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Temperatures of more than 10,000°C and a hail of charged particles from the fusion fuel (plasma): These are extreme conditions that the exhaust wall (divertor) of future fusion power plants will need to withstand. It makes handling the exhaust stream one of the main challenges to realizing clean, safe and affordable commercial fusion power plants.

Initial proof-of-concept studies already show that the "Super-X" design for the divertor can reduce heat loads more than tenfold, compared to conventional designs. Now, new experimental results elevate these initial observations beyond proof-of-concept by demonstrating their key benefits for fusion power plants: enhanced power exhaust control while balancing engineering complexity.

MAST Upgrade, the U.K.'s national fusion experiment, was purpose-built by the United Kingdom Atomic Energy Authority (UKAEA) to develop solutions for the fusion power exhaust. Its Super-X design, developed from a concept originating from the Institute for Fusion Studies at the University of Texas at Austin, features a longer divertor, with extended "legs" of plasma compared to conventional designs offering more space to cool the plasma before it hits the divertor walls.

The new results are a world-first: At MAST Upgrade, the researchers have shown that the Super-X approach enables exhaust control without impacting the opposing divertor or the core of the plasma where fusion energy is produced. The researchers show that it is significantly easier to control the desired, more benign conditions in a Super-X configuration compared to conventional designs.

This increases confidence in achieving a suitable exhaust solution for fusion power plants and builds on previous findings that the Super-X configuration on MAST Upgrade facilitates the integration of a hot plasma core with cold conditions in the divertors.

The experiments further showed that even a modest modification of the divertor legs from the conventional "short-legged" design already offers significant benefits in controlling the fusion heat. This agrees with predictions from computer models which show an improved understanding of divertor design. Future fusion projects can therefore benefit from much-improved divertor conditions and exhaust control while balancing engineering complexity.

The results on the physics and engineering aspects of the Super-X divertor were published in and . Dutch fusion researchers Kevin Verhaegh (formerly at United Kingdom Atomic Energy Authority, currently at the TU/e) and Bob Kool (Dutch research institute DIFFER and TU/e) headed the work with a collaboration between the UKAEA and European EUROfusion research teams. The research builds on the collaborative efforts of the divertor community, for instance in experiments at the Swiss fusion machine TCV.

Verhaegh, TU/e, research group Science and Technology of Nuclear Fusion, says, "These results bode well for a variety of future projects like the U.K.'s STEP machine, the U.S. machine ARC and the European DEMO. We were able to show that even a modest, yet strategic, modification of the divertor can already offer many of the benefits of more extreme divertor geometries. As such extreme geometries are more difficult to realize in a power plant, these results open new pathways to improving the design of future fusion machines."

Kool, DIFFER and TU/e, research section Control Systems Technology, says, "These results clearly demonstrate the many benefits that alternative divertors can offer in maintaining acceptable divertor conditions in fusion power plants. This is major step in solving the exhaust problem and ultimately brings us closer to realizing fusion energy."

James Harrison, Head of MAST Upgrade Science, UKAEA, says, "Demonstrating that the plasma conditions in the divertors of MAST Upgrade can be controlled independently is an important advancement towards developing robust control of plasma exhaust in future machines.

"These exciting results were made possible by strong international collaborations between the UKAEA, TU Eindhoven, DIFFER and EUROfusion teams that will continue pushing the boundaries of our understanding in this important area of research."