Proton-Antiproton Colliding Beams Coming Nearer Work Well Under Way for One of the Experimental Areas at the CERN Proton- Antiproton Collider
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Proton-antiproton colliding beams coming nearer Work well under way for one of the experimental areas at the CERN proton- antiproton collider. The apparatus for the UA1 experiment will be assembled in the cylindrical shaft seen in the picture, and then rolled into position in a second shaft, to be excavated around the SPS tunnel. (Photo CERN 104.2.80) On 1 6 June the CERN SPS will start its 'big shutdown', with no operation for almost a year while the machine and experiments are prepared for proton-antiproton colliding beams. Such a major sacrifice of prime research time has never been seen before and is as sure a pointer as any to the importance of the extension in CERN's research potential which the proton-antiproton collider will bring. This article recalls the major features of the CERN scheme, and of the project at Fermilab, and de scribes the preparations now under way or imminent at CERN for the experiments and for the machines. The CERN scheme Thefirst CERN plans, orchestrated particularly by Carlo Rubbia, emerged in 1976 following crucial advances in accelerator physics — the demonstration at Novosibirsk of Gersh Budker's idea of electron cooling, and at CERN of Simon van der Meer's proposal for stochastic cooling. These beam cooling techni ques enable intense beams of stable particles to be built up, and thus make it feasible to do physics with colliding beams of antiprotons. The system which is now under construction at CERN will take 1013 protons, at 26 GeV and concen trated in five bunches, every 2.4 s from the PS to yield 2.5 x 107 anti- protons at 3.5 GeV from a target for subsequent injection into the Anti- proton Accumulator ring (AA). In the AA ring a fast precooling (about a factor of ten in the beam spread over 2 s) will be carried out. The newly injected pulse will then be The components of the CERN proton- antiproton colliding beam scheme. A ntiproton beams will be cooled and stored in the Accumulator and sent via the PS, for experiments in the SPS and the ISR. CERN Courier, June 1980 143 A plan of the Fermilab proton-antiproton colliding beam project. Two cooling rings are involved in producing a high intensity antiproton beam — the Precoo/er (using stochastic cooling) and the Electron Cooling Ring. With the superconducting Tevatron ring, collision energies of 1000 GeV per beam should ultimately be available. experience in colliding proton and antiproton beams, and allow colli sion energies to be increased up to 2000 GeV in the centre of mass. The antiproton beams will be produced through an intricate se quence. About 2 x 1013 protons per pulse will be accelerated in the Main Ring and ejected at 80 GeV towards an antiproton production target. The antiprotons will be collected at 4.5 GeV in a large aperture ring, called the Precooler, about the size moved into the storage region of the could serve as a stretcher/storage of the present 8 GeV Booster. Here wide aperture of the AA vacuum ring, providing a much higher inten stochastic cooling will be used in chamber by lowering a shutter. In sity of antiprotons in this energy several steps interspersed with de the storage region, stochastic cool range than has ever been available celeration down to 200 MeV, a ing will achieve a further factor of before (see September 1979 issue, process requiring several seconds. 108 in cooling to build up an intense page 260). (Stochastic cooling was achieved at beam of antiprotons. Pulses of the Laboratory for the first time in 6x 1011 antiprotons will be drawn The Fermilab scheme February with the collaboration of from the AA ring every 24 hours. machine physicists from Berkeley.) The 3.5 GeV antiproton beam will The possibility of colliding proton- The 200 MeV beam will then be be sent to the PS for acceleration to antiproton beams at Fermilab was transferred to an Electron Cooling 26 GeV before being transferred to first indicated in a Harvard/Wiscon Ring where cooling will be carried the SPS, distributed in six bunches. sin proposal in 1976, with Dave out while antiprotons are added and Proton injection into the SPS will Cline as spokesman. The latest plans stored. When some 1011 antiprotons then take place and the two beams were assembled in 'The Fermilab have been obtained (a process will be accelerated simultaneously High Intensity Antiproton Source which is expected to take about five to the energy selected for the exper design report', presented in October hours), they will be transferred back iments. In the stored beam mode 1979 by a group with participants to the Precooler, reaccelerated to this could be up