Edge fluctuation measurements in EDA H-mode and QCE plasmas in ASDEX Upgrade using the correlation electron cyclotron emission diagnostic

Abstract

The requirement of a plasma solution without large edge-localized modes (ELMs) for future fusion devices motivates the study of plasmas without Type I ELMs. The enhanced D–α (EDA) and quasi-continuous exhaust (QCE) scenarios are two such Type I ELM free plasmas, but their underlying physics principles are not fully understood. A database of EDA and QCE plasmas from ASDEX Upgrade was compiled and analyzed using the correlation electron cyclotron emission diagnostic in order to study differences between properties of the quasi-coherent mode (QCM) between these two regimes. Investigations into measured spectral properties of the QCM, radi- ated temperature fluctuations δTrad/Trad, and radial correlation lengths Lc showed no differences between the two regimes. Spectral properties using the cross-power spectral density, Gxy were sim- ilar between regimes, with the values measured to be a central frequency, fQCM ≈ 20 − 50 kHz; spectral width, ∆f ≈ 5 − 40 kHz; δTrad/Trad ≈ 4 − 6%; and Lc ≈ 1 − 2 cm. Investigations into a quantity representing the relative bandwidth ∆f/fQCM revealed differences between EDA and QCE plasmas when plotted radially, suggesting some type of interaction between the QCM in QCE plas- mas with the surrounding broadband turbulence that is not present in EDA phases. Additionally, investigations into various drive terms for hypothesized instability drivers for the QCM are per- formed showing no difference in the parameter spaces of collisionality and the ion temperature, electron temperature, or electron pressure scale lengths between each regime indicating that there is no observable difference between the drive behind the QCM in each regime.