An edge-localized mode (ELM) is a plasma instability occurring in the edge region of a tokamakplasma 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]
Issues
ELM is a major challenge in magnetic fusion research with tokamaks, as these instabilities can:
Damage wall components (in particular divertor plates) by ablating them away due to their extremely high energy transfer rate (GW/m2);[4]
Potentially couple or trigger other instabilities, such as the resistive wall mode (RWM) or the neoclassical tearing mode (NTM).[5]
Prevention and control
A variety of experiments/simulations have attempted to mitigate damage from ELM. Techniques include:
Application of resonant magnetic perturbations (RMPs) with in-vessel current carrying coils can eliminate or weaken ELMs.[6]
Injecting pellets to increase the frequency and thereby decrease the severity of ELM bursts (ASDEX Upgrade).[citation needed]
Multiple small-scale ELMs (000s/s) in tokamaks to prevent the creation of large ones, spreading the associated heat over a larger area and interval[7]
Increase the plasma density and, at high densities, adjusting the topology of the magnetic field lines confining the plasma.[8]
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]
