Footprint studies of toroidal field misalignments in DIII-D

Reference measurements suggest that the axis of the toroidal field coil set in DIII-D is slightly shifted with respect to the vacuum vessel center by a few millimeters, and slightly tilted with respect to the vertical axis by a few hundreds of a degree. The effects of such misalignment are investigated for various configurations of DIII-D plasmas, including the new Small Angle Slot (SAS) upper divertor, which requires an accurate placement of the strike point in the slot for easier detachment. Detailed three-dimensional models of the B-field shift and tilt, together with efficient footprinting techniques, yield “vacuum” predictions of the footprint size and position on the lower divertor shelves for Lower Single Null plasmas, as well as on the SAS for Upper Single Null plasmas. The resulting footprints are found to be larger than those produced by typical DIII-D error fields and, in the SAS configuration, have a relatively larger effect, comparable to typical Resonant Magnetic Perturbation (RMP) fields. The consequences of the simultaneous presence of both misalignment errors and RMP fields are then studied and discussed.