In a tokamak with an iron core transformer, vertical and horizontal magnetic fields required for plasma equilibrium are provided not only by poloidal field (PF) coils but also by the surface current flowing on the magnetized iron core. In order to have accurate information on the magnetic field produced by both PF coils and the iron core in the plasma region, knowledge of the magnetic property of the iron core should be preceded.
Magnetic permeability of the KAIST-TOKAMAK iron core was measured by comparing the results of vacuum magnetic field mapping and numerical simulation. The vacuum field mapping was performed by using a 50 channel magnetic pickup loop array in the presence of ohmic heating field only. Numerical simulation result was obtained by using Poisson computer code to include iron core effect. It was found out that the magnetic permeability of the iron core was significantly reduced compared with the value at the time of initial operation of the tokamak. A separate and more direct measurement of the magnetic field produced by a solenoid coil and by using a Gauss-meter showed that the value of the magnetic permeability obtained was reasonable. It was also studied that the role of the iron core on loop voltage was still significant in spite of the reduced permeability.