By Robert Ryne
The 2023 Robert R. Wilson Prize for Achievement in the Physics of Particle Accelerators has been awarded by the American Physical Society (APS) to Alexander J. Dragt, Sr. He received the honors “for pioneering contributions to the development and application of Lie methods in accelerator physics and nonlinear dynamics.”
Alex is emeritus professor in the Department of Physics and Astronomy of the University of Maryland, College Park. He has also had a long affiliation with the University of California and Berkeley Lab. He received his Ph.D. in theoretical physics from UC Berkeley in 1964, studying under Robert Karplus.
During his career Alex was twice a visiting staff member at Los Alamos National Laboratory, then operated by UC. He was a Guest Scientist at the SSC Central Design Group in 1985 when it was located at Berkeley Lab. He was an affiliate for many years in the ATAP Division and its predecessor AFRD. Several of his doctoral students went on to become staff members at Berkeley Lab, including present staff Chad Mitchell and Marco Venturini and former staff David Douglas (retired from Jefferson Lab), Etienne Forest (KEK), and the author.
Among his many professional activities, Alex has chaired the Executive Committee of the American Physical Society Division of Physics of Beams. He has served on many government committees, including the High Energy Physics Advisory Panel (HEPAP), the DOE Committee on High Performance Computing, and the DOE Advisory Panel on Accelerator Physics Computer Codes.
Starting in the 1970s, Alex and his students developed new mathematical and computational tools for modeling dynamical systems including beam dynamics in accelerators. He championed the use of transfer maps in accelerator physics, work that was recognized by a 2013 IEEE Particle Accelerator Science and Technology award. The use of transfer maps led to new methods to characterize, design, analyze, and optimize dynamical systems.
Alex was among the first to emphasize the importance of simulating Hamiltonian systems with tools that maintain the symplectic condition. In 1976 Alex and a graduate student, John M. Finn, developed a method for describing symplectic maps as a factored product of Lie transformations, and published what is now known as the Dragt-Finn factorization theorem. This led to the first beam dynamics code based on Lie algebraic methods, MaryLie. The Lie polynomials in the factored product representation are directly related to aberrations in single-pass systems.
Alex and his students also developed normal form techniques that are crucial to the analysis of long-term dynamics in circular accelerators. Other important developments included new methods for symplectic tracking, as well as techniques for modeling magnets and RF cavities with realistic fringe fields. Also, Alex was also among the earliest beam physicists to espouse the concept of eigen-emittances and to develop numerical methods for their computation.
Alex continues to work on his book Lie Methods for Nonlinear Dynamics with Applications to Accelerator Physics, which can be downloaded for free. The roughly 3000-page book contains considerable background material, along with a very substantial portion—developed by Alex himself and in collaboration with his students—devoted to applications in accelerator physics.
The mathematical and computational methods pioneered by Alex, and developed by him along with his students and collaborators, benefit every new design of a major accelerator today. His emphasis on tools that enforce the symplectic condition had a huge impact on the prediction of long-term dynamics in circular accelerators, and his focus on transfer maps led to new techniques for beamline analysis and optimization. The Lie-algebraic representation of maps has led to new ways to characterize the nonlinear content of dynamical systems in general.
The implications of this work extend far beyond particle accelerators. These methods have applications not only to accelerator physics, but also to electron microscopy, geometric light optics, and plasma physics. ATAP extends its congratulations to Alex on the well-deserved honor of the 2023 Wilson Prize.
ATAP retiree Robert Ryne headed the Accelerator Modeling and Advanced Computing Group in the Center for Beam Physics.