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Boronization-enabled I-mode on EAST tokamak with an expanded density window and favorable-configuration access

This paper presents the first systematic investigation of I-mode on the EAST tokamak under boronized wall conditions, demonstrating that boronization significantly broadens the accessible density range, increases the occurrence of favorable magnetic configurations with enhanced edge shear, and establishes a new energy confinement scaling that supports the development of reactor-relevant, ELM-free fusion scenarios.

Original authors: X. M. Zhong (Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, China), X. L. Zou (CEA, IRFM, Saint-Paul-lez-Durance, France), A. D. Liu (School of Nuclear Science and Technology, Univer

Published 2026-07-16

📖 6 min read🧠 Deep dive

CC BY 4.0

Original authors: X. M. Zhong (Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, China), X. L. Zou (CEA, IRFM, Saint-Paul-lez-Durance, France), A. D. Liu (School of Nuclear Science and Technology, University of Science and Technology of China, Hefei, China), L. Q. Xu (Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, China), B. Zhang (Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, China), C. Zhou (School of Nuclear Science and Technology, University of Science and Technology of China, Hefei, China), J. P. Qian (Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, China), X. Z. Gong (Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, China), Y. T. Song (Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, China), G. Zhuang (School of Nuclear Science and Technology, University of Science and Technology of China, Hefei, China), W. X. Shi (School of Nuclear Science and Technology, University of Science and Technology of China, Hefei, China), L. T. Gao (School of Nuclear Science and Technology, University of Science and Technology of China, Hefei, China), S. F. Wang (School of Nuclear Science and Technology, University of Science and Technology of China, Hefei, China), Y. H. Guan (Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, China), G. Z. Zuo (Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, China), T. Q. Jia (Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, China), Y. X. Cheng (Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, China), S. X. Wang (Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, China), K. N. Geng (Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, China), H. L. Zhao (Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, China), EAST I-mode Working Group, EAST Team

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Quest for the Perfect Fusion Recipe

Imagine trying to build a star right here on Earth. That's the dream of nuclear fusion, the process that powers the sun. Scientists use giant magnetic donuts, called tokamaks, to trap super-hot gas (plasma) and squeeze it until atoms smash together, releasing massive amounts of clean energy. But there's a catch: keeping that gas hot and contained is incredibly hard. If the gas gets too turbulent, it cools down and the reaction stops. If it gets too unstable, it throws off violent bursts of energy that can damage the machine.

For decades, researchers have been hunting for a "Goldilocks" mode of operation—a state where the plasma holds heat perfectly well but doesn't throw tantrums. They found one called "I-mode." Think of I-mode as a magical traffic system for the plasma. In this state, the heat is trapped tightly (like a luxury car with great insulation), but the particles (the "cars" themselves) can still flow out easily. This is crucial because it prevents the buildup of "trash" (impurities) in the center and, most importantly, avoids the violent energy bursts known as Edge Localized Modes (ELMs) that plague other modes. The big question for the future of fusion is: can we keep this perfect state running in different types of machines and under different conditions? Specifically, does the material coating the inside walls of the machine change how well this magic works?

The EAST Experiment: Swapping Lithium for Boron

This paper reports on a series of experiments at the EAST tokamak in China, a cutting-edge machine designed to test these fusion concepts. For a long time, the EAST team had been studying I-mode using a wall coated with lithium. Lithium is great at soaking up stray particles, but it has a downside: it holds onto hydrogen fuel so tightly that it's hard to get the plasma density (the "crowdedness" of the gas) high enough for a real power plant.

The researchers decided to try something new: they coated the inside walls with boron instead. Boron is a different material, often used in future reactor designs to protect the machine from damage. The goal was to see if I-mode could survive and thrive with this new "boronized" wall, or if the magic would disappear.

The Big Discovery: A Wider Playground
The results were surprisingly positive. When the team switched to boron, the "playground" for I-mode got much bigger.

The Density Window: Under the old lithium coating, the plasma density could only be tuned between a Greenwald fraction ( $f_{GW}$ ) of 0.35 and 0.54. It was a narrow lane. With the boron coating, that lane exploded open, stretching from 0.26 all the way to 0.77. This means the machine could handle much denser, "crowder" plasmas without losing the I-mode state.
The "Why": The paper suggests this happens because boron doesn't hold onto hydrogen as tightly as lithium does. This leads to more "edge recycling," where particles bounce off the wall and re-enter the plasma, effectively fueling the system and allowing for higher densities. You can think of it like a room where the walls are less sticky; more air (particles) can circulate and stay in the room without getting stuck to the walls.

