SLAG BEAM LINEI think the saga of CEA is the Book of Jobfor the accelerator builders . .. [but] inthe end the Lord loved them andthey got the right value of R.

Special Issue Number 9 March 1986

COLLIDING BEAM STORAGE RINGSA BRIEF HISTORY

John R. Rees

The discovery of the J/k, announcedin the fall of 1974, resulted in sucha rich flow of new physics and newexperimental technique that physicists callthe beginning of this era the 'NovemberRevolution.'

The Symposium on the Tenth Anniversaryof the November Revolution, held at SLACon November 14, 1984, was a recollectionof the discoveries and a review of theconsequences and included this talk onthe history of storage rings.

The storage ring was crucial to thisrevolution. SLAC Associate DirectorJohn Rees was involved in two such ringsbefore becoming project director of thePEP ring, completed in 1980. He is nowproject director of the SLC, a new kind ofcolliding beam machine.

The other speakers at the symposiumwere James Bjorken, David Hitlin, RoySchwitters, and Samuel C.C. Ting.Professor Bjorken's talk, A TheoristReminisces, was published in 1985 SpecialIssue Number 8 of the Beam Line. Theremaining talks are planned for futureissues.

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2SABemnI , Mac 1986-l

COLLIDING-BEAMSTORAGE RINGS:

A BRIEF HISTORYJohn Rees

This will be-a somewhat personal view of collidingbeams. Although the field is not a very old one, it isquite impossible to cover all of the developments in itin a half-hour talk. Indeed, my own experience doesnot cover all of the developments, so I will concen-trate primarily on what has happened in the field ofelectron-positron dolliding-beam machines from theirbeginnings up until the present time, although I willnot completely neglect the development of hadroncolliders.

To discuss the history of colliding-beam technol-ogy at this time is very appropriate, of course, be-cause of the November Revolution that began withthe discovery, ten years ago, of the J/ib particle inNovember 1974. It is also quite timely from thepoint of view of the state of accelerator technology,because we are probably seeing right now the twi-light of the first epoch of colliding-beam technologyand the dawning of a second epoch based on a newtechnology. But I will come to that later.

The figure at the right is a sort of map of the terri-tory that we are going to cover. This is a graph thatI pinched- I think from Pief. I am not quite sure ofits provenance, but it is a graph of the center-of-massenergy achieved in colliding beam machines plottedagainst time. The machines that are now running orhave run are represented by solid triangles; the onesthat are still a-building or hoped for are representedby open triangles. If one extrapolates this line back-ward to low energy, interestingly enough, and for noreason that I can think of, it comes to the time thatI would identify as the real beginning of the art aswe know it. I mark 1956 as the beginning for reasonsthat will, I hope, become clear in what follows.

To remind us of what was going on in our worldin 1956, I checked into what people were doing atthat time - the people who are involved in thisTenth Anniversary program here today. Sid Drellhad just come to Stanford as an Associate Professor;Sam Ting was an undergraduate at Michigan; RoySchwitters had just grown old enough to join the BoyScouts; B. J. Bjorken had just arrived as a new grad-uate student at Stanford (lured by Sid); and DaveHitlin was in high school in New York. Burt got hisPhD at MIT that year and came here. And Pief wasalready here at Stanford, the Director of HEPL, andprobably the main draw that brought most of theseother people here. Finally 1956 was the year that I

got married.Having thus set the stage, let me now retrace the

salient points of the history of colliding beams as Isee them.

The wish to overcome the handicap imposed bymomentum conservation in stationary-target colli-sions had existed for a long time, but every attemptto imagine a way of overcoming this handicap by col-liding one beam against another was frustrated bythe tenuousness of the beams that were attainable inthe '40s and early '50s. For example, Wider0e hadseriously considered colliding protons with negativehydrogen ions but was discouraged by the poor vac-uum technology of the day; he knew he could notattain useful beam lifetimes. He had proposed us-ing oppositely charged particles so that they couldbe contained in a single magnetic ring for the sake ofeconomy. This idea will recur.

1000

100

81)(9

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10

0.1

2-86

t 1960BIRTH

1970 1980 1990

NOW

2000

4494A29

EVOLUTION OF COLLIDING BEAM MACHINESThis graph summarizes the history of electron collidingbeam machines. From an approximate birth in the mid-1950s, the energy of the machines has increased about tentimes every decade.

I I I

Extrapolated EnergyAt 2000 A.D.

