Electron temperature fluctuation levels of the quasi-coherent mode across the plasma radius

Abstract

EDA H-mode is an ELM-free regime in which the edge quasi-coherent mode (QCM) replaces the ELMs. The estimated location of the quasi-coherent mode is in a partly optically thin region of steep gradients localized between ρpol = 0.96 -1. Relative fluctuations of radiation temperature between 15 and 80 kHz are about 7% with significant density contribution. In the electron cyclotron emission (ECE) channels with resonances in the plasma core, a mode with the same frequency as the quasi-coherent mode is measured. The peak amplitude of both core and edge modes matches the strongest electron temperature gradient in the core and the edge, respectively. The ECE core and edge signals are out of phase. The radiation transport forward model (ECRad) shows that the refraction explains the phase relation between the edge and the core ECE channels. The phase correlates with the sign of the core Te. The amplitude of the fluctuations in the core decreases with decreasing gradients, which is the trend seen in the experiment. The amplitude ratio of the core and edge fluctuation is a factor of five in the experiment; this ratio remains a factor of a hundred in the modeling.

Experimental characterization of the quasi-coherent mode in EDA H-Mode and QCE scenarios at ASDEX Upgrade

“…In both regimes a characteristic edge fluctuation is prominent, called the quasi-coherent mode (QCM), being a known signature of the EDA H-mode [10,19,20], and, as shown here, is also linked to the mode shown by Griener et al [16] and Wolfrum et al [21] in the QCE regime. At Alcator C-Mod, Theiler et al [22] has shown, using the gas puff imaging diagnostic that the QCM moves in ion diamagnetic direction with frequency f ≈ 86 kHz, is localized near the E r minimum and has a poloidal wavenumber of k θ ≈ 1.9 cm −1 .…”