Fusion researcher Robert Goldston, a Princeton University professor of astrophysical sciences and former director of the U.S. Department of Energy's Princeton Plasma Physics Laboratory, received the 2015 Nuclear Fusion Award for the most outstanding paper to appear in the journal Nuclear Fusion during 2012. Presenting the award was Yukiya Amano, director general of the International Atomic Energy Agency (IAEA), during the IAEA's 2016 Nuclear Fusion Energy Conference in Kyoto, Japan. The annual honor, selected by editors ofthe journal published by the Institute of Physics and the IAEA, recognizes papers that have made the greatest scientific impact in the two years following their publication, and includes a $2,500 cash award.
The paper, for which Goldston was sole author, presented a new model for estimating the width of the scrape-off layer, the hot plasma exhausted from the closed magnetic surfaces of doughnut-shaped facilities called tokamaks. Such plasmas must be wide enough to avoid delivering a thin and concentrated blow that could damage the walls of the exhaust -- or divertor -- chamber where the heat is dissipated.
The 2012 paper, titled "Heuristic drift-based model of the power scrape-off width in low-gas-puff H-mode tokamaks," showed that the width of the scrape-off layer depends on how rapidly the plasma drifts across the closed surfaces as it flows into the divertor region. The more rapid the drifts, the greater the width of the layer. This calculation enables operators of future tokamaks like ITER, the international experiment under construction in France, to prepare for the possible dimensions of the layer and its impact on the chamber region.
"A simple yet elegant model"
The editorial board of Nuclear Fusion cited the model as, "Potentially one of the most important results obtained in recent years in fusion" research. The estimate "provides a simple yet elegant model for the scrape-off layer power width," the editors said, "and ultimately could have significant impact on the future direction of the field."
Since publication of the paper, experiments on tokamaks around the world have largely confirmed the predictions of Goldston's model. Confirmations include both the magnitude of the scrape-off-layer width, and its variation with parameters such as machine size, magnetic field strength, plasma shape, and heating power.
The effect has even been seen in plasmas without divertors but with material limiters that bound the hot gas. The model has given rise to additional predictions, such as the pressure gradient in the scrape-off layer, and the limit of plasma density in the best tokamak operating modes due to the limit to this gradient -- both of which have recently been observed.
Prediction of greater ITER scrape-off layers
Recent numerical simulations by PPPL physicist C.S. Chang, reported at the IAEA meeting, predict greater scrape-off layer widths for ITER. Experiments on the international facility will therefore provide a definitive experimental test.
At a separate ceremony, Goldston thanked the many members of the edge physics research community who welcomed him and contributed to his understanding of this area of fusion physics, when he decided to focus his research on this area after stepping down from the PPPL directorship in 2009. The community consists of researchers who study the physical properties at the edge of fusion plasmas.
Goldston is the second researcher at PPPL to receive the award, which began in 2006. Steven Sabbagh, a senior research scientist at Columbia University on long-term assignment to PPPL, won the 2009 award as lead author for a paper titled, "Resistive wall stabilized operation in rotating high beta NSTX plasmas."
PPPL, on Princeton University's Forrestal Campus in Plainsboro, N.J., is devoted to creating new knowledge about the physics of plasmas -- ultra-hot, charged gases -- and to developing practical solutions for the creation of fusion energy. The Laboratory is managed by the University for the U.S. Department of Energy's Office of Science, which is the largest single supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.