Investigation of intrinsic error fields in MAST-U device
In magnetic fusion devices, undesired non-axisymmetric magnetic field perturbations, typically called error fields (EF), have been observed to have a detrimental effect on plasma stability and confinement and can lead to a brute plasma termination, i.e. plasma disruption events (Buttery et al 1999, Nuc. Fus. 39, 1827). The main strategies that can be adopted to minimize the effect of EFs on the plasma dynamics consist in a careful alignment of the coils, when assembling the fusion device, and, most commonly, in the use of EF correction coils which counteract the non-axisymmetric fields by prescribing properly designed correction currents. In this work, an assessment of the n=1 EF source in MAST-U is presented. When mounting MAST-U device, an optimization of poloidal and divertor coil positions has been adopted to reduce the n=1 EF source which consisted in the application of coil shifts and tilts, of the order of mms and mrads, respectively (Piron et al 2020, Fus. Eng. and Des. 161, 111932). To investigate the presence of a residual n=1 EF dedicated studies have been performed during the first MAST-U campaign. The compass scan method (Scoville et al 2003, Nuc. Fus. 43, 25) was employed to identify the n=1 EF, relying on the detection of the locked mode (LM) onset, which proved to be a challenging task. Therefore, a methodology based on transfer functions among each coil and the n=1 radial magnetic field has been developed which allows the detection of LM formation. Such method is described here, complimented with the experimental results achieved, which suggest that the intrinsic n=1 EF source on MAST-U is relatively small with respect to MAST. Indeed, the empirical correction currents for n=1 EF minimization are smaller, about 0.2 kA, than the ones used in MAST, 1 kA range (Kirk et al 2014, Plasma Phys. Control. Fusion 56 104003). This proves that the optimal coil alignment for n=1 EF minimization has been a successful strategy in MAST-U.