradual thermal gradient and longer cooling wave height is being implemented to mitigate the tensile strains. This system employs an inline heater that supplies water at the post shot OH temperature that linearly decreases to the 12C supply chilled water temperature. Results of array testing, and design of the active system are presented. The final decisions regarding acceptance of the testing and implementation of the preheater are presented.