Tokamak nuclear fusion devices use magnetic fields to confine plasma and create the conditions that make fusion reactions possible. Instabilities in these magnetic fields called “edge localized modes” (ELMs) can damage the internal surfaces of tokamaks by exposing them to direct contact with plasma. Researchers respond with a technique called “resonant magnetic perturbation” (RMP). This technique modifies the magnetic confinement field and suppresses ELMs. However, many researchers believe the technique reduces plasma confinement. Now, researchers have applied RMPs to discharges that were deliberately contaminated with helium, the waste byproduct of deuterium-tritium fusion reactions. This approach pushed helium out of the plasma twice as effectively as not using RMP at all.
The new research suggests that future fusion reactors that use RMP for the original purpose of suppressing edge instabilities might also benefit from a large increase in the rate at which helium waste is expelled. This increase in helium exhaust means RMP could help keep internal core regions of the plasma undiluted and hot. These factors would help support fusion reactions. Researchers are now working to identify the physics involved and how to optimize this effect for future fusion devices.
This research, conducted at the DIII-D National Fusion Facility, a Department of Energy Office of Science user facility, is promising because it suggests additional, potentially unanticipated benefits to future fusion devices from the use of RMPs beyond suppression of ELMs. While RMPs can reduce the unwanted confinement of waste helium in the plasma, they also reduce the confinement of heat energy necessary for fusion. The ratio of these effects—the loss of helium to the loss of heat energy—accounts for these competing effects, and is a figure that fusion plasmas must optimize to maintain high fusion gain. The results of this initial work already show net improvement in this ratio, an encouraging sign for future work to understand the effect of RMPs on fusion plasmas. The findings will be important as work progresses on future tokamak fusion devices such as ITER and DEMO.
University of Wisconsin-Madison
This work is supported by the U.S. Department of Energy Office of Science, Office of Fusion Energy Sciences, using the DIII-D National Fusion Facility, a DOE Office of Science user facility.
Hinson, E.T. et al. “Enhanced helium exhaust during edge-localized mode suppression by resonant magnetic perturbations at DIII-D.” Nuclear Fusion 60, 054004 (2020). [DOI: https://dx.doi.org/10.1088/1741-4326/ab7d50]