Effect of Magnetic Materials on the In-Vessel Magnetic Configuration in KSTAR

KSTAR has a nonlinear magnetic material, INCOLOY® alloy 908 (Incoloy), in toroidal field and poloidal field (PF) coil systems. The effect of Incoloy on the magnetic configuration for the plasma initiation was investigated with systematic magnetic field measurements, finite element model (FEM) calculations, and in situ measurements of the magnetic properties. The profile of the vertical field near the field-null center was measured with a vertically movable electron beam (e-beam) probe and Hall sensor arrays in addition to pickup coils in the vacuum vessel. The measured profiles of the additional fields from Incoloy in the PF coils are in good agreement with the FEM calculations. In a typical KSTAR startup configuration, the effect of Incoloy is significant. First, it degrades the connection length significantly due to an additional vertical field in the field-null region, and second, it changes the radial and vertical stabilities by modifying the radial gradient of the vertical field. Initial up-down asymmetry measurements of the vertical fields showed very small static error fields from the PF coils. Calculations suggest that the main sources of the measured downshift of the plasma column are asymmetric eddy currents in the cryostat.