New discoveries about moving electricity could improve fusion devices

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Researchers from the Princeton Plasma Physics Laboratory (PPPL) of the United States Department of Energy (DOE) have found that updating a mathematical model to include a physical property known as resistivity could lead to improved design of ring-shaped fusion facilities known as tokamaks.

“Resistivity is the property of any substance that inhibits the flow of electricity,” said PPPL physicist Nathaniel Ferraro, one of the collaborating researchers. “It’s a bit like the viscosity of a fluid, which prevents things from passing through it. For example, a stone will move slower in molasses than in water, and slower in water than in air.

Scientists have discovered a new way resistivity can cause instabilities in the plasma edge, where temperatures and pressures rise sharply. By incorporating resistivity into models that predict the behavior of plasma, a soup of electrons and atomic nuclei that makes up 99% of the visible universe, scientists can design systems for future fusion facilities that make plasma more steady.

“We want to use this knowledge to understand how to develop a model that allows us to plug in certain characteristics of the plasma and predict whether the plasma will be stable before doing an experiment,” said Andreas Kleiner, a PPPL physicist who was the lead author of the project. an article reporting the results nuclear fusion. “Basically in this research, we saw that resistivity was important and our models should include that,” Kleiner said.

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 – and generates huge amounts of energy . Scientists seek to harness fusion on Earth for a virtually inexhaustible supply of energy to generate electricity.

Scientists want the plasma to be stable because instabilities can lead to plasma flares called edge-localized modes (ELMs) that can damage the tokamak’s internal components over time, requiring those components to be replaced more frequently. Future fusion reactors will have to operate non-stop for repair, however, for months at a time.

“We need to be sure that the plasma from these future facilities will be stable without having to build large-scale prototypes, which is prohibitively expensive and time-consuming,” Ferraro said. “In the case of edge-localized modes and certain other phenomena, failure to stabilize the plasma could lead to damage or reduced component life in these installations, so getting it right is very important. .”

Physicists use a computer model called EPED to predict the behavior of plasma in conventional tokamaks, but the predictions produced by the code for a variety of plasma machines called spherical tokamaks are not always accurate. Physicists are studying spherical tokamaks, compact facilities such as the National Spherical Tokamak Experiment-Upgrade (NSTX-U) at PPPL that resemble hollowed-out apples, as a possible design for a pilot fusion plant.

Using the powerful computers at the National Energy Research Scientific Computing Center, a DOE Office of Science user facility at Lawrence Berkeley National Laboratory in Berkeley, Calif., Kleiner and the team tried to add resistivity to a model of plasma and found that the predictions were beginning to match. observations.

“Andreas looked at data from several previous plasma discharges and found that the resistive effects were very significant,” said Rajesh Maingi, head of PPPL’s ​​tokamak experimental science department. “The experiments showed that these effects were probably the cause of the ELMs we were seeing. The improved model could show us how to modify plasma profiles in future installations to get rid of ELMs.

Using these types of computer models is standard procedure for physicists to predict what the plasma will do in future fusion machines and to design those machines so that the plasma behaves in ways that make fusion more likely. “Basically, a model is a set of mathematical equations that describes the behavior of plasma,” Ferraro said.

“And all the models incorporate assumptions. Some models, like the one used in this research, describe plasma as a fluid. In general, you can’t have a model that includes all of the physics. It would be too difficult to solve. You want a model that is simple enough to calculate but comprehensive enough to capture the phenomenon you are interested in. Andreas discovered that resistivity is one of the physical effects that we should include in our models.

This research builds on previous calculations by Kleiner and others. It adds to these results by analyzing more discharges produced by NSTX, the machine before NSTX-U, and by studying scenarios where ELMs do not occur. The research also helped scientists determine that instabilities caused by resistivity are driven by plasma current, not pressure.

Future research will aim to determine why resistivity produces these types of instabilities in spherical tokamaks. “We don’t yet know what property causes the resistive modes to appear at the edge of the plasma. It could be the result of the geometry of the spherical torus, the lithium that coats the interior of some facilities, or the elongated shape of the plasma,” Kleiner said. “But this needs to be confirmed by further simulations.”

– This press release was originally posted on the Princeton Plasma Physics Laboratory website

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