Simon van der Meer, the inventor of stochastic beam cooling, passed away on 4 March 2011 in Geneva.
Stochastic cooling is essential for the increase in density of rare particle beams to obtain, for example, high interaction rate (luminosity) in a proton–antiproton collider. In 1984 the Nobel Prize in Physics was awarded to Simon and Carlo Rubbia for the discovery of the W and Z bosons in the CERN Super Proton Synchrotron (SPS) collider. Their CERN colleagues described the pair’s accomplishment using a phrase coined by Gösta Ekspong, head of the Nobel Committee: Simon made it possible, Carlo made it happen. Without cooling, scientists would never have reached the density of antiprotons required to provide evidence for the existence of the W and Z bosons.
Simon was born on 24 November 1925 in the Hague, the Netherlands. His parents worked to ensure that Simon and his three sisters received a high-quality education. Despite passing his final school examination in 1943, he stayed in high school for another two years; Dutch universities were closed because of World War II. Simon’s interest in electronics was sparked by his physics teacher, and he filled his home with all sorts of electronic gadgets.
He began his studies in electrical engineering in 1945 at Delft University of Technology and received his degree in 1952. Inspired by his previous practical work, Simon focused on feedback circuit analysis and related RF measurement methods. He told colleagues that those early studies may have paved the way for his main invention, two decades later, of stochastic beam cooling. But during the intervening years, his work experience deepened his knowledge in other fields of electrical engineering. His work on high-voltage equipment and electronics for electron microscopes at the Philips Research Laboratory in Eindhoven during the early 1950s helped him in designing particle accelerators, which later became the center of his professional life.
In 1956 Simon joined CERN, which had been founded in 1953. He was given the task of designing the pole-face windings for the magnets of the 26-GeV Proton Synchrotron, which is still in operation today. Limited computational tools were available then; hence the task required a lot of practice in solving complicated magnetic field problems by analytical means. That art is close to extinction today but was essential in the early days of CERN.
Inspired by discussions with his colleagues and support from supervisors John Adams and Colin Ramm, Simon became interested in particle physics. Using the framework of his magnet design, he looked for methods to produce dense neutrino beams by focusing their parent muons. That work triggered the idea of the magnetic horn, Simon’s first great invention, which is nowadays used worldwide for focusing and separating different kinds of particles near a target station.
The next step in his professional life was the g − 2 experiment in 1965. A small storage ring was used to carry out precision measurements of the magnetic moment of the muon. Simon was a member of both the design and the measurement teams.
In 1967 Simon returned to working on magnets, but at a much higher managerial level: He was responsible for the magnet power supplies of the Intersecting Storage Rings (ISR), the 30-GeV proton–proton collider. There he made his second great contribution to particle physics, the “van der Meer” scan, a method for measuring and optimizing the luminosity of colliding beams.
During that period he devised his third great concept, stochastic cooling. The object was to improve the beam density and thus the interaction rate in the ISR. However, in 1968 the idea looked farfetched to Simon and the few colleagues with whom he shared it. In addition, it was anticipated (and confirmed right after startup in 1971) that the ISR could store already without cooling such dense beams that stochastic cooling was not useful, even impossible, given the density limitations by space charge and beam instabilities. So for another four years the idea was regarded as a curiosity and not further pursued.
In 1972, almost by accident, Schottky noise signals of the beam coasting in the ISR were observed. Those signals are due to the granularity of the beam; they are similar to the noise in a DC electron beam, identified by Walter Schottky in 1918. Stochastic cooling may be interpreted as a damping of Schottky signals. Observation of those signals—together with the progress made in the damping of coherent beam oscillations—finally encouraged Simon to publish his idea in an internal note.
In the following years, scientists grew increasingly interested in stochastic cooling for other applications. After a proof-of-principle experiment in 1974 in one of the ISR rings, CERN set up a dedicated Initial Cooling Experiment (ICE) to which Simon contributed. The experiment consisted of a small storage ring, assembled from components of a former g − 2 ring. Rubbia and colleagues proposed to convert the SPS, which had just started operation, into a proton–antiproton collider capable of producing the W and Z bosons. It needed an antiproton accumulator (AA) ring. The object of ICE was to test all aspects of stochastic cooling necessary to cool and store antiprotons by the immense factor, 105 in intensity and 108 in density, required for the SPS collider to have a decent chance to catch the W and Z bosons.
The theory of stochastic cooling and stacking of relatively low-intensity beams required various extensions and computer codes for optimization. That became one of Simon’s preoccupations, and during 1977–84 he published important papers on the subject, including his seminal 1978 report, Stochastic Stacking in the Antiproton Accumulator, which paved the road to the AA. Guided by his work on stochastic cooling, Simon had another brilliant idea: noise-assisted stochastic extraction, a low-ripple version of resonant extraction. That new technique was successfully applied in the Low Energy Antiproton Ring, which started operation in 1983. It eventually made possible spills of 20-hour duration; conventional extraction gave, at best, a few seconds.
Simon and Roy Billinge became joint project leaders of the AA, which came on line in 1982. Simon participated in its design, construction, and operation. He knew that complex machine the best, as he contributed insatiably to solve problems.
The first discovery of the long-sought W boson was announced in January 1983 and that of the Z boson a few months later, in May. In the following year the Nobel Prize was awarded jointly to Rubbia and Simon.
During the next few years Simon continued working on the AA and later on the Antiproton Accumulator Complex. His expertise was highly prized. He had an amazing capability of determining the value of an idea, and he was always open to ideas from colleagues and students. Although he rarely spoke up during meetings, his words had a huge weight; that was already the case long before he received the Nobel Prize. He approached his work with competence, authority, and an enormous commitment. After his retirement, he left the refereeing and lecturing to “younger people” and spent his time gardening and visiting with friends and colleagues. Despite his success, he remained the humble, kind man he had always been. We miss him!
