Current fusion power plant designs envision using tungsten as a key component in the reactor vessels because of its high resistance to heat and radiation. However, tungsten is also an impurity for the fusion reaction and can impair performance. The new approach researchers are testing will leverage an existing component on DIII-D, known as the Small Angle Slot (SAS) Divertor, to minimize tungsten erosion and release into the plasma.
"Developing robust wall materials is absolutely necessary for economic fusion energy," said Dan Thomas, Ph.D., a scientific manager at DIII-D who is helping to coordinate the multi-institution effort on these new experiments. "The specific way in which this will be accomplished is still an open unsolved question and a very active area of research worldwide. Here at DIII-D, we have an active collaboration, with multiple institutions looking at the many ways plasma and the vessel wall can interact."
Tungsten is the leading candidate to withstand these conditions, but it comes with a significant concern. Wall surfaces exposed to the fusion reaction can erode and form impurities in the plasma. Suppression of tungsten leakage from the divertor into the core of the reactor is especially important for efficient fusion performance.
Tungsten contamination of even 0.001% in the fusion fuel is enough to seriously impair performance. Thus, methods of reducing tungsten erosion in the divertor and preventing its escape from the divertor region are critically important for future designs.
The SAS-VW (V-closure with Tungsten Surface) experiment employs a row of tungsten-coated tiles in the divertor to study these issues. The original SAS divertor design demonstrated the beneficial aspects of improved divertor geometry for better heat dissipation and decreased impurity leakage. Building on this success, and using a unique set of diagnostic instruments and detailed modeling, SAS-VW is designed with improved shape to address three research questions in an uncontaminated plasma environment.
During the experiment, researchers will examine how tungsten emerges from such a divertor, how it moves towards the core, and how it affects core fusion performance. These studies can be used to find solutions with a high-performance fusion core, enabling DIII-D to develop integrated solutions for fusion power plants. The scientific understanding gained from these experiments will support designs and simulations for the next generation of fusion devices.
"Our aim here is to solve a critical challenge to economic fusion energy," Thomas said. "This is another illustration of the flexibility and impact of the DIII-D research program."