Description
In Quiescent H-mode (QH-mode), edge-localized modes (ELMs) are naturally replaced by a low-$n$ edge harmonic oscillation (EHO), yet the self-regulating transport mechanism driven by the EHO remains insufficiently understood. Using high-spatiotemporal-resolution imaging diagnostics on KSTAR—electron cyclotron emission imaging (ECEI) and broadband ECE—we resolve the eigenmode structure of the EHO and elucidate its causal role in edge transport.
The EHO is localized within the pedestal, propagates in the ion-diamagnetic direction, and shifts inward as the negative $E \times B$ shear strengthens. Notably, an inverse correlation is observed between the $E \times B$ shear and the radial wavenumber $k_\mathrm{r}$, where the radial width of the mode expands with increasing shear. This suggests that rotational shear acts as a structural drive for the EHO, distinct from its typical role in suppressing turbulence.
Information-theoretic Transfer Entropy (TE) analysis reveals a distinct "dual-stabilization" feedback loop sustaining the stationary ELM-free phase. While the EHO($A$) actively drives outward energy transport($\Gamma$) to limit the pressure gradient ($A \to \Gamma$), the background shear flow($S$) regulates the EHO saturation amplitude ($S \to A$) and prevents excessive flux ($S \to \Gamma$). The TE analysis indicates that the QH-mode pedestal is sustained by a shear-driven regulation mechanism, where the coupling between rotational shear and mode structure is central to maintaining the ELM-free state. This work provides a direct experimental evidence on the EHO as an active, shear-regulated governor of pedestal transport, revealing the definitive causal links between rotational shear, mode dynamics, and energy exhaust.
This work was supported by the NRF of Korea (Grant Nos. 2019R1F1A1057545, 2022R1F1A1073863, RS-2023-00281272), the KFE Korea-US Collaboration Program (EN2603), and the Princeton Plasma Physics Laboratory (U.S. DOE Contract No. DE-AC02-09CH11466).