Identifying the MHD signature and power deposition characteristics associated with type-II ELMs in ASDEX Upgrade

The properties of ASDEX Upgrade's type-II ELM regime, which combines good confinement with benign temporal variation of the power exhaust, make it an attractive candidate for ITER baseline scenario operation. As yet it is not understood what the physics origin of this regime is. In previous type-II ELM studies (Stober J et al2001 Nucl. Fusion41 1123, Stober J et al2005 Nucl. Fusion45 1213) a whole range of fluctuations have been reported. In this paper the properties of these fluctuations are inspected in more detail, and their relevance for the regime investigated. For the high frequency (∼150–220 kHz) magnetic fluctuation activity, from its absence in certain type-II ELM scenarios and behaviour during regime transitions it is concluded that it is not an essential ingredient of type-II ELM regimes on ASDEX Upgrade. A stronger contender to explain the type-II ELM regime appears to be given by small repetitive electromagnetic bursts, and low frequency (∼5–25 kHz) magnetic precursor activity associated with these bursts. It is demonstrated that these bursts give rise to small but frequent heat loads of the right order to influence ELM cycles. These are best detected in the vicinity of the inner strike point in the lower divertor through infrared thermography and Langmuir probes. Their properties further suggest that the bursts are neither small type-I ELMs nor type-III ELMs. However, it remains to be shown that the losses associated with these bursts are high enough to replace the type-I ELM losses. Other enhanced density fluctuation activity at low frequency (10–30 kHz) is detected by reflectometry near the plasma boundary. The information available about this mode is more limited, but so far a causal implication of this mode in the type-II ELM regime cannot be excluded. Overall it can be concluded that, of the three potential dissipation mechanisms, the low frequency electromagnetic burst behaviour seems to be the most promising candidate to account for type-II ELMs and their energy losses.