Batten down hatches: Preventing heat leaks to help create a star on Earth

PPPL physicist Elena Belova

(Photo by Elle Starkman)
PPPL physicist Elena Belova

The two neutral beam boxes installed next to PPPL's National Spherical Torus Experiment-Upgrade

The two neutral beam boxes installed next to PPPL’s National Spherical Torus Experiment-Upgrade

Creating a star on Earth requires a delicate balance between pumping enormous amounts of energy into plasma to make it hot enough for fusion to occur and preventing that heat from escaping. Now, physicists at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) have identified a method by which instabilities can be tamed and heat can be prevented from leaking from the plasma, giving scientists a better grasp on how to optimize conditions for fusion in devices known as tokamaks.

The findings provide new insight into the loss of heat that makes fusion reactors less efficient. Results could benefit the operation of ITER, the multinational fusion facility being built in France to demonstrate the practicality of fusion energy.

Fusion, the mechanism that drives the sun and stars, is the fusing of light elements that are in the form of plasma, a soup of electrically charged electrons and atomic nuclei, that generates massive amounts of energy. Scientists are seeking to replicate fusion on Earth for a virtually inexhaustible supply of power to generate electricity.

The research began as an exercise to analyze plasma in the National Spherical Torus Experiment-Upgrade (NSTX-U), PPPL’s flagship fusion device. “These simulations were originally meant to help explain observations made on NSTX-U,” said Elena Belova, a principal research physicist at PPPL and lead author of a paper reporting the results in Physics of Plasmas.

The analysis demonstrated a significant fact, originally discovered by PPPL physicist Eric Fredrickson. Belova said it showed “that a second neutral beam, designed to help heat the plasma, also quieted instabilities that might cause heat to leak out of the plasma and reduce the efficiency of the plasma heating.”

Belova created the simulations using a computer code known as HYM, which ran on the National Energy Research Scientific Computing Center (NERSC), a DOE Office of Science user facility. The results confirmed and extended observations made during NSTX-U experiments. “It turns out that the new beam was stabilizing all the modes, or plasma instabilities, of this type that were being driven by the original beam,” said Belova. “That’s a good thing.”

These findings extend previous research into how instabilities affected plasma temperature. “Elena Belova’s prior work on NSTX had shown how these instabilities could be responsible for electron heat loss in NSTX,” said Amitava Bhattacharjee, the head of the PPPL theory department. “Her new work using fresh data from NSTX-U shows how to cure this heat loss mechanism, and it has very interesting implications for ITER.”

The simulations also revealed an explanation for why the second neutral beam so effectively shuts down the instabilities. The beam injects particles into the plasma in a path nearly parallel to the magnetic fields, as opposed to the first neutral beam, which injected particles into the plasma at an angle to the magnetic fields. The new beam, in other words, evened out the distribution of plasma particles with both fast and slow velocities along the magnetic field. The uneven distribution was what was causing the unwanted instabilities in the first place.

The second neutral beam therefore helps keep the heat in the plasma core, the best possible place for fusion to occur. In addition, the simulations show that plasma instabilities could be stabilized with a far weaker second neutral beam – only seven percent of the total beam output is needed to avoid the plasma instabilities.

Belova’s research further shows how simulations can provide insight into physical experiments by allowing a researcher to vary many more conditions than on a physical apparatus. “In experiments, you cannot measure everything,” Belova said. “But when you use a simulation, you can really examine the details of the phenomenon because you have easy access to all the data, and you’re changing numbers instead of hardware.”

Not all details have been revealed, however. The experiments only correlate the instabilities, known as global Alfvén eigenmodes (GAEs), with the heat loss. Experimentalists can now use the second neutral beam to completely shut down the GAEs and observe if that causes an increase in the plasma core temperature. “This is exciting because now we know how to control these modes in future experiments and can definitely establish whether they are causing the heat loss,” Belova said.

She now plans to extend this research by using the HYM code to examine the paths that electrons take within fusion plasmas and how their motion transfers heat within the plasma. She also plans to change the code to account for the crucial rotation of plasma that can affect the frequencies of the modes within a tokamak.

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