Raking out the fusion ashes

New simulations and experiments at the record-breaking Joint European Torus fusion device JET take another step towards sustainable fusion power to the grid. In the scientific journal Nature Communications, EUROfusion and international researchers show how periodic waves in the plasma can 'rake out' unburnable helium while the fusion reaction burns steady.


Future fusion power plants will produce clean, sustainable energy by fusing hydrogen isotopes into energetic helium. A big hurdle has been how to remove the helium ash from the plasma: once it's given off its heat to the surrounding plasma to keep the reaction going, helium just takes up space and dilutes the fuel.

A new collaboration between EUROfusion and QST (Naka, Japan) was established in order to study helium in the plasma of the Joint European Tokamak experiment JET (UKAEA, UK). "In our new publication, we show how we can use so-called sawtooth oscillations to push out helium once it gets colder, while leaving hot helium that's fresh out of the fusion process untouched", explains main author Andreas Bierwage (QST, Japan).

Sawteeth are periodic outbursts inside a fusion plasma, that occur when the magnetic fields get wound up and release stress. During such outbursts, they mix fuel particles and push them outward from the plasma's hot centre where fusion takes place.

"Researchers previously thought that sawteeth affect all the particles in their reach indiscriminately", says co-author Yevgen Kazakov (ERM-KMS, Belgium), who led JET studies that are analyzed in the paper. "However, several past papers hinted that it might be possible that very fast particles - like the hot helium - can actually escape their grasp. After state-of-the-art modeling of our recent JET experiments we now have a good understanding of when this process works, when not, and how."

The work has breakthrough potential, because it allows fusion researchers to keep reducing the amount of cooled helium ashes in their devices without disturbing the hot helium that heats the plasma. The collaboration between European and Japanese teams will continue in the joint Japanese-European tokamak JT-60SA, which will produce a large population of energetic particles, says co-author Jeronimo Garcia (CEA, France), responsible for the JET-QST collaboration. The final proof will occur if future fusion devices like the international experiment ITER can replicate the raking trick. Kazakov: "If so, we'll have taken a major step towards making fusion energy a practical reality!"