Steady progress is advancing in plans for combatting damaging disruptions in experiments that aim to bring to Earth the fusion energy that powers the sun and stars. That was the key finding of the recent online Theory and Simulation of Disruptions workshop hosted by the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL), which drew more than 100 registrants from around the world to the July 19-23 gathering.

The event was the ninth in the series of workshops that PPPL launched in 2013 to help predict and mitigate disruptions on ITER, the international experiment under construction in France, to demonstrate the practicality of fusion energy. The gathering was held in cooperation with the International Atomic Energy Agency (IAEA).

"Leading contributions"
"Significant problems remain but there has been substantial progress," said Amitava Bhattacharjee, lead organizer of the workshops who recently stepped down as head of the PPPL Theory Department to devote full time to teaching and research. "There have been excellent contributions by experimentalists and theorists," Bhattacharjee said. "I was greatly encouraged by the leading contributions of early and mid-career scientists who have become deeply engaged in dealing with disruptions."

Damaging disruptions, which develop when the superhot plasma that fuels fusion reactions grows highly unstable, are the greatest challenge facing tokamak devices such as ITER, doughnut-shaped magnetic fusion facilities that are the most widely used experimental devices for producing fusion reactions.

Such reactions combine light elements in the form of plasma - the hot, charged state of matter composed of free electrons and atomic nuclei that makes up 99 percent of the visible universe - to produce massive amounts of energy. Scientists around the world are seeking to replicate and control fusion on Earth for a virtually inexhaustible supply of safe and clean energy to generate electricity.

Delivering an overview of ITER's disruption mitigation system was physicist Michael Lehnen, who chairs the ITER Disruption Mitigation System Task Force. "The design of the system is progressing at fast pace toward its final stage," Lehnen said.

"And although some questions on the physics of disruption mitigation are still to be answered, the strong expansion of activities in this field over the last few years provides confidence that we will get the answers to help us with the remaining design decisions." Moreover, he added, "the Princeton workshop has been a key event for many years to exchange information between theory, modelling and experiments and has made a strong contribution to the impressive growth in the field."

Runaway electrons
A particularly difficult challenge facing ITER - and all next-generation tokamaks - are runaway electrons, high-energy electron beams that can be formed during disruptions and can melt the plasma facing walls of fusion devices. "The runaway electron problem is expected to become very serious in ITER," said Gabriella Pautasso, a scientist in the tokamak department of the Max Planck Institute for Plasma Physics in Garching, Germany, who chaired a session on dealing with the near light-speed particles.

Among the proposed solutions she described were talks by researchers that outlined the results of massive injections of the hydrogen isotope deuterium, which creates conditions for dissipating the impact of the electrons by scattering them over a wide interior surface. "Whether this method is a viable and reliable way of mitigating the effect of runaway electrons on a device like ITER is under intense experimental and theoretical investigation," she said.

Another presentation Pautasso discussed described the development of tools in recent years for modelling massive gas injection and runaway electron generation and suppression. Researchers provided "convincing examples of the progress made in developing these tools and of their advanced predicting capabilities," she said.

Effective mitigation
Nate Ferraro, a PPPL physicist who chaired the session on the physics of disruptions, offered summaries of several workshop events. "There was clear agreement that disruptions, and particularly the threat of runaway electrons, remain an important risk for ITER and future large-scale tokamak designs," Ferraro said.

At the same time, "rapid progress in the capability to simulate disruptions and disruption mitigation techniques was reported," as were "continued incremental improvements in the ability to predict disruptions in real-time, both using machine learning and physics-based methods." However, he said, "at least for now ITER will rely on effective disruption mitigation systems rather than predictions that can lead to avoiding disruptions altogether."

Also commenting on the workshop was David Campbell, former head of the ITER Science and Operations Department who participated in the first workshop in 2013 and attended sessions in the recent one. The latest gathering "highlighted the remarkable progress that has been made in experimental, theoretical and simulation activities since the first workshop in 2013," Campbell said.

Overall, "it was very pleasing and reassuring" to see that the fusion community has responded wholeheartedly to the significant expansion of the ITER R and D program on disruption mitigation," he said. "Substantial challenges remain in integrating the progress already visible into a reliable disruption mitigation methodology for ITER," he added, "but the quality of the results presented to the workshop gave confidence that considerable progress has been made."

Related Links
Princeton Plasma Physics Laboratory
Powering The World in the 21st Century at Energy-Daily.com

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