Turbulence, the violently unruly disturbance of plasma, can prevent plasma from growing hot enough to fuel fusion reactions. Long a puzzling concern of researchers has been the impact on turbulence of atoms recycled from the walls of tokamaks that confine the plasma. These atoms are neutral, meaning that they have no charge and are thus unaffected by the tokamak’s magnetic field or plasma turbulence, unlike the electrons and ions — or atomic nuclei — in the plasma. Yet, experiments have suggested that the neutral atoms may be significantly enhancing the edge plasma turbulence, hence the theoretical interest in their effects.
Turbulence, the violently unruly disturbance of plasma, can prevent plasma from growing hot enough to fuel fusion reactions. Long a puzzling concern of researchers has been the impact on turbulence of atoms recycled from the walls of tokamaks that confine the plasma. These atoms are neutral, meaning that they have no charge and are thus unaffected by the tokamak’s magnetic field or plasma turbulence, unlike the electrons and ions — or atomic nuclei — in the plasma. Yet, experiments have suggested that the neutral atoms may be significantly enhancing the edge plasma turbulence, hence the theoretical interest in their effects.
In the first basic-physics attempt to study the atoms’ impact, physicists at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) have modeled how the recycled neutrals, which arise when hot plasma strikes a tokamak’s walls, increase turbulence driven by what is called the “ion temperature gradient” (ITG). This gradient is present at the edge of a fusion plasma in tokamaks and represents the transition from the hot core of the plasma to the colder boundary adjacent to the surrounding material surfaces.
Extreme-scale computer code
Researchers used the extreme-scale XGC1 kinetic code to achieve the simulation, which represented the first step in exploring the overall conditions created by recycled neutrals. “Simulating plasma turbulence in the edge region is quite difficult,” said physicist Daren Stotler, who took over authorship of the paper, published in Nuclear Fusion in July, once PPPL computational scientist Jianying Lang joined Intel Corp. in California. “Development of the XGC1 code enabled us to incorporate basic neutral particle physics into kinetic computer calculations, in multiscale, with microscopic turbulence and macroscale background dynamics,” he said. “This wasn’t previously possible.”
Read more at DOE/Princeton Plasma Physics Laboratory
Image: This is physicist Daren Stotler. (Credit: Elle Starkman/PPPL Office of Communications)
