New findings could help scientists tame damaging heat bursts in fusion reactors

Date:
August 17, 2020
Source:
DOE/Princeton Plasma Physics Laboratory
Summary:
Physicists have discovered a new trigger for edge localized modes
(ELMs) -- instabilities that can halt fusion reactions and damage
the tokamaks that house such reactions.



FULL STORY ========================================================================== Picture strong wind blowing against a tree until it's knocked down. Such
action would mimic the process that causes damaging heat bursts called
edge localized modes (ELMs) to flare up in fusion facilities called
tokamaks, which scientists use to develop on Earth the fusion energy
that powers the sun and stars. Such heat bursts normally occur when the pressure at the edge of the hot plasma gas that fuels fusion reactions
reaches a peak, causing heat to erupt against the walls of the tokamak,
much like a tree finally toppling in a growing wind.


========================================================================== However, researchers at the U.S. Department of Energy's (DOE) Princeton
Plasma Physics Laboratory (PPPL) recently found that the heat bursts
sometimes can occur during the recovery from eruptions -- providing
fresh insight into what triggers the bursts. The findings, based on data collected from past experiments on the DIII-D National Fusion Facility
that General Atomics operates for the DOE in San Diego, California,
could help tame the potentially damaging processes.

Network of waves "Piecing the dots together, we found that a network
of interacting waves suddenly plays a role in the ELMs process during recovery," said Julien Dominski, a PPPL physicist and lead author
of a paper describing the results in Plasma Physics and Controlled
Fusion. "These waves come together and abruptly exchange energy," Dominski said, "causing the 'pedestal' at the edge of the plasma to crash and
release bursts of heat." This network, which consists of triads of waves,
is key to the triggering, Dominski said. "Sometimes the network can be
excited without causing the abrupt crash," he said. "This enabled us to
look into the physics governing this new picture." The findings grow
from efforts to reproduce fusion reactions that combine light elements
in the form of plasma -- the state of matter composed of free electrons
and atomic nuclei that makes up 99 percent of the visible universe - - to generate massive amounts of energy in the sun and stars. Tokamaks are the
most popular devices that scientists around the world use in experiments
to capture and control fusion energy, hoping to create a virtually inexhaustible supply of safe and clean power to generate electricity.

Emerging picture "The new findings are an emerging picture for the
triggering mechanism of ELMs," said Ahmed Diallo, a PPPL physicist and co-author of the paper.

"Discovering this network opens a new chapter in the study of ELMs."
Coinciding with these networks, the physicists found, are perturbations at
the edge of the plasma caused by a device called a neutral beam injector
(NBI) that is used to heat and fuel the plasma. The millisecond intervals between NBI perturbations correspond with the intervals between network occurrences, the scientists noticed. "Making this connection was exciting
and gave us an insight into a new way to trigger ELMs," the authors said.

The researchers next aim to conduct experiments to study how triggers for
ELMs may vary and test the use of NBI beams to control the bursts. Support
for this research comes from the DOE Office of Science.


========================================================================== Story Source: Materials provided by
DOE/Princeton_Plasma_Physics_Laboratory. Original written by John
Greenwald. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. J Dominski, A Diallo. Identification of a network of nonlinear
interactions as a mechanism triggering the onset of edge localized
modes.

Plasma Physics and Controlled Fusion, 2020; 62 (9): 095011 DOI:
10.1088/ 1361-6587/ab9c48 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200817191513.htm

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