IL NOSTRO MONDO THE DESIGN, CONSTRUCTION AND PERFORMANCE OF THE CERN INTERSECTING STORAGE RINGS (ISR) A RECOLLECTION OF WORLD’S FIRST PROTON-PROTON COLLIDER KURT HÜBNER CERN, Geneva, Switzerland 1 Design which had a beam energy of 160 MeV. The interaction points to increase the collision rate The concept of colliding beams appeared design of these colliders started in 1957. but without special lattice insertions as one first in a German patent by Rolf Widerøe In 1961, the Accelerator Research Group would use these days. registered in 1943 and published in 1952. Division was expanded into the Accelerator Combined-function magnets were chosen However, at that time the intensity of beams Division as experienced manpower had as in the PS, i.e. the main magnets had a was too low for an exploitable collision rate as become available after the running-in of the magnetic dipole field to bend the beam and a beam accumulation had not yet been invented. PS in 1960. At the same time it was decided quadrupole field to focus the beam. This type The first ideas of a realistic design were to construct a small accelerator to test rf of magnet provided space for the elaborate published in 1956 by Gerard O’Neill and by the stacking, a technique to be experimentally pole-face windings foreseen to control the MURA Group lead by Donald Kerst in the USA. proven, as it was essential for the performance magnetic field to a very high precision. It also MURA had come up with beam accumulation and success of the ISR. This CERN Electron guaranteed good access to the demanding in longitudinal phase space by radio-frequency Storage and Accumulation Ring (CESAR) was vacuum system. The ISR circumference was (rf) stacking and O’Neill proposed rings of the ready in 1964 and demonstrated rf stacking 942.64 m, 1.5 times the circumference of the synchrotron type, first two tangential rings to the satisfaction of the team. This came in PS. Eight crossing points were foreseen with and then intersecting rings, a topology later time for the ISR Design Report presented in one reserved for the beam dumping (I3). The adopted for the ISR to increase the number of 1964. The CERN Council approved the ISR as resulting topology is shown in fig.1 with the interaction points. Supplementary Programme in June 1965, and transfer lines from the PS. CERN Council showing considerable the financing of the project to be lead by Kjell The nominal field of the main magnets was foresight set up an Accelerator Research Group Johnson was decided in December of the same 1.2 T for 28 GeV/c protons, the maximum the in 1956 still during the construction of the year. PS could provide. This implies a bending radius CERN 28 GeV Proton Synchrotron (PS). This of 78.6 m. Each ring contained 60 long magnet group considered various options, particularly, 2 Construction units, about 5 m long, and 72 short units, 2.4 those which would give “added value” to the The construction took four years from 1966 m long. The coils having 32 turns were made PS. The main options which emerged were to 1969. It was rather uneventful as the leading of copper. A number of auxiliary magnets intersecting storage rings for proton-proton team mainly consisted of staff which had been complemented the magnet system. collisions and an accelerator in the 300 GeV part of the PS team, hence bringing with them The figure of merit of a collider apart from range for fixed target experiments. This the expertise and experience from the design, the beam energy is the integrated luminosity approach was by intention complementary to construction and running-in of this accelerator which in the ISR was proportional to positron-electron or electron-electron colliders which was comparable in size and complexity under design in other laboratories. The lineage to the ISR. Nevertheless, no other proton- ∫ (I1 · I2 / heff ) dt , of the e+e– colliders, essentially single rings proton collider had ever been built before and with positrons and electrons circulating in a number of technological challenges had to where Ii are the currents of the counter- opposite directions, which culminated in LEP be mastered. rotating beams and heff is the effective beam at CERN, was initiated in Frascati with ADA The magnet lattice of the ISR had to be height in the interaction point. All three by B.Touschek in 1960. The beam energy was matched to the requirements: e.g. straight variables depend on time t as the beam current 200 MeV. The e+e– colliders consisting of two sections much longer than in the PS in order decays due to nuclear and single Coulomb tangential rings were pioneered by Gerard to create space for the experiments around scattering of the protons on the residual O’Neill and