EU-US Breakthrough Mitigates Effects of Fusion Instabilities

A new braking technique can protect future fusion devices from damage by fast electrons erupting from their 150-million-degree plasma.


A team of researchers from the European fusion research consortium EUROfusion, the international fusion experiment ITER, and US corporation General Atomics (GA) have found a new way of stopping avalanches of fast electrons before they can damage tokamak devices. The new technique may end up protecting the inner walls of future fusion power plants, write the team in top physics journal Physical Review Letters (PRL).

Avalanche of electrons

For fusion to become a dependable energy source, researchers need to mimic the processes at the heart of the sun in their donut-shaped tokamaks. Maintaining a plasma (the super-hot, charged gas of independently moving atomic nuclei and electrons) at 150 million degrees in such a device is challenging enough, but stably confining plasmas is equally important to avoid damaging internal components of the fusion research devices.

One particularly important topic in fusion research is how to handle unruly runaway electrons. These beams of near-light-speed electron particles develop during so-called disruption events where a plasma threatens to slip its magnetic cage. After the initial kick from such a disruption, electrons can accelerate more and more within the magnetically-confined plasma, and knock other slower electrons on in an exponential avalanche process until finally slamming into the fusion device's inner wall. There they can penetrate centimetres into the material, potentially damaging components.

Deuterium quenches runaways

Fusion researcher Cédric Reux from EUROfusion member the French Alternative Energies and Atomic Energy Commission (CEA) has been studying runaway electrons since his PhD research 13 years ago. He explains that the effect may be just a nuisance in current tokamaks, but that the speed of the runaway avalanche process grows exponentially as the tokamak’s size increases. In the significantly larger ITER tokamak and the fusion power plants to follow, runaway electrons would deposit tens of megajoules of energy concentrated onto tiny impact sites if left unchecked – enough to cause substantial damage to the interior of the device.

"Mitigating the impact of runaway electrons is one of the main challenges to developing fusion energy", says Reux. Together with international colleagues from the EU and US, the physicist has now shown that injecting either clouds or frozen pellets of light deuterium atoms can effectively quench the runaways before they become a problem.

The current work was inspired by an earlier observation in a single experiment at the DIII-D National Fusion Facility in the US, which GA operates for the US Department of Energy (DOE) Office of Science. Because the DIII-D work showed promise, researchers wanted to confirm that injecting deuterium atoms can safely slow down runaway electrons. At the EUROfusion JET tokamak, researchers had the benefit of improved camera coverage, an ITER-like wall sensitive to runaway electrons and a whole campaign to investigate the process of braking runaway electrons.

Second line of defence

Disruptions and the runaway electrons they produce have been around since the first Soviet tokamaks in the 1950s. To calm down disruptions and radiate away the power they unleash, researchers favour injecting heavy atoms into their plasma if they cannot avoid a disruption through other control techniques. Unfortunately, these heavy atoms tend to produce precisely the fast electrons that can then become runaways.

"We really needed a second line of defence to handle those runaways, in case we can't mitigate a disruption other than by injecting heavy atoms", says Reux. A second dose of heavy atoms used to be enough to get rid of the runaways in smaller tokamaks. However, in the larger JET tokamak, this technique already proved less efficient. In fact, those heavy atoms destabilized the runaway beam and sent it to the tokamak’s interior wall, and produced even more potentially damaging, fast electrons. With the confirmation that injecting lighter deuterium stops the runaways even better than heavier atoms, the road is clear to start testing this concept for use in the ITER fusion experimental device.

Key issue solved through international cooperation

Previous techniques to slow runaway electrons could lead to so-called regeneration, where additional runaways were created even as fusion researchers were mitigating their effects on plasma facing components. This was identified as a key unresolved issue following a workshop on runaway electrons held at the ITER Organization in July 2008, says Alberto Loarte, Head of the Science Division at ITER: "This work is an excellent example showing how the targeted effort of the Members’ R&D institutes coordinated by the ITER Disruption Mitigation Task Force can advance the validation of the design of ITER’s Disruption Mitigation System."

ITER's Shattered Pellet Injectors

Stopping runaway electrons is just one of the challenges in mitigating plasma disruptions. Researchers also need a way to reduce the heat and resulting strain on structural components that disruptions cause. In ITER, that task will fall to the 27 Shattered Pellet Injectors (SPI) encircling the 30-meter-wide tokamak. These systems will inject shards of frozen hydrogen and neon into the plasma to effectively quench it when it misbehaves.

To confirm that the SPI design can handle this important task, EUROfusion participated in a multi-party research project including Euratom and the UK Atomic Energy Authority (UKAEA), the ITER Organization, the US DOE, US ITER, and the US Oak Ridge National Laboratory. This project installed and tested an SPI on the EUROfusion JET tokamak.

Future projects aim to install an SPI at other fusion devices as well as a second one at JET over the coming years. SPI has also been installed at other fusion devices as part of the international effort of the ITER Disruption Mitigation Task Force to establish a solid physics basis for the ITER Disruption Mitigation System.

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