ELMs in MAST tokamak. (Credit © ITER Organization, www.iter.org)

Instability in the edge part of the tokamak plasma, which can lead to a rapid quasi-periodic release of a significant amount of energy towards the wall of the vacuum vessel. Edge Localised Modes arise due to a strong gradient in temperature and density at the edge of the plasma. This is a feature of a mode with better particle confinement, the so-called H-mode. There are various methods that aim to suppress ELMs or increase their frequency, and thus reduce the amount of energy released in a single ELM. Examples are resonant magnetic perturbation or pellet injection. Since ELMs also play an important role in removing impurities from the plasma core, the possibility of using the Quiscent H-mode, in which ELMs do not occur, is not much considered.