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Fluid Model of Turbulence Transport Shows Small-Scale Fluctuations

A research team from the DIII-D National Fusion Facility, a DOE Office of Science user facility operated by General Atomics, utilized a "reduced physics" fluid model of plasma turbulence to describe the unexpected properties of the density profile inside a tokamak experiment. Modeling the turbulent behavior of plasma could help researchers to improve the tokamak performance in future fusion reactors, such as ITER.

Many interesting phenomena are produced when heat is applied in a tokamak – for instance, changes in plasma density and plasma rotation. The DIII-D team modeled how various types of heating, like neutral beams that generate ion heating or microwaves that create electron heating, affect the behavior of impurities, plasma density, and turbulent transport. The different types of heating techniques drive turbulence at the long (ion) scales as well as the much shorter (electron) scales that are at the forefront of turbulence computer simulations.

The findings, published in AIP Publishing’s Physics of Plasmas, demonstrated that when electrons are heated in a fusion reactor, important changes occur in density gradients inside the plasma. The researchers "trapped gyro-Landau fluid" (TGLF) model estimated that when heat is added, the turbulence is excited at wavelengths between the electron and ion scales, and this would create a particle pinch that changes the overall density profile of plasma. In this paper, the reduced transport model was also used by the researchers to predict the impurity transport in a fusion reactor.

According to Brian Grierson, a Princeton Plasma Physics Laboratory physicist working as a researcher at the DIII-D National Fusion Facility in San Diego, "when you heat the plasma, you don't just change the temperature, you change the type of turbulence that exists, and that has secondary implications on the transport of plasma density and the plasma rotation."

Usually, turbulent diffusion is driven when heat flows from the hot plasma center to the cold plasma edge. This turbulent diffusion should serve to flatten the density gradient. "But the fascinating thing is that sometimes applying heat in a fusion reactor causes it to produce a density gradient rather than flatten it," Grierson said. This density peaking is very important, because as the plasma density increases the fusion reaction that occurs between tritium and deuterium particles in a tokamak experiment also increases. To put this in simpler terms, he said, "fusion power is proportional to the [plasma] density squared."

Grierson praises Gary Staebler, the paper’s co-author, as the General Atomics theoretician behind TGLF, the model analyzed in this paper. TGLF is a reduced physics model of GYRO – the "full physics" gyrokinetic code – for turbulent transport, which has to be run on supercomputers.

By utilizing this more cost-effective TGLF model, the team was able to execute the code with different experimental measurement and inputs hundreds of times to measure how the theoretical interpretation is affected by uncertainties in the experimental data.

In future, Grierson believes that these latest findings will help inform research to improve the fusion community's interpretation of impurity transport and very small-scale fluctuations inside a plasma.

"We need to understand transport under ion and electron heating to confidently project to future reactors because fusion power reactors will have both ion and electron heating," Grierson said. "This result identifies what we need to investigate with the computationally challenging full physics simulations to verify the interaction of particle, momentum and impurity transport with heating."

This material is based upon work supported by the U.S. 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, under Awards DE-AC02-09CH11466, DE-FC02-04ER54698, DE-FG02-08ER54999, DE-FG02-07ER54917, DE- FG03-97ER54415, DE-FG02-04ER54235, and DE-FG02- 08ER54984.

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