Three-Dimensional Analysis of Eddy Current in TOKASTAR-2

The accuracy of magnetic field analysis including eddy current is important in MHD equilibrium reconstruction of tokamak plasmas. In a small toroidal plasma experimental device TOKASTAR-2, the eddy current calculations were done with an axisymmetric model of the vacuum vessel though its vacuum vessel has periodic three-dimensionality every 90 degrees of toroidal angle due to large horizontal ports. The three-dimensional (3D) eddy current magnetic field is evaluated by 3D magnetic field calculations using ANSYS for the first time in TOKASTAR-2. The results are compared with the conventional axisymmetric magnetic field calculation and measurements using magnetic probes located inside and outside of the vacuum vessel. The resistivity of the vacuum vessel model in ANSYS is modified to reduce the error from the experimental values. Using the developed model, the effect of the presence of the port on the eddy current magnetic field is evaluated. The results show that the toroidal-average of the eddy current magnetic field becomes smaller by presence of the ports but the non-uniformity in the toroidal direction is relatively small. This implies that the effects of the port would be introduced in an axisymmetric model by using poloidally nonuniform resistivity to suppress the eddy current on the midplane.