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 radiate into a spherical tokamak, a magnetic facility designed to reproduce the fusion energy that powers the sun and stars, the less the central temperature rises.
A great mystery
“Typically, the more beam power you put in, the higher the temperature gets,” said Stephen Jarden, head of the theory and computational science group that performed the calculations and lead author of a proposed explanation published in Physical examination letters. “So it was a big mystery: Why is this happening?”
Solving the mystery could contribute to worldwide efforts to create and control Earth’s fusion 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 a massive amount of energy.
This mechanism may be common in spherical tokamaks, he said, and the possible destruction of surfaces should be taken into account when planning future spherical tokamaks.
Jardin plans to continue researching the process to better understand the breakdown of magnetic surfaces and why it seems more likely in spherical than conventional tokamaks. And he was invited to present his findings at the annual meeting of the American Physical Society-Division of Plasma Physics (APS-DPP) in October, where early-career scientists could be recruited to tackle the problem and iron out the details. for a proposed mechanism.
Through recent high-resolution computer simulations, Jardin and colleagues have shown what can cause the temperature to remain flat or even drop at the center of the plasma, fueling fusion reactions even as more heating power is emitted. Increasing the power also increases the pressure in the plasma to the point where the plasma becomes unstable and the motion of the plasma equalizes the temperature, they found.
“These simulations likely explain an experimental observation made more than 12 years ago,” Jardine said. “The results show that care must be taken when designing and conducting spherical tokamak experiments to ensure that the plasma pressure does not exceed certain critical values at certain locations in [facility]”, he said. “And now we have a way to quantify those values through computer simulations.”
The findings highlight a key hurdle researchers must avoid when seeking to replicate fusion reactions in spherical tokamaks — devices shaped more like cored apples than the more widely used conventional donut-shaped tokamaks. The spherical devices produce cost-effective magnetic fields and are candidates to become pilot fusion power plant models.
The researchers simulated past experiments at the National Spherical Torus Experiment (NSTX), the flagship fusion facility at PPPL that has since been upgraded and where the puzzling behavior of the plasma was observed. The results are largely consistent with those found in the NSTX experiments.
“Through NSTX, we also got the data through a DOE program called SciDAC [Scientific Discovery through Advanced Computing] we developed the computer code we used,” said Jardine. 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 certain locations to break the nested magnetic surfaces formed by the magnetic fields that surround the tokamak to confine the plasma. The decay equalizes the temperature of the electrons inside the plasma and thus keeps the temperature at the center of the hot, charged gas from rising to fusion-related levels.
“So what we think now is that as you increase the power of the injected beam, you also increase the plasma pressure, and you get to a certain point where the pressure starts to destroy the magnetic surfaces near the center of the tokamak,” Jardin said, “and therefore the temperature stops rising.”
Support for this research came from the DOE Office of Science and the DOE SciDAC program. Co-authors of the paper include PPPL physicists Walter Guttenfelder, Stefano Munnaretto, and Stan Kay, head of the upgraded NSTX study.
Materials provided by DOE/Princeton Plasma Physics Laboratory. Original written by John Greenwald. Note: Content may be edited for style and length.