Quasi-Coherent Modes in the High Density H-mode Regime of W7-AS

摘要

The high density H-mode (HDH) is an ELM-free H-mode regime at the Wendelstein 7-AS (W7-AS) stellarator exhibiting no impurity accumulation. Quasi-Coherent (QC) modes found recently on Mirnov coil signals are thought to be prime candidates responsible for the low impurity concentration in this regime based on their strong correlation to impurity radiation. These modes are high frequency (50-150 kHz), high poloidal mode number (m-40), possibly pressure-driven oscillations, showing strong amplitude modulation. QC modes are occasionally accompanied by low frequency oscillations (principle frequencies <50 kHz plus higher harmonics) not unique for the HDH regimes detected by both the Mirnov coils and Langmuir probes, as well as high frequency oscillations (250-350 kHz) that - when present - decrease the QC amplitude. QC modes on W7-AS show similarities to quasi-coherent modes found on the Alcator C-Mod tokamak, responsible for the low impurity concentration in the similar ELM-free, high density H-mode called Enhanced D-alpha mode, prompting a careful comparison between the two modes