First observation of “E×B drift” effects on plasma edge suggest pathway to improved core-edge integration
To operate effectively over long periods, fusion reactors must be able to remove heat and by-products from the fusion reaction without affecting stable operation of the device. In this study, DIII-D researchers showed for the first time that a specific particle-transport mechanism known as E×B (“E-cross-B”) drift can be used to efficiently remove excess heat and impurity ions from the edge of a plasma without negatively affecting fusion performance in the plasma core. This is an important result for addressing challenges facing stable operation of future fusion devices, such as the ITER experiment under construction in France and the fusion power plants that will follow it.
“This discovery is quite significant, because it represents a key element in understanding particle flow around the edge of the plasma,” said GA researcher Huiqian Wang, who led the study. “It suggests that we have a pathway toward addressing the challenges of maintaining a stable plasma edge and a high-performance core in future devices like ITER.”
Economical fusion energy would be one of the greatest achievements in human history, representing a potentially unlimited source of clean, safe, always-on electricity.
DIII-D is the largest magnetic fusion research facility in the U.S. and is operated by General Atomics as a national user facility for the U.S. Department of Energy’s Office of Science. The heart of the facility is a tokamak that uses powerful electromagnets to produce a doughnut-shaped magnetic bottle for confining a fusion plasma. In DIII-D, plasma temperatures more than 10 times hotter than the Sun are routinely achieved. At such extremely high temperatures, hydrogen isotopes can fuse together and release energy. (See Fusion Energy 101 explainer below for more detail on how fusion works.)
However, plasma particles and their associated heat eventually leak through the tokamak’s magnetic fields. For this reason, tokamaks incorporate a device called a divertor that removes stray particles and heat from the edge regions of the plasma. One challenge is that when the plasma contacts the tokamak interior in and around the divertor, new impurity particles can be liberated and flow into the magnetic fields. If these particles enter the plasma core, they can degrade fusion performance. Because the exact mechanisms of particle movement in the divertor region are not yet fully understood, DIII-D scientists have devoted considerable effort toward studying how these particles, the divertor surfaces, and the magnetic fields interact.
E×B drift is a mechanism of particle movement and is related to the plasma electric field created by the temperature drop at the plasma edge. Wang’s team found experimentally that E×B drift can cause particle flow away from the divertor and up the edge of the magnetically confined core plasma through the boundary layer. The edge region of the high-confinement plasma, known as the pedestal, is a region of steep temperature and pressure gradients. In these experiments, the E×B drift flow caused a localized flattening or “shelf” of density in the pedestal. They also found that this shelf reduced the intensity of plasma instabilities in the edge region that are known as edge-localized modes (ELMs). Essentially, reducing the density gradient in the pedestal reduced the intensity of the ELMs.
The research was aided by new diagnostic capabilities on DIII-D that have much better temporal and spatial resolution. Using significantly improved boundary profile measurements, this density shelf has been broadly seen in the high-power and high-pedestal-temperature regimes that will be used in reactor-grade plasmas, such as in ITER and later devices. That makes this discovery potentially significant for the stable and efficient operation of future fusion power plants.
The researchers also found that they could enhance the effect of the E×B drift on the density shelf by increasing the plasma heating power. This increased the edge temperature gradient, thus increasing the electric field and the drift effects.
“An efficient divertor solution compatible with a high-performance core is highly desired for large fusion devices like ITER,” said DIII-D Director David Hill. “These results are significant for improving understanding of the underlying physics of core-edge integration and laying the groundwork for high-performance operation of future reactor plasmas.”
About General Atomics: General Atomics pioneers technologies with the potential to change the status quo. Since the dawn of the atomic age, GA’s innovations have advanced the state of the art across the full spectrum of science and technology – from nuclear energy and defense to medicine and high-performance computing. Behind a talented global team of scientists, engineers, and professionals, GA delivers safe, sustainable, and economical solutions to meet growing global demands.
About the DIII-D National Fusion Facility. DIII-D is the largest magnetic fusion research facility in the U.S. and 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
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Fusion Energy 101
- Nuclear fusion occurs when light elements such as hydrogen are brought together at extremely high temperatures and pressures, causing the nuclei to fuse into heavier elements such as helium. This process powers stars like our sun and releases vast amounts of energy.
- Fusion differs from nuclear fission, where heavy elements split into lighter elements, releasing energy. Fission is the process used in existing commercial nuclear power plants.
- Fusion power plants will be fueled by a mixture of hydrogen isotopes: deuterium (the nucleus comprises a proton and a neutron) and tritium (the nucleus comprises a proton and two neutrons).
- Deuterium can be extracted from seawater and tritium can be created from small amounts of lithium in the reactor, making fusion a nearly limitless, carbon-free source of energy that leaves no long-lived radioactive waste.
- One way to achieve fusion on earth is in a tokamak (a doughnut-shaped metal vacuum chamber) surrounded by extremely powerful magnets that create strong magnetic fields.
- Creating fusion in a tokamak requires that the fuel be converted into a plasma by heating it to over 100 million degrees.
- Plasma is the “fourth state of matter” in which electrons are stripped from the nuclei of their atoms. This creates an electric charge that allows the plasma to be confined by the magnetic fields within the tokamak without touching the inside walls.
- Plasma is the most common state of matter in the universe. It can be seen all around us in places such as the stars, lightning, and fluorescent light bulbs.
- Tokamaks are inherently safe – any loss of control causes the plasma to touch the inside wall, immediately cooling it and stopping the fusion reaction.