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Harnessing energy from plasma requires a precise understanding of its behavior during nuclear fusion to keep it hot, dense and stable. A plasma's edge can become unstable and bulge. A new theoretical model brings the prospect of commercial fusion power closer to reality.
Jason Parisi is a staff research physicist at PPPL. He said, “The model refines the thinking on stabilizing the edge of the plasma for different tokamak shapes.” Parisi is the lead author of three articles describing the new model that were published in the journals Nuclear Fusion and Physics of Plasma. The primary paper focuses on a part of the plasma referred to as the pedestal which is located at the edge. The pedestal is prone to instabilities because the plasma's temperature and pressure often fall sharply across this area.
The new model is important because it is the first to match pedestal behaviors that were seen in the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL) National Spherical Torus Experiment (NSTX). Conventional tokamaks are shaped like donuts. However, NSTX is one of several tokamaks that are shaped more like a cored apple. The difference in tokamak proportions impacts plasma and the pedestal.
Parisi and his team explored the limits of pedestals and investigated how much pressure could be applied to plasma inside a fusion reactor before instabilities appeared. They examined disruptions in the pedestal called ballooning instabilities. These are bulges of plasma that jut out, like the end of a long balloon when squeezed.
Parisi said, “The model is an extension of a model that people have used in the field for maybe 10 years, but we made the ballooning stability calculation a lot more sophisticated.”
To create their model, the researchers looked at the relationship between the pedestal measurements and ballooning instabilities. Parisi said the new model fit well on the very first try. He continued, “I was surprised by how well it works. We tried to break the model to ensure it was accurate, but it fits the data really well.”
The existing model is known as EPED. It is known to work for donut-shaped tokamaks but not for the spherical variety. Parisi said, “We decided to give it a go, and just by changing one part of EPED, now it works really well.” The results also give researchers a better picture of the contrast between the two tokamak designs.
Parisi added, “There is certainly a big difference between the stability boundary for the apple shape and the standard-shaped tokamak, and our model can now somewhat explain why that difference exists.” The findings could help minimize plasma disruptions in fusion reactors.
Tokamaks are designed to increase the pressure and temperature of plasma, but instabilities can thwart those efforts. If plasma bulges out and touches the walls of the reactor vessel, it can erode the walls over time.
Instabilities can also carry energy away from plasma. Knowing how steep a pedestal can be before instabilities occur could assist researchers to find ways to optimize plasmas for fusion reactions based on the proportions of the tokamak.
While Parisi added that it's not yet clear which shape is most advantageous, the model suggests other experiments that would try to exploit the positive aspects of the apple shape and see how much benefit they could provide.
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