Calming Plasma's Stormy Seas

Energy researchers continue to make headway in their quest to better understand what makes a fusion reaction “tick.”

For decades, controlled nuclear fusion has held the promise of a safe, clean, sustainable energy source that could help wean the world from fossil fuels. But the challenges of harnessing the power of the sun in an Earth-based nuclear fusion reactor have been many, with much of the progress over the last several years coming in incremental advances.

One of the key technical issues that has puzzled physicists is actually a common occurrence in fusion reactions: plasma turbulence. Turbulence inside a reactor can increase the rate of plasma heat loss, significantly impacting the resulting energy output. So researchers have been working to pinpoint both what causes this turbulence and how to control or even eliminate it.

Now simulations run at the National Energy Research Scientific Computing Center (NERSC) have shed light on a central piece of the puzzle: the relationship between fast ion particles in the plasma and plasma microturbulence.

Ion-Temperature-Gradient Instabilities

In a fusion reaction, energy is released when two hydrogen isotopes (tritium and deuterium) are fused together to form a heavier nucleus, helium. To achieve high enough reaction rates to make fusion a useful energy source, the hydrogen gas must be heated to extremely high temperatures -- more than 100 million degrees centigrade — which ionizes the gas and transforms it into plasma.

A tokamak is a reactor configuration considered by many to be the best candidate for producing controlled thermonuclear fusion power. It uses a torus (doughnut-shaped) vessel and extremely strong magnetic fields to confine the energy of the heated plasma by a sufficient degree to ensure a net fusion energy gain.

But in a tokamak, turbulence caused by microinstabilities in the plasma -- particularly ion-temperature-gradient (ITG) instabilities – can significantly impact the energy confinement.

“The ITG mode is a ubiquitous driver of tokamak microturbulence, which in general is driven by the very large gradients of temperature and density in the plasma,” explained Jonathan Citrin of the Dutch Institute for Fundamental Energy Research and lead author of a study published in Physical Review Letters. “This is essentially one of the limiting factors of the total fusion power one can achieve with tokamaks. The more we can stabilize these ITG modes, the more we can reduce the size and cost of the machine for the same total fusion power.”

Recent ion heat transport experiments led by Paola Mantica at the Joint European Torus (JET) tokamak in Oxfordshire, UK demonstrated that in certain regimes, the ITG turbulence in the tokamak core could be reduced, leading to higher ion temperatures. But the mechanism for this stabilization was not fully understood. What was stabilizing the turbulence? And could these results be replicated in reactor-scale tokamaks such as the International Thermonuclear Experimental Reactor (ITER) currently under construction in Cadarache, France?

Suprathermal Ions Key

These questions prompted Citrin and collaborators from the EU and the U.S. to run a series of computer simulations on NERSC’s newest supercomputer, Edison, to test the findings of the JET experiments.

Plasma with fast ions (left) and without (right). Images: Jonathan Citrin

“An important goal with these kinds of studies is to gain a more complete understanding of the basic underlying turbulence physics. This then informs the approximations we can make in constructing reduced turbulence models that allow faster predictions of tokamak temperature and density profiles over the full plasma radius,” Citrin explained.

Following extensive nonlinear gyrokinetic turbulence simulations with the GENE code — a software package written by Frank Jenko and his group at the Max Planck Institute for Plasma Physics — Citrin and his colleagues were able to explain the improved energy confinement seen in the JET experiments.

“The hypothesis following these experiments was that the relatively high rotation found in these plasmas was responsible for the reduced turbulence, but we found that this is not actually the case,” Citrin explained. “Rather, it was a combination of reduced turbulence due to magnetic fluctuations and the inclusion of suprathermal ion species. When you heat the plasma, you accelerate beams of particles and inject them into the plasma. This also makes the plasma rotate, but correlated with the rotation is a significant source of fast ions that coexist with the thermal ions, and we found that this changed the thermal regime significantly.”

Thanks to the capabilities of Edison and the large amount of computing time available, the researchers were able to simulate and explain this observation.

“This is an excellent example of experimental measurements forcing an extensive validation effort in direct numerical simulation,” Citrin said.

Edison’s computing capabilities were central to enabling these calculations to be run efficiently, he added. These sorts of simulations are “very expensive numerically,” he noted; for example, this particular study used 8 million CPU hours, about half on Edison, and this was only to analyze a few tokamak discharges at two radial points.

“In the past, most simulations were done where only the electric fields fluctuate. But with Edison we were able to include the magnetic fluctuations as well as additional ion species, which leads to less turbulence,” Citrin said. “This is exciting because in future reactors like ITER, one does not expect to have strong rotation because of the large size and the amount of torque you can put in. But there will be fast ions due to the fusion reactions themselves. Increased ion temperature due to rotation does not extrapolate well to future devices, but this explanation does extrapolate well.”