Edge-Localized Mode
Encyclopedia
An edge-localized mode 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.
Research is underway as to how to prevent formation of edge localized modes. 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; apparently, this decreases 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, from its onset, the highly non-linear phase and its decay. However, this would not constitute a 'true' ELM cycle, since a true ELM cycle would require to also model the slow growth after the crash, in order to have a second ELM.
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...
occurring in the edge region of a tokamak
Tokamak
A tokamak is a device using a magnetic field to confine a plasma in the shape of a torus . Achieving a stable plasma equilibrium requires magnetic field lines that move around the torus in a helical shape...
plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
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
ASDEX Upgrade
ASDEX Upgrade is a divertor tokamak, that went into operation at the Max-Planck-Institut für Plasmaphysik, Garching in 1991...
in 1981.
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 is underway as to how to prevent formation of edge localized modes. 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; apparently, this decreases 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
Magnetohydrodynamics
Magnetohydrodynamics is an academic discipline which studies the dynamics of electrically conducting fluids. Examples of such fluids include plasmas, liquid metals, and salt water or electrolytes...
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, from its onset, the highly non-linear phase and its decay. However, this would not constitute a 'true' ELM cycle, since a true ELM cycle would require to also model the slow growth after the crash, in order to have a second ELM.