Physicist Greg Hammett honored for his work advancing understanding of fusion plasmas

Written by
John Greenwald
Sept. 23, 2021

Theoretical and computational physicist Greg Hammett, a leader in advancing understanding of the complex turbulence that controls the performance of fusion plasmas and a dedicated educator, has been named a 2021 Distinguished Scientist Fellow by the Office of Science of the U.S. Department of Energy (DOE). The prestigious and highly competitive award honors the outstanding achievements of Hammett, a physicist in the DOE’s Princeton Plasma Physics Laboratory (PPPL), for work he has done with colleagues to push forward the development of increasingly realistic computer simulations of turbulence in fusion plasmas and in the cosmos.

The award, authorized by the America COMPETES Act, recognizes exceptional scientists at the 17 DOE national laboratories who have substantially advanced innovation in their fields and provides Fellows with $1 million in direct funding over three years to devote to projects of their choosing.

“The talented scientists that make up DOE’s Distinguished Scientist Fellowship Program are advancing the scientific solutions for some of our nation’s greatest challenges, from measuring the impact of climate change to developing the bioeconomy and pursuing fusion as a future energy source,” said U.S. Deputy Secretary of Energy David Turk. “I look forward to their continued success in achieving impactful results as they also work to inspire and guide a diverse new generation of scientists.” A virtual ceremony will be held on Oct. 20, 2021.

The citation honors Hammett, a physicist at PPPL for 35 years, “For leading the development of the quantitative theory and simulation of plasma turbulence in fusion and astrophysics, and for educating and mentoring a diverse group of graduate students and early career researchers.” He is one of three honored.

For Hammett, the honor deserves to be shared. “I was pleased and surprised at the award and I hope that it is viewed as recognition of the excellent work done by people I collaborate with and others in the fusion program,” said Hammett. “It’s really been a team effort with brilliant colleagues and graduate students so I hope that this award is a way for them to gain recognition as well.

“We have also benefited from scientists we don’t directly collaborate with, as we have learned from the advances they’ve made and from interacting with them at seminars and conferences,” he said. “This reflects on the high quality of research done to understand the complex nonlinear dynamics of fusion plasmas.”

Theoretical and computational physicists use computer codes to simulate and predict the impact of turbulence on the effort to reproduce fusion, which combines light elements in the form of plasma — the hot, charged state of matter composed of free electrons and atomic nuclei— to power the sun and stars. Scientists around the world are seeking to reproduce and harvest fusion energy for a virtually inexhaustible supply of safe and clean power to generate electricity.

Turbulence occurs when small fluctuations in fusion plasmas produce eddies that can shift heat from the hot core of the plasma to its outer edge. The transport degrades the performance of the plasma by lowering the temperature of the multi-million-degree core where fusion reactions take place. While this does not hinder stars in producing energy, Hammett and his colleagues and students have developed and applied cutting-edge computer codes to comprehend the highly complex turbulent process and enable experimentalists to deal with it on Earth.

Said Steve Cowley, PPPL director: “From his work in the 1990s with Bill Dorland and other students on gyro-fluid models to his present-day leadership on gyrokinetic models with the Gkeyll group, Greg Hammett has been a world leader in the development of the theory of turbulence in confined plasmas. It is an astonishing contribution that has revolutionized our understanding. Greg has always been gracious and generous about the contributions of others so I am particularly gratified that his immense personal impact has been recognized by this prestigious award.”

Hammett and collaborators will conduct the award projects in several tasks aimed at reducing the small-scale turbulence that causes hot particles to leak from a fusion device. These tasks range from investigating possible multi-scale algorithms to significantly speed up cutting-edge codes to studying the effects of plasma shape on turbulence, and to understanding the dramatic reduction in turbulence caused by coating some of the interior walls of a device with lithium liquid metal as the Lithium Tokamak Experiment-beta has done at PPPL.

Their work has opened new doors on this major challenge to the production of fusion energy, said Jon Menard, deputy director for research at PPPL. “Greg is a world-leading expert on plasma turbulence theory and simulation, and is highly deserving of this award. I especially look forward to Greg and his collaborators investigating the role of lithium wall coatings in reducing plasma turbulence. We do not yet have a fundamental understanding of how the turbulence is modified, and further improving confinement will be key to reducing the scale and cost of fusion energy,” Menard said.

