Plasma Turbulence Generates Flow in Fusion Reactors

Heating the core of fusion reactors causes them to develop sheared rotation that can improve plasma performance.

plasma
Simulation of plasma turbulence generating positive (red) and negative (blue) residual stress that drives rotation shear. Comparison between measured and simulated rotation profile (inset). Image courtesy of W.X. Wang

The Science

Improved stability and confinement in fusion reactors comes through generation of sheared flow. High-energy particle beams commonly drive the flow. Researchers ran new simulations that are showing how the plasma can self-organize to generate its own sheared flow even when there is no external torque applied by particle beams.

The Impact

Rotation and rotation shear benefits confinement in fusion plasmas. Such rotation is provided primarily by injecting momentum into the plasma with energetic neutral particles. In future, larger fusion reactors, the amount of torque that can be applied to the plasma is more limited than in present devices. Creating plasma self-generated flows could improve confinement and stability in fusion reactors. This work is a vital step towards definitively predicting the physics behind these self-generated flows. The potential impact of this research is better fusion reactor performance in terms of improved stability.

Summary

New measurements and simulations of plasma rotation in DIII-D National Fusion Facility, a U.S. Department of Energy Office of Science user facility, show that self-organized “intrinsic rotation” in tokamaks is generated by turbulence. Such self-organized flow can be beneficial for fusion reactor performance by suppressing turbulent energy loss and magnetohydrodynamic instabilities. The present work shows that by simply heating the plasma core, we can cause it to generate a sheared flow. The modeling shows that we now have a quantitative understanding of the amount of sheared flow that can be generated using this self-generated intrinsic torque. The plasma flow is generated by the variation of the turbulence strength across the radius of the plasma, which the plasma sees as a source of momentum flux. The plasma reacts to this momentum flux, causing acceleration from rest and driving differential flow, like the atmospheric jet stream or the bands of Jupiter. This final steady flow represents the balance between the intrinsic torque driven by the turbulence, and the viscosity of the plasma that keeps the gas from spinning arbitrarily fast. Theoretical simulations are able to predict the intrinsically generated torque and plasma rotation, in very good agreement with observations.