Integrating hot cores and cool edges in fusion reactors
Future fusion reactors have a conundrum: maintain a plasma core that is hotter than the surface of the sun without melting the walls that contain the plasma. Fusion scientists refer to this challenge as "core-edge integration." Researchers working at the DIII-D National Fusion Facility at General Atomics have recently tackled this problem in two ways: the first aims to make the fusion core even hotter, while the second focuses on cooling the material that reaches the wall. Protecting the plasma facing components could make them last longer, making future fusion power plants more cost-effective.
Just like the more familiar internal combustion engine, vessels used in fusion research must exhaust heat and particles during operation. Like a car's exhaust pipe, this exit path is designed to handle high heat and material loads but only within certain limits. One key strategy for reducing the heat coming from the plasma core is to inject impurities—particles heavier than the mostly hydrogen plasma—into the exhaust region. These impurities help remove excess heat in the plasma before it hits the wall, helping the plasma-facing materials last longer. These same impurities, however, can travel back into regions where fusion reactions are occurring, reducing overall performance of the reactor.
Past impurity injection experiments have relied on gaseous impurities, but a research team from the U.S. Department of Energy's Princeton Plasma Physics Laboratory experimented with the injection of a powder consisting of boron, boron nitride, and lithium (Figure 1). The use of powder rather than gas offers several advantages. It allows a larger range of potential impurities, which can also be made purer and less likely to chemically react with the plasma. Experiments using powder injection on DIII-D are aimed at cooling the boundary of the plasma while maintaining the heat in the core of the plasma. Measurements showed only a marginal decrease in fusion performance during the heat production.
The experiments developed a balanced approach that achieved significant edge cooling with only modest effects on core performance. Incorporating powder injection or the use of the Super H-mode into future reactor designs may allow them to maintain high levels of fusion performance while increasing the lifetime of divertor surfaces that exhaust waste heat. Both sets of experimental results, coupled with theoretical simulations, suggest that these approaches would be compatible with larger devices like ITER, the international tokamak under construction in France, and would facilitate core-edge integration in future fusion power plants.