One major problem with creating a controlled fusion reaction that could be used to generate power is confinement of the superheated plasma. Powerful magnetic fields are used in fusion reactors to confine the plasma and prevent it from striking the walls of the reaction chamber which can quench the fusion reaction and even damage the walls of the reaction vessel.
Physicists at the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL) have been working to verify computer simulations of energy loss caused by turbulent transfer of plasmas in fusion experiments. General Atomics (GA) in San Diego has developed sophisticated computer models which are being compared with the experimental findings from the compact—or "low-aspect ratio"—National Spherical Torus Experiment-Upgrade (NSTX-U).
Low-aspect ratio tokamaks have a shape like an apple with the core removed instead of the donut-shape of common tokamaks. Physicist Walter Guttenfelder is the lead author of a Nuclear Fusion paper that reports the findings of the PPPL researchers. He said, “We have state-of-the-art codes based on sophisticated theory to predict transport. We must now validate these codes over a broad range of conditions to be confident that we can use the predictions to optimize present and future experiments.”
Analysis of the ion transport in the NSTX-U experiments revealed that a major reason for the energy losses was turbulence. This caused the transport of electrons be considered anomalous which means that the electrons spread rapidly. The GA computer model predicted that energy losses could be attributed to a combination of three different types of turbulence.
The observation of energy losses in the NSTX-U marks the beginnings of a new phase in developing models of transport in low-aspect ratio tokamaks. The PPPL research team’s next goal is to identify the mechanisms that give rise to anomalous electron transport in a compact tokamak. Simulations predict that energy losses in these tokamaks are the result of two types of turbulence with relatively long wavelengths as well as a third type of turbulence with wavelengths that are tiny compared to the other two types of turbulence.
It is difficult to simulate the combined effect of all three of the types of turbulence. Normally, researchers simulate different wavelengths of turbulence separately. Researchers at MIT have utilized significant time on their supercomputer to carry out multi-scale simulations.
Next researchers must test additional simulations in order to arrive at a more complete agreement between computer simulation of transport and the results of experiments on transport in low-aspect ration tokamaks. Measurements of actual turbulence in low-aspect ratio tokamaks will be done by the University of Wisconsin-Madison authors of the paper published the in Nuclear Fusion journal. This will help to further refine the predictions made by the computer models.
Billions of dollars are being poured into fusion research by national governments and private companies. If commercial fusion can be achieved, it will have a major impact on the global energy market. But this will only be possible if the problems with leaking electrons in confined plasmas can be solved.