Numerical study on toroidal mode coupling and triggering of neoclassical tearing modes by sawteeth

Abstract

Numerical simulations have been carried out to understand the seeding of neoclassical tearing modes (NTMs), using a single fluid model and the four-field equations. The paper starts with considering a simplified physics situation: the destabilization of a linearly stable m/n=3/2 mode by an unstable m/n=2/2 mode (m/n being the poloidal/toroidal mode number). Within single fluid model the mode coupling is found to be weakened by relative mode rotation and enhanced by finite plasma β value. For the four-field equations in case of a sufficiently low electron diamagnetic drift frequency, the results are similar to those from the single fluid equations. For a sufficiently large diamagnetic drift frequency, three regimes are found: (a) Suppression regime for moderate plasma β values, in which the originally unstable 2/2 mode is stabilized by its coupling to the originally stable 3/2 mode; (b) Oscillation regime for a sufficiently large β value, in which the 3/2 mode has the feature of an ideal mode, and the diamagnetic drift frequency reaches the shear Alfven frequency outside the tearing layer; (c) NTM regime for sufficiently large β value and bootstrap current density. Consistent with these findings, in our simulations 3/2 modes are triggered by sufficiently strong sawtooth crashes at high β value and/or a low electron diamagnetic drift frequency. A sufficiently large bootstrap current density is then required for the 3/2 island to grow into the NTM regime.

Fast magnetic reconnection and driven plasma rotation in reversed central magnetic shear configuration

“…It should be mentioned that the coupling of the modes of different helicities in toroidal geometry, which could lead to the magnetic field line stochasticity, has not been included in our calculations. The toroidal mode coupling is known to be more important for a larger mode amplitude, higher plasma beta value and lower differential plasma rotation frequency [54]. Similar to the triggering of neoclassical tearing modes by sawtooth crash [54], low-m magnetic islands of different helicities could be driven during a core crash when further taking into account the toroidal mode coupling, which might induce a larger degradation in plasma confinement or even possibly the disruption as observed in experiments [6,9].…”

“…The toroidal mode coupling is known to be more important for a larger mode amplitude, higher plasma beta value and lower differential plasma rotation frequency [54]. Similar to the triggering of neoclassical tearing modes by sawtooth crash [54], low-m magnetic islands of different helicities could be driven during a core crash when further taking into account the toroidal mode coupling, which might induce a larger degradation in plasma confinement or even possibly the disruption as observed in experiments [6,9]. High-m modes of different helicities could also be destabilized by the DTM, to affect the local plasma transport if the island width is larger than the critical width for the plasma pressure flattening across the island [70,71].…”

“…The black, red and blue (magenta, cyan and green) curves show the FSA EM, inertia and viscous torque density perturbations with Δρ = 0.295 and Δω E = 1.8 × 10 6 /τ R (Δρ = 0.055 and Δω E = 3.4 × 10 5 /τ R ). The maximum EM torque density is slightly shifted away from the inner q = 3 surfaces, caused by the shear Alfven resonance outside the resistive layer for a sufficiently large plasma rotation frequency [54].…”

“…To obtain the radial profile of the torque density, the fourfield equations are solved in the single fluid limit using the initial value code TM1, which was used earlier for studying the nonlinear growth of the drift-tearing and the internal kink modes and RMP-related physics issues [50][51][52][53][54][55].…”

Influence of aspect ratio, plasma viscosity, and radial position of the resonant surfaces on the plasmoid formation in the low resistivity plasma in Tokamak

“…Figures 8(e) and ( f ) confirm that the amplitude of the m/n = 2/2 and m/n = 3/3 modes can be larger than or at least comparable to the m/n = 1/1 mode, but other non-harmonic modes are small. This is because, with a small aspect ratio R 0 /a = 2/1, the toroidal effect is strong and the mode with (m ± 1, n) will be generated by the (m, n) [51]. However, with a large aspect ratio, the toroidal effect is weak, and the (m ± 1, n) cannot be generated.…”