Disconnection of edge coherent modes between the outer midplane and divertor target in EAST H-mode plasma

Abstract

Abstract The edge coherent mode (ECM) is considered a highly attractive pedestal mode as it prolongs the duration of edge-localized modes (ELMs), increases particle and impurity transport without significantly affecting energy transport, and operates compatibly with high-performance plasma discharges. The ECM can also be detected by Langmuir probes on the divertor target plate, indicating that it extends from the pedestal region into the SOL, resulting a connection to the divertor target plate through magnetic field line. In this work, the distribution of ECM on the divertor target plates are investigated through the analysis of 215 upper single null discharges on the EAST tokamak. The coherence analysis of plasma fluctuation between the ECE signal at pedestal region of outer midplane and the ion saturation current measured by Langmuir probes in the divertor region reveals that the ECM is hardly detected by the divertor probe close to the outer strike point (OSP) but can be observed at far SOL. This indicates that there exists an ECM quiescent region in the near SOL on divertor plate and the extent of the quiescent region in poloidal flux coordinate (Δ) has been statistically analyzed. A pronounced relation between Δ and triangularity (δ) has been observed, i.e. Δ increasing with δ. Further analysis shows that this relation could be related to averaged magnetic shear in the SOL. This result is consistent with the physical picture that the strong magnetic shear close to X-point significantly squeezes the cross-section of flux-tubes down to scales where collisions dominate.