Identification of an Island-induced Alfv\'{e}n Eigenmode in MST plasmas

Recent theoretical work analytically computes the effect of a magnetic island on the shear Alfv\'{e}n continuum and may explain unresolved Alfv\'{e}nic activity observed in neutral beam-heated MST plasmas. Consideration of the previously-ignored core-localized n=5 island leads to theoretical Alfv\'{e}n continua that provide a gap in which the observed n=4 Alfv\'{e}nic bursts reside. Numerical simulations using the STELLGAP/AE3D codes, as well as a new code called SIESTAlfv\'en have identified the bursts as the first observation of an Island-induced Alfv\'{e}n Eigenmode (IAE). The IAE arises from a helical coupling of mode numbers, similar to the helicity-induced Alfv\'{e}n eigenmode, but occurs in the core of an island. The observed frequency of bursting n=4 Alfv\'{e}nic modes fall within the island-induced gap over a wide range of MST operating parameters. Characteristics such as mode frequency, width and damping rate are measured as a function of experimentally-varied magnetic island width. Coincident bursts with toroidal mode number n=1 may exhibit frequency scaling of an Alfv\'{e}nic eigenmode; the possibility of an island induced extremum mode is explored as an explanation. Work supported by US DoE under grants DE-FG02-99ER54546, DE-SC0006103 and DE-FC02-05ER54814.