Magnetic analysis of ohmic discharges in the superconducting tokamak TRIAM-1M

A magnetic analysis to determine the plasma cross-sectional shape and equilibrium parameters in the superconducting tokamak TRIAM-1M is discussed using magnetic data obtained in ohmic discharges. First, it is shown that the eddy currents in the tokamak structures should be taken into account. Two magnetic analysis codes are applied. One is the SHP code in which the plasma current is replaced by several filamental loop currents and the other is the FITEQ code in which the plasma current density is consistent with the MHD equilibrium. The magnetic fields due to eddy currents are expanded into multipole fields. The central safety factor q(0) is fixed at 1.0. The eddy currents reduce the vertical field by 10%. Taking account of them, the accuracy of the magnetic analysis is improved and estimation of the stored thermal energy in a plasma becomes possible. The SHP code has the difficulty that the local plasma surface position can be distorted by the eddy current magnetic field. In addition, it is also noted that the pressure profile analysed by the FITEQ code is consistent with the Thomson scattering measurement and that the energy confinement characteristics are consistent with the neo-Alcator scaling.