2.6 TeV-

LEP 11

ALEP I,SLC

PETRA R A PEP

/ CESRA/ DORIS II -

/SPEAR II

ADONE ySPEAR, DORISA/ ACO 11

AVEPPII,ACOPR INCETON- STANFORD

/

/ A ADA

I I I I

SLAC Beam Line, March 1986II

- a - l I · - -- l --

I I I I J-- ;- -

2

SLAC Beam Line, March 1986 3

In my view, the main historical precondition forthe development of colliding beams that we have seenin the past two decades was the alternating gradient(AG) principle put forward in 1952 by Courant, Liv-ingston and Snyder (and invented two years earlierby Christofilos but not then published). Of course,the AG principle alone was not enough. It did not, initself, open the door to the higher density of beamsrequired to do physics, but it was a precondition thatinspired other developments, in particular the fixed-field, alternating-gradient (FFAG) accelerator andthe technique of beam stacking. First the AG prin-ciple was applied to fixed field devices with the ideathat very high-intensity beams could be achieved.

The early cyclic AG synchrotrons were limited intheir beam intensity, because particles could be in-jected for only a brief time at the bottom of the cycle.This limitation could be overcome in fixed-field (dc)machines of a type developed primarily at the Mid-western Universities Research Association (MURA),initially under the direction of Donald Kerst. Sincethe main advantage pursued was higher intensity, theMURA scientists had to learn more about stackingparticles in machines than had ever been known be-fore. By the way, although it is not obvious fromwhat I have said, the accelerated particle in the plansof the MURA group for a big machine was the proton.

THE CEA TEAM, 1959. The group that led the Cambridge Electron Accelerator (CEA) in Cambridge,Massachusetts. The machine was later converted for colliding beam experiments, testing the technique of'low-beta' that proved so important in storage rings. Seated from left: Thomas Collins and David Jacobus.Standing from left: Fred Barrington, CEA Director Stanley Livinston, Robert Cummings, Lee Young, JohnRees, William Jones, Janez Dekkra, and Kenneth Robinson (deceased).

4 SLAC Beam Line, March 1986

In the course of their studies Kerst and his MURAco-workers realized that, with stacking, they couldhope for beam densities sufficient to do colliding-beam physics, and at the International Accelera-tor Conference at CERN in 1956, Kerst made thefirst proposal for a colliding-beam facility, a pairof clashing-beam accelerators. Although the figureof merit we now use, the luminosity, had not beeninvented at that time, the luminosity of the ma-chine proposed was about 103 2cm-2sec- 1 (probablythe first time that nearly universal constant of naturefor accelerator builders was cited).

Although MURA was the most active center, itwas not the only place where colliding-beam systemswere being devised. At Princeton, Gerry O'Neill wasalso absorbed in this matter. At the same CERNconference, he also gave a paper about storage ringsin which he suggested that one need not approachcolliding beams via clashing accelerators but rathervia storage rings as separate machines - separatefrom the accelerators that would feed them. In otherwords any kind of an accelerator could feed particlesinto two static storage rings in which the particlescould be stacked and could collide.

Now I believe (this is a speculation on my part)that O'Neill was somewhat frustrated by the factthat things were not moving very rapidly towardthe realization of a big FFAG machine. Such ma-chines were inherently very large and therefore veryexpensive. Indeed, the high-energy FFAG machineproposed by MURA was never built. In any case,inspired by frustration or not, O'Neill realized thatelectrons could be used instead of protons. In fact,electrons had advantages over protons. The radia-tion damping that came automatically with electronsbecause of their low rest mass would make stackingeasier.

In the case of protons stacking required putting anew swarm of particles into a region of phase spacevery close to an old swarm of particles. But theswarms could not overlap, and in the end the den-sity of the particle points in the full six-dimensionalphase space could never exceed the density of the in-jected swarms. With electrons, on the contrary, theparticle points move inward in phase space becauseof radiation damping, so that the density increasesgreatly. That made stacking an easier process. Theradiation damping also had the potential of amelio-rating instabilities that clearly might arise.

O'Neill believed that a relatively inexpensive pairof rings could not only test colliding beam principlesbut could also do forefront electromagnetic physics,because the center-of mass-energies that could bereached in the electron-electron system far exceededthat available at that time by any other means. Ofcourse, these inexpensive storage rings had to be cou-

pled to a suitable source of electrons.

This is where Stanford entered the picture. Stan-ford's High Energy Physics Laboratory had an idealsource in the Mark III linear accelerator, so O'Neillcame to Stanford with Bernie Gittleman, his stu-dent, and discussed these ideas with Pief Panofsky,Burt Richter, and Carl Barber. And the Princeton-Stanford colliding-beam experiment (which came tobe known around Stanford as the CBX, although Ifind now there are few people who remember that)was proposed in a report dated in 1958, and workbegan in that same year.

