A team lead by Korean researchers from Seoul National University, Prof. Yong-Su Na and Prof. Taik Soo Hahm has comprehensively demonstrated that fast ions present inside a fusion reactor, contrary to previous understanding, do not degrade fusion performance but can actually enhance it.

Fusion energy is attracting attention as a next-generation, eco-friendly energy source because it produces no carbon emissions and relies on virtually unlimited fuel. In particular, with the recent rapid development of artificial intelligence technologies, the construction and maintenance of data centers now require massive amounts of electricity. Fusion energy is increasingly recognized as a promising solution to meet such demands. As a result, major investors and tech giants, including Microsoft’s Bill Gates, ChatGPT’s Sam Altman, Amazon’s Jeff Bezos, and PayPal’s Peter Thiel, have been making large-scale investments in the fusion energy sector.

To achieve fusion, hydrogen ions must be maintained at ultra-high temperatures exceeding 100 million degrees Celsius for an extended period in a stable state. Korea’s fusion research device, KSTAR, recently set a world record by maintaining plasma at 100 million degrees for 48 seconds. However, turbulence arising inside the plasma has remained a major challenge as it significantly hinders fusion performance, making effective turbulence suppression a key goal in fusion research.

Conventionally, methods to suppress turbulence in fusion plasmas have relied on inducing flows within the plasma or manipulating the current distribution. However, in future large-scale fusion reactors, forming such flows becomes increasingly difficult, and the current distribution itself tends to change over time. These limitations have raised concerns about the feasibility of turbulence control using conventional approaches.

This study took a new, comprehensive approach to controlling plasma turbulence by utilizing fast ions and systematically uncovered the underlying physical mechanisms. 

In general, particles in a plasma tend to equalize their energy through collisions. However, when external heating is applied or when alpha particles are produced through deuterium-tritium fusion reactions, some particles can acquire much higher energy than the average. These particles, with significantly above-average energies, are known as fast particles which include fast ions and fast electrons.

Fast ions have long been regarded as one of the primary sources of instabilities in fusion devices. They can interact with waves inside the plasma, rapidly causing energy loss or disrupting the sustained stability of fusion reactions. However, this study revealed through extensive experiments and simulations that fast ions can, in fact, suppress turbulence and thereby improve fusion performance.

Based on experimental and simulation data from a variety of tokamak devices, the research team classified the interaction between fast ions and plasma turbulence into four primary mechanisms and clearly explained how these ions suppress turbulence. These mechanisms are as follows:

Suppression via Magnetic Structure Modification: Fast ions alter the magnetic field structure, thereby weakening turbulence.

Suppression via Ion Dilution: Fast ions dilute the ion density within the plasma and change its distribution, leading to reduced turbulence.

Resonant Interaction with Turbulence: Under certain conditions, fast ions resonate with turbulence, diminishing its intensity.

Excitation of Instabilities and Cross-Interaction: Fast ions generate additional instabilities, which interact with preexisting turbulence and ultimately suppress it.

Notably, these mechanisms were found to be linked to the enhancement of zonal flows, band-like, symmetric flows commonly observed in nature and laboratory environments. In plasmas, small-scale wave interactions gradually concentrate energy, eventually leading to the spontaneous formation of zonal flows. These flows play a critical role in suppressing turbulence and thus contribute significantly to plasma stability and improved performance.

Using this new understanding, the research team successfully interpreted experimental results from major tokamak devices around the world, including Korea’s KSTAR, Germany’s ASDEX Upgrade, the U.S.'s DIII-D, the EU’s JET, and China’s HL-2A and EAST. This integrated framework allows for the unified explanation of phenomena that were previously understood only in isolation, thereby advancing the understanding of the complex interactions between fast ions and turbulence in fusion plasmas.

The research team, in collaboration with leading scholars from the U.S., Germany, and France, systematically and comprehensively demonstrated that fast ions inside a plasma are not merely sources of instabilities, but can actually suppress turbulence and enhance fusion performance.

This study opens new possibilities for utilizing fast ions to suppress turbulence and increase fusion output in future reactors. By applying these physical principles to the design of next-generation fusion reactors, this research is expected to make significant contributions toward the commercialization of fusion energy.

Comparison of Turbulence and Energy Loss With and Without Fast Ions:

A cross-sectional analysis of tokamak plasma reveals that in the presence of fast ions, the area dominated by blue regions significantly expands, indicating a marked reduction in turbulence. In terms of energy loss as well, the presence of fast ions leads to a substantial decrease in loss levels. These findings are based on experiments conducted using the ASDEX Upgrade device at the Max Planck Institute for Plasma Physics in Germany and were simulated using the GENE gyrokinetic code.

[Reference] Yong-Su Na, T. S. Hahm, P. H. Diamond, A. Di Siena, J. Garcia & Z. Lin, "How fast ions mitigate turbulence and enhance confinement in tokamak fusion plasmas", Nature Reviews Physics, volume 7, pages190–202 (2025)

https://doi.org/10.1038/s42254-025-00814-8 

[Main Author] Yong-Su Na (Seoul National University), Taik Soo Hahm (Seoul National University)

* Contact : Prof. Yong-Su Na, ysna@snu.ac.kr