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 (https://iopscience.iop.org/article/10.1088/1361-6587/aafd15/meta) 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.
The Shattered Pellet Injector at the EUROfusion fusion device JET shooting a pellet of frozen hydrogen into the plasma to quickly brake runaway electrons. Two ultra-fast cameras show how the pellet arrives in the right image at 8.445 sec (counter at top) and how the typical crescent-shaped synchrotron light emitted by runaway electrons disappears around 8.637 s, showing complete and benign dissipation of runaways. The plasma then produces streams of cold filaments as it settles down. The video is in slow motion, 75 times slower than reality. Credit: EUROfusion consortium
EUROfusion is a consortium of 30 research organisations, and behind them about 150 affiliated entities including universities and companies, from 25 European Union member states plus the United Kingdom, Switzerland and Ukraine. Members receive funding for fusion projects in accordance with the European Research Roadmap to the Realisation of Fusion Energy. The EUROfusion programme has two aims: preparing for ITER experiments and developing concepts for the future European demonstration fusion power plant DEMO.
This research project involved scientists from international partners and from the EUROfusion members CEA (FR), IPP-CAS (CZ), IPP (DE) and UKAEA (UK). The experiments were performed at the premiere EUROfusion fusion device JET (Joint European Torus), which is managed on behalf of EUROfusion by UKAEA.
The CEA is a key player in research, development and innovation in four main areas: energy transition, digital transition, technology for the medicine of the future and defense and security. With a workforce of 20,000 people, based in nine sites equipped with very large-scale research infrastructures, the CEA actively participates in collaborative projects with a large number of academic and industrial partners, in France, Europe and worldwide.
ITER—designed to demonstrate the scientific and technological feasibility of fusion power—will be the world’s largest experimental fusion facility. Fusion is the process that powers the Sun and the stars: when light atomic nuclei fuse together to form heavier ones, a large amount of energy is released. Fusion research is aimed at developing a safe, abundant and environmentally responsible energy source.
ITER is also a first-of-a-kind global collaboration. Europe is contributing almost half of the costs of its construction, while the other six Members to this joint international venture (China, India, Japan, the Republic of Korea, the Russian Federation and the USA), are contributing equally to the rest. The ITER Project is under construction in Saint-Paul-lez-Durance, in the south of France.
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