Influences of multiple low-n modes on n=1 resistive wall mode identification and feedback control

Abstract

It is well known in theory that even after the n =1 resistive wall mode (RWM) is suppressed, the other low-n modes, such as n =2 or 3, can appear sequentially, as beta increases. In recent DIII-D experiments [J. L. Luxon, Nucl. Fusion 42, 614 (2002)], we found such an example that supports the theoretical prediction: while the n =1 mode was suppressed, an n =3 mode grew dominant, leading to a beta collapse. The n = 1 RWM suppression was likely due to a combination of rotational stabilization and n = 1 RWM feedback. The multiple RWM identification was performed using an expanded matched filter, where n = 1 and n = 3 RWM basis vectors are simultaneously considered. Taking advantage of the expanded matched filter, we found that an n = 3 mode following an edge-localized-mode burst grew almost linearly for several milliseconds without being hindered. This n = 3 mode appeared responsible for the beta collapse (down to the n = 3 no-wall limit), as well as for a drop in toroidal rotation. A preliminary analysis suggests that the identity of the n= 3 mode could be related to the n = 3 RWM (possibly the first observation in tokamak experiments), while the impact of the n = 3 mode was not as destructive as that of n = 1 RWM. A numerical postprocessing of Mirnov probes showed that the n = 2 mode was also unstable, consistent with the theoretical prediction. In practice, since the presence of an n = 3 mode can interfere with the existing n = 1 RWM identification, multiple low-n mode identification is deemed essential not only to detect n > 1 mode, but also to provide accurate n = 1 RWM identification and feedback control.