Measuring static and quasi-static non-axisymmetric magnetic fields in MAST-U

To detect static and quasi-static non-axisymmetric (3D) magnetic fields in MAST-U, a novel connection scheme for the toroidal saddle coil arrays has been designed, implemented, and tested. In tokamaks, non-axisymmetric magnetic fields are typically several orders of magnitude smaller than the axisymmetric (2D) fields. To overcome the dynamic range limitations of the analog-to-digital converter system, the signals from identical sensors at different toroidal positions are subtracted in the analog stage. This removes the dominant 2D component, allowing the residual 3D signal to be digitized with a 20-fold gain in amplitude relative to single saddle coil measurements. These differential signals are then used to fit the data to a sum of Fourier harmonics via singular value decomposition. The connection scheme is optimized using the condition number of the design matrix as a metric. Although fabrication and assembly fall within prescribed tolerances, they result in small asymmetries that cause residual 2D field pickup even after subtraction. Dedicated corrections are applied to reduce this undesired pickup, enabling the identification of 3D fields of only a few Gauss, even in real time.