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Bruno Zotter passed away on 22 December 2015. He was one of the leading theoretical accelerator physicists who made essential contributions in the recent period of development of particle accelerators after single-particle stability became well understood, and after which the interaction of the beams of increasing intensity with themselves and their environment moved into focus. Bruno was the right man at the right time for this. His thesis at the Technische Hochschule in Vienna had dealt with the calculation of electromagnetic fields in high-frequency cavities, one of the topics that accompanied him for the rest of his professional career which, after a short stint at the International Patent Office in The Hague, started with work on low-noise travelling-wave tubes in a US Government laboratory in New Jersey. The predictable decline of this line of research due to the emergence of semiconductor applications led him to move into the related particle accelerator field at CERN’s ISR where the importance of high-intensity beam phenomena had already been realized during the construction phase. Bruno refined existing models and developed new ones favouring an analytical approach in his many contributions laying the foundations for the steady increase in circulating proton beam current up to typically 40 A. He had a propensity for theory and mathematics, a sign of this is his work on the summation of infinite algebraic and Fourier series, but he also participated in experimental work in parallel to advancing theoretical understanding. This included topics as diverse as space-charge phenomena, beam-beam effects, definition and determination of the accelerator coupling impedance characterizing the potential of the adverse interaction of a vacuum chamber with the beam, depending on the frequency spectrum of the latter. He applied his insight also when the SPS became affected by high-intensity effects after the running-in phase and he actively participated in the CERN studies of the options for a post-ISR accelerator at the high-energy frontier, the most prominent examples being LEP, LHC and CLIC. LEP offered an ideal playground for Bruno, in particular, the interaction of the very short, intense electron bunches with the vacuum enclosure and the long array of RF cavities. An impressive set of publications illustrates his tenacious investment where he put the enormous increase in computer power to good use to refine the comprehensive simulations of collective effects. Although being an appreciated lecturer at Accelerator Schools and Workshops, he found time to summarize the main part of his work in the book “Impedances and Wakes in High-Energy Accelerators” written together with his friend Sam Kheifets from SLAC. His competence, steady focus on high-level electrodynamics-related accelerator physics and an extraordinary gift for tutoring attracted an amazing number of students and visitors to work with him. Many visitors from abroad remember with gratitude his generous help in overcoming bureaucratic hurdles and getting around in the Geneva area. His colleagues remember him as an open but independently minded, often sarcastic discussion partner. He was known for perseveringly pondering over problems seemingly too difficult or tedious for the rest of us but tricky and, therefore, interesting enough to be worth his investment. After some gestation he would discreetly put his solution on the table. He stayed active in the field even after his retirement in 1997, remaining an appreciated tutor, discussion partner and co-author with a vivid interest in the latest developments and measurement results obtained in the CERN accelerators under the continuous push for higher performance. Having enjoyed over the years his competence and his benevolent, unassuming attitude, we are proud to have had the chance to work with him, be it as colleague, visitor or student. His friends and colleagues. |