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

The edge coherent mode (ECM) is considered a highly attractive pedestal mode because it extends the duration of edge-localized modes and increases particle and impurity transport without significantly affecting energy transport. Moreover, it operates compatibly with high-performance plasma discharges. The ECM can also be detected using Langmuir probes on the divertor target plate, indicating that it extends from the pedestal region into the SOL and resulting in a connection to the divertor target plate via magnetic field lines. In this work, the distribution of ECM on divertor target plates is investigated by analyzing 215 upper single null discharges on the EAST tokamak. The coherence analysis of plasma fluctuations between the electron cyclotron emission signal in the pedestal region of the 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 but can be observed at far SOL. This finding indicates the presence of an ECM quiescent region near the SOL on the divertor plate, and the extent of the quiescent region in the poloidal flux coordinate (Δ) has been statistically analyzed. A pronounced relationship between Δ and triangularity (δ) has been observed, that is Δ increasing with δ. Further analysis reveals that this relationship can be attributed to the average magnetic shear in the SOL. This result is consistent with the physical picture, which states that strong magnetic shear close to X-point significantly squeezes the cross-section of flux tubes down to scales dominated by collisions.