Stewart C. Prager, professor of astrophysical sciences and an affiliated faculty member of the Program on Science and Global Security, will transfer to emeritus status on July 1, 2022, having served fourteen years on the Princeton faculty and as the director of the Princeton Plasma Physics Laboratory (PPPL) for seven years. 

Stewart grew up in the Bronx, New York City. In 1970, he earned a bachelor’s degree in liberal arts from Queens College of the City University of New York and a bachelor’s degree in electrical engineering from Columbia University. Stewart earned his Ph.D. in plasma physics from Columbia in 1975. He then conducted plasma physics research at the San Diego-based General Atomic Company before joining the University of Wisconsin’s physics department faculty in 1977. During his thirty-one years at Wisconsin, Stewart taught physics and led research teams studying basic plasma physics and magnetic confinement for fusion energy, and became the Dexter Professor of Physics. As part of his research on the levitated octupole experiment, he was a co-discoverer of the bootstrap current, which is created by the plasma pressure, and can be used to improve the plasma confinement. For this work, Stewart co-received the American Physical Society’s (APS) John Dawson Award for Excellence in Plasma Physics Research in 2008. 

Stewart co-proposed, constructed, and directed the Madison Symmetric Torus experiment (MST), funded by the Department of Energy (DOE). This experiment explored the use of a weak magnetic field to confine plasma, instead of the usual strong magnetic field. The weak magnetic field became chaotic due to plasma turbulence. It selforganized, generating additional magnetic field in a manner similar to the generation of the earth’s magnetic field. The researchers used MST to study many aspects of magnetic chaos and turbulence that occur in both laboratory and astrophysical plasmas. They developed an understanding of the cause of this magnetic chaos and used that understanding to reduce turbulence and suppress the plasma heat loss, making the magnetic confinement more efficient. These insights and approaches were adopted by laboratories around the world to study these types of plasma configurations. 

Stewart joined the Princeton faculty in 2009 and became the sixth PPPL director. He strengthened the Laboratory programs researching plasma physics both connected to fusion energy and connected to other areas, including laboratory astrophysics and the interaction between plasma and materials. He shepherded the application process that won DOE approval for a substantial upgrade and enhancement of the National Spherical Torus Experiment user facility at PPPL. He strengthened the Laboratory’s theoretical plasma physics program with new leadership and a formal partnership with the experimental programs. Stewart also led a revitalization of the Laboratory’s research staff, emphasizing a vibrant post-doctoral fellowship program. To develop a stronger fusion energy program, he instigated a set of “pilot plant” fusion system studies comparing designs based on different magnetic confinement configurations. These studies produced highly influential papers, generated a new series of international conferences organized by the International Atomic Energy Agency, and have led the community to call for a series of national pilot plant studies as the basis for future facilities and programs. 

Stewart has constantly strived to strengthen the scientific community, particularly for plasma physics, fusion, and plasma-astrophysics research. He organized multi-organization collaborations and cooperation. While at Wisconsin, he was the founding director of the Center for Magnetic Self-Organization in Laboratory and Astrophysical Plasmas (CMSO), a multi-institutional National Science Foundation (NSF) Physics Frontier Center. At Princeton, Stewart co-founded the Max-Planck-Princeton Center for Plasma Physics (MPPC), a collaboration between Princeton, PPPL, the Max Planck Society in Germany, and three Max Planck Institutes. In the United States, this was supported by the DOE, the NSF, and Princeton. The MPPC grew to include other US, EU, and Japanese research institutions and researchers. He also led periodic strategy discussions among the US plasma physics research institutions to promote collaboration and cooperation. 

Stewart’s service also included being the chair of the DOE Fusion Energy Sciences Advisory Committee, president of the University Fusion Association, chair of the APS Division of Plasma Physics, vice-chair of the APS Forum on Physics and Society, and a member of the President’s Committee of Advisors on Science and Technology (PCAST) panel on the US Fusion Energy R&D Program. He received the Leadership and Distinguished Career awards from Fusion Power Associates and is a fellow of the APS. 

After focusing his career on plasma physics with application to fusion energy and astrophysics, Stewart switched in 2018 to studying security policy issues. In 2020, he co-founded (with Zia Mian and Frank von Hippel) the Physicists Coalition for Nuclear Threat Reduction, under the auspices of the APS. After transferring to emeritus status, he plans to continue his current project, together with colleagues in the Program on Science and Global Security, to engage and activate physicists on the issue of nuclear threat reduction. 

Written by members of the Department of Astrophysical Sciences faculty.