Proton Synchrotron

When early in the 1950s the plans for a European laboratory of particle physics began to take shape, two different accelerator projects emerged. One machine was to be of standard type, easy and relatively fast and cheap to build: the synchrocyclotron, achieving collisions at a center-of-mass energy of 600 MeV. The second device was a much more ambitious undertaking: an accelerator bigger than any other then existing, a synchrotron that could accelerate protons up to an energy of 10 GeV – the PS.

By May 1952 a design group was set up with Odd Dahl in charge.[4] Other members of the group were among others Rolf Widerøe, Frank Kenneth Goward, and John Adams. After a visit to the Cosmotron at Brookhaven National Laboratory in the US, the group learnt of a new idea for making cheaper and higher energy machines: alternating-gradient focusing. The idea was so attractive that the study of a 10 GeV synchrotron was dropped, and a study of a machine implementing the new idea initiated.[5] Using this principle a 30 GeV accelerator could be built for the same cost as a 10 GeV accelerator using weak focusing.[5] However, the stronger focusing the higher a precision of alignment of magnets required. This proved a serious problem in the construction of the accelerator.

A second problem in the construction period was the machines behavior at an energy called "transition energy". At this point the relative increase in particle velocity changes from being greater to being smaller, causing the amplitude of the betatron oscillation to go to zero and loss of stability in the beam. This was solved by a jump, or a sudden shift in the acceleration, in which pulsed quadruples made the protons traverse the transition energy level much faster.

The PS was approved in October 1953, as a synchrotron of 25 GeV energy with a radius of 72 meter, and a budget of 120 million Swiss franc.[6] The focusing strength chosen required a vacuum chamber of 12 cm width and 8 cm height, with magnets of about 4000 tonnes total mass. Dahl resigned as head of the project in October 1954 and was replaced by John Adams. By August 1959 the PS was ready for its first beam, and on 24 of November the machine reached a beam energy of 24 GeV.[4]

Aerial view of the 28 GeV Proton Synchrotron. The underground ring of the 28 GeV proton synchrotron in 1965. Left, the South and North experimental halls. Top right, part of the East hall. Bottom right, the main generator room and the cooling condensers.

By the end of 1965 the PS was the center of a spider's web of beam lines: It supplied protons to the South Hall (Meyrin site) where an internal target produced five secondary beams, serving a neutrino experiment and a muon storage ring; the North Hall (Meyrin site) where two bubble chambers (80 cm hydrogen Saclay, heavy liquid CERN) were fed by an internal target; when the East Hall (Meyrin site) became available in 1963, protons from the PS hit an internal target producing a secondary beam filtered by electrostatic separators to the CERN 2 m bubble chamber and additional experiments.[7]

Together with the construction of the Intersecting Storage Rings (ISR), an improvement program for the PS was decided in 1965, also making space for the Gargamelle and the Big European Bubble Chamber experiments. The injection energy of the PS was raised by constructing an 800 MeV four ring booster — the Proton Synchrotron Booster (PSB) — which became operational in 1972.[7]

In 1976 the Super Proton Synchrotron (SPS) became a new client of the PS. When SPS started to operate as a proton–antiproton collider — the SppS — the PS had the double task of producing an intense 26 GeV/c proton beam for generating antiprotons at 3.5 GeV/c to be stored in the Antiproton Accumulator (AA), and then accelerating the antiprotons to 26 GeV/c for transfer to the SPS.

The linear accelerator, now serving the PSB, was replaced in 1978 by Linac 2, leading to an further increase in intensity.[7] During this period acceleration of light ions entered the scene. Linac 1, which was replaced by Linac 2, was equipped to accelerate deuterons that were accelerated in the PS, and transferred to the ISR where they collided with protons or deuterons.

When the Low Energy Antiproton Ring (LEAR), for deceleration and storage of antiprotons, became operational in 1982, PS resumed the new role of an antiproton decelerator. It decelerated antiprotons from the AA to 180 MeV, and injected them into LEAR. During this period the PS complex truly earned its nickname of "versatile particle factory".[7] Up to 1996, PS would regularly accelerate ions for SPS fixed-target experiments, protons for the East Hall or antiproton production at AA, decelerate protons for LEAR, and later accelerate electrons and positrons for the Large Electron–Positron Collider (LEP).

During its long operation the PS has increased its proton density many times

To provide leptons to LEP, three more machines had to been added to the PS complex: LIL-V electron linear accelerator, the LIL-W electron and positron linear accelerator, and the EPA (Electron-Positron Accumulator) storage ring. A modest amount of additional hardware had to be added to modify PS from a 25 GeV proton synchrotron to a 3.5 GeV lepton synchrotron.

During this period the demand for heavier ions to be delivered as a primary beam to the SPS North experimental hall (Prévessin site) also increased. Both sulfur and oxygen ions were accelerated with great success.

After the end of operation as a LEP injector, the PS started a new period of operation in preparation as LHC injector and for new fixed-target experiments. New experiments started running in the East area, such as the CLOUD experiment. The PS complex was also remodeled when the AA area was replaced by the Antiproton Decelerator and its experimental area.

By increasing the energy of the PSB and the Linac 2, the PS achieved record intensities in 2000 and 2001. During the whole of 2005 PS was shut down: radiation damage had caused aging of the main magnets. The magnets, originally estimated to have a lifetime of less than 10 years, had exceeded the estimate by more than a factor of four, and went through a refurbishment program. The tunnel was emptied, magnets refurbished, and the machine realigned. In 2008 PS started operating as a pre-accelerator to the LHC. Simultaneously the ion operation changed: LEAR was converted into a storage ring — the Low Energy Ion Ring (LEIR) — and the PSB stopped being an ion injector.