“Super H Mode” experiments demonstrate record fusion performance
San Diego, June 24, 2019 – It’s called “Super-H Mode,” and it could mean a dramatic reduction in the cost and size of future fusion reactors.
In a paper released today in the journal Nuclear Fusion, General Atomics (GA) researchers and collaborators working at the DIII-D National Fusion Facility describe experiments exploring a new advanced operating mode for fusion reactors that could represent a major step toward realizing a nearly limitless source of carbon-free energy.
Super-H Mode, as the researchers dub the approach, allows tokamaks to achieve higher fusion performance than previously possible. In recent experiments operating in and near the Super H-mode regime, researchers have achieved record-breaking values of fusion gain for a device of DIII-D’s size. Fusion gain is the ratio of fusion power generated to heating power.
“Our latest experiments have shown very encouraging results, so it’s becoming clear there is significant potential to move practical fusion energy a step closer to reality,” said Phil Snyder, director of the Theory and Computational Science Group at GA, the San Diego-based contractor that operates DIII-D as a national user facility for the U.S. Department of Energy. “It’s very exciting from both a scientific and a fusion energy perspective.”
Professor Steven Cowley, director of the Princeton Plasma Physics Laboratory, said the new discovery is highly important for the fusion energy community.
"Super-H Mode promises to reduce the cost and scale of future fusion reactors, thereby bringing the realization of fusion power closer," said Cowley, who was not involved in the research. “It couldn’t be more significant.”
A tokamak is a doughnut-shaped device with strong magnetic fields that confine matter at temperatures exceeding 100 million degrees. Inside the tokamak, matter transitions to a plasma state where electrons are stripped from their nuclei. The resulting electrically charged plasma can be shaped and controlled by the magnetic fields. Within a sufficiently hot plasma, atoms collide and fuse together, producing fusion energy in a manner similar to the sun. (See Fusion Energy 101 explainer below for more detail on how fusion works.)
Scientists have been working to develop more effective methods for confining plasmas at the temperatures and pressures necessary for cost-effective fusion energy. Snyder and his colleagues at GA – along with Professor Howard Wilson and his research group at the University of York and Culham Centre for Fusion Energy (UK) – developed the theoretical model that predicted Super H-Mode. The findings developed by the model were confirmed by a series of record-breaking experiments on DIII-D and the Alcator C-Mod tokamak at MIT. The work was conducted by a multi-institutional team from GA, MIT, Princeton, LMU Munich, Lawrence Livermore National Laboratory, University of Wisconsin, College of William and Mary, and the University of California, Irvine.
Super-H Mode works by increasing temperature and pressure in the outer region of the plasma, called the pedestal. The experiments showed – as the theory predicted – that proper tuning of the plasma cross-sectional shape and density leads to pedestal temperatures and pressures that are more than twice as high as those of typical pedestals.
Because plasma conditions in the core – where fusion takes place – are dependent on conditions at the edge, Super-H Mode enables as much as a four-fold increase in fusion performance.
“When our calculations first predicted the existence of the Super-H regime, we thought it might just be a theoretical curiosity,” Snyder said. “But we soon realized that with appropriate control, it could be realized experimentally.”
Though more research needs to be conducted, the advantages offered by Super-H Mode mean commercial fusion reactors could be substantially smaller and less expensive than once thought.
“Fusion energy research historically advances with steady and marked improvements over time,” said David Hill, director of DIII-D. “It is not often you see a significant leap in results like we have seen with Super-H Mode. This discovery has significant ramifications for future fusion energy plants, and we’re excited to see how far it will carry the field forward.”
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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.
- DIII-D, like most scientific experiments exploring fusion, operates with a pure deuterium (or “DD”) fuel, because it is inexpensive and easy to work with. Fusion power plants will employ a deuterium-tritium (or “DT”) fuel mix, because it has a much higher probability of fusing than its DD counterpart.
- The DIII-D plasmas in Super H Mode experiments, if converted to DT fuel, would produce more than 4 million watts of fusion power. This corresponds to a fusion gain, the ratio of fusion power produced to heating power injected, of about ½, the highest ever achieved on DIII-D or other tokamaks of similar size.
- Such high values of fusion gain in a modest-scale device, together with emerging technologies such as high-field superconducting magnets, enhance prospects for developing fusion energy as an essentially limitless and environmentally attractive power source.
- Confinement of the fusion plasma in a tokamak is enabled by three key ingredients: the physical size of the device (characterized by the minor radius), the magnetic field, and the plasma current. Multiplying these three together gives a measure of the “effective size” of the device. Increasing this effective size increases performance, but also increases the cost and technical challenges associated with building and operating the device. Cost-effective fusion power is facilitated by achieving high fusion performance per effective size. Recent DIII-D experiments have achieved ratios of DT equivalent fusion gain per effective size as high as 21 (in units of %/m MA T) for brief periods, and sustained values as high as 8.