On 8 September, Fermilab senior scientist and world-leading beam physicist Yuri I Alexahin died from a sudden stroke.
Yuri was born in 1948 in the Russian town of Vorkuta. After studying physics and graduating from Moscow State University, from 1971 to 1988 he worked at the Joint Institute for Nuclear Research in Dubna and, in 1980, received his PhD in physics from the Institute of High Temperatures of the USSR Academy of Sciences. In Dubna, Yuri developed an interest in the physics of accelerators and beams and, especially, of charged-particle colliders, which remained the focus of his work throughout his career. He generated brilliant ideas and made critical contributions to a number of facilities and projects. He proposed a new scheme for a tau-charm factory based on monochromatisation to reduce the collision energy spread, addressed a problem of limited dynamic aperture at high energies faced by CERN’s LEP collider, and recommended the low-emittance option for LEP operation at the W± production energies.
Yuri published pioneering works on the theory of coherent beam–beam oscillations and their stabilisation with Landau damping, laying the foundation for a parameter optimisation of the LHC. Among many other highlights, Yuri ingeniously predicted the loss of Landau damping for the two beams colliding in the LHC, derived analytical formulae describing the emittance growth in collision with transverse feedback and noise, and produced some of the most thought-provoking articles related to the LHC design.
In 2000, Yuri joined Fermilab’s accelerator division, where he made seminal contributions to the theory of nonlinear beam–beam compensation by electron lenses and was deeply engaged in Run II of the Tevatron, playing a critical role in the luminosity increases of what was then the world’s most powerful accelerator. Widely recognised is Yuri’s leading role in the design and implementation of optimal helical orbits to minimise Tevatron beam–beam effects at injection, acceleration and squeeze, and his optimisation of the beam lifetime at injection energy via reduction of the differential chromaticity.
From 2007 to 2018 Yuri led the accelerator theory group at Fermilab. These years were another extremely productive period, as he steered the interaction-region lattice development for an energy-frontier muon collider within the US Muon Accelerator Program. He also invented the so-called “helical FOFO-snake” muon ionisation cooling channel concept. Yuri was closely involved in the operation and upgrade of the existing Fermilab accelerator complex and in other intensity-frontier accelerators worldwide. Not only was he actively taking part in many experimental beam studies, but he also proposed new theoretical and numerical algorithms for space-charge dominated beams, for the Landau damping of beam instabilities provided by electron lenses and for novel space-charge compensation techniques.
Yuri will be remembered by his colleagues, friends and family as a highly intelligent, kind and soft-spoken person. An excellent mentor, he generously shared his knowledge with students and younger colleagues, and many world-renowned physicists are happy to call him their teacher. While being a workaholic, in his leisure time he loved to ski and was an avid sports fan.