Antiprotons in the big machine Right the beam line which injects 26 GeV antiprotons into the SPS. Centre, the beam fine which sends high energy protons from the SPS towards the West Experimental Area. The main ring is on the left. (Photo CERN 38.4.81) On 7 July a pulse of antiprotons was sent to the CERN Super Proton Synchrotron, accelerated to 270 GeV and (briefly) stored. Two days later the exercise was repeated with greater success and the first evi­ dence obtained for proton-antipro­ ton collisions at 540 GeV — by far the highest collision energies ever achieved. Although there is still much to be done before the scheme becomes fully operational, this achievement is a major milestone in the CERN antiproton story. The origins of the project Antiprotons, since they presuma­ bly have the same properties as protons except for the sign of their electric charge, can be accelerated and stored in the same magnet ring as protons. Thus colliding beam sys­ tems, like the familiar electron-posi­ tron machines, are, in principle, fea­ sible. However until recently it was not possible to produce antiproton tion, the antiprotons emerge with a for colliding beam physics. It in­ beams of sufficient intensity and wide range of momenta, distinctly volves using electron beams travel­ density to give sufficient collisions in unsuitable for a magnet system in a ling along with the antiproton beam a reasonable enough time for useful beam transfer line, an accelerator or at the same velocity. The electron physics to be done. a storage ring which is designed to beam, which is much easier to con­ This situation has changed with handle a well defined particle mo­ trol, has particles at precisely the the invention of 'beam cooling'. The mentum. Only antiprotons at or very desired momentum. In the subse­ technique gets its name from the near to the design momentum could quent antiproton-electron collisions, relationship between temperature be guided — the rest would stray off the antiprotons transfer momentum and particle energy — the higher the into the walls of the vacuum cham­ to the electrons in such a way that temperature of a body, the higher the ber. the antiproton momenta are concen­ particle energies. However beam The advent of beam cooling ena­ trated around the desired value. cooling refers not to a process bles an antiproton beam with an This idea of 'electron cooling' was whereby all the particle energies in initially quite wide span of momenta successfully tested at Novosibirsk the beam are reduced (so that the to be concentrated around a particu­ in 1 975, and has subsequently also beam is made cold), but to a process lar momentum so that a dense beam been demonstrated at CERN and at which concentrates all the particle can be built up and the subsequently Fermilab. However at CERN, an energies around a particular value. It encountered magnet systems can alternative idea called 'stochastic is the variation of energies around hold the beam. cooling' was invented by Simon van this value which is 'cooled down'. A cooling technique was proposed der Meer in 1968. The word sto­ Antiprotons are produced in the in 1966 by Gersh Budker and his chastic means random — stochastic collisions of protons with a metal colleagues at the Institute for Nu­ cooling operates by reducing the target at relatively low rates — typi­ clear Studies at Novosibirsk in the random motion of the particles in a cally it needs about a million protons USSR with the specific aim of beam. It was invented to improve the to produce each antiproton. In addi­ achieving intense antiproton beams density (and hence the luminosity) of CERN Courier, September 1981 289 The ICE Ring, scene of much of the early development work on beam cooling at CERN. (Photo CERN 80.9.78) dered. In tests in the ICE ring in 1 979 (see October 1979 issue, page 312), it was also clearly established that antiprotons live long enough to make the whole scheme feasible. The role of the Antiproton Accumu­ lator The heart of the antiproton project is the Antiproton Accumulator (AA) where the stochastic beam cooling technique is applied to build up anti­ proton beams tens of thousands of times more intense than have ever been achieved before. Protons are first accelerated in the Proton Synchrotron. Instead of be­ ing evenly distributed in twenty bunches around the PS ring, as in a normal acceleration cycle, they are crowded into five bunches occupy­ ing a quarter of the ring. As the 50 m diameter AA ring is one quarter the size of the PS, these protons, when the beams in the Intersecting Stor­ electron cooling. The results from they