Off-axis Fishbone Modes and Excitation of RWM in DIII-D

摘要

In neutral beam injection (NBI) heated plasmas above the no-wall limit of the external kink and central safety factor q(0) >1, bursting MHD modes often trigger the resistive wall mode(RWM) in JT-60U and DIII-D [1-3]. The RWM is considered a potentially-disruptive global MHD that would be unstable even in the presence of a conducting wall. Present theoretical understanding is that the stabilization above the no-wall kink-limit is sensitive to the plasma rotation and kinetic effects, in particular to a significant contribution of precession drift of trapped energetic particles (EPs). Significant losses of trapped EP reduce the population of precession-drifting particles, possibly leading to the RWM onset. Accompanying EP losses, the non-ambipolar radial electric field causes a sudden reduction in toroidal plasma rotation, reducing the stabilizing effect. Thus, the bursting modes are thought to impact the stability in two serious ways. There arise several issues in the process of bursting modes triggering RWM. One of them is the mode character of bursting mode and another is the transient process or kwivi onset, This paper will briefly describe the bursting mode and one aspect of nonlinear behavior by discussing the relationship of poloidal/toroidal components of magnetic perturbations. The toroidal component is useful to identify the characteristics of EP-driven bursting mode.