ELMs can resemble small solar flares.

Plasma is inherently susceptible to a variety of instabilities. Instead of maintaining an ideally smooth boundary, it can develop distortions, oscillations and localised eruptions that affect confinement and plasma stability.

Some plasma instabilities can grow rapidly once triggered, amplifying perturbations in the magnetic configuration and degrading confinement. They can increase energy and particle losses and therefore have to be detected, controlled or avoided during tokamak operation.

The most serious instabilities are those that can release large amounts of energy and particles towards plasma-facing components, potentially causing excessive thermal and mechanical loads.

Edge Localised Modes

Edge Localised Modes (ELMs), first observed on the German ASDEX tokamak in 1981, are repetitive instabilities that can occur at the edge of H-mode plasmas, where particle and energy confinement is significantly improved. ELMs produce short bursts of heat and particles towards the divertor and other plasma-facing components. Although they can help expel impurities from the plasma edge, large ELMs may impose damaging heat loads. ITER will therefore use techniques such as resonant magnetic perturbations and pellet pacing, while also studying plasma regimes in which ELMs are naturally small or suppressed.

Resonant Magnetic Perturbation

Pellet injection nozzle for plasma fuelling and ELM control.

Resonant magnetic perturbations will be generated by 27 in-vessel ELM-control coils. These coils produce small three-dimensional perturbations of the magnetic field at the plasma edge, modifying edge transport and, depending on plasma conditions, reducing the size of ELMs or suppressing them altogether.

Pellet Injection

Pellet pacing uses small frozen deuterium pellets injected into the plasma edge to trigger ELMs at controlled intervals. This can produce more frequent but smaller ELMs, reducing the peak heat load associated with each event while retaining some of the impurity-control benefits of edge transport.

Disruptions

ELMs in the MAST tokamak.

A more serious threat to ITER is a plasma disruption — a sudden loss of plasma stability and magnetic confinement that terminates the discharge and releases stored thermal and magnetic energy on a very short timescale. Disruptions can produce intense heat loads, large electromagnetic forces and, in some cases, high-energy runaway electrons. During the rapid current quench of a disruption, a strong toroidal electric field can accelerate some electrons faster than collisions can slow them down. These runaway electrons can reach relativistic energies and form intense beams capable of severely damaging plasma-facing components if they strike the wall.

Disruption Mitigation

ITER’s Disruption Mitigation System is based on shattered pellet injection (SPI). Large cryogenic pellets containing hydrogen, neon or mixtures of the two are accelerated to high speed and shattered just before entering the plasma. The resulting cloud of small fragments is injected from several toroidal locations, allowing the material to penetrate and spread rapidly through the plasma. This increases plasma density and radiative cooling, helping to distribute the stored energy more uniformly, reduce thermal and electromagnetic loads and suppress damaging runaway-electron beams. SPI has been selected as the baseline ITER disruption-mitigation technique, and the system passed its final design review in 2024.