Scientists at a US government plasma lab have discovered a missing component in nuclear fusion equations that could speed up development of a working reactor.
Specifically, the discovery could improve the design of the donut-shaped fusion reactors known as tokamaks.
Nuclear fusion is the process of joining two atomic nuclei together in order to form a single, larger nucleus whilst releasing energy in the process. It’s the same process that powers our sun, where hydrogen atoms are fused together to form helium.
Scientists have been working on nuclear fusion reactors for decades since fusion promises to be a clean, safe, and virtually limitless energy source. However, scientists have yet to achieve a stable reaction that gives out more energy than it consumes.
Tokamaks work by creating a material known as plasma, in which an element—usually hydrogen—is heated so much that it becomes an electrically-charged soup of electrons and atomic nuclei. Powerful magnets then contain this plasma into a safe, stable flow, creating conditions where fusion should be possible.
In order to perfect tokamak designs, scientists use computer models to predict how the plasma will act under certain conditions. Now, scientists at the Princeton Plasma Physics Laboratory (PPPL), a US Department of Energy lab managed by Princeton University, have found that the equations used to create these computer models have been missing an important detail—resistivity.
Resistivity refers to the ability of any material or substance to prevent the flow of electricity. Just like how a rock will move more easily through air than through water, electricity moves more easily through some things than others.
In a study, PPPL scientists have found that resistivity is an important property of plasma since it can cause instabilities known as edge-localized modes (ELMs), which are essentially small eruptions of plasma. If left unchecked, these eruptions could cause damage to fusion reactors which would mean they’d need to be taken offline more often for repairs.
“We need to have confidence that the plasma in these future facilities will be stable without having to build full-scale prototypes, which is prohibitively expensive and time-consuming,” said Nathaniel Ferraro, a PPPL researcher, in a press release. “In the case of edge-localized modes and some other phenomena, failing to stabilize the plasma could lead to damage or reduced component lifetimes in these facilities, so it’s very important to get it right.”
This is where the computer models come in. By adjusting the models so that they incorporate resistivity, Ferraro and colleagues, including lead study author and PPPL researcher Andreas Kleiner, found that the models more accurately predicted observations.
Having accurate computer models is important, since it means scientists can use time and money as efficiently as possible to build a reactor they know will probably work well, rather than wasting resources on a trial-and-error approach.
“You want a model that is simple enough to calculate but complete enough to capture the phenomenon you are interested in,” said Ferraro. “Andreas found that resistivity is one of the physical effects that we should include in our models.”
The PPPL study was published in the journal Nuclear Fusion in May.
Future research will investigate which specific tokamak properties cause these resistive plasma eruptions to occur, which could result in improved designs.
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