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

Abstract

Abstract. 2D 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 20-60kHz 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.

Edge localized mode rotation and the nonlinear dynamics of filaments

“…ECEI measurements in the KSTAR tokamak [3] show that the ELM evolu- the ELM crash occurs. These measurements are in agreement with AUG ECEI measure-ments [4,5]. In AUG, the rotation of the ELM precursors is also found in the electron 48 diamagnetic direction but the first-expelled ELM filament is observed to reverse rotation 49 and to propagate in the ion diamagnetic direction.…”

From Phase Locking to Phase Slips: A Mechanism for a QuiescentHmode

“…The phase locked solution provides a robust route for thermal energy release. With locked phase, δP will grow large, leading to collapse of the edge pressure profile and the formation of filamentary structures [18]. This violent thermal energy eruption phenomenon corresponds to the so-called ELMy H mode.…”

Study of the ELM fluctuation characteristics during the mitigation of type-I ELMs

“…The highest mode amplitudes are seen for small ELMs in the mitigated regime. The reason for this is that after the transition to the small ELM regime, there are always some background temperature fluctuations present, so-called 'inter-ELM mode' [9]. The inter-ELM mode will be discussed in more details in subsection 3.8.…”

“…At ASDEX Upgrade it is commonly observed that above a certain threshold in density, a transition from the large type-I ELMs to much smaller ELM crashes occurs. The application of active in-vessel off-midplane saddle coils ('B-coils') [7] leads to an increase of density which cause the type-I ELMs to disappear [8] while smaller and more frequent crash events start to dominate [9]. In contrast to type-I ELMs, in the small ELM regime there is an almost continuous succession of small crashes.…”