Burt Richter for the Princeton- the interaction points; a large horizontal beam gas and the effective height gets blown up Stanford experiment with a beam energy of aperture to accommodate the momentum by multiple Coulomb scattering. Hence, the 500 MeV and by Gersh Budker proposing VEP-1 spread required by stacking. Attention vacuum pressure was a key parameter for the was given to maximize the focusing in the ISR performance. Requiring that the luminosity 46 < IL NUOVO SAGGIATORE Fig. 1 Schematic layout of the ISR, its transfer lines and its Fig. 2 ISR Intersection point 5 showing the crossing of the injector, the PS (courtesy CERN Courier, January/February, 2011). rings (courtesy CERN). does not drop to less than 20% in 12 h lead to were increased later from currents around for the first experimental test of transverse the constraint that the vacuum pressure had to 10 A in 1971 to 30 to 40 A per beam for physics stochastic cooling of the beam. This cooling –9 be less than 10 Torr (N2 equivalent) averaged runs. The luminosity was further boosted by a method had been put forward by Simon around the ring. The pressure in the interaction low-beta section composed of conventional van der Meer in 1972 based on theoretical points had to be less than 10–11 Torr to limit quadrupoles and later by a more advanced considerations. It was extended later to the background for the experiments. Thus, the section consisting of superconducting longitudinal cooling and, after verification in vacuum system extending over about 2 km quadrupoles providing 1.4 × 1032 cm–2s–1, a the Initial Cooling Experiment (ICE), was the became the principal technological challenge. world record until 1991 when it was broken by basis for the antiproton source of CERN and, The choice of the vacuum system was the Cornell electron-positron storage ring. in turn, for the antiproton-proton program at guided by results from CESAR which had The vacuum system benefited from a the CERN Super-Proton-Synchrotron and the been used also to test advanced vacuum gradual but continuous upgrade to keep pace Low-Energy-Accumulation-Ring. An antiproton technologies: a stainless-steel vacuum with the increasing currents. Typical loss-rates beam permanently cooled with this technique chamber bakeable in situ to 300 °C; flanges during physics runs could be kept to one part was also used in the ISR for antiproton-proton with metal seals; powerful sputter ion pumps per million per minute exceeding by far the collisions. The CERN antiproton programme (350 l/s) with Ti-sublimation pumps (2000 l/s) initial specifications and providing the required continues these days with the Antiproton in critical spots. low-background condition for the experiments. Deceleration (AD) ring providing very low- The construction was terminated in 1970 Since the magnets were far from saturation energy antiprotons (100 MeV/c) for trap and the project was officially completed in at 26.5 GeV/c and since the poleface experiments. March 1971 within schedule and budget. windings permitted a precise compensation The ISR were decommissioned as collider in Figure 2 shows the ISR tunnel at one of the of saturation effects, the beams could be 1983 to free resources for the Large Electron intersection points. accelerated slowly to 31.4 GeV/c by rf phase Positron Ring (LEP). They will be remembered displacement also pioneered by MURA. as the facility which allowed for an early 3 Performance As soon as deuterons and alpha particles peeping at the highest energies in the Commissioning started in October 1970 with became available from the PS, they were used centre-of-mass energy and for the discovery the first beam injected into ring 1. In January for p-d, d-d, p-α and α-α collisions from 1976 of a number of advanced technologies 1971, when both rings had become available, onwards. indispensable for hadron accelerators and of the first proton-proton collisions were The ISR not only served a large physics storage rings of today and of the future. observed. The beam-beam effect did not lead community but will also be remembered to discernible losses though some had feared for a number of advances in accelerator that this effect would create a strong beam physics and technology. The most important Kurt Hübner blow-up exacerbated by the lack of damping achievement was the discovery of Schottky Kurt Hübner has been at CERN since 1964 through synchrotron radiation in proton rings. noise in the beams. Since the number and he served as the laboratory’s director Operation for physics started in February with of particles per beam was finite and the of accelerators from 1994 until 2001. He beams at 15 GeV/c.
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