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.

Suppression of toroidal Alfvén eigenmodes by the electron cyclotron current drive in KSTAR plasmas

“…The reason why the difference between TRANSP calculated and measured neutron rates increases is not clear yet, but one reason that can be inferred is the substantial fast-ion loss observed by the FILD. FILD measurement depicted in figure 4 shows the large amount of fast-ion loss Fast-ion loss measured by the scintillator-based detector [41]. Unlike the typical H-mode discharges, fast-ion loss in the high q min or high q 0 discharge is significant even in the H-mode inter-ELM period.…”

“…Nominal sampling rate of Mirnov coil probe is 2 MS s −1 (Nyquist frequency ∼ 1 MHz) and the toroidal MHD modes are analysed with maximum 15 toroidal magnetic probes (Mirnov coils). Frequency spectrogram and toroidal mode number are obtained from the phase shifts between the Fourier-decomposed toroidal magnetic probe signals [32,40], and the maximum toroidal mode number detected by the signal process is about 10 [41] with irregular spacing between the toroidal probe locations. Mode amplitude is obtained from one of toroidal Mirnov coil signal, which is band-passed in TAE frequency range and smoothed with the best-fit.…”

“…Figure 4.Fast-ion loss measured by the scintillator-based detector[41]. Unlike the typical H-mode discharges, fast-ion loss in the high q min or high q 0 discharge is significant even in the H-mode inter-ELM period.…”

Role of fast-ion transport manipulating safety factor profile in KSTAR early diverting discharges

“…Mirnov coils were used to estimate the toroidal mode number n of each MHD mode, following the procedure in the appendix. Since the allocation of the poloidal MC array has limitations for direct poloidal mode number m extraction [16], the estimation of m number was only done by analysis on the ECEI, valid only for relatively wide and low m number modes such as broad 2/1 tearing modes.…”

“…The toroidal mode number n of MHD mode can be calculated by using times series of the set of Mirnov coils irregularly toroidally spaced [16]. This approach is generally used in KSTAR toroidal mode number analysis and the NSTX methodology [36] was referenced.…”