Tokamak Physics Advance

May 19, 2020

General Atomics has issued a press release that reads in part: San Diego, May 15, 2020 – Scientists at the DIII-D National Fusion Facility have for the first time observed a phenomenon that could help improve the stability and performance of large fusion devices and reduce the impact of potentially damaging high heat loads on the surrounding vessel wall. Published in an article in the journal Physical Review Letters, the work represents an important step toward practical fusion energy and understanding of plasma boundaries.

To operate effectively over long periods, fusion reactors must be able to remove heat and by-products from the fusion reaction without affecting stable operation of the device. In this study, DIII-D researchers showed for the first time that a specific particle-transport mechanism known as E×B (“E-cross-B”) drift can be used to efficiently remove excess heat and impurity ions from the edge of a plasma without negatively affecting fusion performance in the plasma core. This is an important result for addressing challenges facing stable operation of future fusion devices, such as the ITER experiment under construction in France and the fusion power plants that will follow it.

“This discovery is quite significant, because it represents a key element in understanding particle flow around the edge of the plasma,” said GA researcher Huiqian Wang, who led the study. “It suggests that we have a pathway toward addressing the challenges of maintaining a stable plasma edge and a high-performance core in future devices like ITER.”

However, plasma particles and their associated heat eventually leak through the tokamak’s magnetic fields. For this reason, tokamaks incorporate a device called a divertor that removes stray particles and heat from the edge regions of the plasma. One challenge is that when the plasma contacts the tokamak interior in and around the divertor, new impurity particles can be liberated and flow into the magnetic fields. If these particles enter the plasma core, they can degrade fusion performance. Because the exact mechanisms of particle movement in the divertor region are not yet fully understood, DIII-D scientists have devoted considerable effort toward studying how these particles, the divertor surfaces, and the magnetic fields interact.

E×B drift is a mechanism of particle movement and is related to the plasma electric field created by the temperature drop at the plasma edge. Wang’s team found experimentally that E×B drift can cause particle flow away from the divertor and up the edge of the magnetically confined core plasma through the boundary layer. The edge region of the high-confinement plasma, known as the pedestal, is a region of steep temperature and pressure gradients. In these experiments, the E×B drift flow caused a localized flattening or “shelf” of density in the pedestal. They also found that this shelf reduced the intensity of plasma instabilities in the edge region that are known as edge-localized modes (ELMs). Essentially, reducing the density gradient in the pedestal reduced the intensity of the ELMs.

The research was aided by new diagnostic capabilities on DIII-D that have much better temporal and spatial resolution. Using significantly improved boundary profile measurements, this density shelf has been broadly seen in the high-power and high-pedestal-temperature regimes that will be used in reactor-grade plasmas, such as in ITER and later devices. That makes this discovery potentially significant for the stable and efficient operation of future fusion power plants.

The researchers also found that they could enhance the effect of the E×B drift on the density shelf by increasing the plasma heating power. This increased the edge temperature gradient, thus increasing the electric field and the drift effects.

“An efficient divertor solution compatible with a high-performance core is highly desired for large fusion devices like ITER,” said DIII-D Director David Hill. “These results are significant for improving understanding of the underlying physics of core-edge integration and laying the groundwork for high-performance operation of future reactor plasmas.”