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A sustained high-temperature fusion plasma regime facilitated by fast ions

Abstract

Nuclear fusion is one of the most attractive alternatives to carbon-dependent energy sources1. Harnessing energy from nuclear fusion in a large reactor scale, however, still presents many scientific challenges despite the many years of research and steady advances in magnetic confinement approaches. State-of-the-art magnetic fusion devices cannot yet achieve a sustainable fusion performance, which requires a high temperature above 100 million kelvin and sufficient control of instabilities to ensure steady-state operation on the order of tens of seconds2,3. Here we report experiments at the Korea Superconducting Tokamak Advanced Research4 device producing a plasma fusion regime that satisfies most of the above requirements: thanks to abundant fast ions stabilizing the core plasma turbulence, we generate plasmas at a temperature of 100 million kelvin lasting up to 20 seconds without plasma edge instabilities or impurity accumulation. A low plasma density combined with a moderate input power for operation is key to establishing this regime by preserving a high fraction of fast ions. This regime is rarely subject to disruption and can be sustained reliably even without a sophisticated control, and thus represents a promising path towards commercial fusion reactors.

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Fig. 1: Tokamak geometry and the parameter evolution of a FIRE mode.
Fig. 2: Comparison between a FIRE mode and a hybrid mode.
Fig. 3: The ratio of the fast-ion density to the electron density.
Fig. 4: The gyrokinetic simulation results of a FIRE mode.

Data availability

Raw data were generated by the KSTAR team. The data that support the findings of this study are available from the corresponding author upon reasonable request.

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Acknowledgements

We express our gratitude to R. Nazikian at Princeton Plasma Physics Laboratory, H. Zohm at Max-Plank-Institute for Plasma Physics, W. Choe at Korea Advanced Institute of Science and Technology and J. Candy and E. Belli at General Atomics for fruitful discussions. We also thank all members of the KSTAR centre for their support and assistance in our studies. This work was supported by the Ministry of Science and ICT under the Korea Institute of Fusion Energy R&D Program KSTAR Experimental Collaboration and Fusion Plasma Research (KFE-EN2101-12), the National R&D Program through the National Research Foundation of Korea (NRF) funded by the Korea government (Ministry of Science and ICT) (NRF-2021M1A7A4091135), and the National Supercomputing Center with supercomputing resources including technical support (KSC-2020-CRE-0364) and by the US Department of Energy under contract number DE-AC02-09CH11466 (Princeton Plasma Physics Laboratory). We gratefully acknowledge The Research Institute of Energy and Resources and The Institute of Engineering Research at Seoul National University.

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Authors and Affiliations

Authors

Contributions

Y.-S.N., H.H., J.C., Y.H.L., S.J.P. and Y.S.P. conceived the experiments in KSTAR. H.H., J.C., J. Kang, S.H.H., S.J.P., Y.H.L. and Y.-S.N. conducted all the experiments in KSTAR. J.G.B. diagnosed the magnetic perturbations in the experiments using the Mirnov coil measurements. W.H.K. and J.K.L. diagnosed the ion temperature in the experiments using charge-exchange spectroscopy. J .H. Lee (Korea Institute of Fusion Energy and Korean University of Science and Technology) diagnosed the plasma electron and density profiles using TS. K.D.L. diagnosed the electron temperature in the experiments using the ECE. J. Ko diagnosed the radial magnetic pitch angle profile using the MSE. J.J. measured the emission lines from neutralization of ion species to diagnose the impurity intensity as well as the plasma interaction. K.C.L. diagnosed the plasma density using the TCI. J.H.K. diagnosed and analysed the fast-ion properties. M.J.C. and J. H. Lee (Korea Institute of Fusion Energy) diagnosed the temperature fluctuation using the ECE imaging. S.J.P., Y.H.L., C.Y.L. and G.J.C. performed the power balance and the linear gyrokinetic simulations. C.S. performed the nonlinear gyrokinetic simulations and investigated the impacts of fast ions on energy transport through these simulations. T.S.H., J.P.L., C.S., G.J.C., S.M.Y., S.K.K. and Y.-S.N. analysed the simulation results. H.H, S.J.P., J.-K.P., J.S., B.K., J.G., M.S.C., C.S. and Y.-S.N. prepared the manuscript, figures and video. W.C.K. and S.W.Y. supported all of this work as the KSTAR project managers. Y.-S.N. designed and led the whole research including coining the new confinement regime as FIRE mode. All authors analysed the results and contributed to the compilation and review of the manuscript.

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Correspondence to Y.-S. Na.

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Extended data figures and tables

Extended Data Fig. 1 External and internal view of the KSTAR device.

a, The Korean Superconducting Tokamak Advanced Research (KSTAR) device at KFE, Daejeon, Republic of Korea. b, Plasma composite image of the KSTAR vacuum vessel.

