The role of temperature fluctuations in the dynamics of type-Ⅰand type-Ⅱ edge localized modes at ASDEX Upgrade

摘要

Two-dimensional temperature measurements using the electron cyclotron emission imaging diagnostic at ASDEX Upgrade revealed a variety of temperature fluctuations associated with type-Ⅰ and type-Ⅱ edge localized modes (ELMs). The characteristics and dynamics of these modes, and their role in the ELM cycle, are presented. During type-Ⅰ ELMs, different phases of distinct mode activity have been identified. At the onset of the ELM crash, a short lived mode is observed in the pedestal region. During the actual crash phase, multiple filamentary structures are observed just outside the separatrix. A third type of fluctuation, the inter-ELM mode, is often observed in between type-Ⅰ crashes. The occurrence of this mode tends to lengthen the ELM period. During type-Ⅰ ELM suppression with magnetic perturbation coils, smaller crash events become more frequent, replacing the large type-Ⅰ crashes. In type-Ⅱ ELMs, temperature crashes are absent altogether, and a continuous broadband fluctuation in the 20-60 kHz range, showing beat wave like behaviour, is observed. The similarities between the characteristics of this mode and the inter-ELM mode suggest that it is the same instability. In type-Ⅰ ELMs it delays the next crash, in type-Ⅱ ELMs it might be responsible for the complete absence of crash events.