A nominally axisymmetric plasma configuration, such as a tokamak or a spherical torus, is highly sensitive to nonaxisymmetric magnetic perturbations due to currents outside of the plasma. The high sensitivity means that the primary interest is in the response of the plasma to very small perturbations, i.e., ∣b⃗∕B⃗∣≈10−2 to 10−4, which can be calculated using the theory of perturbed equilibria. The ideal perturbed equilibrium code (IPEC) is described and applied to the study of the plasma response in a spherical torus to such external perturbations.

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A new method for correcting magnetic field errors in the ITER tokamak is developed using the Ideal Perturbed Equilibrium Code (IPEC). The dominant external magnetic field for driving islands is shown to be localized to the outboard midplane for three ITER equilibria that represent the projected range of operational scenarios. The coupling matrices between the poloidal harmonics of the external magnetic perturbations and the resonant fields on the rational surfaces that drive islands are combined for different equilibria and used to determine an ordered list of the dominant errors in the external magnetic field. It is found that efficient and robust error field correction is possible with a fixed setting of the correction currents relative to the currents in the main coils across the range of ITER operating scenarios that was considered.