The Surprise: The "Good" Configuration
Usually, I-mode is hard to find in a specific magnetic setup called the "favorable configuration." In the past, with lithium, only 8% of their attempts (4 out of 48 shots) managed to get I-mode in this favorable setup. It was like finding a needle in a haystack.
However, with the boron coating, the needle suddenly appeared in 51% of the attempts (19 out of 37 shots). The paper notes that these successful "favorable" cases happened mostly at high densities. The researchers suspect that the higher density raises the threshold for the plasma to jump into a chaotic state (H-mode), giving I-mode a wider window to exist. In these cases, the plasma also showed a deeper "electric field well" and stronger shearing forces, which act like a shield to keep turbulence in check.

The "ETRO" Mystery
The team also looked at a specific wobble in the plasma called ETRO (Edge Temperature Ring Oscillation). In previous lithium experiments, this wobble was a key ingredient for keeping I-mode steady. With boron, the wobble still happened and worked the same way when it appeared, but it was much rarer, showing up in only 15% of the boron shots. This suggests that while ETRO is a helpful tool, it's not the only way to keep I-mode stable; the plasma found other ways to stay calm under the boron coating.

The Rules of the Game
Finally, the team created a new mathematical rule (scaling law) to predict how well the energy is held in this new setup. They found that the energy confinement time ( $\tau_E$ ) follows this formula:
$\tau_E = 3.29 I_p^{0.51} P_{loss}^{-0.53} \bar{n}_e^{0.08}$
(Where $I_p$ is current, $P_{loss}$ is power loss, and $\bar{n}_e$ is density).
The most exciting part of this rule is the power number: -0.53. In the standard "H-mode" used by most fusion machines, this number is -0.69. A less negative number means that as you add more heating power, the efficiency doesn't drop as fast. This is a huge deal for future power plants because it means you can crank up the heat without losing your grip on the plasma. Also, the density number (0.08) is almost zero, confirming that I-mode doesn't care much about how crowded the plasma is, which is a very stable trait.

What's Next?
The paper is careful to note that while these results are promising, they are based on specific experiments at EAST with a magnetic field of 2.47 T. The authors suggest that future work needs to do more direct comparisons between lithium, boron, and no coating at all to understand exactly why boron works so well. They also need to map out the exact "power thresholds" where the plasma switches modes. But for now, the message is clear: swapping lithium for boron didn't break the I-mode; it actually made it more flexible, more accessible, and potentially more ready for the real-world demands of a fusion power plant.

Technical Summary: Boronization-enabled I-mode on EAST Tokamak

Problem Statement
The I-mode is a promising confinement regime for future fusion reactors, offering enhanced energy confinement with L-mode-like particle transport and naturally ELM-free operation. However, previous experimental studies on the EAST tokamak were conducted exclusively under lithium-conditioned wall conditions. While lithium effectively controls impurity radiation and reduces edge recycling, it suffers from excessive tritium retention. Conversely, boronization is the preferred wall conditioning method for next-step devices like ITER to suppress tungsten sputtering and reduce radiative losses. A critical gap existed in understanding whether I-mode could be sustained under boronized wall conditions, particularly regarding its operational density window, access to favorable magnetic configurations, and compatibility with reactor-relevant scenarios.

Methodology
This study presents the first systematic experimental investigation of I-mode under boronized wall conditions on the EAST tokamak. The research compares a dataset of 37 boronized-wall I-mode discharges with 48 existing lithium-conditioned I-mode discharges. Key experimental parameters were matched, specifically the toroidal magnetic field ( $B_t = 2.47$ T). The boronization process utilized carborane ( $C_2B_0H_{12}$ ) assisted by ICRF plasma, while lithium coating was achieved via vacuum evaporation.

