More than a dozen years ago, the physicists at the DOE'sPPPL were startled by the paradoxes. The more heat they beamed into a spherical tokamak, a magnetic facility designed to reproduce the fusion energy that powers the sun and stars, the cooler the central temperature was.
There's a big mystery.
Stephen Jardin, head of the theory and computational science group that performed the calculations, was the lead author of a proposed explanation published in Physical Review Letters. This was a big mystery.
Efforts to create and control fusion on Earth to produce a virtually inexhaustible source of safe, clean and carbon-free energy to generate electricity while fighting climate change could be helped by solving the mystery. It is possible to combine light elements in a way that releases a lot of energy.
Jardin and colleagues used recent high-resolution computer simulations to show what can happen if more heating power is beamed in. They found that increasing the power causes the plasma to become unstable and flatten out the temperature.
Jardin said that the simulations likely explain an experiment made over a decade ago. The results show that when designing and operating spherical tokamak experiments, care must be taken to ensure that the pressure doesn't go over critical values. We can use computer simulations to quantify these values.
A key hurdle for researchers to avoid when trying to reproduce fusion reactions in spherical tokamaks is the device's shape. Magnetic fields produced by spherical devices can be used as a model for a fusion power plant.
The National Spherical Torus Experiment (NSTX), the flagship fusion facility atPPPL that has since been upgraded, was the subject of a simulation. The results were similar to the ones found on the experiments.
Jardin said that they developed the computer code after getting the data through the DOE program.
The co-author of the book said that the program was very important in the development of the code.
The mechanism is discovered.
The discovered mechanism caused heightened pressure at certain locations to break up the nested magnetic surfaces. The center of the hot, charged gas was kept from rising to fusion due to the flattening of the temperature of the electrons inside.
Jardin said that when the injected beam power is raised, the pressure on the magnetic surface of the tokamak will destroy it.
He said that the mechanism might be general in spherical tokamaks and that the destruction of surfaces must be taken into account when planning for future spherical tokamaks.
Jardin plans to investigate the process to better understand why it appears more likely in spherical form than in conventional tokamaks. He will present his findings to the annual meeting of theAPS-DPP in October, where early career scientists can be recruited to flesh out the details of the proposed mechanism.
More information: S. C. Jardin et al, Ideal MHD Limited Electron Temperature in Spherical Tokamaks, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.128.245001 Journal information: Physical Review Letters
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