One of the great challenges in fusion tokamaks is how to keep the core of a plasma hot enough that fusion can occur while maintaining a temperature at the edge of the plasma low enough that it doesn’t melt the tokamak’s walls. This requires dissipating the heat and particles flowing towards the wall without reducing the performance of the core.
Physicists refer to this challenge as core-edge integration. Researchers recently developed a pathway to improving core-edge integration. They did so by injecting impurities at the edge of the plasma and optimizing the divertor, a device that removes excess heat and particles from the plasma’s edge. Once injected, the impurities convert the heat flux into electromagnetic radiation. This allows the heat to dissipate before contacting the walls.
This work is an important step toward practical fusion energy. It gives scientists improved understanding of the mechanisms that govern the interaction of the core and edge of the plasma. As tokamaks become more powerful and efficient, core-edge integration will become an even more important area of research. This study offers a promising solution to reduce divertor power loads without degrading the core performance. The results are significant for ITER and for the design and operation of future commercial fusion power reactors.
Researchers developed a new way to address the core-edge integration challenge by combining a new divertor geometry recently installed at the DIII-D National Fusion Facility with impurity radiation. For the first time, impurities were used in DIII-D’s new small angle slot (SAS) divertor to reduce divertor heat flux. If unmitigated, heat and particle flows at the edge of the plasma are so intense that the material surfaces would melt. A closed divertor like SAS combined with impurity seeding allows maximum cooling across the divertor area dissipating the heat flow before it reaches the material surface.
This study was enabled by an unprecedented level of diagnostic coverage in a closed divertor. This allowed the team to perform the first simultaneous observation of plasma cooling on the entire suite of diagnostics without degrading core performance. The experiments showed that plasma cooling and impurity leakage have a strong relationship with divertor geometry and the type of impurity injected in the edge. These insights were made possible by state-of-art edge simulation tools that, for the first time at DIII-D, included impurities and full treatment of certain plasma flows known as drifts. The researchers analyzed different impurity species and found that neon showed improved radiative performance as long as the content of the impurity is well controlled. Neon is the impurity being considered for use in ITER. The research indicates that the feasibility of fusion reactors can be improved with a combination of the appropriate radiative impurity species, an optimized divertor geometry, and tailored drifts for controlling particles.
This work was supported by the Department of Energy Office of Science, Office of Fusion Energy Sciences, using the DIII-D National Fusion Facility, a DOE Office of Science user facility.
Casali, L. et al. “Improved core edge compatibility using impurity seeding in the small angel slot (SAS) divertor at DIII-D,” Physics of Plasmas 27, 062506 (2020) Featured Article.
Casali, L. et al, “Neutral leakage, power dissipation and pedestal fueling in open vs closed divertors,” Nucl. Fusion 60 076011 (2020). [DOI: 10.1088/1741-4326/ab8d06]