Advance in Tokamak Science

May 13, 2020

Scientists at the DIII-D National Fusion Facility in the U.S. have made a significant advancement in physics understanding that represents a key step toward practical fusion energy. The research answers a key question about the relationship between plasma turbulence and core electron density in a tokamak. “This work substantially improves the understanding of electron behavior in the plasma core, which is an area of great importance for increasing fusion gain,” said David Hill, Director of DIII-D. “This is another important step toward practical fusion energy in future commercial reactors.” The research involved scientists from General Atomics, the College of William & Mary, the University of California Los Angeles, the VTT Technical Research Centre of Finland, General Atomics, the University of Wisconsin - Madison, and Chalmers University of Technology in Sweden.

The work, published in an article in the journal Nuclear Fusion, helps better explain the relationship between three variables - plasma turbulence, the transport of electrons through the plasma, and electron density in the core. Because these factors are key elements of the fusion reaction, this understanding could significantly improve the ability to predict performance and efficiency of fusion plasmas, a necessary step toward achieving commercial fusion power plants.

“We've known for some time that there is a relationship between core electron density, electron-ion collisions and particle movement in the plasma,” said Saskia Mordijck, who led the multi-institutional research team at DIII-D. “Unfortunately, until now research has not been able to untangle that relationship from the other components that affect electron density patterns.”

Because electron density in the plasma core is a critical element of fusion gain, scientists are developing methods to achieve greater peak densities. One previously identified approach that shows promise is reducing electron-ion collisions, a parameter that plasma physicists refer to as collisionality. However, previous research was not able to establish the exact relationship between density peaking and collisionality, nor isolate the effect from other characteristics of the plasma.

The DIII-D team conducted a series of experiments in which only plasma collisionality was varied while other parameters were held constant. The results demonstrated that low collisionality improves electron density peaking through the formation of an internal barrier to particle movement through the plasma, which in turn altered the plasma turbulence. Previous work had suggested the effect might be due to plasma heating by neutral beam injection, but the experiments show that it was linked to particle transport and turbulence.

For more information, contact David Hill: hilldn@fusion.ga.com