The edge decides: How turbulence determines the success of fusion power plants

Two IPP papers in the journal Physical Review Letters explain for the first time, from first principles, what happens in the extremely thin edge layer of a fusion plasma. In doing so, they provide a foundation for the design of future reactors.
 

A fusion power plant must achieve two things at the same time that appear fundamentally contradictory: it must confine a plasma at 100 million degrees Celsius well enough to reach ignition, while exhausting the resulting heat over a sufficiently broad area to prevent the walls from melting. Whether these two requirements can be reconciled is determined within a layer only a few centimetres thick at the plasma edge. Two research teams at the Max Planck Institute for Plasma Physics (IPP) have now used high-performance computers to calculate phenomena in this edge layer from the fundamental equations of physics for the first time – without prior assumptions derived from experiments. Both papers appear in Physical Review Letters; one of them was highlighted by the journal’s editors as being of particular significance.

 

The gatekeeper of the plasma

At the edge of a tokamak – the toroidal magnetic confinement device used to contain fusion plasmas, such as ASDEX Upgrade in Garching – lies an invisible boundary known as the separatrix. Inside it, the magnetic field lines are closed and the plasma remains confined; outside it, they lead towards actively cooled components. Ideally, a steep pressure gradient known as the pedestal forms precisely in this region. It acts as a kind of thermal insulation that determines the efficiency of the entire device. There is, however, a catch: if this barrier becomes too effective, it can discharge in sudden bursts that deposit enormous amounts of energy onto small areas of the wall. Such events would be intolerable in a power plant because they could damage the tokamak wall.

 

A valve instead of an eruption

Physicist Dr Kaiyu Zhang and his colleagues in IPP’s Tokamak Theory division have now simulated an operating regime that avoids precisely this problem: the quasicontinuous exhaust regime, or QCE. Rather than releasing its energy in infrequent eruptions, the plasma continuously exhausts heat in small portions. The simulations show for the first time how this works: a wave-like structure travels along the separatrix and causes the pedestal boundary to oscillate rhythmically across it. In the process, it continuously pinches off finger-like packets of plasma – “blobs” measuring only around one centimetre across, but extending for more than ten metres along the magnetic field lines. These blobs propagate outwards at approximately 1,000 metres per second and distribute the heat over a broad area.

For the first time, the researchers were able to demonstrate that this behaviour is caused by the interplay of two very different instabilities that meet at the separatrix. When this effect is removed from the calculation, the heat-carrying plasma blobs disappear. The density and temperature profiles agree with measurements from IPP’s ASDEX Upgrade tokamak in Garching, without the theoretical model having been adjusted to match the experiment.  The editors of Physical Review Letters highlighted the particular significance of the paper by selecting it as an “Editors’ Suggestion” and „Featured in Physics“.
Dr Kaiyu Zhang says: “We can now derive from physical principles why heat is exhausted in small portions.”

 

Why the direction of the magnetic field matters

The second paper, led by Dr Baptiste Frei, resolves a puzzle that has accompanied fusion research for decades. If only the direction of the magnetic field in a tokamak is reversed, the heating power required to reach the operating regime with good thermal insulation – the H-mode – doubles, even though all other conditions remain identical.

The simulations of ASDEX Upgrade now provide an explanation: turbulence at the plasma edge itself generates the flow that subsequently shears the turbulent structures apart and thereby brings the turbulence under control. This feedback loop, however, operates efficiently in only one of the two magnetic-field directions – namely when the turbulent structures are tilted in such a way that they transfer their energy particularly effectively to the flow. In the unfavourable direction, this transfer remains weak, the turbulence is stronger and the stabilising shear flow is shallower. The simulations also pointed out the importance of diverted geometry in this mechanism. Similar signatures are found in simulations of other devices, indicating that the researchers have identified a generally applicable mechanism.
Dr Baptiste Frei says: “Reversing the magnetic field changes, in a sense, the choreography of the turbulence – and with it how readily the plasma can access H-mode.”

 

From explanation to prediction

Although the two studies address different questions, they reveal the same underlying pattern: at the plasma edge, turbulence is not merely a disruptive influence that researchers seek to suppress. It self-organises and, in doing so, helps regulate how effectively a plasma is insulated and how it exhausts its heat.

Designing devices such as ITER or a demonstration fusion power plant requires a quantitative understanding of this interplay. Until now, such predictions have largely relied on empirical scaling laws derived from present-day experiments – a risky basis for extrapolation to power plants that have yet to be built. The two new studies replace part of this empirical knowledge with physics founded on first principles.

 

Links to the original publications:

Turbulent Nature of the Quasicontinuous Exhaust Regime for Fusion Plasmas: https://doi.org/10.1103/j44y-5dp6

First-Principles Explanation of the Drift Configuration Dependence of the Radial Electric Field and High-Confinement Access in Tokamaks: https://doi.org/10.1103/m911-g6kc