Reconstruction of the eddy current profile on the vacuum vessel in a nuclear fusion device using only external magnetic sensor signals

Abstract

In a reversed field pinch device, the ordinary Cauchy-condition surface (CCS) method cannot reconstruct the magnetic flux profile accurately due to the strong eddy current flow on the shell (vacuum vessel). Boundary integrals of the eddy current density along the shell are added to the original CCS method formulation. The eddy current profile is unknown in advance but identified using only the signals of magnetic sensors located outside the plasma. As the sensors are closely adjacent to the shell, the near singular boundary integrals along the shell should be accurately evaluated. This singularity can be damped out with the algorithm based on the distance function proposed by Ma and Kamiya. The modified truncated singular value decomposition technique of Hansen et al. is an effective way to solve an ill-conditioned matrix equation when a large number of nodal points exist on the shell. The capability of the new method is demonstrated for a test problem modelling the RELAX device.