Résumé

CESAR : le premier anneau de stockage du CERN

Le 18 décembre 1963, le premier faisceau circulait dans CESAR, une petite machine qui a été pour le CERN une première étape sur la voie des anneaux de stockage à intersection et, à terme, du Grand collisionneur de hadrons. Le succès du démarrage du Synchrotron à protons en 1959 a incité les chercheurs à utiliser cette machine comme injecteur d’un collisionneur protons-protons à deux anneaux, permettant d’accumuler des impulsions successives de faisceaux injectés . Il a été décidé d’expérimenter ce procédé au moyen d’un petit anneau de stockage : CESAR, dont la conception a commencé en 1960. L’expérience a été suffisamment probante pour amener l’approbation du projet des anneaux de stockage à intersection (ISR).

Mention CESAR today in accelerator circles and the likely reaction will be "Caesar who?" However, the CESAR we are writing about was not a person. It was the CERN Electron Storage and Accumulation Ring – a small machine, just 24 m in circumference, but of decisive importance for the direction in which CERN’s accelerators evolved. To understand why, we have to go far back in CERN’s history, to well before the first beam in the 26 GeV Proton Synchrotron (PS).

In 1956, when components for the PS were starting to be assembled, thoughts were already turning to what should come after. So in 1957, a group was constituted within the PS Division for research on new ideas for high-energy accelerators. An intensive exchange on such ideas ensued between CERN, the US and Novosibirsk (in what was then the USSR). Theoretical studies were supplemented by building prototypes for experimental studies and plans were made for entire model accelerators.

Apart from accelerators with energies well beyond that of the future PS, the concept of colliding beams – where the centre-of-mass energy would be orders of magnitude higher than achievable with beams on stationary targets – was gaining interest. The problem, however, was in obtaining sufficient beam intensity. The novel idea of "beam stacking", i.e. accumulation of many beam pulses of low intensity into a beam of high intensity, pioneered by a group at the Midwestern University Research Association (MURA) in the US, showed the way to go.

The PS started up brilliantly in November 1959, soon far exceeding its design intensity of 1010 protons per pulse and promising to go much further (CERN Courier December 1999 p15). That opened the possibility for the PS to be the injector for a proton–proton collider consisting of two synchrotron rings, in which successive PS pulses would be accumulated through beam stacking at 26 GeV, without the need for further acceleration. However, experience with beam stacking needed to be gained and important aspects of the collider rings had to be verified experimentally. To this end, the design of a small strong-focusing synchrotron-type model started in 1960 – and so CESAR was conceived.

As a model, CESAR had to be small – 24 m in circumference – and yet the particles had to be highly relativistic, which meant the use of electrons. On the other hand, effects from synchrotron radiation had to be negligible, which meant low magnetic fields – 130 G (13 mT) in the bending magnets – and a corresponding kinetic energy of 1.75 MeV. The 2 MV van de Graaff generator already ordered for the fixed-field alternating-gradient (FFAG) model that CERN had previously intended to build therefore fitted the bill as injector.

In 1961, the group that had been formed in 1957 was extended to become the Accelerator Research Division. It had groups to design the Intersecting Storage Rings (ISR) and the 300 GeV machine, which was to become the Super Proton Synchrotron. The CESAR group completed both the design of the storage-ring model and the ordering and building of components that had begun in 1960, and prepared for construction in a new experimental hall. Construction, installation of the magnet system and, in particular, the preparation of the vacuum system took place in 1962–1963. During this time, the long-awaited van de Graaff generator arrived. Its conditioning took months, through which loud bangs from spark-overs rang around the hall.

Finally, in summer 1963, the first beam was injected into the completed CESAR (figure 1). However, it would not circulate. To make it do so turned out to be a tedious job. The cause was that the magnetic fields were extremely low – 130 G in the bending magnets and a mere 15 G (1.5 mT) at the poles of the quadrupole magnets – compounded by the fact that the magnets were not laminated but made of massive soft iron. After powering a bending magnet, it took more than a day for its magnetic field to settle down to within 10–4 of its final value. The overhead crane had always to be parked at the end of the hall, as its position influenced the path of the electrons. Jokingly, we even evoked the phase of the Moon! Every power failure was a catastrophe, from which it took days to recover. Nevertheless, we finally made it. Early in the morning of 18 December 1963, the beam circulated.

A challenging experimental and technical programme lay ahead. Foremost, we had to demonstrate RF-capture of the injected beam and beam stacking and measure the stacking efficiencies for various modes of stacking. Of equal importance, we had to prove that a vacuum of 10–9 Torr, as required for the ISR, could be achieved in an extended accelerator system. We also had to measure beam lifetime in terms of number of turns, as an input to the considerations about long-term stability of the ISR beams. Later, there were also studies of the influence of higher-order resonances on emittance and beam lifetime.

Through 1964 and 1965, beam stacking was the dominant topic. Measurements showed that the stacking efficiency depended on various parameters more or less as theory and simulations predicted. Several variants of the stacking process were successfully developed, all with high efficiency and some approaching 100%. The vacuum system reached pressures of 2 × 10–9 Torr and clearly showed that lower pressures could be achieved. The beam lifetime of about 1 s was consistent with the calculated scattering on the residual gas.

By early 1965, we therefore had enough positive results to bolster the conviction that the ISR could achieve sufficiently intense beams with sufficiently long lifetimes. In June 1965, CERN Council approved the ISR Project. CESAR had done its job.

Experiments with CESAR, however, continued until the end of 1967, delivering a host of results that were useful later for the ISR, its vacuum system and its stacking operation. And there was another benefit from CESAR. It was an excellent accelerator school, from which several accelerator physicists emerged to play important roles in CERN’s subsequent projects.

One can muse about the course that CERN’s accelerator history might have taken without CESAR and its results. The ISR would not have been built. Would we then have dared to convert the SPS to a proton–antiproton collider? And without the competence and experience gained with these two colliders, would we have dared to propose the LHC? We opine that CESAR was decisive in setting CERN on the collider course – a course of great success – and that tiny CESAR is actually the great-grandfather of the giant LHC.