Extended Data Fig. 2 Comparison between an NBI only and an ECH applied FIRE mode.

a, Overview of an ECH injected FIRE mode (shot 26043). b, The ion temperature, electron temperature, electron density, toroidal rotation velocity, ion heat diffusivity, and fast ion density profiles in ρn at 4.2 s (NBI only) and 8.7 s (NBI+ECH). The fast ion density profiles calculated by the NUBEAM code and the ion heat diffusivity profiles calculated from the power balance analysis with TRIASSIC incorporating NUBEAM, NCLASS, and ASTRA. The blue shaded region indicates the region of the ITB from the foot to the shoulder while ECH was applied.

Extended Data Fig. 3 Nonlinear gyrokinetic simulation results with the CGYRO code in the case of Ti/Te ~ 0.91.

a, The energy fluxes without considering the fast ions versus the simulated time, b, the energy fluxes with considering the fast ions versus the simulated time. Each energy flux of thermal ions (Qi), fast ions (Qfast), and electrons (Qe) normalised to the gyro-Bohm energy flux (QGB) is coloured by navy, dark cyan, and wine, respectively. Here, the simulation setup and the notations are the same as those used in Fig. 4c and d except the Ti/Te ratio ~ 0.91, where the experimental Ti/Te ratio is 1.42. Note that only latter part of the nonlinear simulation results, indicated by the arrow double end line, should be considered since a certain simulation time requires until the simulated turbulence is saturated by nonlinear effects.

Extended Data Fig. 4 ITB characteristics of a FIRE mode (KSTAR Shot 22663).

a, The time evolution of the thermal ion heat diffusivity in ρn calculated from the power balance analysis with TRIASSIC incorporating NUBEAM, NCLASS, and ASTRA. ρITB,foot is plotted in the black line to show the time evolution of the ITB region. b, Fluctuation of the electron temperature measured by Electron Cyclotron Emission Imaging (ECEI) for ITB characteristics of a FIRE mode (Shot 22663). The coherence between poloidally adjacent channels near Z = 0 m is summed over frequency 0–250 kHz to represent the amplitude of turbulence, where Z is the vertical position of an ECEI channel. As ITB expands outwards to ρn ~ 0.6, the edge fluctuations are significantly reduced. The black dashed line indicates the NBI heating timing. c, the inverse normalised ion temperature gradient length profile, d, the thermal ion heat diffusivity profile calculated from the power balance analysis at 3.75 s, 4.55 s, and 5.35 s. The black, red, and blue curves correspond to the time points indicated by arrows in a. The error bars are estimated from standard deviation of ion temperature diagnosed by Charge Exchange Spectroscopy (CES) for each channel. e, 3-D landscape view for ITB characteristics of a FIRE mode (shot 22663). The normalised ion energy flux to the thermal ion density versus the ion temperature gradient is plotted at ρn = 0.3, 0.4, 0.5, and 0.6 from 3.7 s to 5.7 s. The transport bifurcation occurs at ρn ~ 0.3 where the ITB foot locates when the ITB was formed.

Extended Data Fig. 5 The time evolution of main parameters of a FIRE mode (KSTAR Shot 25477) during the transition from L-mode to an I-mode like via H-mode.

The plasma is in L-mode, H-mode including the dithering phase, and I-mode like phase up to 2.0 s, 2.1 s to 2.7 s, and after 2.7 s, respectively. A strong weakly coherent modes are observed in the I-mode like phase in the magnetic fluctuation detected by Mirnov coils. The ion temperature close to the edge (Ti,95%) in the I-mode like phase is higher compared with the L-mode phase which could imply the formation of an ETB.

Extended Data Fig. 6 The stationary operation window in terms of the normalised plasma pressure to the magnetic pressure βN and confinement enhancement factor H89 versus internal inductance li.

They are calculated with magnetic EFIT. Yellow squares present H-modes at the divertor configuration including hybrid modes. Grey circles are conventional ITB discharges with the L-mode edge at the limited configuration and blue triangles are FIRE modes with the L-mode edge at the diverted configuration. Red diamonds present FIRE modes with the I-mode like edge at the diverted configuration.

Supplementary information

Supplementary Video 1

Ion temperature (3D) with the sound transformed from diagnostics in a FIRE mode (shot 25,860) presented in Fig. 1. The ion temperature taken from charge-exchange spectroscopy is plotted on the magnetic flux surface for 3D visualization. Charge-exchange spectroscopy, ECE, TS and Mirnov coil signals are transformed into audible sound, so that one can diagnose the plasma through the sound. An outstanding sound around 6 s corresponds to the plasma instability.

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Han, H., Park, S.J., Sung, C. et al. A sustained high-temperature fusion plasma regime facilitated by fast ions. Nature 609, 269–275 (2022). https://doi.org/10.1038/s41586-022-05008-1

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