Piece by piece JET is being adapted to be as close as possible to a mini-ITER, to enable as much advance testing of the next step fusion experiment’s design and operation as possible. The last experimental campaign was based around the ITER-Like-Wall, testing out the materials and behaviour of the plasma in a metallic vessel. Future plans involve moving to different fuels and testing other subsystems planned for ITER such as ELM coils, which could prove crucial to ITER’s success.

ITER of course aims to operate in the plasma regime with the highest energy confinement. Known as the H-mode, this regime of operation was discovered at the Asdex tokamak in Germany in 1982. When the right combination of heating and gas fuelling is used, the H-mode occurs, characterised by the plasma developing an outer layer which prevents particles from escaping. However, with the H-mode came ELMs, edge localised modes. The confinement in the H-mode is so good that pressure builds up inside the plasma until an ELM develops: a burst of turbulence that spews out particles and energy, much like a solar flare. Depending on the amount of energy ejected these ELMs can damage components and erode or melt wall tiles. ITER will have a plasma an order of magnitude larger than any device before, so the scale of its ELMs is a bit of an unknown. Thus the designers have taken a precautionary approach and opted to include technologies to tackle them head on. One of the most successful approaches has been to use coils to perturb the magnetic field on the edge of the confined plasma to create resonant magnetic perturbations (RMPs).

The exact mechanism of the process is still being explored. The H-mode’s confinement layer relies on particles circulating around the tokamak in distinct layers, a bit like lanes on a motorway. The innermost layers have the fastest moving particles, while the outer layers are the slowest – in the H-mode confinement is good because the particles do not change lanes much. The RMP coils induce regular wiggles in the outside layers of the plasma. These wiggles cause the concentric layers to overlap, so particles then have a way to change lanes between the layers without completely breaking down the structure. In this way the confined particles can leak out, relieving the pressure build-up which would otherwise lead to an ELM.

This technique was pioneered in the nineties on the DIII-D tokamak in San Diego and has since been replicated on other similar sized fusion experiments. However testing on JET, currently the world’s largest tokamak, is required to ensure the successful scaling to ITER’s large dimensions.

Phil Dooley, EFDA