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  • Review Article
  • Published:

How fast ions mitigate turbulence and enhance confinement in tokamak fusion plasmas

Nature Reviews Physics volume 7, pages 190–202 (2025)Cite this article

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Abstract

Along with high temperature and density, magnetic fusion requires good confinement and a degree of transport control for thermal plasmas. Meanwhile, fast ions are generated by the external heating used to raise plasma temperature and by the fusion reactions. As a result, the fusion plasmas are effectively rendered into systems with two coexisting populations of main interest — namely, the fast ions and the thermal plasma. Interestingly, several recent experiments indicate that the fast-ion population can improve the confinement of the thermal plasmas by mitigating turbulence. In this Review, we describe the physical mechanisms that underpin the improved confinement and discuss recent experimental results in terms of these mechanisms.

Key points

  • Recent tokamak experiments have shown that fast ions, generated both by external heating used to raise plasma temperature and by fusion reactions, can enhance the confinement of thermal plasmas by mitigating turbulence.

  • Fast ions can modify the magnetic field structure, thereby reducing turbulence by decreasing the drive for curvature-type microinstabilities and increasing the E × B flow shear. Confinement improvements observed in hybrid modes in JET (Joint European Torus) and KSTAR (Korea Superconducting Tokamak Advanced Research) are examples of this mechanism.

  • Fast ions can lead to thermal ion dilution and changes in the thermal ion density gradient, which have a stabilizing effect on microinstabilities by changing the nonlinear saturation level of microinstabilities and by enhancing zonal flow generation. This mechanism could explain internal transport barrier experiments in KSTAR (FIRE (fast-ion-regulated enhancement) mode), HL-2A (Huan Liuqi-2A), and ASDEX (Axially Symmetric Divertor Experiment) Upgrade with neutral beam injection.

  • Fast ions can interact with microinstabilities through wave–particle resonant interactions, extracting energy from the instabilities and, thereby, weakening their linear drive. The internal transport barrier formation in ASDEX Upgrade (F-ATB (fast-ion-induced anomalous transport barrier)) with ion cyclotron resonance heating can be understood through this mechanism.

  • Furthermore, fast ions can generate fast-ion-driven instabilities that interact with microturbulence via zonal flows, phase-space coherent structures, and nonlinear mode coupling, leading to turbulence mitigation. This mechanism could explain internal transport barrier formation in DIII-D (Doublet III-D) and EAST (Experimental Advanced Superconducting Tokamak), wherein fast-ion-driven instabilities are present.

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Fig. 1: The effect of dilution by fast ions on the drift wave turbulence–zonal flow system.
Fig. 2: Interplay between wave–particle resonant interactions and ion-scale plasma instabilities.
Fig. 3: Nonlinear simulation results on zonal flows (ZFs) for cases with and without fast ions in FIRE mode on KSTAR.
Fig. 4: Simulations of the poloidal cross section showing fluctuations in the electrostatic field ϕ1 for the F-ATB (fast-ion-induced anomalous transport barrier) discharge in ASDEX Upgrade6, comparing cases with and without fast ions.
Fig. 5: Interacting reversed-shear Alfvén eigenmode (RSAE) and ion temperature gradient (ITG) turbulences.
Fig. 6: Core confinement feedback loops.

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Acknowledgements

The authors thank G. Choi (KAIST), A. Ishizawa (Kyoto Univ.), S. Park (Seoul Nat. Univ.) and J. Lee (Seoul Nat. Univ.) for the fruitful discussions. This work was supported by 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). P.H. Diamond supported by the U.S. Department of Energy, Office of Science, Office of Fusion Energy Sciences under Award No. DE-FG02-04ER54738 and Award No. DE-SC0024651. Z. Lin acknowledges support by US DOE SciDAC and INCITE. This work has been partially carried out within the framework of the EUROfusion Consortium, funded by the European Union via the Euratom Research and Training Programme (Grant Agreement nos. 101052200 — EUROfusion). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Commission. Neither the European Union nor the European Commission can be held responsible for them. The authors also 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

