Abstract
Strongly shaped (-0.55 < δ < -0.45) negative triangularity (NT) DIII-D discharges show evidence of both edge localized tearing and external kink modes with complex, coupled dynamics that significantly modify the edge plasma. These edge instabilities, found in the majority of strongly shaped NT discharges, have a magnetic signature in frequencies ranging from 0-10kHz and a toroidal mode number of n=-1 as determined by the crossphase of the mode. In addition to the magnetics, 2D electron cyclotron emission imaging (ECE-I) is utilized to measure mode-driven fluctuations in the edge region. The ECE-I data shows large kink-like perturbations localized to the LCFS that extend into the SOL, indicative of an external kink mode. Additionally, evidence of phase mixing and observed tearing-parity perturbations localized near the edge rational surfaces are observed in certain shots. Utilizing a database of strongly shaped NT discharges, the behavior of the edge instabilities was found to be strongly dependent on the value of the edge safety factor and normalized plasma pressure, with three unique mode regimes being identified. The main regime is the “bursty” state, in which short (~5ms) magnetic bursts occur every 5-10ms. Each burst drives a small reduction in the edge electron temperature as measured by ECE and ECE-I, which then rapidly recovers between the bursts. These fluctuations do not have a large impact on global confinement, but may play an important role in regulating the transport in the NT edge, providing a potential explanation to the observations of edge gradients well below known infinite-n stability limits. Finally, linear MHD modeling with MARS-F shows agreement with the experimental observations, with both an external kink mode and edge localized tearing being found using model inputs representative of the experiments. Nonlinear modeling will likely be necessary to understand the complex dynamics and interactions of these instabilities and their ability to regulate the NT edge.
