Scientists propose solution to long-puzzling fusion problem
The paradox shocked scientists at the U.S. Department of Energy (DOE)’s Princeton Plasma Physics Laboratory (PPPL) more than a dozen years ago. The more heat they radiated into a spherical tokamak, a magnetic facility designed to reproduce the fusion energy that powers the sun and stars, the less its central temperature rose.
“Normally, the more jet power you put in, the higher the temperature gets,” said Stephen Jardin, head of the theory and computational science group that performed the calculations, and lead author of a proposed statement published in Physical Assessment Letters† “So this was a big mystery: why is this happening?”
Solving the mystery could contribute to efforts around the world to create and control nuclear fusion on Earth to produce a virtually inexhaustible source of safe, clean and carbon-free energy to generate electricity while fighting climate change . Fusion combines light elements in the form of plasma to release massive amounts of energy.
Using recent high-resolution computer simulations, Jardin and colleagues have shown what allows the temperature to remain flat or even drop in the center of the plasma fueling fusion reactions, even as more heating power is radiated inward. Increasing power also increases the pressure in the plasma to the point where the plasma becomes unstable and the plasma motion smoothes out the temperature, they found.
“These simulations probably explain an experimental observation made more than 12 years ago,” Jardin said. “The results indicate that when designing and conducting spherical tokamak experiments, care should be taken to ensure that the plasma pressure reaches certain critical values at certain locations in the [facility]’, he said. “And we now have a way to quantify these values through computer simulations.”
The findings highlight an important hurdle researchers must avoid in reproducing fusion reactions in spherical tokamaks — devices that are more similar to cored apples than more commonly used donut-shaped conventional tokamaks. Spherical devices produce cost-effective magnetic fields and are candidates to become models for a pilot fusion power plant.
The researchers simulated previous experiments at the National Spherical Torus Experiment (NSTX), the flagship fusion facility at PPPL that has since been upgraded, where the puzzling plasma behavior had been observed. The results were largely consistent with those of the NSTX experiments.
“We got the data through NSTX and through a DOE program called SciDAC [Scientific Discovery through Advanced Computing] we developed the computer code we used,” said Jardin.
PPPL physicist and co-author Nate Ferraro said, “The SciDAC program was absolutely essential in developing the code.”
The discovered mechanism caused increased pressure at certain locations to fracture the nested magnetic surfaces formed by the magnetic fields that wrap around the tokamak to trap the plasma. The breakup dropped the temperature of the electrons in the plasma and prevented the temperature at the center of the hot, charged gas from rising to fusion-relevant levels.
“So what we’re thinking now is that when you increase the jet power injected, you also increase the plasma pressure, and you get to a certain point where the pressure starts destroying the magnetic surfaces near the center of the tokamak,” Jardin said, “and that’s why the temperature doesn’t rise anymore.”
This mechanism may be common in spherical tokamaks, he said, and the possible destruction of surfaces should be considered when planning future spherical tokamaks.
Jardin plans to explore the process further to better understand the destruction of magnetic surfaces and why it seems more likely in spherical than conventional tokamaks. He has also been invited to present his findings at the American Physical Society-Division of Plasma Physics (APS-DPP) annual meeting in October, where early scientists may be recruited to address the issue and provide details of the proposed mechanism. .
State-of-the-art computer code could advance efforts to harness fusion energy
SC Jardin et al, Ideal MHD Limited Electron Temperature in spherical tokamaks, Physical Assessment Letters (2022). DOI: 10.1103/PhysRevLett.128.245001
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