Schematic of a tokamak showing exhaust from the divertor area. Zoomed-in view of the divertor's geometry and camera view taken during the experiments. Images taken from the experiments reveal that heat fluxes were reduced during p powder injection. This is indicative of the lower heat fluxes during powder inject. Credit: F. Effenberg. Princeton Plasma Physics Laboratory. T. Wilks. Massachusetts Institute of Technology.
Future fusion reactors will have to solve a problem: keep the plasma core hotter than the sun's surface without melting the plasma walls. This problem is known as core-edge integration by fusion scientists. The DIII-D National Fusion Facility at General Atomics has recently addressed this issue in two ways. The first attempts to heat up the core of fusion, while the second seeks to cool down the material that reaches wall. Future fusion power plants will be more cost-effective if they are protected from the elements that face plasma.
Like the familiar internal combustion engine, vessels involved in fusion research must also exhaust heat and particles. This exit pipe is similar to a car's exhaustpipe. It can handle high heat and heavy material loads, but 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. The impurities are used to remove excess heat from the plasma, thereby extending the life of plasma-facing materials. However, these same impurities can also travel into areas where fusion reactions occur, which could reduce the reactor's overall performance.
In the past, impurity injection experiments relied on gaseous impurities. However, a team of researchers from the U.S. Department of Energy's Princeton Plasma Physics Laboratory tested the injection of a powder containing boron, lithium, and boron nitride (Figure 1). There are many advantages to using powder over gas. This allows for a wider range of possible impurities that can be purified and less likely to react chemically with plasma. Powder injection on DIII–D is used to cool the plasma's boundary while maintaining heat in its core. The heat production caused a slight decrease in fusion performance, but measurements showed no significant change.
This graph shows plasma performance against divertor temperatures. It illustrates the progress in core-edge integration in Super H mode plasmas. The black data points represent standard Super H-modes that have no external impurities. The pink points inject nitrogen to cool down the divertor. They also overlap the core-edge integration target area, which is shaded green in upper left. Credit: F. Effenberg. Princeton Plasma Physics Laboratory. T. Wilks. Massachusetts Institute of Technology.
The experiment developed a balanced approach to achieve significant edge cooling while having minimal effects on core performance. Future reactor designs could incorporate powder injection or use the Super H-mode to increase fusion performance and the lifespan of divertor surfaces that expel waste heat. These approaches, based on both theoretical simulations and experimental results, would work well with larger devices such as ITER, an international tokamak currently under construction in France. This would allow for core-edge integration in future Fusion Power Plants.