New small angle slot (SAS) divertor offers a promising solution for power exhaust in future fusion power plants
San Diego, July 11, 2019 – Scientists at the DIII-D National Fusion Facility have taken a step toward advancing fusion energy to a practical reality with a technology that enables more effective cooling of fusion plasmas, reducing the risk of damage to the interior walls of fusion tokamaks.
The development is known as a small angle slot (SAS) divertor. Described in an article published this week in the journal Nuclear Fusion, it takes a new approach for reducing the intensity of heat and particles that come into contact with the tokamak walls during operation.
“Advanced divertor solutions are critical to future fusion reactors because there are limits to the energy that plasma-facing components can absorb,” said General Atomics (GA) physicist Houyang Guo, who led the multi-institutional team that developed the SAS technology at DIII-D. “To make such reactors reliably operate for many years, we need ways to more efficiently dissipate heat for steady-state operation.”
DIII-D is the largest magnetic fusion research facility in the U.S. and is operated as a national user facility by GA for the U.S. Department of Energy 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. Plasma is the fourth state of matter in which electrons are stripped from the atoms, producing a highly ionized “soup” of nuclei and electrons that can be controlled by magnetic fields. 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.
The heat and particle flow along the magnetic field lines in the edge region is intense enough to damage the inside surfaces of the tokamak, as heat levels can exceed those found in a rocket engine nozzle. To avoid this damage, the heat flow must be dissipated, and the escaping plasma made several orders of magnitude cooler, before it reaches the material surfaces.
Various methods have been developed to achieve this cooling, but if these methods reduce temperatures in the core region, fusion performance will be degraded. Thus, efficient divertor cooling methods compatible with high core fusion performance will be critical for sustained operation of future fusion power plants.
Solutions to this challenge, however, have been elusive.
One approach for lowering edge temperatures and dissipating the heat flux is to produce a “gas cushion” between the incoming heat flux and the divertor surface. In 2017, the team led by Guo developed a model suggesting that this effect could be optimized by directing the open magnetic field lines into a specially designed divertor structure that features a slightly angled, slot-shaped geometry. The goal was to trap cold particles in this slot, thereby producing a gas cushion that allows maximum cooling across the area subject to the intense heat flux.
Recent experiments on DIII-D have validated the concept developed by the modeling. The experiments showed several key improvements compared to other divertor designs.
“We found that SAS allows for effective cooling of hot plasmas at the plasma-materials interface over a wider range of ‘high confinement,’ or ‘H-mode’ plasmas, which are used to model operational conditions on future fusion reactors,” Guo said. “In addition, the performance of the core plasma was either preserved or the usual deterioration was reduced.”
Work is ongoing at DIII-D to further optimize the SAS, and additional research is necessary to better understand plasma behavior in the divertor under more reactor-like conditions. However, the success of the SAS design bodes well for addressing the significantly larger cooling demands that will be present with future fusion reactors, and it represents another positive step on the road to commercial fusion energy.
About General Atomics: General Atomics pioneers technologies with the potential to change the world. 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.