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PPPL physicist Brian Grierson wins DOE Early Career Research Program grant

Physicist Brian Grierson of the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) has won a highly competitive Early Career Research Program award sponsored by the DOE’s Office of Science. The five-year grant will total some $2.5 million and fund exploration of the mechanisms that govern the formation and maintenance of the hot edge of fusion plasmas — the electrically charged gas that results in fusion reactions in facilities called tokamaks. The work will be carried out on the DIII-D National Fusion Facility in San Diego.

Understanding and controlling the edge of hot fusion plasma is crucial for achieving high performance in devices like the ITER tokamak, the world’s largest fusion experiment currently under construction in the south of France.

The award for Grierson, who is on long-term assignment to the DIII-D National Fusion Facility operated by General Atomics for the DOE, marks the second Early Career grant to PPPL physicists in as many years. Ahmed Diallo, who serves as deputy boundary-group leader for PPPL’s National Spherical Torus Experiment (NSTX), won a five-year award for research on the plasma edge last year.  

Exceptional ability

The Early Career grants fund scientists who have demonstrated exceptional ability. This year’s awards went to 35 researchers who were chosen from among some 750 applicants from across the country. “By supporting our most creative and productive researchers early in their careers, this program is helping to build and sustain America’s scientific workforce,” said Patricia M. Dehmer, Acting Director of DOE’s Office of Science.

Grierson’s research focuses on a thin, roughly thumb-wide slice of the plasma edge called the “pedestal” in which conditions change very rapidly, becoming far hotter and denser toward the plasma core. “It’s like going from the surface atmosphere to the bottom of the sea,” Grierson said of the transition, “only this change happens in a very small distance.”

Conditions within the little-understood pedestal can strongly affect the core of the plasma where fusion reactions take place. Comprehending what happens in the pedestal can thus lead to better confinement, greater stability and more fusion power in tokamak plasmas.

Funds from the Early Career award will enable Grierson to install new diagnostic equipment on the DIII-D tokamak and bring in two postdoctoral fellows to assist with the research. Included in the new equipment will be 16 fiber-optic lines to relay light from the edge of the plasma to new spectrometers — devices that measure and record shifts in in the spectrum of light — and to high-speed scientific cameras.   Grierson will work closely with researchers from General Atomics, Oak Ridge National Laboratory, Lawrence Livermore National Laboratory and several U.S. universities to leverage this new measurement capability towards an improved understanding of the pedestal region.  

Testing computer models

Results of Grierson’s experiments will test computer models that seek to predict conditions such as the temperature and pressure of the deuterium atomic nuclei, or ions, the main fusion fuel. The behavior of these fueling ions has proven far more difficult to measure than the behavior of impurities that enter the edge of the plasma from the walls of a tokamak and have been studied in detail. “Our research combines diagnostic development with model validation,” Grierson said.

Current methods infer the behavior of the main fueling ions from the properties of impurity ions. However, there is significant uncertainty in these inferences because of the complexity of the physics at the plasma edge. So direct measurement of the fueling ions is essential to advance understanding of edge-plasma physics and to test emerging plasma models.

Those building edge models include theorists from PPPL and General Atomics who are developing complex, large-scale simulation codes to run on supercomputers. “There’s a lot of theoretical work out there and the question is how to tie it all together with state-of-the-art measurements,” noted PPPL physicist Richard Hawryluk, who oversees the Laboratory’s collaboration with General Atomics on DIII-D.

Grierson’s new research will employ techniques that he previously developed to measure the properties of main ions in the plasma core. “The challenges at the core were overcome by comprehensive modeling and detailed spectral analysis,” he said. “As we get to the edge there will be new challenges as well.”

While such challenges could be great, the scientific results could be greater. Combining measurement of the main ions with validation of models of the plasma edge could help pave the way for high-performance operation of ITER and future tokamaks, Grierson said.

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