DIII-D Scientists to Work with PPPL to Find a Path to Sustained Fusion Energy

Researchers from the DIII-D National Fusion Facility are preparing to support their colleagues at the National Spherical Tokamak Experiment-Upgrade (NSTX-U) at the U.S Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) in a quest to develop sustained fusion energy. Under recently announced DOE funding programs, two teams at DIII-D will perform research on physics and instrumentation for NSTX-U as the facility’s staff work to restart operations late next year.

“One of the great strengths of the fusion energy field is its collaborative nature, and this program is a great example of that,” said DIII-D Director Richard Buttery. “Joint DIII-D/NSTX activities over the past two decades have been very productive and we look forward to continuing our collaborations on NSTX-U.”

DIII-D, which General Atomics operates as a national user facility for DOE’s Office of Science, is the largest magnetic fusion research facility in the country. It hosts researchers from more than 100 institutions across the globe, including 40 universities. The heart of the facility is a tokamak that uses powerful electromagnets to produce a doughnut-shaped, or toroidal, magnetic containment for confining a fusion plasma. In DIII-D, plasma temperatures more than 10 times hotter than the Sun are routinely achieved. At such extremely high temperatures, hydrogen isotopes can fuse together and release energy. (See Fusion Energy 101 explainer below for more detail on how fusion works.)

NSTX-U is also a tokamak, but it is more spherical in shape, looking like a cored apple with a much smaller central hole than the doughnut of DIII-D. The NSTX-U tokamak, after completing upgrades and repairs, is expected to resume operations as early as late 2021. The research that DIII-D scientists will focus on will determine whether the spherical shape of the tokamak, coupled with its heating and electric current-drive systems, can produce benefits that could help with the design of the next generation of fusion reactors.

DIII-D researcher Kathreen Thome is leading one of the teams that will explore ways to sustain longer periods of operation using heating and current-drive techniques in the spherical device. Thome’s team will use its extensive computer modeling and simulation expertise to predict the best approaches to utilizing NSTX-U capabilities, then work to optimize those scenarios once the facility begins operations.

“It’s great to get a chance to apply your research to another tokamak with a different configuration,” Thome said. “Exploring both tokamak configurations brings us closer to understanding the underlying physics, moving us closer to our goal of practical fusion energy.”

In tokamaks, the plasma must carry a current to generate the magnetic structure that holds the heat. However, plasmas can self-generate this current in a process known as the “bootstrap effect.” This could greatly reduce the need for externally driven current in future fusion power plants, and thus cut costs or increase electrical output. These techniques have been experimentally evaluated on DIII-D, successfully validating state-of-the-art theory-based simulations. Thome’s team will apply knowledge gained from DIII-D to NSTX-U scenarios to understand how plasmas with high bootstrap currents could benefit future reactors.

The other DIII-D team will be working on sensors that will explore how energetic “fast” ions in NSTX-U transfer power to the plasma as they whirl around the device. This understanding will be key to keeping those fast ions contained in the magnetic fields of the tokamak, thereby allowing sustained high performance. The team will build sensors to measure how the fast ions interact with magnetic waves in the plasma, which can reduce performance by kicking the fast ions out of the magnetic containment.

GA researcher Luke Stagner will be working on the fast-ion project with Bill Heidbrink, his doctoral thesis advisor from the University of California, Irvine.

“This is an exciting time to be joining the U.S. fusion energy community,” Stagner said. “If all goes according to plan, we will have both of our major tokamaks online and working toward the same goal of creating a stable and sustainable approach to fusion energy.”