Setting the Scene The history of CERN and its accelerators dates ...

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as the Secretary General. Four Study Groups were established: Theory under N. Bohr, Laboratory services under L. Kowarski, Synchrocyclotron under C. Bakker, and Proton Synchrotron under O. Dahl with F. Goward as deputy. Further members of the PS Group were H. Alfvén, J. Fry, W. Gentner, K. Johnsen, E. Regenstreif, Chr. Schmelzer and R. Wideröe. From here on we shall follow in some detail the activities of the latter group. At end June 1952 the PS Group presented a first proposal for the construction of a 10 to 15 GeV proton synchrotron, intended to be an up-scaled version of the 3 GeV Cosmotron at Brookhaven, which just had accelerated protons to more than 1 GeV in May of that year. Council asked that a detailed design and planning for a 10 GeV machine be presented by November 1953. Three members of the group, Dahl, Goward and Wideröe, visited Brookhaven during the first two weeks of August 1952 so as to learn about details of the Cosmotron. The Cosmotron team discussed, in anticipation of the visit, possible improvements to their machine design, and discovered during these reflections a new way of beam focusing (5). The visitors were thus introduced, to their great surprise, to a brand-new idea, alternatinggradient or ‘strong’ focusing of particle beams, which would reduce by a large factor the size of the vacuum chamber containing the beam and hence the magnet providing the guide field. An accelerator of higher energy could thus be built within a given budget. Upon returning home the PS Group was faced with a very difficult decision: going ahead on the proven trail (which anyway was new territory for most of them) or changing course and proceed with the new and untried idea. It might have been a deadly blow to the young team if they found themselves in a dead alley. But one could also hope that a machine of classical design might in this case‘ only’ be delayed by a year or so. O. Dahl strongly advocated an intense study of the strong focusing system and obtained Council approval at its third session in October 1952 at Amsterdam, which also saw the decision for Geneva as the location of the future laboratory. O. Dahl in his presentation to Council gave due warning of the risks involved in that choice but pointed out that a project based on the ‘classical’ design could not now be defended in view of the possible advantages inherent in the new invention. In fact, it was expected that a synchrotron of 30 GeV might be built at the same cost as a Cosmotrontype machine of 10 GeV (for which however no reliable cost estimate existed at that moment). The successful development of CERN during more than 5 decades was largely due to the surprising flexibility of the alternating-gradient PS. CERN would surely have developed in a very different way, had this courageous decision not been taken. In fact, as the theory of the alternating gradient synchrotron was developed, two serious problems were soon discovered: first, the stronger the focusing, the higher a precision of the magnet field and of the alignment of the magnets is required. If the tight tolerances were not met, the beam would be lost due to resonance blow-up of the ‘betatron’ oscillations of the protons about the equilibrium orbit (6). Secondly, the beam looses
longitudinal stability at a point of the acceleration cycle, the ‘transition energy’, when the relative increase in particle velocity changes from being greater to being smaller than the variation of orbit length. Several mathematical physicists (7,8,9,10,11) joined in to analyze the sensitivity of the betatron oscillations to field errors and the limits of their stability. A mechanical analogue model for some experimental work was developed as well (12). The problem of the loss of phase stability was to first approximation solved by a jump of the acceleration phase angle and has been studied in great detail K. Johnsen (13-5,15-6). Still, it remained a subject of deeper study and several improvements as the beam intensity was increased during the whole life of the PS. In the meantime more scientists were seconded by their home laboratories to the PS Group, amongst them J. B. Adams, A. Citron, P. Grivet, M. G. N. Hine, J. C. Jacobsen, J. Lawson and G. Lüders, and, very importantly, two colleagues from the Brookhaven Laboratory, Hildred and John Blewett. Several design options with different focusing gradients were explored resulting in magnet masses between 800 and 10 000 tons or more for a 30 GeV synchrotron. However, the precision required in machining of the pole profile and of the overall alignment of an accelerator with the smallest magnet turned out quite impossible. As result of several iterations, the design converged on a machine of 30 GeV beam energy at a magnet field of 12 kG (1.2 T), and an average radius of 112 m. The focusing strength chosen required a vacuum chamber of 12 cm full width and 8 cm height, with magnets of about 4000 tons total mass. As from January 1953 work on the Convention of a final organization (as opposed to the provisional convention in force since nine months) was launched. It was signed and submitted for ratification by the member states by end June 1953. So as to keep track of the developments, the PS group was asked to organize a meeting with international accelerator experts. Thus in October 1953 the ‘Conference on the Alternating Gradient Proton Synchrotron’ was held at Geneva, which was attended by Council members and many European and American scientists. In addition to the papers by the PS Group (13) on the design of the 30 GeV machine and aspects of beam dynamics, W. Heisenberg was asked to present the users’ view as to the beam energy required. He recommended to ‘ build a machine that could be operated without difficulty in the region somewhat above 20 GeV but which could, in the limit, be extended in energy to 30 GeV’. After the conference, Council, satisfied by the very positive response to the work of the PS Group, gave green light to building an AG proton synchrotron with nominal beam energy of 25 GeV at 12 kG and 3 s cycle time (which could in fact at reduced repetition rate be driven up to 28 GeV) at a cost provisionally estimated at 130 MSF. Council also approved the proposal that the PS Study Group should move to Geneva and nominated a ‘chief architect’ (R. Steiger from Zürich) so that construction work on the site could be organized without delay. Group members could use temporary offices and a hall for experimental work made available at the Institut de Physique (where some barracks were added) and – with the exception of Dahl – began moving their families to Geneva. Collaboration between them was of course enormously improved.
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