New measurements of coil-related magnetic field errors on DIII-D

Non-axisymmetric (error) fields in tokamaks lead to a number of instabilities including so-called locked modes [J.T. Scoville, R.J. La Haye, Nucl. Fusion 43 (4) (2003) 250-257] and resistive wall modes (RWM) [A.M. Garofab, R.J. La Haye, J.T. Scoville, Nucl. Fusion 42 (11) (2002) 1335-1339] and subsequent loss of confinement. They can also cause errors in magnetic measurements made by point probes near the plasma edge, error in measurements made by magnetic field sensitive diagnostics, and they violate the assumption of axisymmetry in the analysis of data. Most notably, the sources of these error fields include shifts and tilts in the coil positions from ideal, coil leads, and nearby ferromagnetic materials excited by the coils. New measurements have been made of the n = 1 coil-related field errors in the DIII-D plasma chamber. These measurements indicate that the errors due to the plasma shaping coil system are smaller than previously reported and no additional sources of anomalous fields were identified. Thus they fail to support the suggestion of an additional significant error field suggested by locked mode and RWM experiments.