Surface current approximation of magnetization currents of magnetic materials in axisymmetric tokamak devices for the correction of their effects on magnetic diagnostics

Magnetic materials such as ferritic steel are planned to be installed in fusion DEMO reactors to support blankets and in-vessel components with low activation. Since a magnetic material disturbs magnetic measurement, it is difficult to perform equilibrium control when plasma and sensors are placed between magnetic materials. To calculate the position of plasma current centroid with the disturbed magnetic measurement, we devised a simple scheme to correct their effects on magnetic diagnostics by approximating the magnetization currents as surface currents. Estimated errors of the position by this scheme, especially in the R position, are about 1%. Those with positionally dependent surface currents become smaller by relocating the filament positions for plasma current. Additionally, flux surfaces reconstructed by this correction show agreement with the true flux surfaces calculated by finite element method. (author)