Boronise your Tokamak

Tokamaks are at the forefront of nuclear fusion research, serving as experimental devices designed to achieve controlled thermonuclear fusion. By confining hot plasma using powerful magnetic fields, tokamaks aim to replicate the energy production processes found in stars, potentially offering a virtually limitless and clean energy source.

To maintain optimal plasma conditions and performance, researchers employ special techniques to condition the walls of their machines. These techniques are essential in reducing impurities that can cool the plasma and degrade its performance. Among these, boronization stands out as a particularly effective method. By depositing a thin boron layer on the plasma-facing components, boronization significantly reduces impurities and enhances plasma performance.

Boronization

The boronization process involves a glow discharge, similar to the effect seen in neon tubes, during which a boron-rich gas such as diborane is introduced into the tokamak. This gas decomposes and deposits a boron-hydride layer on the inner wall of the machine. Different boronization techniques include glow discharge boronization and Ion Cyclotron Range of Frequencies (ICRF) boronization.

For instance, in the Swiss fusion device TCV (Tokamak à Configuration Variable) at École Polytechnique Fédérale de Lausanne (EPFL)’s Swiss Plasma Center, the process begins with heating the device to 240°C. This is followed by a helium glow discharge for about an hour, and then a 60-minute glow discharge with 10% diborane in helium. Initially, the diborane glow discharge faced instability issues, which were addressed by increasing the glow power and reducing the boronization time from 60 minutes to 15 minutes. These adjustments have stabilized the process and improved its effectiveness. In WEST, the diborane injection phase lasts for as long as 4 to 5 hours, with a total injected boron mass of about 10 grams per boronization.

Benefits of Boronization

Boronization offers several significant benefits:

1. Reduction of Impurities: The boron layer effectively reduces oxygen (and carbon for machines with carbon walls) impurities, crucial for maintaining plasma purity. This gettering effect of boron helps in achieving a cleaner plasma environment.
2. Improvement in Plasma Performance: By minimizing impurities, boronization reduces radiative losses, maintains higher plasma temperatures, and increases the rate of fusion reactions.
3. Pumping Wall Effect: Boronization acts as a pumping wall, reducing hydrogen recycling and improving density control, enabling operation at lower plasma edge densities otherwise not possible in present  small tungsten-wall devices.

Case Studies: Boronization in EUROfusion Devices

1. TCV (Switzerland)
   • Overview: Operated by the Swiss Plasma Center at EPFL, the TCV focuses on advanced plasma configurations and heating methods.
   • Boronization Practices: TCV employs a thorough process of heating, helium glow discharge, and diborane glow discharge. Stability issues were mitigated by increasing glow power and reducing boronization time.
   • Perspective: The Swiss Plasma Center emphasizes the importance of a preceding helium glow for stability and highlights the positive impact of boronization on impurity levels and plasma performance.
2. WEST (France)
   • Overview: Operated by the Alternative Energies and Atomic Energy Commission (CEA)  Institute for Magnetic Fusion Research, The Tungsten (W) Environment in Steady-state Tokamak (WEST) explores the use of tungsten as a plasma-facing material in a long-pulse environment (over 364 pulses so far).
   • Boronization Practices: WEST follows a similar boronization procedure involving helium and diborane (He (>85%) and B₂D₆ (<15%)) glow discharges.
   • Insights: CEA highlights boronization’s role in reducing tungsten erosion and enhancing plasma stability. The long pulse capability is crucial to study the lifetime of the boron layers and project their effects towards the 900-second pulses expected in ITER.
3. ASDEX-Upgrade (Germany)
   • Overview: Operated by the Max Planck Institute for Plasma Physics (IPP), ASDEX Upgrade is a key research device in studying plasma-wall interactions. Tungsten plasma facing components and a flexible high power heating systems allow for ITER relevant conditions.
   • Boronization Practices: ASDEX Upgrade routinely applies glow discharge boronization with helium and diborane. This is vital for low-density scenarios and advanced plasma configurations in ASDEX Upgrade, an important fraction of the operation scenarios.
   • Perspective: Insights from the ASDEX Upgrade team reveal that boronization is essential for reducing oxygen and maintaining stable low  collisional  edge plasma conditions, especially with tungsten plasma-facing components.

Collaborative Efforts and Future Directions

EUROfusion plays a critical role in coordinating boronization efforts across different tokamaks, facilitating knowledge exchange and best practices. Ongoing research aims to improve boronization techniques, such as optimizing glow discharge parameters and exploring new boron sources.

Collaboration between research institutes like EPFL, CEA, and IPP is essential for advancing boronization practices and enhancing tokamak performance. These collaborations ensure that the collective experience and findings from various tokamaks are shared, driving forward the development of more efficient and effective boronization techniques.

Conclusion

Boronization significantly reduces impurities and enhances plasma performance. As research and techniques continue to evolve, boronization will further improve fusion reactor performance and contribute to the success of future fusion energy projects. The collaborative efforts within the EUROfusion community are key to optimizing this critical wall conditioning technique, ensuring that tokamaks can operate safely and efficiently under demanding conditions.

Further Reading

For those interested in delving deeper into the topic of boronization and its role in tokamak operations, the following resources are highly recommended:

Journal Papers:

1. Bortolon, A., et al. (2019). Real-time wall conditioning by controlled injection of boron and boron nitride powder in full tungsten wall ASDEX Upgrade. Nuclear Materials and Energy, 19,384-389. Link to Paper.
   • This paper explores an innovative method for wall conditioning in the ASDEX Upgrade tokamak. The researchers implemented real-time injection of boron and boron nitride powders into the plasma, aiming to deposit boron layers on tungsten surfaces. This method proved effective in reducing tungsten erosion, mitigating impurity accumulation, and improving plasma performance. The study demonstrates the feasibility and benefits of this real-time conditioning technique for maintaining optimal conditions in fusion devices with tungsten walls.
2. Wauters, T., et al. (2019). Wall conditioning in fusion devices with superconducting coils. Plasma Physics and Controlled Fusion, 61(6), 066013. Link to Paper.
   • This paper presents an overview of wall conditioning techniques for fusion devices with superconducting coils, focusing on W7-X, JT-60SA, and ITER. It discusses the challenges and solutions for conditioning in the presence of a magnetic field, including RF conditioning and diverted plasmas. The study highlights the effectiveness of various techniques for impurity control and tritium recovery, providing insights into optimizing plasma performance and maintaining safety standards in advanced fusion devices.

Articles:

• ITER Organization. (2023). Wall conditioning – A coat of boron to capture impurities. ITER Newsline, 2023.10.16. Link to Article.
  • The article discusses the significance of boronization as a wall conditioning technique in fusion reactors. It highlights how a thin layer of boron effectively captures impurities, such as oxygen and carbon, enhancing plasma performance and maintaining optimal conditions within the tokamak. The article also covers the practical aspects of implementing boronization in ITER and its expected benefits for future fusion experiments.

Presentations:

• Vanó, L., et al. (2023). W7-X Workshop OP2.1 Results. Max Planck Institute for Plasma Physics. Link to Presentation.
  • W7-X Workshop OP2.1 Results, presented by L. Vanó and colleagues in 2023, details various wall conditioning techniques used in W7-X, including baking, glow discharge, and boronization. The presentation highlights the effectiveness of these methods in reducing impurities and improving plasma performance, with detailed findings on boronization parameters and their impact on plasma operations.

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