Roger Calder (1934 – 2014)
Roger joined CERN’s Accelerator-Research Division in May 1963, at the beginning of the Intersecting Storage Rings (ISR) project. The challenge was to evacuate the two beam pipes of about 1 km each to 10-7 Pa and to obtain a beam lifetime of 20 hours. An ultra-high vacuum system of this size and complexity required the most advanced technologies available.
Among the outstanding contributions by Roger were: the choice of turbomolecular- and ion-pumps, low outgassing rates and an all-metal system bakeable to 300°C. Still today, Roger’s name is linked to "900°C vacuum degassed stainless steel" for very low hydrogen outgassing.
The unexpected dynamic pressure run-away during the ISR start-up, at only a few A of beam, was quickly identified as the "ion-induced pressure bump instability". Among Roger’s efforts to mitigate this effect, the pumping system was supplemented with titanium sublimation pumps. In a second step, the ion induced molecular desorption yield was reduced by a novel cleaning method, known as argon-oxygen glow discharge. Together, these measures introduced by Roger resulted in the ISR operating with stable beam currents well above 2x40 A, more than twice the design value, and beam lifetimes of many days.
Detailed understanding of beam-induced outgassing required skilful experiments. Roger contributed greatly to creating the necessary laboratory facilities for surface analysis. For his profound understanding of underlying physical processes, including the interaction with the proton beam itself, his advice was sought wherever storage rings were built. At CERN, his insight contributed greatly to the adoption for LEP of rigorous cleaning methods and “distributed pumping”, without which the vacuum requirements could not have been met.
The latest contribution by Roger concerned the LHC vacuum. Based on his earlier work at CERN, he demonstrated that cryo-pumping large quantities of hydrogen at low saturation pressure would require a temperature of less than 3 K combined with a ‘beam screen’ inserted into the cold bore of the LHC magnets.
Roger’s way of studying everything in detail, looking at a problem from all possible angles and verifying twice before declaring a result, was much appreciated. It was his painstaking attitude that, in 1980, made him a valued Assistant Editor of the British journal “Vacuum”.
Roger had the invaluable aptitude to ask pertinent questions tactfully and to guide his colleagues towards the best solution to a problem, even in fields not directly related to physics. Roger was an ideal teacher for young recruits, whom he patiently guided to competence and accompanied in a fatherly role during their career. Even today, long after his retirement in 1997, their gratitude is still evident.
His friends and colleagues
