Phase-locking of magnetic islands diagnosed by ECE-imaging

Abstract

Millimeter-wave imaging diagnostics identify phase-locking and the satisfaction of 3-wave coupling selection criteria among multiple magnetic island chains by providing a localized, internal measurement of the 2D power spectral density, S(ω, kpol). In high-confinement tokamak discharges, these interactions impact both plasma rotation and tearing stability. Nonlinear coupling among neoclassical tearing modes of different n-number, with islands not satisfying the poloidal mode number selection criterion ⟨m, m('), m - m(')⟩, contributes to a reduction in core rotation and flow shear in the vicinity of the modes.

Phase locking of multi-helicity neoclassical tearing modes in tokamak plasmas

“…Finally, the phase-locked state is characterized by a modified toroidal angular velocity profile, internal to the m, n rational surface, which is either flattened or inverted (depending on the amount of poloidal flow present in the plasma). The nonlinear toroidal coupling of the 2, 1 and the 3, 2 neoclassical tearing modes has been observed on both JET and DIII-D. 21,22 In accordance with our analysis, the phase-locking bifurcation is observed to take place when the frequency mismatch between the 3, 2 and the 4, 2 modes has been reduced to about one half of its original value. Moreover, the phase-locked state is characterized by a toroidal angular velocity profile, internal to the 2, 1 rational surface, which is either flattened or inverted.…”

“…21,22 To be more exact, in both cases, the 3, 2 and 2, 1 neoclassical tearing modes are observed to phase lock to one another. This phase locking is inferred to take place in two stages.…”

“…For example, 2, 1 and 3, 2 neoclassical modes have been observed simultaneously in both DIII-D and JET hybrid discharges. 21,22 In addition, 4, 3 and 5, 4 modes have been observed simultaneously in JET hybrid discharges. 23 In all cases, the various modes are eventually observed to phase lock to one another, giving rise to a significant flattening, or even an inversion, of the core toroidal plasma rotation profile.…”

“…23 In all cases, the various modes are eventually observed to phase lock to one another, giving rise to a significant flattening, or even an inversion, of the core toroidal plasma rotation profile. 22,23 This behavior is highly undesirable because the loss of core plasma rotation is known to have a deleterious effect on plasma stability (because it facilitates locked mode formation).…”

Microwave Imaging Reflectometry for the study of Edge Harmonic Oscillations on DIII-D

“…Since the forwarding modeling indicates that neither misalignment between the transmitter and receiver antennas nor non-sinusoidal time dependence of the EHO modes (n > 2) accounts for the broadening of the measured spectra and thus counter-propagation, we are led to speculate as to whether the counter-propagation of the dominant EHO mode is a similar diagnostic artifact as seen in ECE-I data [16], which is known to correspond with a fluctuation phase jump due to magnetic islands. This weak tearing parity has been overlooked in the past, but a similar feature has now been observed in beam emission spectrometer (BES) and other ECE data, as shown in figure 7.…”