The beam energy of this experiment was plannedto be 500 MeV; the radii of the rings were about 1.4meters and they were weak focusing. (These figuresare approximate; I will continue to be approximateabout energies and radii throughout this talk.)

At the same time, Budker's Institute for NuclearPhysics in Novosibirsk started work on a pair of ringscalled VEP-1 to collide electrons of 140 MeV. The sci-entists at Frascati, Italy, noting what was being pro-posed for the Princeton-Stanford experiment, werethinking along a slightly different line. Their ideawas to put electrons and positrons in a single stor-age ring for both beams, which offered an economicadvantage. But perhaps a more important advan-tage was the fact that the events that ensued fromcollisions would be dominated by a well-known andwell-understood intermediate state, namely the vir-tual photon. They became extremely enthusiasticabout the idea, and in 1960, just two years after thePrinceton-Stanford proposal, they proposed a pro-gram that began with the construction of a smallmachine called ADA and continued to the construc-tion of a much larger one. The first storage ring wasto be a 250-MeV machine, with a radius of only 60centimeters, and like the CBX, weak focusing. But atthe outset they stipulated that their plans called forproceeding to a larger machine which they even thennamed - ADONE, for 'big ADA'- with an energyof 1.5 GeV, if possible. So ADA was to be a proof-of-principle, and ADONE was to follow on right away.Thus in the four years from 1956 to 1960 the firstgeneration of colliding-beam storage rings was welland truly launched. I want to emphasize that, al-though protons were the particles that were contem-plated originally, they had by this time disappearedtemporarily from the scene. Furthermore, although Ihave asserted that the strong-focusing principle wasessential to this development, it was not used in thedesign of any of these first three machines. (Its usebegan in the second generation.) It was essential toget FFAG thinking started, but these first machineswere attempts to find out whether this vein had anygold in it, and they were not pushed to the limits ofaccelerator technology, which I think was very wise.

SLAC Beam Line, March 1986 5

The builders of the first generation faced what allpioneers have to face: the banes of the first gener-ation, the unanticipated problems that plague newmachines, to which their builders have to find solu-tions - solutions which became part of their heritageto the next generation.

In the case of the Princeton-Stanford machine avery fast kicker was required, a magnet that switchedon and off very rapidly to inject the electron beamfrom the linear accelerator, and that was a techno-logical challenge that occupied the attention of thebuilders for some time. They recognized that thismagnet would be difficult to realize, and they metthe challenge.

But then they encountered their first real sur-prise. At very low stored currents - far belowwhat they required - they observed a large evolu-tion of gas accompanied by an intolerable pressurerise, and they were led to learn about the desorptionof molecules from the surfaces of the vacuum system.It is noteworthy that the vacuum system for this ma-chine was, at the time, the largest ultrahigh vacuumsystem in the world. They learned that they couldnot pump it with oil pumps, and they learned thatthey had to bake it very well. They overcame theproblem, but it took a long time, because rebuildingan ultrahigh vacuum system is very difficult.

luminosity measurea=i.) x i-u'sec cm

SPEAR - 1972. This photo of the first storage ring at SLAC was proudly completedwith typed-on performance figures. The view is from the east end of the site looking over the

Research Yard.

6 SLAC Beam Line, March 1986

And finally, after they had solved that problem,the Princeton-Stanford physicists came up againstwhat may fairly be characterized as the FundamentalLimit, the beam-beam limit of the colliding-beamstorage ring. This phenomenon has proved to beinsurmountable and continues to place the basic limiton the performance of colliding-beam storage rings.The theorem deserves to be capitalized.

ADA, the first Frascati ring that I mentioned ear-lier, being more limited in its goals, never reachedthe beam-beam limit, although it did uncover a basiclimitation on the low-energy beam-storage capacityof storage rings. It was at first curbed by its injec-tion scheme. A gamma-ray beam was directed ontoa target at the edge of the aperture, and some of theelectrons and positrons produced went into the ring.Subsequently a more sophisticated system of time-varying inflection equipment was installed, but thepower of the Frascati synchrotron was inadequate,so ADA was eventually moved to Orsay, France, andput at the end of the Orsay linac. Then it was ableto accumulate enough current to find a new limita-tion on storage-ring behavior, the Touschek effect, inwhich Coulomb scattering within the bunch transferstransverse momenta into longitudinal momenta andcauses intensity-dependent particle losses.

