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CERN's Low Energy Antiproton Ring CERN’s Low Energy Antiproton Ring L • E • A • R by GORDON FRASER Beam transfer line ganglion at CERN. On the left are to LEAR. Across these is the bulky U-turn to steer linac feeds from linacs towards the booster, and center, the particles directly towards LEAR. On the right is a section antiproton ejection line from the PS proton synchrotron of the PS. 22 WINTER 1999 N SHAKESPEARE’S TRAGEDY, King Lear falls victim to his own misjudgment and dies from grief after a series of Job-like misfortunes. I Another LEAR, CERN’s Low Energy Antiproton Ring, was also buffeted by the stormy waves of destiny. Coming in the wake of CERN’s push for a high energy proton-antiproton collider, LEAR, a machine physicists’ concert platform, eventually fell victim to an even larger scheme—the Large Hadron Collider. Andy Warhol said that anyone can be famous for fifteen minutes. For LEAR, this fame came in January 1996, when newspapers and media all over the world carried the news that an experiment had synthesized the first atoms of chemical antimatter. But LEAR will also go down in science history as a stage which saw remarkable machine physics performances. In the mid-1970s, antiprotons were just another item on a long menu of secondary beams. But the idea was taking root that new techniques using antiprotons could open up another route to physics. The demonstration of electron cooling, by Gersh Budker’s team at Novosibirsk, and the invention of stochastic cooling, by Simon van der Meer at CERN, promised that high fluxes of antiprotons could be produced. In addition, the antiparticles would be free of contamination by other particles, and extremely “cold”—well col- limated in momentum and energy. At the time, the first electron-positron colliders were making their mark on the world physics stage. Budker sug- gested doing annihilation physics with protons and antiprotons. But antipro- tons are more difficult to produce than positrons, and to feed them into a collider needed additional control via cooling. With the demonstration of cooling, the door to proton-antiproton collider physics was unlocked. The key was turned by Carlo Rubbia, and at CERN a working group was estab- lished to look into the possibility of using the new SPS proton synchrotron as a proton-antiproton storage ring. Still untouched by these collider ideas, Kurt Kilian, then at Heidelberg, was doing experiments using secondary beams of antiprotons from CERN’s 11 GeV PS proton synchrotron. A mere five antiprotons per PS cycle were available at low momentum, but the special conditions of particle-antiparti- cle annihilation are always a fruitful source of physics. Kilian was impressed when his machine physics colleague Dieter Möhl at the PS told him that in principle the new cooling techniques could open up the antiproton sluice BEAM LINE 23 gates and provide a million antipro- chemical antimatter, atoms con- CERN, however, was justifiably tons per second. taining nuclear antiprotons and or- very protective of its high energy Rubbia and his colleagues were bital positrons. proton-antiproton scheme. LEAR was looking at how to produce antiprotons While the big push at CERN con- only approved subject to the condi- and accelerate them to high energies. tinued for a high energy proton- tions that it should not interfere with For this, one initial idea was to use a antiproton collider, the low energy the PS commitment to the high small ring to decelerate the antipro- splinter group was joined by Pierro energy antiproton scheme, that it tons so that electron cooling could be Dalpiaz, then at Ferrara, and many could use only six percent of CERN’s applied. This scheme was subse- other enthusiastic antiproton physi- antiproton production, and that it quently abandoned in favor of sto- cists. The machine side was joined should have overall “low priority.” chastic cooling at higher energies, but by Pierre Lefevre (later to become It was a deprived childhood, but the possibility of decelerating anti- project and eventually group leader) nevertheless introduced a completely protons had been discussed. and Werner Hardt of CERN. new way of life for the low energy Looking at the dynamics of an- To extract particles slowly from a antiproton physics community. Very tiproton production, Kilian soon re- circulating beam, the classic method nice results emerged—meson spec- alized that if secondary antiprotons is to excite a beam resonance and use troscopy, antiprotonic atoms, low produced by the PS could be decel- a magnet to remove a thick layer of energy annihilation, reaction mech- erated, several orders of magnitude beam. With expensive antiprotons, anisms on protons and nuclei, and more antiparticles could be made such brutal treatment would be strangeness production. available than via a standard sec- wasteful. However a new technique It was also a lengthy childhood, ondary