High-frequency fluctuation and EHO-like mode in the H-mode pedestal on the EAST tokamak

Abstract

In the pedestal region of the Experimental Advanced Superconducting Tokamak (EAST) during high confinement mode plasma operations with radio-frequency heating, two distinct fluctuations are observed: high-frequency fluctuations (HFFs) and edge harmonic oscillation-like (EHO-like) modes. The HFFs are characterized by intermittent fluctuations with a broadband frequency range of $1{-}3{\text{ MHz}}$ and a poloidal wave number ( ${k_\theta }$ ) greater than $0.9{\text{ c}}{{\text{m}}^{ - 1}}$ . On the other hand, the EHO-like mode exhibits characteristics similar to magnetohydrodynamics (MHD)-like modes with n = 1−5 and lower poloidal wave numbers ( ${{\text{k}}_{{\theta }}} \unicode{x2A7D} 0.12{\text{ c}}{{\text{m}}^{ - 1}}$ ). During the pedestal establishing phase following the L–H transition, a significant concurrent presence of HFF and EHO-like modes in high-density pedestal regions has been noted. In this phase, the EHO-like mode not only modulates the amplitude of the HFF but also engages in nonlinear interactions. The occurrence of EHO-like mode and HFF is associated with particle transport toward the divertor, though it is notably less than that caused by edge coherent modes. During the inter-edge localized mode (ELM) period, a significant decrease in the ${D_\alpha }$ baseline is observed whenever the low frequency fluctuation (LFF) weakens and the HFF grows, prior to each large ELM. One possible explanation is that the rapid increase of $E \times B$ shear stabilizes the LFF and destabilizes the HFF, which lowers the pedestal transport and enables the further growth of the pedestal until the onset of the ELM.

Keywords

• EHO
• high frequency fluctuation
• pedestal turbulence
• interaction
• H-mode