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Direct observation of ion acceleration from a beam-driven wave in a magnetic fusion experiment

Nature Physics volume 15, pages 281–286 (2019)Cite this article

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Abstract

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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Fig. 1: Illustration of the FRC.
Fig. 2: Basic plasma and NBI parameters, and a comparison of the measured and calculated neutron emission rates.
Fig. 3: Summary of measurements of fluctuations in the electron density and magnetic field associated with the beam-driven mode.
Fig. 4: Measurements of the energy spectra of charge exchange neutrals at the edge of the plasma reveal ion acceleration coincident with a rise in neutron emission.
Fig. 5: Simulation of the beam-plasma system reveals the three features observed in experiment: fluctuations at harmonics of the ion cyclotron frequency, a high-energy tail on the main ion species and enhanced neutron production.
Fig. 6: Controlling mode activity by reducing velocity space gradients.

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Data availability

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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Acknowledgements

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.

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Authors and Affiliations

  1. TAE Technologies, Inc., Foothill Ranch, CA, USA

    R. M. Magee, A. Necas, R. Clary, S. Korepanov, S. Nicks, T. Roche, M. C. Thompson, M. W. Binderbauer & T. Tajima

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  1. R. M. Magee
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  2. A. Necas
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  7. M. C. Thompson
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  8. M. W. Binderbauer
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  9. T. Tajima
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Contributions

R.M.M. contributed neutron measurements and neutron calculations, created all the figures and wrote the majority of the text. A.N. ran the PIC simulations, provided output data, and provided text for the ‘Simulation and theory’ section. R.C. provided NPA measurements. S.K. provided neutral beam injection. S.N. provided the analytical theory for benchmarking. T.R. and M.C.T. provided magnetic data. M.W.B. is the driving force behind the C-2U device and helped edited the text. T.T. provided theoretical interpretation of the experimental and simulation data and contributed significantly to the editing of the text.

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Correspondence to R. M. Magee.

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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

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