PPPL Finds Lithium Walls Influence Fusion Fuel Retention in Tokamak Reactors

Achieving practical fusion energy depends on mastering the interactions between fusion fuel and the walls of tokamak reactors, especially when those walls are coated with lithium. 

Recent collaborative research led by the Princeton Plasma Physics Laboratory (PPPL) is transforming our understanding of this critical relationship, offering fresh strategies for the next generation of fusion power plants.

Why Lithium Matters in Fusion Reactors

Lithium isn't just a protective surface, it's a dynamic player in plasma management. Its unique characteristics allow it to absorb stray fuel atoms, stabilize the plasma boundary, and handle intense heat loads. When applied to reactor walls, lithium can form a self-repairing barrier or even a vapor shield, safeguarding internal components from extreme conditions.

• Plasma-facing lithium actively absorbs fuel, reducing unwanted reflection and promoting plasma stability.
  
  
• This absorption, however, leads to fuel retention, especially with tritium, a rare and radioactive isotope vital for fusion reactions.

Fuel Retention: The Co-Deposition Effect

The study shows that co-deposition (where fuel atoms are trapped with lithium during plasma operations) is the main cause of fuel retention. This occurs whether lithium is introduced directly during plasma runs or previously deposited and redeposited by plasma exposure.

• Injecting lithium during plasma shots enhances both temperature control and fuel trapping compared to pre-coating.
  
  
• The thickness of pre-applied lithium coatings has minimal effect on overall fuel retention; most trapping happens during active plasma-lithium interactions.

This insight is crucial since excessive tritium retention can limit fuel supply, complicate reactor operations, and introduce safety risks especially if tritium accumulates in cooler, hard-to-reach areas.

Testing Lithium Delivery: Pre-Coating vs. Powder Injection

Researchers compared two main methods in the General Atomics DIII-D tokamak:

• Pre-coating reactor components with lithium before plasma exposure
  
  
• Injecting lithium powder during active plasma operation

Results reveal that co-deposition during operation captures more fuel than pre-existing coatings. Ongoing work is also exploring liquid lithium, which could offer superior thermal performance and potentially simplify tritium recovery by enabling purification flows within the reactor system.

Design Strategies for Future Fusion Devices

As tokamaks move from graphite to tougher materials like tungsten, lithium emerges as a crucial tool for managing heat and erosion. The research team recommends several approaches to control tritium retention:

• Prioritize lithium injection during plasma operation over thick pre-applied coatings
  
  
• Design reactors to minimize cold wall regions where fuel can become trapped
  
  
• Utilize flowing liquid lithium, maintain higher wall temperatures, and develop advanced methods to prevent co-deposition in inaccessible spots

PPPL is driving innovation with new reactor concepts, including the National Spherical Torus Experiment-Upgrade (NSTX-U) and its proposed successor, STAR, both of which may leverage advanced lithium application technologies.

The Fuel Management Balancing Act

Careful control of fuel retention is vital: tritium is scarce, costly, and generated in limited amounts inside the reactor. While some retention helps stabilize plasma, too much reduces available fuel, undermining reactor efficiency and complicating operation.

By refining lithium delivery and reactor design, scientists can better manage fuel retention and safety, paving the way for practical, reliable, and cost-effective fusion energy.

About the  Princeton Plasma Physics Laboratory (PPPL)

The U.S. Department of Energy's Princeton Plasma Physics Laboratory (PPPL) is a leading national center for fusion energy and plasma research. Managed by Princeton University, PPPL's primary mission is to develop the scientific and technological foundation for fusion as a clean, safe, and abundant energy source. 

The laboratory is renowned for its pioneering work in tokamak and stellarator designs, which are devices that use powerful magnetic fields to confine plasma and create the conditions for fusion. Beyond fusion, PPPL's research extends to a wide range of plasma applications, including nanotechnology, microelectronics, and astrophysics, contributing to innovations across various scientific and industrial fields.

Conclusion

This research spotlights the delicate balance between maximizing fusion output and minimizing fuel loss in lithium-lined reactors. With actionable recommendations and ongoing experiments, the work brings us closer to safe, efficient, and commercially viable fusion power.