Unique PPPL-led workshop assesses research crucial to the success of ITER
Leading experts from around the world gathered at the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) in July to focus on a key issue for the development of fusion energy: Improving ways to predict and mitigate disruptions that can destroy magnetically confined plasmas that are needed for fusion reactions.
Confronting this challenge will be crucial for ITER, the huge international fusion facility that is being built in the shape of a donut, or torus, in France. “The future of magnetic fusion research hangs on ITER being successful,” said Amitava Bhattacharjee, head of the Theory Department at PPPL, which convened the July 17-19 workshop. “And ITER will need to avoid disruptions or mitigate them when they occur.”
Disruptions take place when heat or electric current are suddenly reduced during fusion experiments. This quenching allows the hot, electrically charged plasma gas that fuels the experiments to dissipate and can release forces that damage fusion facilities. Controlling disruptions will thus be essential to meeting ITER’s goal of producing a “burning plasma,” or sustained fusion reaction, to demonstrate the feasibility of fusion energy.
The PPPL-led workshop brought together some 35 theorists and experimentalists from the U.S. Department of Energy Laboratory and other fusion research centers, including two representatives from ITER. Also attending were some 15 Princeton University graduate students studying plasma physics at PPPL.
Developing a unified approach to disruption research was a major goal of the workshop, and the success of the effort could set a precedent. The event produced “a very good degree of analysis from both experimentalists and theorists,” said David Campbell, director of plasma operations at ITER. “I hope that we can build on the links developed here.”
Such links allow scientists to work hand-in-glove, with theorists using experimental data to design codes that predict and simulate disruptions, and experimentalists testing the codes with further experiments. “By making sense of existing data, theory and simulation can be predictive for ITER,” Bhattacharjee said. “This will enable ITER to plan its operations much better.”
He stressed the urgency of such research since plans call for the ITER Organization to review systems for mitigating disruptions in 2017. “That’s a little less than four years away,” Bhattacharjee said. “The mitigation system has to be based on a good understanding of the underlying causes of disruptions.”
Current mitigation strategies call for injecting gas into the plasma to control the rate of disruption. But the gas can have both good and bad effects since the amount that mitigates the heat loss can intensify the impact of the current quench. Physicists likened the situation to finding the precise path between Scylla and Charybdis, the twin dangers that menaced Odysseus and his crew in Homer’s “Odyssey.”
Speakers assessed the full range of disruption physics and zeroed in on critical areas for research. These ranged from improved early warning of the onset of disruptions to better understanding of the electromagnetic forces involved.
The workshop served as a teaching tool for graduate students as well. “Understanding and mitigating disruptions in fusion plasmas are among the grand challenges in plasma physics,” Bhattacharjee said. “Graduate students can have tremendous impact in the fusion program by bringing their imagination and skills to bear on these problems.”
Students will have fresh learning opportunities over the next year. Bhattacharjee hopes to coordinate research on disruptions with organizations such as the International Tokamak Physics Activity, which plans and conducts research relevant to ITER, and with the U.S. Burning Plasma Organization, which plays a leading role in the design of the disruption mitigation system for ITER. Bhattacharjee also expects coordination between theory and experiment to be further developed when the Laboratory sponsors the second workshop on disruptions next year.