to 300 GeV, once from Argonne, Berkeley, Fermilab, 8 GeV and injected into the Main transformers in the power supplies Novosibirsk and Wisconsin. Given Ring — in the opposite direction, of are changed. The anticipated lumi the different machine configurations course, to the protons. nosity is 1 030 per cm2 per s. Two and energies, etc., the Fermilab Further acceleration will be carried long straight sections will be modi scheme has taken a considerably out before transfer to the supercon fied for the installation of detectors different form from that at CERN. ducting ring. Some 1012 protons will for experiments with the colliding The overall aim is to achieve then be accelerated and fed to the beams. proton-antiproton collisions at a superconducting ring, and both Provision is also being made to luminosity of 1030 per cm2 per s, and beams could then be raised in energy send antiprotons to the Intersecting to have collision energies as high as simultaneously and collided. Storage Rings, where the present 1000 GeV per beam in the super The scheme thus makes use of detection systems can then study conducting magnet ring of the Teva- both types of cooling technique in proton-antiproton collisions at up to tron. situations where their application is 26 GeV. Because of lower energy Preparations at Fermilab are by no appropriate. Stochastic cooling is physics interests, the PS will also be means as far advanced as those at most effective for high energy used to decelerate the antiproton CERN, either in experimentation in beams with large momentum beam from 3.5 GeV to 0.6 GeV for cooling techniques or in machine spread, while electron cooling is injection into an additional small construction. Nevertheless use of better suited to low energy beams Low Energy Antiproton Ring (LEAR) the Tevatron could follow on very with comparatively small spread in in the South Hall at the PS. LEAR logically from CERN's first years of momentum. 144 CERN Courier, June 1980 The Antiproton Accumulator ring (AA) approaching completion. The ring as been designed and built very rapidly to accomplish the most complex and crucial stage of the antiproton scheme at CERN. (Photo CERN 258.4.80) 10 Preparations at the CERN SPS with 2.5 x 10 protons per bunch, installed in the SPS to simulate one and stored at 270 GeV. These bunch of particles passing through The SPS has had to endure major figures will change to 26 GeV and another. In addition, computer simu civil engineering work at two 1011 per bunch for proton-antipro lation can be tried, but is very heavy straight sections, which are being ton operation. Studies have been on computer time. The large Cray enlarged to become colliding beam carried out on beam-gas interac computer at the Daresbury Labora experimental halls, plus the con tions, on possible resonances, on tory is being used initially with data struction of the new TT70 beam instabilities, on simulation of the from the PETRA storage ring at transfer tunnel. It also has to low-beta insertions and on r.f. noise. DESY. confront many machine physics refi It was r.f. noise which initially looked A lot of thought has gone into nements in order to achieve the like being a major problem, limiting beam monitoring systems and three design luminosity. The vacuum will beam lifetime to minutes in the first techniques will be used. Synchro be improved from 8 x 10"9 to 2 x 1 0"9 machine physics experiments. This tron light from the beams as they by the 'brute force' addition of more is being overcome by reduced band pass through discontinuities in the pumps. A special low-beta insertion width and frequency spread which magnetic field is one novel method has been designed to concentrate gives shorter bunches requiring (see January/February issue, page the beams in the collision region. The higher r.f. voltage (hence the interest 446). However it is limited at r.f. cavities will be applied two per in standing wave cavities). By now, present to high energies (over 250 beam; there may be a need to proton beams have been held for GeV) and requires a fairly intense increase the r.f. voltage and the 1 8 hours. beam. For the injection energy possible advantages of using stand The possible disturbance to the region, a rapid wire scanner will be ing wave cavities are under study. stored beams by beam-beam inter operated (50 |im beryllium wire Machine physics studies have actions is also being considered. passing across the beam). In addi been carried out using the present Some practical experience can be tion the Schottky scan technique, proton beams injected at 10 GeV gained by having a non-linear lens developed at the ISR, will be used. CERN Courier, June 1980 145 Assembly of the 800 ton magnet for the UA1 experiment at the CERN proton- antiproton collider.