Promising approaches to combatting tokamak disruptions presented at PPPL-hosted workshop

Written by
John Greenwald
Aug. 22, 2023

Researchers are launching novel new approaches to combatting powerful disruptions, the greatest challenge facing the production of fusion energy in doughnut-shaped tokamak devices. Experts detailed approaches at a  wide-ranging July 19-21 Theory and Simulation of Disruptions Workshop that drew more than 60 researchers from around the world to the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL).

“Some really exciting ideas are being experimentally fleshed out and theoretically simulated,” said PPPL physicist Amitava Bhattacharjee, who has hosted the annual workshops since 2013 to contend with damaging disruptions on ITER, the international tokamak under development in France. “We now must find out whether these ideas will actually work when implemented in ITER or SPARC ,” he said. SPARC is the new tokamak reactor under construction by Commonwealth Fusion Systems, a spinoff from the Massachusetts Institute of Technology (MIT).

Fusion, the power that drives the sun and stars, releases vast energy by combining 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. Scientists around the world seek to reproduce the process to harvest a virtually inexhaustible supply of safe and clean power.

Shattered pellets

One new approach calls for injecting  shattered pellets of impurities such as argonne and neon in two stages rather than one into the plasma to subdue damaging runaway electron beams produced by tokamak disruptions. Researchers are testing this technique on the U.K.’s Joint European Torus (JET) at the Culham Centre for Fusion Energy. ITER, which is presently set up to use single-stage shattered pellet shots to cope with disruptions, could adapt the two-stage technique if JET finds it to be effective.

The validation of such efforts benefit from the increasing capability of high-fidelity computer codes and the use of more powerful computers, Bhattacharjee said, which is one of PPPL’s strengths. “We are getting closer and closer to the point where computer simulations can begin to be predictive for experiments,” he said. “This is important because ITER will operate in a regime of runaway electrons that has not been realized in any existing experiment.”

While progress has been made,  the fusion community has not yet found a way to completely mitigate  disruptions in tokamaks, the most widely used fusion device throughout the world. “The term we used during this workshop was a disruption ‘resilient’ tokamak rather than a disruption ‘mitigated’ one,” he said. “In other words, we might have to learn ways in which we can live with disruptions rather than mitigate them completely in tokamak fusion reactors.”

Early-career scientific talent flowing into fusion development in universities and national laboratories are adapting the new attitude, he said. “The greatest hope to find a solution to the tokamak disruption problem may be the ingenuity and new ideas that are pursued unrelentingly by experienced researchers who have stayed the course and inspired talented early-career people to join in,” he said.

Workshop speakers ranged from physicists Michael Lehnen, head of the ITER Disruption Mitigation System, to researchers from PPPL, universities and other national laboratories. More than 25 panels detailed a variety of topics such as computer simulations of runaway electrons and the use  of artificial intelligence to forecast disruptions.

The International Atomic Energy Agency now cosponsors this workshop under Lehnen’s leadership, which has become a biennial event.

TSD workshop

Participants in the disruption workshop. PPPL host Amitava Bhattacharjee is second from bottom row at right. (Photo by Michael Livingston.)

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