Efficiently heating a magnetically confined plasma to thermonuclear temperatures remains a central issue in fusion energy research. One well-established technique is to inject beams of neutral particles into the plasma, a process known as neutral beam injection. In the classical picture, fast ions generated from neutral beam injection predominantly heat electrons as they are slowed by friction. This electron heat is then collisionally coupled to the plasma ions, which comprise the fusion fuel. Fast ions can also drive plasma waves, which divert energy from the fuel and can degrade confinement. Here we present new observations from a field reversed configuration plasma in which a beam-driven wave in the open field line region couples directly to fuel ions, drawing a high-energy tail on subcollisional timescales that dramatically enhances the fusion rate. This mode therefore allows the beam energy to bypass the electron channel and does so without having a deleterious effect on global plasma confinement. Our results demonstrate a means of directly and non-destructively coupling energy from fast ions to plasma ions, which may pave the way for improved neutral beam injection heating efficiency or the prevention of ash accumulation with alpha channelling.
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The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.
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The authors thank the investors for their support of TAE Technologies and the TAE and Budker teams for their contributions to this project. Special thanks go to E. Granstedt and E. Trask for help with the design of the experiment. This research used resources of the Oak Ridge Leadership Computing Facility, which is a DOE Office of Science User Facility supported under contract no. DE-AC05-00OR22725.
TAE Technologies, Inc. is a private corporation owned and financially supported by its shareholders. Some or all of the authors of this manuscript may have a financial interest in the company.
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Magee, R.M., Necas, A., Clary, R. et al. Direct observation of ion acceleration from a beam-driven wave in a magnetic fusion experiment. Nat. Phys. 15, 281–286 (2019). https://doi.org/10.1038/s41567-018-0389-0