beam. of ultra slow extraction developed at extended by the drama of the last ex- The 1977 International Accelera- CERN enabled particles to be deli- periment at CERN’s Intersecting Stor- tor Conference at Serpukhov heard a cately scraped off the surface of the age Rings (ISR) before this machine paper by Kilian, Möhl, and Ugo stored antiproton beam. This made was closed in 1984. This study, which Gastaldi of CERN for the deceleration it feasible to propose an additional used a gas jet target, clamored for a of antiprotons for physics experi- ring, grafted onto the low energy side stored beam of antiprotons in just ments at a low energy antiproton of the new CERN antiproton complex. one of the two ISR rings. factory. From the outset, the idea was This ring was initially called by very much overshadowed by the the unimaginative name of APR AT THE RINGSIDE imaginative schemes pushed by (Anti-Proton Ring), before Helmut Rubbia for high energy proton- Poth of Karlsruhe suggested the LEAR Before injection, LEAR’s meager antiproton reactions. Rubbia’s aim low energy antiproton ring acronym. ration (typically 109 antiprotons) was to use the proton-antiproton The scheme had the strong backing would be skimmed off from the route to obtain sufficient energy to of CERN’s PS Division under Gordon Antiproton Accumulator once every synthesize the long-sought W and Z Munday and Gunther Plass, and the 15 minutes or longer, and first be de- carriers. enthusiastic support of a wide phys- celerated in the parent PS from In contrast, the objective of the ics community. 3.5 GeV/c to 600 MeV/c to benefit low energy scheme was to explore in The project was finally approved from phase space optimization. For depth the annihilation process, in June 1980, two years after CERN’s this, the PS had to learn some new where many kinds of particles could major high energy proton-antiproton tricks, in addition to the repertoire be created. This would greatly extend scheme had been given the green needed to handle all CERN’s differ- the exploration of hadron spec- light. By this time a substantial low ent beams on complicated “super- troscopy. In addition, low energy energy antiproton physics program cycles.” antiprotons on tap would open up the had begun to crystallize, leading to Although LEAR’s R stands for ring, study of antiprotonic atoms and even 16 experiments involving 240 physi- it is in fact four 10-meter straight the possibility of synthesizing cists from 44 research centers. sections joined by 90 degree bends. It 24 WINTER 1999 was built inside the existing South Hall of the PS in just 16 months. Although a modest machine, LEAR had some interesting technical curiosities. Strong focusing proton rings have to face up to the problem of transition. When accelerated par- ticles attain a certain energy, rela- tivity effects come into play, and the radiofrequency accelerating field has to be adjusted to maintain the vital phase stability of the circulating par- ticle bunches. With conventional magnetic optics, LEAR’s transition energy To shape LEAR’s beams at injec- CERN’s LEAR low energy antiproton ring, would have occurred right in the tion and at higher energy required the site of remarkable accelerator middle of its physics range. How- stochastic cooling. Assuring syn- physics. ever, the design ensured that high chronization of the cooling signals momentum particles follow a over a range of energies called for spe- shorter orbit rather than the larger cial solutions, and this variable one normally encountered. Transi- energy stochastic cooling became tion was thereby totally side- another LEAR trademark. Once stepped, a feature which has been cooled, the low energy beam was adopted in the design of the pro- “frozen” by electron cooling. Taking posed Japanese Hadron Facility. the beam down to 100 MeV/c re- Having to contend for the crumbs quired just one minute, with inter- of CERN’s antiproton supply had a mediate cooling at 600 (stochastic), major influence on LEAR’s design 300 and 200 MeV/c (electron cooling). and operation. But there was not LEAR was the first machine to use only bad news. With the PS war- electron cooling as an integral part ranting a new linac injector, LEAR of its operations, using the electron inherited the old one and was thus cooler inherited from CERN’s Initial able to extend its range of beams. Cooling Experiment and refurbished LEAR was tested and later routinely in collaboration with a group headed set up for physics using expendable by Helmut Poth from KfK Karlsruhe. test particles (protons and negative LEAR was foreseen from the out- hydrogen ions) and in principle set as providing antiprotons for a could even store antiprotons and wide range of experiments. As well other particles at the same time, us- as the possibilities with co-rotating ing colliding or overlapped co- beams, the straight sections were de- rotating beams.
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