Hammett explained his work: “When computers are used to solve equations to predict the performance of a fusion plasma, it is a lot like using computers for weather or hurricane predictions,” he said. “The equations describe how conditions like airspeed and temperature are affected at millions of locations by conditions at adjacent points, and are solved to predict what things will look like one minute from now, two minutes from now, and so on. Weather predictions are pretty good for one day, but get harder the farther into the future we go. Thankfully, for fusion we don't need to predict the exact plasma ‘weather’ on each day. We only need to know the average amount of plasma rain and sunshine that will occur averaged over time.”

Hammett also has been instrumental in mentoring physics students for much of his tenure at PPPL. “Complementing his research at PPPL, Professor Hammett has been a superb mentor and teacher, serving for many years as Lecturer with Rank of Professor in the Program in Plasma Physics at Princeton University,” said Nat Fisch, director of the Program in Plasma Physics, and Princeton University professor of astrophysical sciences. “Professor Hammett is a natural teacher, and has been an absolutely essential member of the faculty of this graduate program. On a personal note, having co-taught a course with Professor Hammett, I can say from up close what a joy it is to teach with him; his enthusiasm is infectious; his dedication to students is unlimited; and his ability to capture and explain complex processes is simply a gift. It is my hope that this highly deserved recognition will facilitate the further profound contributions of Professor Hammett not only in his research, but also in teaching and in mentoring the next generation of researchers.”

The son of a U.S. Air Force fighter pilot, Hammett was born in Japan and attended 13 schools from Idaho to Georgia before graduating from a small-town Georgia high school. “There were a number of things in physics that my father was the first to tell me about that inspired me,” Hammett said, "and my mom was an avid encourager of all my academic interests. Dad was a hands-on guy who did drag racing as a 1950’s teenager and souped up his car. When I was a kid I helped him a lot repairing our cars, replacing brake pads and hydraulic systems, stuff like that.”

The era also drew Hammett to science. “I was a child of the Apollo moon landing era and I felt a little closer to all that because my father flew the F-106 airplane that could reach high altitudes and he sometimes wore a spacesuit. Not just an oxygen mask but a full-pressure suit, similar to what the astronauts wore, so there were people around me who were involved with things like that.”

Hammett earned his bachelor’s degree in physics from Harvard University in 1980 and his doctorate from Princeton in 1986, the year he joined PPPL. He has been a Lecturer with Rank of Professor in the Princeton Program in Plasma Physics since 2001 and is an associated faculty member with the Program in Applied and Computational Mathematics at Princeton. As a visiting scientist or research fellow he has done research at institutions ranging from the Joint European Torus (JET) laboratory and the University of Oxford in the United Kingdom to the University of California, Berkeley. An American Physical Society Fellow, he has taught graduate courses since 1995 and has supervised 11 doctoral dissertations and mentored many other students

Hammett has written or co-authored more than 180 papers in scientific journals and proceedings of national and international conferences. His work on fluid models of a process called Landau damping, which extends fluid equations to handle kinetic effects, has been cited in more than 400 published papers and has found applications in fields as diverse as turbulence in the ionosphere and the plasma processing of semiconductors.

When not pursuing plasma turbulence, Hammett enjoys biking, and taking walks and watching TV mysteries with his wife, Kate, an artist turned craftsman who designs innovative furniture. They share their home with their tiny Maltipoo, Lily — a retired therapy dog who visited hospital patients and schools for eight years with Kate — and an orange tabby cat named Walter. The couple is also a “professional aunt and uncle. I’ve got six nieces and nephews on my side and four on my wife’s side and that keeps us busy,” he said.

Hammett eagerly looks forward to the work ahead. “I think this is an exciting time for fusion,” he said. “There are some really cool ideas on how to improve fusion reactor designs, and computers are 100 million times more powerful now than when I was a graduate student. I’m just kind of blown away with the capabilities that are on them. Just as people used computers to design better airplanes, we hope to use them to design better fusion devices.”

For more information about the ceremony and contributions the 2021 Distinguished Scientist Fellows has made to U.S. leadership in energy, science and security, please visit the Office of Science’s Distinguished Scientist Fellows website.

PPPL, on Princeton University's Forrestal Campus in Plainsboro, N.J., is devoted to creating new knowledge about the physics of plasmas — ultra-hot, charged gases — and to developing practical solutions for the creation of fusion energy. The Laboratory is managed by the University for the U.S. Department of Energy’s Office of Science, which is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit

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