But now I want to come back to the Fundamen-tal Limit and its elucidation. It is sometimes calledthe incoherent limit and more often the beam-beamlimit. I think it was appreciated early by people atMURA, at Stanford and at other places that therewas some inevitable limit to be encountered simplyowing to the beam dynamics that result from the in-teraction of the bunches with each other - the effecton the particles of one bunch of the macroscopic orcollective field of the other bunch, which is highlynon-linear. But it remained for Fernando Ammanand Dave Ritson (who was visiting Frascati) in 1961to elucidate the consequences of this interaction ina way that made it fairly clear. I don't believe thatthe outlines of the problem have changed since theystated them.

Amman and Ritson said that the transverse beamdensity - the number of particles in the bunch di-vided by its interacting area - would be restrictedby a 'tune shift,' namely the shift of the betatron fre-quency of the individual particles of one bunch by thecollective electromagnetic field of the other bunch,treated in the linear approximation, i.e., treated asa lens. They wrote their formula this way:

N < Avv,A - reRFv

where Av, is the vertical betatron-frequency tuneshift, and v, is the vertical betatron frequency itself.(I've used the American notation, v rather than Q.)

Here - is the energy, re is the classical radius of theelectron and R is the gross radius of the machine.F, is a function called the beat function, and in thisformula it is evaluated at the interaction point. F (s)is the 'focusing function' and is dimensionless. Itis more common today to use af(s) as the focusingfunction. More on that in a moment.

Amman and Ritson pointed out that the tuneshift could not increase in magnitude beyond thevalue that would carry the particle's betatron fre-quency to the nearest proper resonance. There, theparticle's motion would become unstable. This wascertainly an upper limit on the tune shift.

Later Ernest Courant at Brookhaven did simula-tions and suggested that the tune shift would proveto be limited to something of the order of 0.1 regard-less of the location of the nearest proper resonance.His conclusion was not surprising, since the actualdynamical problem is strongly non-linear, and theAmman-Ritson result follows from a linearized treat-ment. Courant's estimate remained for a long timethe accepted statement of the Fundamental Limit.

Now I want to note before leaving this subjectthat, in the Amman-Ritson formula, the combina-tion of the v on top and the R and Fv on the bot-tom could have been combined together into a singlefunction which I have already mentioned, the betafunction. Had this been done, the formula wouldhave appeared this way:

N < AvA - reP

We shall see in a few moments why it is impor-tant that it could have been written that way. I havetalked to Dave Ritson to find out whether he or Am-man had any notion that this alternative form of theequation held the key to getting higher luminositiesthan they thought in 1961 were in prospect, and hehas told me, "No." They really were thinking of F,as a periodic function which is intrinsically of orderone and about which little could be done.

In 1962, construction began on the second genera-tion of machines, ACO at Orsay, ADONE at Frascatiand VEPP-2 at Novosibirsk.

ACO was designed to operate at beam energiesup to 500 MeV; it had a radius of 3.5 meters. Itwas a strong-focusing ring which reached luminosi-ties up to 102 9 cm-2sec-1 . ADONE was larger; astrong-focusing machine with a radius of 16.5 me-ters, it operated at energies up to 1.5 GeV.

ADONE was designed to take advantage of justabout everything that had been learned in the firstgeneration. ACO was somewhat too crowded to takeadvantage of everything; it was a rather small ma-chine, no larger really than the Princeton-Stanford

SLAC Beam Line, March 1986 7

machine. But ADONE was a big machine as its nameimplied. It was a machine with six periods. The ver-tical beta value at the interaction region, the thingI just mentioned, was 3.2 meters. Advantage hadbeen taken of the knowledge that a small value ofbeta at the interaction region was desirable, and thesmallest value of the vertical beta function anywherein the machine occurred at the interaction region.The achievements of ADONE were that it reached6x 102 9 cm-2sec 1 in luminosity and in fact achieveda tune-shift of 0.06 which is as good a value as hasever been achieved in any machine.

A new and unexpected feature arose in ADONE:the head-tail instability. This single-beam instabil-ity curtails the current that can be stored in a singlebunch. Its nature was elucidated by Claudio Pelle-grini of Frascati and Matt Sands of SLAC who wasvisiting Frascati at that time. It can be greatly ame-liorated by the use of sextupole magnets, and it hasbeen avoided as a performance limitation in existingmachines. Nevertheless it has been a design influenceon all of them.

Then came CEA and 'low beta.' The CambridgeElectron Accelerator physicists had a synchrotronand wanted to get into the colliding-beam business.

They were prevented from doing what they firstproposed: building a new storage ring and filling itfrom the synchrotron.

Then two of them, Ken Robinson and Gus Voss,had a brilliant insight. They looked at the beam-beam limit in the form of the second equation above,and they recognized that the beta function couldbe made extraordinarily small - not just somewhatsmaller than it was in other places in the machine,but extraordinarily small - in one or a few specialplaces around the machine. And if they made it tiny,they then had the wherewithal to get high luminosityeven with a machine that was limited by its complex-ity to rather small currents. (In terms of the beatfunction, they realized that it did not have to be oforder one everywhere; it could much smaller locally.)