strike a target, produce a length age Rings. ICE, particularly on stochastic cool­ of antiproton beam which fills the The distribution of particles across ing, were so spectacular that in circumference of the AA. a beam is observed at 'pick-up sta­ 1978 CERN was able to give the When the protons reach 26 GeV tions' in the ring, and from this green light to an antiproton project they are ejected from the PS and information the centre of gravity of which had been promoted particu­ strike a target in front of the AA ring. the beam density is calculated. As it larly by Carlo Rubbia. This was then From the spray of secondary parti­ passes further around the ring, the authorized in the confidence that, for cles, a focusing system selects anti- same section of the beam is sub­ the first time, antiproton beams of protons of energy around 3.5 GeV jected to an electric field which sufficient intensity and density could for injection into the AA. This energy nudges the measured centre of grav­ be achieved to make proton-antipro­ gives the maximum yield of antipro­ ity towards the desired value. Be­ ton physics possible. tons — for each pulse of 1013 pro­ cause of the random distribution of According to our present under­ tons on target, some 2 x 107 anti- particles, this push acts unfavoura­ standing of particle physics, the pro­ protons are injected. (In other words, bly on some particles, taking them ton and its antiparticle should be for every million protons hitting the further from the required conditions, equally stable. The proton lives for at target only two antiprotons are col­ but works favourably on the majori­ least 10 30 years, if not for ever. lected!) This operation is repeated ty. Over millions of repetitions, the However up till quite recently physi­ every 2.4 s. beam is progressively cooled. cists had never been able to test the Eventually the AA is required to The technique was first success­ stability of the antiproton. If it had provide beams containing 6x10 11 fully demonstrated at the ISR in turned out that the antiproton were particles, and about 30 000 PS 1 975. This led to the construction of nowhere near as stable as the pro­ pulses (representing about a full a small storage ring aptly known as ton, theorists would have had to day's operation of the machine) will ICE— Initial Cooling Experiment — have a major rethink, and the whole be needed to supply all these anti­ specifically to study stochastic and antiproton project might have foun­ protons. 290 CERN Courier, September 1981 One of the magnets of the AA ring, showing its very large aperture. (Photo CERN 49.3.80) The AA's vacuum chamber is unu­ sually large (70 cm wide) to give space for all the necessary ma­ noeuvres and is held at high vacuum (1 0~10 torr) to minimize loss of anti­ protons due to collisions with resi­ dual gas molecules. The first pulse is injected and bent by 'kicker magnets' so that the anti­ protons orbit on the outside of the vacuum chamber. During injection, this region is shielded from the rest of the chamber by a mechanically operated shutter. This protects the antiprotons subsequently stored in the main body of the chamber from the magnetic fields of these kickers, and makes it possible to cool the low density injected beam without being swamped by the much stronger sig­ nals from the high density stack. The first injected pulse is monitored at pick-up stations and other kicker magnets act upon it to cool the antiprotons. In two seconds the pulse is pre- cooled so that the momentum 60 000 injected pulses later, some AA, the injection and ejection sys­ spread of the particles is reduced by 1012 antiprotons would be orbiting tems are located in the same section a factor of ten. The shutter is then in the stack, with the majority (6 x of the ring. The antiproton beam lowered and the precooled antipro­ 1 0 11) of them concentrated in the therefore has to be turned around in tons moved into the stack position in core, thanks to the cooling system. It a loop of magnets before it can be the main body of the chamber. The is this core which will provide the injected into the PS, where it circu­ shutter rises again and the second intense antiproton beam for colliding lates in the opposite direction to the pulse is injected to receive the same beam physics. protons. The antiprotons are then treatment. A residue of some 4 x 10 11 anti­ accelerated in six bunches from 3.5 While this sequence of injection, protons would remain in the AA to 26 GeV, when they can be sent precooling and transfer to the stack stack to start the next core. Injection either to the SPS or the ISR.