Measurement, correction and implications of the intrinsic error fields on MAST

The misalignment of field coils in tokamaks can lead to toroidal asymmetries in the magnetic field, which are known as intrinsic error fields. These error fields often lead to the formation of locked modes in the plasma, which limit the lowest density that is achievable. Measurements on MAST suggest that the dominant source of the intrinsic error field is due to the P4 and P5 poloidal field coils. A direct measurement of the toroidal asymmetry of the fields from these coils has been made, which has then been parametrized in terms of distortions to the coils. Empirically the error fields are corrected using error field correction coils, where the optimum correction is found by determining the current required to ensure that the discharge is furthest from the onset of a locked mode. Assuming that the dominant n = 1 error field is produced by the P4 and P5 coils, the empirically derived corrections have been compared with the known distortion of these coils. In the vacuum approximation there is a factor of similar to 3 difference between the predicted and empirically determined correction. When the plasma response is included better agreement is obtained, but there are still some cases where the agreement is not good. The results suggest that other effects may be important. These include on the experimental side additional unmeasured sources of the error field or on the theory side the non-linear coupling of the error field to the plasma.