If you put enough heating power into a confined plasma, then it will self-organize into a state of improved confinement. The amount of input power required for this transition depends on the species of plasma. It takes more power to reach this higher confinement in a proton plasma compared to a deuterium plasma. As we design fusion power plants, being able to access this higher confinement regime is an important part of the system. The mechanism for this behavior is not fully understood, making this one of the mysteries we need to solve on our path to producing fusion energy.
UCLA Ph.D. student Kyle Callahan studied this behavior in DIII-D experiments and identified a counter-intuitive result. Proton plasmas featured a lower impurity level, but that led to increased plasma turbulence, which increased power fluxes to the wall, thereby requiring more input power to reach the improved confinement regime. Deuterium plasmas, by contrast, feature a lower power loss, even as deuterium impacts with the graphite wall cause a larger amount of carbon impurity to get into the plasma.
This work is helping to show how we might optimize controlled impurity introduction to get our plasmas producing the most fusion power possible.
Kyle’s thesis work brought together co-authors from UCLA, General Atomics, Princeton Plasma Physics Laboratory (PPPL), University of Wisconsin-Madison, Fusion and Fission at ORNL, and UC San Diego.
K.J. Callahan, et al., Nuclear Fusion 63, 126009 (2023), https://doi.org/10.1088/1741-4326/acf86c
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