Effect of bootstrap current on the onset of neoclassical tearing mode

Abstract

The effect of bootstrap current perturbation on the onset of neoclassical tearing modes (NTMs) is studied numerically based on two-fluid equations, using externally applied resonant magnetic perturbations (RMPs) as triggers. With increasing the bootstrap current density, a larger RMP is found to be required to excite the NTM's onset when the bi-normal electron fluid velocity is in the ion drift direction, being contrary to the conventional understanding that the bootstrap current perturbation is always destabilizing. For the electron fluid velocity in the electron drift direction, however, the opposite results are found.

Influence of Micro-turbulence on Neoclassical Tearing Mode Onset

“…In fact, from NTM theory [19], neoclassical effect (bootstrap current) always plays a destabilizing role in NTM triggering. From a recent simulation [35], bootstrap current shows a stabilizing effect on NTM onset because of particle pumping-in effect caused by RMP when magnetic island rotates in co-current direction before mode penetration. Nevertheless, in our case, seed island has been locked on vacuum field and island rotation remains nearly zero for a long time before 2nd-BF happening.…”

Stabilization of double tearing mode growth by resonant magnetic perturbations

“…It is well known that the externally applied RMPs can suppress or mitigate edge-localized modes [57][58][59][60][61][62][63][64][65], tearing modes [19,20,[36][37][38][39] and sawteeth [66]. Rotating RMPs have been utilized to avoid mode locking and disruption [67][68][69].…”

“…When the value of |ω n | is sufficiently large, the stabilization is also found for −ω n < 0. Due to the dependence of the modified current density profile on the ω n value [38,39], the stabilization regions are asymmetric on the two sides of ω n = 0. When the local electron fluid velocity is not sufficiently large in the electron drift direction, the change in the local current density gradient is destabilizing [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.…”

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

“…It is shown in this paper that due to nonlinear effects, the local 𝑚/𝑛 = 0/0 component plasma current density gradient can be changed by the helical shielding currents coupled to magnetic and plasma velocity perturbations, as seen from equation ( 1). The change of the local electron density gradient and bootstrap current density is also caused by the coupling of helical shielding currents and magnetic perturbations [18,44]. The current density profile modification is significant for RMPs of the order 𝜓 𝑎 = 10 −5 − 10 −4 below their penetration threshold, being larger for a higher electron temperature and depending on the plasma rotation velocity.…”