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An edge-localized mode ("ELM") is a disruptive instability occurring in the edge region of a tokamak plasma due to the quasi-periodic relaxation of a transport barrier previously formed during an L --> H transition. This phenomenon was first observed in the ASDEX tokamak in 1981.[1]

Impact

The development of edge-localized modes poses a major challenge in magnetic fusion research with tokamaks, as these instabilities can damage wall components, particularly divertor plates, due to their extremely high energy transfer rate.

Research involving prevention of edge localized mode formation is underway. A paper was recently published that suggested a novel method of countering this phenomenon by injecting static magnetic noisy energy into the containment field as a containment-stabilization regime; this may decrease ELM amplitude. ASDEX Upgrade has had some success using pellet injection to increase the frequency and thereby decrease the severity of ELM bursts.

The underlying MHD instabilities have been identified, but at present no numerical MHD simulations exist of a full cycle of the ELM instability. In 2006 an initiative (called Project Aster) was started to simulate a full ELM cycle including its onset, the highly non-linear phase, and its decay. However, this did not constitute a 'true' ELM cycle, since a true ELM cycle would require modeling the slow growth after the crash, in order to have a second ELM.
Control in practice

As of late 2011, several research facilities including KSTAR [2] have demonstrated active control (suppression) of some types of this phenomenon.
References

Wagner, F. et al. (1982). Recent results of H-mode studies on ASDEX, Proceedings of the Thirteenth Conference on Plasma Physics and Controlled Nuclear Fusion Research (IAEA, Vienna), vol. I, pp. 277-90.

Kwon, Eunhee (2011-11-10). "KSTAR announces successful ELM suppression". Retrieved 2011-12-11.

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Edge-localized mode