Frequency chirping of neoclassical tearing modes by energetic ions

Abstract

The mechanism of rapid frequency chirping for neoclassical tearing modes (NTMs) is studied. Resonance between NTMs and trapped energetic ions can provide an additional torque to change the evolution of frequency. Whether the frequency rises or falls depends on the direction of island propagation. If the island propagates in the direction of ion diamagnetic drift, the frequency will be increased dramatically and rapidly. If the island propagates in the direction of electron diamagnetic drift, the frequency will be reduced to a lower value. The predicted chirping time is consistent with experimental results in DIII-D (Liu et al 2020 Nucl. Fusion 60 112009).

Nonlinear growth of magnetic islands by passing fast ions in NSTX

“…Fast ions can affect NTMs. Theoretically, fast ions can affect the NTM rotation frequency [5], which qualitatively explains the NTM frequency 'chirping' in correlation with bursts of fast ion losses, as observed in ASDEX [6] and TFTR [7]. Another theory, however, is that fast ions can affect the NTM growth rate via the formation of a parallel current that alters either the bootstrap [8] or polarization [9] current.…”

“…We assume that the single fluid approach is appropriate for NTM simulation, because the theories of the fast ion effect on NTM we tested in this paper are developed using the single fluid model. An improved two fluid approach without fast ions [15] and a drift kinetic approach for fast ions [5] will be included in the future. We also assume that free parameters can be useful in NTM simulations, not because the theory is inaccurate/overly simplified or the physics constants can change, but rather because of the measured quantities input to the model have uncertainties.…”

Effects of plasma radiation on the nonlinear evolution of neo-classical tearing modes in tokamak plasmas with reversed magnetic shear

“…The neo-classical tearing mode (NTM), as one of the typical modes of MHD instabilities, has been widely explored theoretically and experimentally for decades. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] NTM can be driven by the perturbed helical bootstrap current because of the pressure flattening inside the magnetic island, leading to the magnetic island destabilization. [1][2][3][4][5][6][7][8][9][10][11] Moreover, large NTM islands on different rational surfaces can interact with one another, affecting the transport and equilibrium of the plasma, and even causing the plasma disruptions in a highbeta plasma.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] NTM can be driven by the perturbed helical bootstrap current because of the pressure flattening inside the magnetic island, leading to the magnetic island destabilization. [1][2][3][4][5][6][7][8][9][10][11] Moreover, large NTM islands on different rational surfaces can interact with one another, affecting the transport and equilibrium of the plasma, and even causing the plasma disruptions in a highbeta plasma. [13,14] To avoid NTMs, numerous studies have explored various methods for triggering them to prevent the occurrence of seed magnetic islands.…”