No abstract

A new model for the edge harmonic oscillations (EHOs) in the quiescent H-mode regime has been developed, which successfully reproduces the recent observations in the DIII-D tokamak. In particular, at high E × B flow shear only a few low-n kink modes remain unstable at the plasma edge, consistent with the EHO behavior, while at low E × B flow shear, the unstable mode spectrum is significantly broadened, consistent with the low-n broadband electromagnetic turbulence behavior. The model is based on a new mechanism for destabilizing low-n kink/peeling modes by the E × B flow shear, which underlies the EHOs, separately from the previously found Kelvin-Helmholtz drive. We find that the differential advection of mode vorticity by sheared E × B flows modifies the 2D pattern of mode electrostatic potential perpendicular to the magnetic field lines, which in turn causes a radial expansion of the mode structure, an increase of field line bending away from the mode rational surface, and a reduction of inertial stabilization. This enhances the kink drive as the parallel wavenumber increases significantly away from the rational surface at the plasma edge where the magnetic shear is also strong. This destabilization is also shown to be independent of the sign of the flow shear, as observed experimentally, and has not been taken into account in previous pedestal linear stability analyses. Verification of the veracity of this EHO mechanism will require analysis of the nonlinear evolution of low-n kink/peeling modes so destabilized in the linear regime.

A quasi-coherent high-frequency mode (HFM), with frequency ∼300 kHz, has been observed in EAST H-modes obtained by lower hybrid current drive or combined heating of lower hybrid wave and ion cyclotron resonance frequency wave after lithium wall conditioning. The HFM with an initial growing phase in frequency and amplitude appears both following L–H transition and between edge-localized modes at a relatively high-collisionality . Detailed analysis from edge Langmuir probe has revealed that the HFM propagates in the electron diamagnetic direction in both the lab frame and the plasma frame. The decorrelation rate of HFM is smaller than the maximum E × B shearing rate at the steep gradient region by an order of magnitude. Furthermore, the growth, saturation and oscillations of HFM, including both frequency and amplitude, are strongly correlated with the evolution of pressure gradient. Thus, the limit-cycle-like oscillations between the amplitude and the frequency of HFM may play an important role in the formation and saturation of pedestal. During the saturation phase of pedestal pressure gradient, the HFM, sometimes accompanied by a mid-frequency (∼80 kHz) quasi-coherent mode, drives strong cross-field transport resulting in small crashes of pressure gradient and finally limits the pressure gradient.