NTM Localization by Correlation of T_e and dB/dt

Abstract

For electron cyclotron current drive-based stabilization of neoclassical tearing modes (NTMs), it is crucial that the current deposition occurs as close to the island as possible; hence, its location needs to be accurately known. An NTM, rotating in the laboratory frame, causes fluctuations of magnetic flux measurable by Mirnov coils (dB/dt). Temperature perturbations in the vicinity of an NTM are caused by displaced flux surfaces and thus have the same frequency as the Mirnov signal but show a constant phase difference, which depends on the mode topology (poloidal and toroidal periodicity), on the toroidal displacement of the Mirnov coil with respect to the temperature measurement, and on the sign of the temperature change between the X-point profile and the O-point profile, which inverts somewhere inside the island. The sign flip of ΔT_e is equivalent to a change of the phase difference between T_e and magnetic reference by π and therefore can be localized using the presented correlation method. Using the suggested algorithm, we can determine the rational surface that coincides with the radial island location with low latency and good reliability in real time from electron cyclotron emission temperature profiles when correlated with the appropriate magnetic fluctuations on a modern workstation computer.