Creating an efficient fusion plasma in a tokamak requires balancing the plasma’s competing temperature needs. It must be extremely hot in the core to allow fusion to occur, but cool enough at the edge to not damage the walls of the device. An international research team at the DIII-D National Fusion Facility has developed an innovative solution. It uses the active injection of gases to cool the edge region of a plasma coupled with enhanced core confinement from an internal transport barrier. This reduces the heat before it reaches the walls of the tokamak. Even better, they have done this while maintaining high plasma performance in the core region.
This new physics approach, making use of improved core confinement, will be critical for future fusion reactors. These reactors will feature higher-power plasmas and operate for extended periods. Current tokamaks like DIII-D operate for short pulses, after which the armor in the divertor – a region of the wall that dissipates heat and energetic particles along the top and bottom edges of the device – has time to cool. With sustained operations at higher power levels, future reactors will have to be even better at keeping the core hot while the divertor stays cool. This ability will be essential to future reactors that provide practical fusion energy.
The ability to fully cool plasma in the divertor without reducing the performance of the core has eluded researchers for decades. To illustrate the scale of the challenge, temperatures in excess of 100 million degrees Celsius are necessary for fusion in the core, but the solid walls of the tokamak can tolerate temperatures of only a few thousand degrees before the armor erodes and shuts down the fusion process. In the past, researchers tried injecting radiative gases into the plasma and achieved strong divertor cooling. Unfortunately, that same cooling effect often moves from the divertor to the plasma edge to impair a region of high confinement referred to as an edge transport barrier (ETB), thus reducing fusion performance. A transport barrier is a region of low plasma turbulence that functions as an insulating layer, significantly slowing the passage of particles or energy.
The DIII-D team, working with colleagues at China’s Experimental Superconducting Tokamak (EAST), has shown promising results with a new approach that uses an additional transport barrier, further inside from the edge of the plasma, to isolate the hot core from the cooler edge. This internal transport barrier (ITB) is created by tailoring the shape of the plasma current in a way that is known to reduce plasma turbulence. When the injected radiative gases dissipate divertor heat and cool the edge transport barrier, this internal transport barrier is actually enhanced, improving the thermal isolation of the high-temperature core from the walls.
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. It was also supported by the National Natural Science Foundation and the National Magnetic Confinement Fusion Science Program, both of China.