The paradox startled scientists at the US Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) more than a dozen years ago. The more heat they radiated into a spherical tokamak, a magnetic device designed to reproduce the fusion energy that powers the sun and stars, the less the central temperature rose.
“Typically, the more beam power you put in, the higher the temperature gets,” said Stephen Jardin, leader of the theory and computational science group that did the calculations and lead author of a proposed explanation published in Physical Verification Letters. “So that was a big mystery: Why is this happening?”
Solving the mystery could contribute to efforts around the world to create and control fusion on Earth to create a virtually endless 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 unleash massive amounts of energy.
Through the latest high-resolution computer simulations, Jardin and colleagues showed what can cause the temperature at the center of the plasma that drives fusion reactions to remain flat or even decrease, even when more heating power is injected. Increasing the power also increases the pressure on the plasma, to the point where the plasma becomes unstable and plasma motion flattens the temperature, they found.
“These simulations likely explain an experimental observation made over 12 years ago,” Jardin said. “The results indicate that when designing and operating spherical tokamak experiments, care must be taken to ensure that the plasma pressure at certain points in the plasma does not exceed certain critical values [facility]he said. “And we now have a way to quantify those values through computer simulations.”
The results highlight a key hurdle researchers must avoid when attempting to reproduce fusion reactions in spherical tokamaks — devices shaped more like pitted apples than the more commonly used conventional donut-shaped tokamaks. Spherical devices generate inexpensive magnetic fields and are candidates to become models for a pilot fusion power plant.
Researchers simulated previous experiments at the National Spherical Torus Experiment (NSTX), the flagship fusion facility at PPPL, which has since been upgraded, and where the enigmatic plasma behavior was observed. The results were broadly 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 that we use,” Jardin said.
Physicist and co-author Nate Ferraro of PPPL said, “The SciDAC program was absolutely instrumental in developing the code.”
The discovered mechanism caused increased pressure at specific locations to break up the nested magnetic surfaces formed by the magnetic fields wrapping around the tokamak to confine the plasma. The breakup flattened the temperature of the electrons in the plasma, thereby preventing the temperature at the center of the hot, charged gas from rising to levels relevant to fusion.
“What we’re thinking now is that as you increase the injected beam power, you also increase the plasma pressure, and you reach a certain point where the pressure starts to destroy the magnetic surfaces near the center of the tokamak,” Jardin said, “and that’s why the temperature doesn’t rise any more.”
This mechanism could be common in spherical tokamaks, he said, and the potential destruction of surfaces needs to be considered when planning future spherical tokamaks.
Jardin plans to study the process further to better understand the destruction of magnetic surfaces and why it appears more likely in spherical than conventional tokamaks. He was also invited to present his findings at the American Physical Society-Division of Plasma Physics (APS-DPP) annual meeting in October, where early-stage researchers could be recruited to delve into the topic and flesh out the details of the proposed mechanism.
State-of-the-art computer code could fuel efforts to harness fusion energy
SC Jardin et al, Ideal MHD Limited Electron Temperature in Spherical Tokamaks, Physical Verification Letters (2022). DOI: 10.1103/PhysRevLett.128.245001
Provided by Princeton Plasma Physics Laboratory
Citation: Scientists suggest solution to long-puzzling fusion problem (2022, July 13), retrieved July 14, 2022 from https://phys.org/news/2022-07-scientists-solution-long-puzzling-fusion-problem .html
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