Effect of resonant magnetic perturbations on local plasma current density gradients and neoclassical tearing modes

Abstract

The effect of externally applied resonant magnetic perturbations (RMPs) on the local equilibrium plasma current density profile is studied numerically based on two-fluid equations in simplified cylindrical geometry. It is found that a moderate RMP below its penetration threshold, via non-linear mode coupling, induces a parallel electric field around its rational surface that can significantly change the local flux-surface-averaged current density gradient. At a given RMP amplitude, the modification of the current density profile increases with increasing electron temperature, and it significantly depends on the bi-normal electron fluid velocity at the resonant surface. The effect of this modification on the magnetic island growth is demonstrated by the example of small m/n = 2/1 islands (m/n being the poloidal/toroidal mode numbers), driven by an unfavorable plasma current density profile and bootstrap current perturbation. The 2/1 mode growth is stabilized by moderate static 4/2 or 6/3 RMPs if the local electron fluid velocity is in the ion drift direction or sufficiently large in the electron drift direction. These results reveal that a weakly three-dimensional equilibrium, containing a moderate 4/2 RMP and the associated shielding current, can be more stable against the 2/1 mode, which often causes tokamak plasma major disruptions.

Stabilization of double tearing mode growth by resonant magnetic perturbations

“…When the local bi-normal electron fluid velocity at the resonant surface is sufficiently large, especially for plasma rotation in the ion drift direction, the growth of m/n = 2/1 NTMs is found to be suppressed in a similar way by moderate static m/n = 2/1, 4/2 or 6/3 RMPs for an electron temperature of a few keV. The stabilization is dominated by the change in the local plasma current density gradient at the q = 2 surface caused by RMPs, resulting from the nonlinear coupling of the helical shielding current to magnetic and plasma velocity perturbations [38].…”

“…In addition, some results are obtained by solving the electron heat transport equation together with the four-field equations, to further take the electron temperature perturbations into account. These equations are described in appendix A of reference [38].…”

“…The following input parameters are used if not otherwise mentioned: the equilibrium electron temperature T e = 2 keV, electron density n e = 3 × 10 19 {0.8[1 − (r/a) 2 ] 2 + 0.2} m −3 , a = 0.5 m, plasma major radius R = 1.7 m, and toroidal magnetic field B t = 2 T. The perpendicular plasma momentum and particle transport coefficients are assumed to be μ = 0.2 m 2 s −1 and D ⊥ = μ/5, and a larger plasma viscosity for the m/n = 0/0 component, 20 m 2 s −1 , is used, considering the neoclassical damping of poloidal plasma rotation [19,38]. The electron diamagnetic drift frequencies at the two resonant surfaces, f * e1 and f * e2 , are around the level of 1 kHz for all the q-profiles shown in figure 1, calculated from the above parameters and equation (B6) of reference [38].…”

“…Recently, the effect of the RMPs on the NTM growth was studied numerically using two-fluid equations [37][38][39]. When the local bi-normal electron fluid velocity at the resonant surface is sufficiently large, especially for plasma rotation in the ion drift direction, the growth of m/n = 2/1 NTMs is found to be suppressed in a similar way by moderate static m/n = 2/1, 4/2 or 6/3 RMPs for an electron temperature of a few keV.…”

Screening of resonant magnetic perturbation penetration by flows in tokamak plasmas based on two-fluid model

“…However, considering the plasma rotation playing a significant role in the screening process of RMP [28], the two-fluid model, retaining the electron diamagnetic drift as well as the E × B flow, is more suitable to account for more complex physics. Recently, in the frame of the two-fluid drift-MHD theory, plenty of researches were carried out to investigate the interaction of the RMP and magnetic islands in tokamak plasmas [29][30][31][32]. Using a two-fluid model, Hu et al found that the two-fluid effects can give significant modifications to the scaling law of mode penetration for different plasma parameters.…”