Abstract
Experiments in the DIII-D tokamak show that the plasma responds to resonant magnetic perturbations (RMPs) with toroidal mode numbers of n = 2 and n = 3 without field line reconnection, consistent with resistive magnetohydrodynamic predictions, while a strong nonlinear bifurcation is apparent when edge localized modes (ELMs) are suppressed. The magnetic response associated with this bifurcation is localized to the high field side of the machine and exhibits a dominant n = 1 component despite the application of a constant amplitude, slowly toroidally rotating, n = 2 applied field. The n = 1 mode is born locked to the vacuum vessel wall, while the n = 2 mode is entrained to the rotating field. Based on these magnetic response measurements and Thomson scattering measurements of flattening of the electron temperature profile, it is likely that these modes are magnetic island chains near the H-mode pedestal. The reduction in del T-e occurs near the q = 4 and 5 rational surfaces, suggesting five unique islands are possible (m = 8, 9, or 10 for n = 2) and (m = 4 or 5 for n = 1). In all cases, the island width is estimated to be 2-3 cm. The Chang-Callen calculated confinement degradation due to the presence of an individual island of this size is 8%-12%, which is close to the 13%-14% measured between the ELMs and suppressed states. This suggests that edge tearing modes may alter the pedestal causing peeling-ballooning stability during RMP induced ELM suppression. (C) 2015 AIP Publishing LLC.
