As part of an initiative to support the development of nuclear fusion as a future practical energy source, the U. S. Department of Energy is renewing three-year funding for two Plasma Science and Fusion Center (PSFC) projects on the Wendelstein7-X (W7-X) stellarator at the Max Planck Institute for Plasma Physics in Greifswald, Germany.
The largest stellarator in the world, W7-X was built with helically-shaped superconducting magnets to investigate the stability and confinement of high temperature plasma in an optimized toroidal configuration, ultimately leading to an economical steady state fusion power plant. With plasma discharges planned to be up to 30 minutes long, researchers anticipate W7-X will demonstrate the possibility of continuous operation of a toroidal magnetically-confined fusion plasma.
PSFC principal research scientist Jim Terry is being funded to build and install on the stellarator a new diagnostic called “Gas-Puff Imaging,” which measures the turbulence at the boundary of the hot plasma by taking images in visible light at 2 million frames per second. The light is emitted as the plasma interacts with gas that is introduced locally at the measurement location. This fast frame rate allows researchers to see the dynamics of the turbulence. Observing plasma turbulence in fusion devices will help researchers understand how to better confine the plasma, while at the same time handling the plasma’s exhaust heat.
The new funding of $891,000 is a renewal of a three-year grant that ran from 2015 to 2018, during which time this diagnostic was designed. Terry’s team includes PSFC research scientist Seung Gyou Baek, as well as graduate student Sean Ballinger of the Department of Nuclear Science and Engineering and undergraduate physics major Kevin Tang, both of whom have had extended stays on-site at W7-X.
Over the past three years, professor of physics Miklos Porkolab and his team have designed and installed a “phase contrast imaging” (PCI) diagnostic on W7-X. PCI is a unique interferometer method using a continuous wave coherent carbon dioxide laser and additional specialized optical components that allow it to measure instantaneously the turbulent density fluctuations in the core of the hot plasma.
Using data collected over the past year, the team is analyzing the measured turbulence levels and comparing them with predictions of state-of-the-art gyrokinetic codes, assessing how turbulence contributes to the loss of energy and particles in an optimized stellarator. The renewal of this three-year grant, for $900,000, will fund not only personnel to continue analysis of experimental data, but also necessary upgrades to allow simultaneous imaging of core and edge fluctuations, making the PCI diagnostic versatile in its ability to measure a wide range of waves and instabilities.
In addition to Porkolab, members of the team include former PSFC staff scientist Eric Edlund, now an assistant professor at SUNY Cortland, who played a key role in the design of this diagnostic; and PSFC postdoc Zhouji Huang, who is stationed onsite in Greifswald. PSFC research physicist Alessandro Marinoni and postdoc Evan Davis (both stationed at DIII-D, an MIT collaboration in San Diego) also contributed to the project during the summer of 2018.