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 modelled 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.
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", stated physicist Daren Stotler, who took over authorship of the paper, published inNuclear Fusionin 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 stated. "This wasn't previously possible."
The results, reported in July in the journalNuclear Fusion, showed that neutral atoms enhance ITG turbulence in two ways:
Going forward, researchers plan to compare results of their model with experimental observations, a task that will require more complete simulations that include other turbulence modes. Findings could lead to improved understanding of the transition of plasmas from low confinement to high confinement, or H-mode - the mode in which future tokamaks are expected to operate. Researchers generally consider lower recycling, and hence fewer neutrals, as conducive to H-mode operation. This work may also lead to a better understanding of the plasma performance in ITER, the international fusion facility under construction in France, in which the neutral recycling may differ from that observed in existing tokamaks.
This research was performed under the supervision of PPPL physicist C.S. Chang. Modelling was done on the Titan supercomputer at the Oak Ridge Leadership Computing Facility and the Edison supercomputer at the National Energy Research Scientific Computing Center with support from the DOE Office of Advanced Scientific Computing Research and the DOE Office of Science (FES).
PPPL, on Princeton University's Forrestal Campus in Plainsboro, New Jersey, is devoted to creating new knowledge about the physics of plasmas - ultra-hot, charged gases - and to developing practical solutions for the creation of fusion energy. The Laboratory is managed by the University for the U.S. Department of Energy's Office of Science, which is the largest single supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time.