Study Of Eddy Current Effects On Magnetic Diagnoses And Equilibrium Reconstructions

Temporal-varying magnetic field induces eddy currents in conductive structures.When a tokamak device is in discharging,the varying external field and plasma results in kiloampere eddy currents surrounding in the vacuum vessel and other conductive structures.These eddy currents influence the stress distribution of components,increase the difficulties of breakdown and start-up,reduce the accuracy of equilibrium reconstruction and real-time control,interact with instabilities and result in disruptions then make a serious threat to the safety of tokamak machines.Especially in small / modest size tokamak with short-pulse discharges and complex vacuum vessel design,such as in the Sino-UNIted Spherical Tokamak(SUNIST),the eddy current effects are very important in machine operation,control,heating,equilibrium reconstruction,density measurements,MHD analysis and so on.This dissertation is devoted to study the effect of eddy currents on magnetic signals and equilibrium reconstruction.A new method,named as the magnetic diagnostic transfer function,has been developed to calculate the response to eddy currents in magnetic signals and to integrate eddy current effects into equilibrium reconstruction.This method is applied in analyzing the magnetic sensor signals and improving the reconstruction results.For comparison,a 3D FEM simulation has been applied to investigate the asymmetrical eddy currents in the vacuum vessel of SUNIST in this work.The eddy currents due to sine oscillating poloidal field currents have been simulated,and the effect of a vertical slit in a conductive torus to the eddy current toroidal circuit has been analyzed.It is found that some magnetic signals at the top and bottom of the vacuum vessel have a leading phase to the excitation source,which we called it “abnormal” eddy current effect.This abnormal effect has been interpreted though the simulation results.Then,a new approach to obtain the transfer function of magnetic diagnostics is introduced in this dissertation.Using this method,the absolute response to active currents and that to induced eddy currents can be extracted from the magnetic signals without detailed simulation of the conductive structures.The physical meanings of parameters in the transfer function are well analyzed,as well as verified by some specific experiments.Appling the transfer function method in analyzing SUNIST discharges,the contribution of vertical field,ohmic field and their separated eddy currents in magnetic signals can be decomposed.Then,the toroidal distribution of their eddy currents can be fitted by their separate contributions in flux loop signals with least squares.The fitted distribution of eddy current agrees with that from the FEM simulation,naturally,the “abnormal” eddy current effect can be clearly explained here.Finally,the eddy current effects are integrated in the equilibrium reconstruction code(SU-EFIT code).Firstly,the PF coil currents and their eddy currents calculated by the transfer function method are considered as background field.Then,plasma current density and plasma-induced eddy current profile are iterated separately for reducing the limitation of available magnetic diagnostic number.As a result,the profiles of plasma current density,pressure and self-consistent its eddy currents can be obtained together.Comparing with the picture from a fast camera,the reconstruction results have been improved significantly when the toroidal eddy currents in vacuum vessel are considered.