An edge-localized mode (ELM) is a plasma instability occurring in the edge region of a tokamak plasma due to periodic relaxations of the edge transport barrier in high-confinement mode. Each ELM burst is associated with expulsion of particles and energy from the confined plasma into the scrape-off layer. This phenomenon was first observed in the ASDEX tokamak in 1981. [1] Diamagnetic effects in the model equations expand the size of the parameter space in which solutions of repeated sawteeth can be recovered compared to a resistive MHD model. [2] An ELM can expel up to 20 percent of the reactor's energy. [3]
ELM is a major challenge in magnetic fusion research with tokamaks, as these instabilities can:
A variety of experiments/simulations have attempted to mitigate damage from ELM. Techniques include:
In 2003 DIII-D began experimenting with resonant magnetic perturbations to control ELMs. [9]
In 2006 an initiative (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 produce a second ELM.
As of late 2011, several research facilities had demonstrated active control or suppression of ELMs in tokamak plasmas. For example, the KSTAR tokamak used specific asymmetric three-dimensional magnetic field configurations to achieve this goal. [10] [11]
In 2015, results of the first simulation to demonstrate repeated ELM cycling was published. [12]
In 2022, researchers began testing the small ELM hypothesis at JET to assess the utility of the technique. [7] [3]