An artist’s interpretation of plasma particles flowing inside a doughnut-shaped fusion system known as a tokamak. Credit: Kyle Palmer/ PPPL Communications Department. CC BY.

Future fusion reactors may be better equipped to handle intense heat thanks to some new computer simulations.

Scientists have long seen a puzzling pattern in tokamaks, the doughnut-shaped machines that could one day reliably generate electricity from fusing atoms.

When plasma particles escape the core of the magnetic fields that hold the plasma in its doughnut shape, they stream down toward the exhaust system, known as the divertor.

There, plasma particles strike metal plates, cool down and bounce back. (The returning atoms help fuel the fusion reaction.)

But experiments consistently show that far more particles hit the inner divertor target than the outer one.

Understanding what drives this lopsided distribution matters for designing future fusion systems: engineers need to know where exhaust particles will land to build divertors that can handle the heat.

The leading explanation centred on what’s known as cross-field drifts within the divertor itself, the sideways movement of particles across magnetic field lines.

However, computer simulations that included only this kind of drift couldn’t reproduce the uneven striking pattern in experiments, making it difficult to trust that simulations could reliably guide divertor design for future machines.

The science

New simulations show that the toroidal rotation – the motion of the particles as they move around the tokamak – plays a key role in determining exactly where the plasma fuel lands in the machine’s exhaust system.

A team of researchers used the modelling code SOLPS-ITER to simulate the path of the particles under different conditions.

Their findings show that when plasma core rotation is combined with cross-field drifts, simulations finally match experimental measurements.

Aligning simulations with experimental results is critical to designing fusion power plants that can withstand the demands of real-world operations.

Study lead author Eric Emdee, an associate research physicist at the US Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL), said: “There are two components to flow in a plasma: there’s cross-field flow, where particles drift sideways across the magnetic field lines, and parallel flow, where they travel along those lines.

“A lot of people said cross-field flow was what created the asymmetry. What this paper shows is that parallel flow, driven by the rotating core, matters just as much.”

Scenarios

The team modelled plasma in the DIII-D tokamak in California, testing four scenarios: with and without cross-field drifts and with and without plasma rotation.

The simulations couldn’t come close to matching experiments until the team added one ingredient: the measured core rotation of 88.4 kilometres per second. 

The combined effect proved far greater than either component alone.

The finding suggests that accurately predicting exhaust behaviour in future fusion systems will require accounting for how the rotating plasma core influences edge flows, a connection that could help engineers design divertors that can better handle reality.

The findings are published in Physical Review Letters.