Magnetic field error characterization and mitigation in tokamaks

Two questions will be addressed: (1) How should control coils be chosen to optimally mitigate the effects of the magnetic field errors? (2) How should the expected properties of the magnetic field errors be characterized while deciding on the importance of various control coils? Although magnetic field errors are caused by misplacements in coil locations and winding patterns, the actual source of the error in an as-built tokamak is not relevant to the mitigation of the error field effects unless coils are to be repositioned or replaced. Nevertheless, certain properties of the errors are important for the choice of control coils. These properties can be estimated using a Monte Carlo model of likely errors, but it is not clear that existing Monte Carlo calculations, which take coil displacements as independent random variables, predict these properties correctly or in accordance with the actual field errors in existing tokamaks. (paper)