Investigation of the Magnetic Field Configuration for Magnetic Surface Measurements in the CFQS Quasi-Axisymmetric Stellarator

It has been found that magnetic surfaces in the CFQS quasi-axisymmetric stellarator in a standard magnetic configuration are robust against error fields caused by misalignments of the modular coils. While this property is advantageous for preserving the magnetic field line structures for the plasma confinement as designed, it prevents the detection of the misalignments of the coils by magnetic surface measurements. Thus, a magnetic configuration specialized in measuring magnetic surfaces to detect the error fields is proposed. Calculations by tracing magnetic field lines reveal that the magnetic surfaces in the specialized magnetic configuration are sensitive to the error fields compared to those in the standard magnetic configuration. Additionally, the calculations demonstrate that a weighted electric current on some modular coils can make the magnetic surfaces more sensitive to the error fields, which enables the magnetic surface measurements to detect smaller misalignments of the modular coils in the CFQS.