High frequency magnetic fluctuations correlated with the inter-ELM pedestal evolution in ASDEX Upgrade

Abstract

In order to understand the mechanisms that determine the structure of the high confinement mode (H-mode) pedestal, the evolution of the plasma edge electron density and temperature profiles between edge localised modes (ELMs) is investigated. The onset of radial magnetic fluctuations with frequencies above 200 kHz is found to correlate with the stagnation of the electron temperature pedestal gradient. During the presence of these magnetic fluctuations the gradients of the edge electron density and temperature are clamped and stable against the ELM onset. The detected magnetic fluctuation frequency is analysed for a variety of plasma discharges with different electron pressure pedestals. It is shown that the magnetic fluctuation frequency scales with the neoclassically estimated E × B velocity at the plasma edge. This points to a location of the underlying instability in the gradient region. Furthermore, the magnetic signature of these fluctuations indicates a global mode structure with toroidal mode numbers of approximately 10. The fluctuations are also observed on the high field side with significant amplitude, indicating a mode structure that is symmetric on the low field side and high field side. The associated fluctuations in the current on the high field side might be attributed to either a strong peeling part or the presence of non-adiabatic electron response.

Toroidal mode number determination of ELM associated phenomena on ASDEX Upgrade

“…As already mentioned the second branch starts with n = −7, −8 already in phase II whereas the other branches show up in this phase first. As it was shown by Laggner et al [14] the here discussed inter-ELM modes are placed in the strong pressure gradient region close to the plasma boundary. Some of the following assumptions used for the localization are only valid in this edge region.…”

On the phase velocity in between weak and strong plasma edge turbulence

“…This requires detailed investigations in the future. For example, in H-mode quasi-coherent modes with low frequency broadening are observed [52], while in I-mode a weakly coherent mode with high frequency broadening is observed [53,54]. In general, it is recommended to verify phase velocity measurements based on a frequency-dependent technique with a wavenumber-dependent technique, and vice versa.…”

Insights into type‐I edge localized modes and edge localized mode control from JOREK non‐linear magneto‐hydrodynamic simulations

“…The EPED model predicts that, when a critical pressure gradient for kinetic ballooning modes is reached, these modes lead to a clamping of the pressure gradient. A correlation of this clamping with high‐frequency modes has been confirmed experimentally, although Laggner et al could not confirm the ballooning character of the observed modes. Typically, while the maximum pressure gradient remains clamped, the pedestal grows further inwards until a large ELM crash appears.…”