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Preventing damaging heat bursts at the edge of fusion plasmas

In a fusion energy device that creates a “star in a jar,” bursts of intense heat can damage the walls of the jar that holds the superhot plasma fueling fusion reactions. Fusion scientists now have shown that an innovative new model can serve as the basis for predicting the suppression of such outbursts in the DIII-D National Fusion Facility that General Atomics operates in San Diego. The scientists are now using the model to predict the conditions for suppressing these intense bursts in ITER, the international fusion device being built in the south of France, that will demonstrate the practicality of fusion energy.

Successful simulations

The scientists, working at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) and General Atomics, have successfully simulated experiments performed on the doughnut-shaped DIII-D tokamak. The research has revealed how weak magnetic ripples act to suppress the intense bursts called edge localized modes (ELMs) on DIII-D plasmas that mimic plasmas that will form in ITER. The researchers now aim to extrapolate their discovery to ITER itself.  

Fusion, the power that drives the sun and stars, combines light elements in the form of plasma — the hot, charged state of matter composed of free electrons and atomic nuclei — to generate massive amounts of energy. Scientists around the world are seeking to replicate fusion on Earth for a virtually limitless supply of safe and clean energy to generate electricity.

The simulations, performed by PPPL postdoctoral researcher Qiming Hu, explored how the application to the plasma of small magnetic ripples called resonant magnetic perturbations prevent the occurrence of these ELM bursts. The ripples act like the valve on a pressure cooker to release just enough heat and particles to prevent the bursts. But an understanding of the physics behind this technique has been missing.

Magnetic ripples

Hu's work demonstrates that the magnetic ripples form tiny magnetic bubbles, or magnetic islands, just inside the plasma edge that allow the excess heat and particles to escape. His simulations have for the first time reproduced the experimental conditions that have suppressed the bursts in DIII-D.

The simulations show that the magnetic islands develop when the ripples act as a brake on the edge flow, or rotation, of the plasma. When the flow comes to a halt the islands emerge and serve as a release valve that keeps ELMs from occurring. A paper by Hu and PPPL physicist Raffi Nazikian that details the success of the model is published in the journal Physics of Plasmas and selected as an Editor's Pick.

Nazikian, a lead author in the study and Hu's supervisor, said that “Hu's simulations have proven remarkably accurate in capturing key features of ELM suppression in the DIII-D tokamak, raising the possibility that we may have a quantitatively accurate model for predicting ELM suppression in ITER.

“However,” Nazikian added, “a great deal of work is still required to strengthen confidence that our model can reliably predict the conditions for ELM suppression in ITER." Reliable predictions will enable ITER physicists to develop strategies for optimizing the magnetic ripple used for controlling these heat bursts prior to the startup of operations in 2025.

The DOE Office of Science supported this work. The DIII-D National Fusion Facility provided the user facility.

About the DIII-D National Fusion Facility. DIII-D, the largest magnetic fusion research facility in the U.S operated by General Atomics for the DOE, has been the site of numerous pioneering contributions to the development of fusion energy science. DIII-D continues the drive toward practical fusion energy with critical research conducted in collaboration with more than 600 scientists representing over 100 institutions worldwide. For more information, visit www.ga.com/diii-d.

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 single largest 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, visit energy.gov/science.

U.S. Department of Energy
Princeton Plasma Physics Laboratory is a U.S. Department of Energy national laboratory managed by Princeton University.

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