Using Radio Waves to Control Fusion Plasma Density

Recent fusion experiments on the DIII-D tokamak at General Atomics and the Alcator C-Mod tokamak at Massachusetts Institute of Technology (MIT) showed that beaming microwaves into the center of the plasma can be used to control the density in the center of the plasma, where a fusion reactor would produce most of its power. Several megawatts of microwaves mimic the way fusion reactions would supply heat to plasma electrons to keep the "fusion burn" going.

The new experiments reveal that turbulent density fluctuations in the inner core intensify when most of the heat goes to electrons instead of plasma ions, as would happen in the center of a self-sustaining fusion reaction. Supercomputer simulations run at the Department of Energy’s National Energy Research Scientific Computing Center (NERSC) at Lawrence Berkeley National Laboratory closely reproduced the experiments, showing that the electrons become more turbulent as they are more strongly heated and that this transports both particles and heat out of the plasma.

The findings were presented October 27 at the 2014 APS Division of Plasma Physics Meeting.

"We are beginning to uncover the fundamental mechanisms that control the density, under conditions relevant to a real fusion reactor," said Darin Ernst, a physicist at MIT who led the experiments and simulations, together with co-leaders Keith Burrell (General Atomics), Walter Guttenfelder (Princeton Plasma Physics Laboratory) and Terry Rhodes (University of California, Los Angeles).

The experiments were conducted as part of a National Fusion Science Campaign, a new program that enables research on one fusion experiment to be expanded to another device with complementary instrumentation and capabilities.

"The National Campaign has increased the impact of our work, with added benefit to the fusion program," said Ernst. "Comparing Alcator C-Mod and DIII-D tests our new predictions that particle collisions strongly reduce this type of turbulence. The collision rate varies by a factor of 10 between the two machines."

A Band of Fluctuations

The experiments and simulations suggest that trapped electron turbulence becomes more important under the conditions expected in self-heated fusion reactors. The simulations run at NERSC closely matched detailed measurements of the actual turbulence in the 20cm diameter inner core.

"We discovered that sheared flows also drive turbulence in the inner plasma core, but as we approached conditions where mainly the electrons are heated, the usual plasma flow is reduced and pure trapped electron turbulence begins to dominate," said Guttenfelder, who ran the simulations for the DIII-D experiments with Andris Dimits of Lawrence Livermore National Laboratory.

Measurements revealed a band of fluctuations separated by a constant frequency interval, like harmonics in a musical note.

"These new coherent fluctuations appear to be consistent with the basic trapped electron instability that grows stronger during heating, " said Rhodes.

In a self-heated fusion reactor, fusion reactions produce very energetic alpha particles that collide with electrons as they move through the plasma. The collisions heat the electrons by imparting random thermal motion. The electrons in turn collide with and heat cooler deuterium and tritium fuel ions to fusion temperatures. However, turbulent eddies can swirl the particles and energy away from the hot core toward the cooler edge, where they eventually are lost to the walls of the chamber.

These experiments are part of a larger systematic study of turbulent energy and particle loss under fusion-relevant conditions.

"It's important to understand what drives the turbulence and how it can be controlled and minimized to find new ways of operating tokamaks that exploit that knowledge," said Burrell. By comparing detailed turbulence measurements with simulations, the researchers hope to understand how turbulence controls the core temperature under fusion conditions.

This article was adapted from materials provided by the APS.