Scientists have found that adding a common household cleaning agent – the mineral boron found in cleaners such as borax – can dramatically improve the ability of some fusion energy devices to hold the heat needed to produce fusion reactions on Earth like the sun and stars do.
Physicists from the Princeton Plasma Physics Laboratory (PPPL) of the United States Department of Energy (DOE), in collaboration with Japanese researchers, made the observation on the Large Helical Device (LHD) in Japan, a winding magnetic installation that the Japanese call it a “heliotron”. The results demonstrated for the first time a new regime of heat confinement in facilities called stellarators, similar to the heliotron. The findings could advance the twisty design as a model for future fusion power plants.
The researchers produced the upper confinement regime by injecting tiny grains of boron powder into the LHD plasma that powers the fusion reactions. Injection through a dropper installed in PPPL greatly reduced eddies and turbulent eddies and increased the confined heat that produces the reactions.
“We could see this effect very clearly,” said PPPL physicist Federico Nespoli, lead author of a paper that details the process in the journal Nature Physics. “The more power we put into the plasma, the greater the increase in heat and confinement, which would be ideal under real reactor conditions.”
Said David Gates, Senior Research Physicist at PPPL who heads the Advanced Projects Department who oversaw the work: “I am very excited about these excellent results that Federico has written in this important paper about our collaborations with the team on the large helical device. When we started this project – the LHD Impurity Powder Dropper – in 2018, we had hopes that there might be an effect on energy confinement. Observations are even better than expected with the suppression of turbulence over a large part of the plasma radius I am very grateful to our Japanese colleagues for giving our team the opportunity to participate in these experiments.
The results also delighted the Japanese researchers. “We are very happy and excited to achieve these results,” said Masaki Osakabe, executive director of the LHD project and scientific adviser for nuclear fusion research for MEXT, the Japanese ministry responsible for nuclear energy. “We are also honored to be collaborators with PPPL,” Osakabe said. “The results revealed by this collaboration will provide a nice tool for controlling high-performance plasma in a fusion reactor.”
Stellarators, first built in the 1950s under the direction of PPPL founder Lyman Spitzer, are a promising concept that long followed symmetrical magnetic installations called tokamaks as the main device for producing fusion energy. A relatively poor thermal confinement history has played a role in the restraint of stellarators, which can operate in a stable state with little risk of plasma disturbances that tokamaks face.
Fusion combines light elements in the form of plasma – the hot, charged state of matter composed of free electrons and atomic nuclei, or ions, which makes up 99% of the visible universe – to release huge amounts of ‘energy. Tokamaks and Stellarators are primary magnetic designs for scientists seeking to harvest safe, clean, and virtually limitless fusion energy to generate fusion power for humanity.
Although boron has long been used to condition walls and improve containment in tokamaks, scientists have never before seen “a widespread reduction in turbulence and increase in temperature like that reported in this paper,” according to the item. Additionally, puffs of heat and damaging particles, called edge-localized modes (ELMs), which can occur in tokamaks and stellarators during high-confinement or H-mode fusion experiments, were absent from the observations.
The remarkable improvement in heat and confinement in the LHD plasma may have resulted from the reduction of so-called ion temperature gradient (ITG) instability, according to the paper, which produces turbulence that causes leakage of plasma out of containment. The reduction in turbulence contrasts with a type of heat loss called “neoclassical transport”, the other main cause of particles escaping from the Stellarator’s confinement.
A new series of LHD experiments is currently underway to test whether the improvement in heat and confinement continues for an increased range of mass injection rate, plasma density and heating power. Nespoli and his colleagues would also like to see if carbon powder can perform as well as boron. “The boron creates a coating on the wall which is good for containment and the carbon won’t,” he said. “We want to see if all the powder is good or if it’s the boron that improves the conditions.”
Additional objectives include evaluating the ability of boron to improve plasma performance during steady-state LHD operation, which is capable of extremely long plasma discharges of up to one hour. Such experiments could yield new evidence of the value of the Stellarator design in the future.
Nespoli F, Masuzaki S, Tanaka K, et al. Observation of a regime with reduced turbulence with injection of boron powder in a stellarator. Nat Phys. Published online January 10, 2022. doi:10.1038/s41567-021-01460-4
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