  1. Department of Nuclear Engineering, Seoul National University, Seoul, Republic of Korea

    Yong-Su Na

  2. Nuclear Research Institute for Future Technology and Policy, Seoul National University, Seoul, Republic of Korea

    T. S. Hahm

  3. Department of Astronomy and Astrophysics, University of California San Diego, La Jolla, CA, USA

    P. H. Diamond

  4. Department of Physics, University of California San Diego, La Jolla, CA, USA

    P. H. Diamond

  5. Max Planck Institute for Plasma Physics, Garching, Germany

    A. Di Siena

  6. IRFM, CEA, Saint-Paul-lez-Durance, France

    J. Garcia

  7. Department of Physics and Astronomy, University of California, Irvine, CA, USA

    Z. Lin

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Contributions

The authors contributed equally to the article as first authors. Y.-S.N. formulated the overall structure of the article, invited co-authors, and coordinated the entire writing activities. Y.-S.N. primarily contributed to ‘Introduction’, ‘Outlook’, and the section of ‘Experimental observations’. T.S.H. contributed to organizing the structure of the article and primarily contributed to the sections ‘Thermal ion dilution and change in the thermal ion density gradient’ and ‘Interaction between microturbulence and fast-ion-driven instabilities’. P.H.D. mainly contributed to the ‘Introduction’, ‘Outlook’ and ‘Thermal ion dilution and change in the thermal ion density gradient’ sections. A.D.S. primarily contributed to the ‘Introduction’ and ‘Resonant interaction between fast ions and microturbulence’ sections. J.G. mainly contributed to the sections ‘Change of magnetic field structure by fast ions’ and ‘Experimental observations’. Z.L. primarily contributed to the section ‘Interaction between microturbulence and fast-ion-driven instabilities’. All authors equally contributed to the compilation and review of the manuscript.

Corresponding authors

Correspondence to Yong-Su Na, T. S. Hahm, P. H. Diamond, A. Di Siena, J. Garcia or Z. Lin.

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Glossary

Drift waves

A universally occurring type of collective excitation, driven by a pressure gradient in magnetized plasmas. Differences between ion and electron motion can destabilize it, known as drift wave instability.

Fishbone

An instability characterized by a rapid burst of magnetohydrodynamic activity owing to interaction between fast ions and internal kink instability with the shape of a fishbone when plotted as a function of time from diagnostics, sometimes observed when neutral beam injection is used in tokamaks.

Gyrokinetics

A theoretical framework to study plasma nonlinear behaviour on perpendicular spatial scales comparable to the Larmor radius and frequencies much lower than the particle cyclotron frequencies.

H-mode

A high-confinement regime develops when a tokamak plasma is heated above a characteristic power threshold, which increases with density, magnetic field and machine size. It is characterized by a sharp pressure gradient near the plasma edge, the so-called edge transport barrier (ETB), resulting in an edge ‘pedestal’.

Hybrid mode

A high-performance, long-duration plasma confinement mode that have favourable fusion and neutron fluence characteristics for ITER. It is characterized by low magnetic shear or flat q-profile in the central region of the plasma.

Internal transport barriers

Radially localized plasma regions in which energy and/or particle transport is reduced and driven mainly by collisional transport owing to suppression of turbulence.

Ion cyclotron resonance heating

A method to heat up a plasma confined in a magnetic fusion device using electromagnetic radio frequency waves with frequencies about 20–50 MHz, matching the frequency at which ions gyrate around the magnetic field lines, ion cyclotron frequency, is used. The ions in the plasma absorb the electromagnetic radiation and, as a result, increase in kinetic energy.

Long-lived mode

A type of magnetohydrodynamic instability that persists for a significantly long duration within the plasma, often characterized by a steady-state oscillation with a helical structure, unlike other rapidly fluctuating instabilities. It is primarily driven by the interaction between fast ions and the plasma, occurring in conditions of low magnetic shear and substantial plasma rotation.

Modulational instability

A process whereby a test large-scale, slow perturbation on an ensemble of waves or modes grows by triggering a flow of energy which reinforces the original perturbation. Classic examples of modulational instability are self-focusing of a beam and amplification of test shears by drift wave turbulence. Modulational instability can be computed using either wave kinetics (that is, quasi-particle method) or the envelope formalism.

Neutral beam injection

A method to heat up a plasma confined in a fusion device with a beam of high-energy neutral particles injected into the plasma. These neutral particles are ionized by collision with the confined plasma particles and become fast ions. They transfer their energy to plasma particles mainly by collisions.

Tokamaks

Magnetic confinement devices in which magnetic fields are generated both by external coils and by currents flowing in the plasma to confine plasma in the shape of an axially symmetrical torus. The magnetic field generated in the toroidal direction, following a large circular ring around the torus encircling the central void, by external coils wound along the torus is called the toroidal field. The magnetic field generated in the poloidal direction, following a small circular ring around the surface, by currents flowing in the plasma is called the poloidal field.

Transport bifurcation

An abrupt change in the plasma transport when a critical value of a control parameter is exceeded. Typically, the control parameter is external heating power.

Zonal flows

Azimuthally symmetric band-like shear flows; an ubiquitous phenomenon in nature and the laboratory. In plasma physics, a zonal flow is a plasma flow within a magnetic surface primarily in the poloidal direction arising via a self-organization phenomenon driven by low-frequency drift-type modes, in which energy is transferred to longer wavelengths by modulational instability or turbulent inverse cascade.

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Na, YS., Hahm, T.S., Diamond, P.H. et al. How fast ions mitigate turbulence and enhance confinement in tokamak fusion plasmas. Nat Rev Phys 7, 190–202 (2025). https://doi.org/10.1038/s42254-025-00814-8

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