Development of equilibrium fitting code using finite element method in versatile experiment spherical torus

An equilibrium fitting code using finite element method is developed for various plasma studies, especially on non-inductive start-up and ECH heating & current drive experiments in Versatile Experiment Spherical Torus. The magnetic diagnostics are applied to provide the basic equilibrium fitting constraints. As severe eddy currents are induced by not only the PF coils but also the plasma current itself, the eddy current component from the magnetic signals is dealt with the vessel wall model in the eddy current solver and carefully compensated, which is essential for accurate equilibrium fitting. The fast camera image analysis method is adopted and the limiter current monitoring system is developed for validating the results of the plasma position and shape from the code. The equilibrium fitting code is applied to the limited and diverted plasma configuration in VEST and the time evolution of the plasma position and shape and even averaged magnetohydrodynamic parameters such as the edge safety factor and the internal inductance are extracted. Additionally, the equilibrium fitting code is used to analyze the non-inductive plasma start-up experiment using the direct current helicity injection in VEST. Especially, it is the first time to apply the finite element method to direct current helicity injection experiment and ECH heated plasmas which have significant amount of plasma current flowing along the open field line.