Modeling of the stabilization of neoclassical tearing modes by localized radio frequency current drive

Abstract

Numerical modeling of the stabilization of neoclassical tearing modes by localized radio frequency (rf) current drive is presented. The bootstrap current is self-consistently calculated from the pressure evolution equation, the rf current source is obtained from the ray-tracing code and the Fokker–Planck code, and the change of the driven current profile due to the change of the magnetic island width has also been taken into account by modeling the two-dimensional transport of the fast electrons induced by the rf wave. It is found that both parallel and the perpendicular transport of the fast electrons play important roles in the stabilization, and modulated and nonmodulated rf current drive have approximately the same stabilizing effect. The neoclassical tearing modes is shown to be stabilized by a continuous rf current drive. The simulation results essentially agree with experimental observations.

Numerical modeling of diffusive heat transport across magnetic islands and local stochastic field

“…TM1 is a new version of our previous code TM for modelling the nonlinear evolution of NTMs and their stabilization by rf wave current drive [30]. In TM1 the new numerical scheme described in Ref.…”

“…Since the region k || λ e <1 is very narrow for a high T e case, a modified form of χ || , χ || =χ ||c [1+(3.16λ e k || ) 2 ] 1/2 was used before [30], which reduces to χ ||c in the limit k || λ e <<1…”

“…Away from this region the heat flux is carried by free-streaming electrons and is non-local [29]. For such a case the "flux limit" form for χ || was used before [e.g., 16,30], but this is only a crude approximation [29].…”

Control of neoclassical tearing modes

“…This could be done, as for most of the experiments a deposition width 2d dep smaller than the marginal island width W marg could be easily achieved. The different behaviour for O and X-point phasing could be well understood with the help of an MHD code which includes self-consistently the local bootstrap current [64,65,66]. Later experiments with an increased amount of unmodulated co-ECCD power showed for the first time the possibility of a complete removal of a (3/2)-NTM [67].…”

Computational modeling of fully ionized magnetized plasmas using the fluid approximation

“…67 Although this effect is likely very small in the Rutherford regime, 46 it is possible that a modified viscosity coefficient could be introduced. The goal of the closures to date has been to reproduce the analytic results that have been successfully used [72][73][74][75][76][77][78] to analyze experimental results, and to allow the extension of analytic studies to include effects such as geometry and mode coupling.…”