Simulations Show How Recycled Atoms Boost Plasma Turbulence

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.

Their work represents the first step in exploring the overall conditions created by recycled neutrals. The results, reported in the journal Nuclear Fusion, showed that neutral atoms enhance ITG turbulence in two ways:

• First, they cool plasma in the pedestal, or transport barrier, at the edge of the plasma and thereby increase the ITG gradient.
• Next, they reduce the sheared, or differing, rates of plasma rotation. Sheared rotation lessens turbulence and helps stabilize fusion plasmas.

The PPPL team used the XGC1 code—an extreme scale edge gyrokinetic particle code, with the turbulence, background plasma and neutral particle dynamics solved together in multiscale—to achieve the simulation. The study began on the Titan supercomputer at the Oak Ridge Leadership Computing Facility (OLCF) but was then moved to the Edison supercomputer at the National Energy Research Scientific Computing Center (NERSC). Edison was more efficient in solving this problem, according to C.S. Chang, head of the SciDAC Center for Edge Physics Simulation at PPPL who oversaw this research and was a co-author on the Nuclear Fusion paper.

“XGC1 scales almost perfectly to the maximal number of cores on Edison, in both weak and strong scaling,” Chang said.

Simulating plasma turbulence in the edge region is quite difficult, added physicist Daren Stotler, a principal research physicist at PPPL and lead author on the Nuclear Fusion paper. “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. This wasn’t previously possible.”

Going forward, the team plans 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.

NERSC and OLCF are both DOE Office of Science User Facilities.

This article is based on resources provided by PPPL.