The effects of the misalignment of Mirnov coils in mode analysis based on the phase detection method

Abstract

Mirnov coils (MCs) are used to identify the toroidal and poloidal mode number of tearing mode instabilities in the KSTAR tokamak. The mode number is analyzed by accumulating the phase of MC signals from a Morlet wavelet transform. There is ambiguity in identification of the poloidal mode number due to the phase distortion in the MC measurements. The extreme case is so-called phase folding. Although the phase distortion caused by the eddy currents in the in-vessel components and by the coupling of multimodes has been studied intensively, little attention is given to the effect of the orientation of MCs. The poloidal MCs are not aligned to the plasma shape but to the vacuum vessel. The effect of this misalignment is quantitatively analyzed by simulating the MC measurements of magnetic field fluctuations in a tokamak with mathematical modeling of perturbed current filaments on rational surfaces. The maximum MC response to a perturbed current filament occurs when the current filament is located at an angle different from the geometric installation angle of the MC. This angle is defined as the effective angle. The characteristics of the effective angle are investigated depending on the plasma shape, mode number, and distribution of the perturbed currents. The ambiguity in the identification of the poloidal mode number is reduced when the MC measurements are analyzed in the effective angular coordinate instead of the geometric coordinate. The only requirement for the calculation of the effective angles is an equilibrium reconstruction to identify the rational q surfaces.