An edge-localized mode (ELM) is a disruptive

instability In numerous fields of study, the component of instability within a system is generally characterized by some of the outputs or internal states growing without bounds. Not all systems that are not stable are unstable; systems can also be mar ...

occurring in the edge region of a

tokamak A tokamak (; russian: токамáк; otk, 𐱃𐰸𐰢𐰴, Toḳamaḳ) is a device which uses a powerful magnetic field to confine plasma in the shape of a torus. The tokamak is one of several types of magnetic confinement devices being d ...

plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral), a green translucent silica mineral * Quark–gluon plasma, a state of matter in quantum chromodynamics Biology * Blood pla ...

due to the quasi-periodic relaxation of a transport barrier previously formed during a transition from low to

high-confinement mode High-confinement mode, or H-mode, is an operating mode possible in toroidal magnetic confinement fusion devices mostly tokamaks, but also in stellarators.ASDEX tokamak in 1981.

Diamagnetic Diamagnetic materials are repelled by a magnetic field; an applied magnetic field creates an induced magnetic field in them in the opposite direction, causing a repulsive force. In contrast, paramagnetic and ferromagnetic materials are attracted ...

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. An ELM can expel up to 20 percent of the reactor's energy.

Issues

ELM is a major challenge in magnetic fusion research with tokamaks, as these instabilities can: * damage wall components (in particular

divertor In nuclear fusion power research, a divertor is a device within a tokamak or a stellarator that allows the online removal of waste material from the plasma while the reactor is still operating. This allows control over the buildup of fusion prod ...

plates) by ablating them away due to their extremely high energy transfer rate (GW/m²); * potentially couple or trigger other instabilities, such as the resistive wall mode (RWM) or the neoclassical tearing mode (NTM).

Prevention and control

A variety of experiments/simulations have attempted to mitigate damage from ELM. Techniques include: * injecting static magnetic noisy energy into the containment field as a containment-stabilization regime to decrease ELM amplitude. * injecting pellets to increase the frequency and thereby decrease the severity of ELM bursts (

ASDEX Upgrade ASDEX Upgrade (''Axially Symmetric Divertor Experiment'') is a divertor tokamak, that went into operation at the Max-Planck-Institut für Plasmaphysik, Garching in 1991. At present, it is Germany's second largest fusion experiment after stellarat ...

). * multiple small-scale ELMs (000s/s) in

tokamaks A tokamak (; russian: токамáк; otk, 𐱃𐰸𐰢𐰴, Toḳamaḳ) is a device which uses a powerful magnetic field to confine plasma in the shape of a torus. The tokamak is one of several types of magnetic confinement devices being ...

to prevent the creation of large ones, spreading the associated heat over a larger area and interval * increase the plasma

density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...

and, at high densities, adjusting the topology of the

magnetic field lines A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...

confining the plasma.

History

In 2003

DIII-D DIII-D is a tokamak that has been operated since the late 1980s by General Atomics (GA) in San Diego, USA, for the U.S. Department of Energy. The DIII-D National Fusion Facility is part of the ongoing effort to achieve magnetically confined fusio ...

begn experimenting with

resonant magnetic perturbations Resonant magnetic perturbations (RMPs) are a special type of magnetic field perturbations used to control burning plasma instabilities called edge-localized modes (ELMs) in magnetic fusion devices such as tokamaks. The efficiency of RMPs for contr ...

to control ELMs. 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. In 2015, results of the first simulation to demonstrate repeated ELM cycling was published. In 2022, researchers began testing the small ELM hypothesis at JET to assess the utility of the technique.

Issues

Prevention and control

History

See also

References

Further reading