The analysis employed a comprehensive suite of diagnostics, including:

Density and Temperature: Three-channel HCN laser interferometer, 11-channel POINT polarimeter-interferometer, multi-channel reflectometer, and 32-channel ECE radiometer.
Turbulence and Fluctuations: E-band multi-channel Doppler reflectometry (DR) to measure turbulence rotation, intensity, and the edge temperature ring oscillation (ETRO).
Radiation and Emission: EUV spectrometers for Boron radiation and Filterscope systems for $D_\alpha$ line emission.
Statistical Analysis: The study utilized multiple log-linear regression (log-OLS) on 85 stationary time windows to derive an empirical energy confinement scaling law, incorporating plasma current ( $I_p$ ), loss power ( $P_{loss}$ ), and chord-averaged electron density ( $\bar{n}_e$ ).

Key Contributions and Results

Expanded Operational Density Window:
The most significant finding is the substantial broadening of the accessible density range under boronization. The Greenwald fraction ( $f_{GW}$ ) for boronized-wall I-mode extends from 0.26 to 0.77, compared to a narrower range of 0.35 to 0.54 under lithiation.
- Mechanism: The study attributes this extension to enhanced edge recycling. The boronized wall exhibits lower hydrogen isotope retention compared to lithium, leading to higher edge neutral recycling. This is supported by statistical data showing higher normalized $D_\alpha$ emission in boronized cases, which correlates with increased edge density and overall chord-averaged density.
Frequent Access to Favorable Configuration:
I-mode is typically accessed in the unfavorable configuration (where the $B \times \nabla B$ ion drift points away from the X-point) because the favorable configuration usually has a lower L–H transition power threshold, making the I-mode power window narrow.
- Result: Under boronization, 51% of discharges (19 out of 37) were achieved in the favorable configuration, a stark contrast to the 8% (4 out of 48) observed under lithiation.
- Characteristics: These favorable-configuration cases are concentrated at high densities ( $f_{GW} = 0.6–0.77$ ). They exhibit a deeper radial electric field ( $E_r$ ) well and stronger $E_r \times B$ velocity shear. The authors suggest that the high density raises the L–H transition power threshold, thereby widening the power window and allowing I-mode to be sustained in the favorable configuration.
Turbulence and ETRO Dynamics:
The study confirms that the Edge Temperature Ring Oscillation (ETRO), which modulates transitions between electron and ion turbulence, persists under boronization. However, ETRO occurs less frequently (approx. 15% of boronized discharges) compared to lithium-conditioned plasmas.
- Implication: Steady-state I-mode can be sustained without ETRO, as evidenced by the presence of Type I (GAM + WCM) and Type IV (WCM only) regimes in the boronized dataset. The reduced ETRO frequency is hypothesized to be linked to lower edge electron temperatures in boronized plasmas.
Empirical Confinement Scaling:
An empirical energy confinement scaling law was derived for EAST I-mode at fixed $B_t$ :
$\tau_E = 3.29 I_p^{0.51 \pm 0.10} P_{loss}^{-0.53 \pm 0.05} \bar{n}_e^{0.08 \pm 0.07}$
- Significance: The scaling indicates a weak dependence on density (exponent $\approx 0.08$ ) and a power degradation exponent ($-0.53$) that is significantly weaker than the standard IPB98(y,2) H-mode scaling ($-0.69$). This suggests I-mode confinement degrades more slowly with increasing heating power, a favorable trait for reactor operation.

Significance and Claims
The paper claims that boronization successfully broadens the operational space of EAST I-mode, specifically enabling access to higher densities and the favorable magnetic configuration, which were previously difficult to achieve. The results demonstrate that I-mode is compatible with boronized wall conditions, a critical step toward validating reactor-relevant, ELM-free scenarios for future devices like ITER and DEMO.

The authors modestly conclude that while the expanded density window and favorable-configuration access are promising, the underlying causal mechanisms—specifically the quantitative impact of wall recycling on transition thresholds—require further validation through dedicated power scans, fueling studies, and high-resolution edge diagnostics. The study establishes a foundation for future work to determine if high-density, favorable-configuration I-mode can be robustly extrapolated to full-metal-wall, reactor-relevant conditions.

The Quest for the Perfect Fusion Recipe

The EAST Experiment: Swapping Lithium for Boron

Technical Summary: Boronization-enabled I-mode on EAST Tokamak

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