Spatial decoupling of the n = 1 plasma response during $n$ = 2 RMP ELM suppression on DIII-D

Experiments in the DIII-D tokamak show a strong $n$ = 1 mode appears when edge localized modes (ELM) are suppressed via an applied $n$ = 2 resonant magnetic perturbation (RMP). The poloidal structure of this $n$ = 1 mode, as it unlocks from the vacuum vessel wall, is discussed. An $n$ = 2 mode is found to be entrained by the rotating RMP and an estimate of the modes m is presented. Previous work suggests these $n$ = 1, 2 modes are magnetic island structures located at the top of the H-mode pedestal.\footnote{R. Nazikian et al., Phys. Rev. Lett., 114, 105002 2015.} The width of these islands is estimated to be 2 $\sim$ 3 cm and the calculated confinement degradation due to their presence is 8 $\sim$ 12\%, which is close to the 13 $\sim$ 14\% measured between the ELMing and RMP suppressed states. This suggests island energy transport may be sufficient to explain the change in peeling-ballooning stability during RMP induced ELM suppression.