Researchers at a recent worldwide conference on fusion¹（融合，融化） power have confirmed the surprising accuracy of a new model for predicting the size of a key barrier to fusion that a top scientist at the U.S. Department of Energy's Princeton Plasma² Physics Laboratory (PPPL) has developed. The model could serve as a starting point for overcoming the barrier. "This allows you to depict³（描述） the size of the challenge so you can think through what needs to be done to overcome it," said physicist⁴ Robert Goldston, the Princeton University professor of astrophysical（天体物理学的） sciences and former PPPL director who developed the model. Goldston was among physicists⁵ who presented aspects of the model in late May to the 20th Annual International Conference on Plasma Surface Interactions in Aachen, Germany. Some 400 researchers from around the world attended the conference. Results of the model have been "eerily⁶ close" to the data, said Thomas Eich, a senior scientist at the Max Planck Institute for Plasma Physics in Garching, Germany, who gave an invited talk on his measurements. The agreement appears too close to have happened by chance, Eich added.

 

Goldston's model predicts the width of what physicists call the "scrape-off layer" in tokamaks, the most widely used fusion facilities. Such devices confine hot, electrically charged gas, or plasma, in powerful magnetic fields. But heat inevitably⁷ flows through the system and becomes separated, or scraped off, from the edge of the plasma and flows into an area called the divertor chamber⁸.

 

The challenge is to prevent a thin and highly concentrated layer of heat from reaching and damaging the plate that sits at the bottom of the divertor chamber and absorbs the scrape-off flow. Such damage would halt fusion reactions, which take place when the atomic nuclei⁹, or ions, inside the plasma merge¹⁰ and release energy. "If nothing was done and you took this right on the chin, it could be a knockout blow," said Goldston, who published his model in January in the journal Nuclear Fusion.

 

Solving this problem will be vital for future machines like ITER, the world's most powerful tokamak, which the European Union, the United States and five other countries are building in France to demonstrate fusion as a source of clean and abundant energy. The project is designed to produce 500 megawatts of fusion power in 400 second-long pulses, which will require researchers to spread the scrape-off heat as much as possible to protect the divertor plate.

 

Goldston's model could help guide such efforts. He began pondering the width of the heat flux¹¹ during an international physics conference in South Korea in 2010. Looking at the latest scrape-off layer data based on improved measurements, he estimated -- literally¹² on an envelope -- that the new widths could be produced without plasma turbulence¹³, a factor that is typically considered but is notoriously difficult to calculate. This led him to search for a way to estimate the width of the surprisingly thin layer, and to gauge¹⁴ how the width would vary as conditions such as the amount of electrical current in the plasma varied¹⁵.

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