Russell M. Kulsrud, 97, a pioneering theoretical physicist whose work helped define the modern fields of plasma physics and astrophysics -- connecting the physics of fusion reactors to the forces that shape the cosmos -- died on September 23, 2025, after a short illness.
In a career spanning more than six and a half decades at Princeton University, Dr. Kulsrud helped lay the theoretical foundations for understanding plasmas -- the ionized gases that make up stars, solar winds, and much of the visible universe. His insights ranged from the behavior of charged particles in fusion reactors to the origins of magnetic fields in galaxies.
"He possessed exceptional physical intuition," said the late astrophysicist John Bahcall, his longtime colleague at Princeton University and the Institute for Advanced Study. "He could see to the heart of a problem and help the rest of us see it, too."
Dr. Kulsrud's research advanced both fusion energy and astrophysics, fields that prior to his work rarely intersected. At the Princeton Plasma Physics Laboratory (PPPL), he explored the stability of magnetically confined plasmas -- the key to achieving controlled fusion, the process that powers the sun. Indeed, he and his colleagues at PPPL pioneered much of modern stability theory. In astrophysics, he investigated cosmic rays, black holes, magnetic turbulence, and galactic dynamos -- the mechanisms that generate magnetic fields across the universe.
His blend of mathematical rigor and physical intuition made him a natural teacher. Over his career, he advised 19 doctoral students who then spawned over 54 "grand-students." "He made complex phenomena understandable," said a former student. "He taught us to think about the universe as a single, connected system." Steven Cowley, Director of PPPL and a former student of Kulsrud said, "He was extraordinarily generous with his time. During countless hours together at the blackboard, he showed me that, when viewed from the right perspective, everything becomes simple."
In 1993, the American Physical Society awarded Dr. Kulsrud its highest honor in the field, the James Clerk Maxwell Prize for Plasma Physics, recognizing his "pioneering contributions to basic plasma theory, to the physics of magnetically confined plasmas, and to plasma astrophysics."
He later distilled his lifetime of knowledge in the 2005 textbook Plasma Physics for Astrophysics, praised for its rare combination of accessibility and intellectual depth. It remains a staple in graduate programs around the world.
From Kansas to the Cosmos
Russell Marion Kulsrud was born on April 10, 1928, in Lindsborg, Kansas, a small town founded by Swedish immigrants, to Carl J. Kulsrud, a Norwegian immigrant and Harvard educated historian, and Mary Hughell Kulsrud, an English teacher and grammarian, whose pioneer family traveled to Iowa by covered wagon to stake a farm. Kulsrud showed an early fascination with both music and mathematics -- twin passions that would stay with him throughout his life.
He earned a bachelor's degree in physics and astronomy from the University of Maryland in 1949 and completed his master's and doctorate degrees in physics at the University of Chicago in 1952 and 1954, respectively. His doctoral thesis committee included four Physics Nobel Laureates: Enrico Fermi, Murray Gell-Mann, Hannes Alfvén, and Subrahmanyan Chandrasekhar, who served as his adviser.
Studying at University of Chicago's Yerkes Observatory, where he met his future wife, aspiring astrophysicist Helene he was mentored by celebrated Danish astronomer Bengt Strömgren. Dr. Kulsrud joined a young, secret government effort at Princeton known as Project Matterhorn. Founded by astrophysicist Lyman Spitzer, under the auspices of the Atomic Energy Commission, the project was dedicated to exploring controlled nuclear fusion -- a potential source of clean and limitless energy.
Dr. Kulsrud was among its earliest and most influential theorists, tasked with describing how magnetism could hold ultra-hot plasma in place long enough to release fusion energy. His elegant theoretical work helped form the basis for modern plasma confinement research.
He later became head of the Princeton Plasma Physics Laboratory's Theory Division and, in 1967, joined Princeton's Department of Astrophysical Sciences, where he taught and conducted research as a tenured professor until his retirement in 2004.
A Life of Science and Music
Outside the laboratory, Dr. Kulsrud was a devoted pianist who gave public recitals well into his eighties. He and his wife, Laney, were ardent supporters of The Princeton Festival. In Helene, Dr. Kulsrud found a partner who shared his drive to make the world a better place.
"He was always wise, kind, patient and always humble about his accomplishments," his daughter, Pamela Corey, said. "He had a deep gentleness -- a curiosity about everything, and a love of music and nature that balanced his scientific brilliance. His sense of humor was his calling card."
Dr. Kulsrud continued to pursue research after retirement, focusing on magnetic reconnection -- the process by which magnetic field lines break and reconnect, releasing massive amounts of energy, both in laboratory plasmas and in solar flares. Since 2006, he served as one of the science advisors to TAE Technologies, a fusion power company co-founded by Microsoft's Paul Allen's Vulcan Energy Group, that is developing sustainable and economically competitive alternative energy solutions.
Dr. Kulsrud is survived by his daughter, Pamela Corey, and son-in-law, Troy Corey, of Westchester, N.Y. Recently, he was predeceased by his wife of nearly 70 years, Helene Kulsrud (March 2025); a daughter, Suzanne Allison Gammon; and a son, Peter Clifford Kulsrud.
A memorial service and endowed lecture series in his honor will be announced by Princeton University's Department of Astrophysical Sciences. Flags at the university were lowered to half-staff in his memory.
A funeral service will be held at the Kimble Funeral Home, 1 Hamilton Avenue, Princeton, NJ on October 25, 2025 at 11 a.m. Donations in his name may be made to the Princeton Public Library or to the Michael J. Fox Foundation for Parkinson's Research.
In the words of a former colleague, "Russell Kulsrud spent his life illuminating the hidden order of the universe -- and did so with grace, patience, and music in his heart."