Error field correction strategies in preparation to MAST-U operation

In magnetic fusion devices, unwanted non-axisymmetric magnetic field perturbations, known as error fields (EF), can have detrimental effects on plasma stability and confinement. Such EFs may originate from several sources, such as axi-symmetric coil misalignments, coil feeds, 3D structures in the wall surrounding the plasma, presence of ferromagnetic materials near the plasma surface, blanket materials in future devices. To minimize their impact on plasma performance and on the available operational space, it is important to identify the EF sources and develop EF control strategies. MAST Upgrade (MAST-U) is a spherical tokamak which will operate in the near future after a series of enhancements from the previous MAST experiment (Morris A.W. et al., MAST accomplishments and upgrade for fusion next-steps. IEEE Transactions on Plasma Science, April 2014). To deliver a machine with EF amplitude low enough to allow a high quality experimental programme, systematic analysis of the intrinsic EF sources has been carried out for poloidal field (P) and divertor (D) coils. To deliver a machine with EF amplitude low enough to allow a high quality experimental programme, systematic analysis of the intrinsic EF sources has been carried out for poloidal field (P) and divertor (D) coils, whose magnetic field measurements were available when writing this paper. Such measurements reveal that P and D coils are 3D deformed and thus are responsible for intrinsic EFs, with mainly n = 1 and n = 2 toroidal mode numbers. In preparation to MAST-U operation, both passive and active EF control strategies have been adopted for n = 1 and n = 2 EFs compensation. Passive EF control consisted of installing finely each D and P coil within MAST-U device so as to minimize the intrinsic n = 1 EF amplitude. The optimal coil alignment has been determined based on magnetic field measurements and the corresponding 3D electro-magnetic modelling, and envisaged coil shift and tilt of 3.2 mm and 0.7 mrad, respectively, in the case of P coil named P4. Conversely, active control will be use during MAST-U operation to reduce the n = 2 EF which is associated mainly with P4 and P5, as well (Kirk A. et al. 2014 Plasma Phys. Control. Fusion 56, 104003). Since these coils have been re-used from the MAST device, studies attempting n = 2 EF control, based on MAST plasmas, have been modelled utilizing the MARS-F code (Liu Y.Q. et al. 2000 Phys. Plasmas 7, 3681) to interpret experimental results in MAST and to give hints for future n = 2 EF control studies in MAST-U. A model-based control set for n = 2 EF control has been identified which would allow for the minimization of rotation braking, of the resonant magnetic field at th=agand of the plasma displacement in MAST-U, simultaneously.