Using this idea, the CEA synchrotron itself wasconverted into a special 'bypass' storage ring. Theresult was - and I can't think of a better way ofsaying this - a machine of staggering complexity.It was made to work and used to do colliding-beamphysics in 1973 at a maximum energy of 2.5 GeV perbeam, 5 GeV in the center of mass. They succeededin making the beta function as small as 5 centimeters,sixty-five times smaller than in ADONE.

PEP - 1980. The most recent storage ring at SLAG fills an underground tunnel nearly 1.4miles in circumference.

8 SLAC Beam Line, March 1986

And even then the luminosity of CEA was notlimited by the beam-beam limit; it was limited bythe incredible complexity and difficulty of the CEAoperating cycle. I think that the saga of CEA is theBook of Job of the accelerator builders. They wereafflicted by every handicap that could have been vis-ited upon them, yet they persevered, and in the endthe Lord loved them and they got the right value ofR. Of course nobody believed it. The machine wastoo hard to operate.

In the period we have reached now in the his-tory of colliding-beam storage rings, the first - andto date the only - proton-proton colliding-beamsystem was built, the Intersecting Storage Rings ofCERN. Construction took place between 1966 and1971. The ISR was a great success in accelerator-physics terms. It exceeded its design goals in termsof energy and luminosity, the two most importantparameters. Designed for a maximum energy of 28GeV, it reached 31.4 GeV; and designed to pro-duce a luminosity of 4 x 10 3 0 cm-2 sec- 1 it attained1032cm-2sec- 1, the highest luminosity yet reachedby any colliding-beam system.

Now we shall turn to the third generation ofelectron storage rings. (I have treated the CEAnot as a generation, but as a special event.) Thethird-generation machines were SPEAR at Stanford,DORIS at DESY and VEPP-2M at Novosibirsk. Wehave been through the first and second generations,and I shall not dwell on the characteristics of thesethird-generation machines. I shall merely remarkthat, historically speaking, the third generation tookfull advantage of what had been learned at such greatexpense of effort in the first two generations. Thethird generation used good high-vacuum technique;it used low-beta interaction regions; and it had head-tail-instability control built in. These advances paidoff - especially in SPEAR where a luminosity of103 1 cm-2 sec-lwas achieved at 3.7 GeV, the highestluminosity achieved up to that time in any machine(the ISR had not yet bettered that figure). The tuneshift reached by SPEAR was approximately equal tothat which had been achieved at Frascati, but witha much lower beta at the interaction region, SPEARproduced higher luminosity.

In my view, this third generation of storage ringsindicated that the storage ring technique had ma-tured. There were no profound new difficulties en-countered with these machines. There was detailedunderstanding of the problems that had been facedbefore.

The fourth generation further showed the matu-rity of the technology. It could be characterized as as.aling up by an order of magnitude in energy of al-ready successful techniques, and as the table shows,the luminosities that were achieved (which I state to

Machine Beam Energy Maximum Luminosity

CESR 6 GeV 2 x 1031 cm-2sec- 1

PEP 15 GeV 3 x 103 1 cm-2sec- 1

PETRA 20 GeV 2 x 1031 cm- 2 sec- 1

THE FOURTH GENERATION - Scaling up a suc-cessful technology.

only one significant figure) are a little bit better thanthe best that were achieved in the preceding gener-ation. These machines worked reasonably well whenwe got them working.

There are a couple of other things that should bementioned before I close. One of them is the ideaof building a system of rings to collide protons withelectrons. That possibility was studied as early as1971 by an international group which assembled hereat SLAC. It consisted of Dieter MShl from CERN,Claudio Pellegrini from Frascati, Andy Sessler fromLBL, and the SLAC bunch of Burt Richter, MelSchwartz and me. Our paper, which I presented atthe 1971 International Accelerator Conference, ex-cited the formation of several national projects: Epicin England, SuperAdone in Italy, and PEP in theUnited States. none of these ever got funded, builtor even carried very far, but PEP metamorphosedinto the electron-positron project I have already men-tioned. Now, finally, that dream is being pursued atDESY in the Hera project.

Now, of course, the largest of all the colliding-beam storage rings, LEP, is being built at CERN. Atthe same time linear colliders are being touted as thewave of the future for economic reasons. Is LEP thelast of the rings? If it is, then the whole history ofdesigning and building colliding-beam storage ringswill have spanned just over three decades.

(The author gratefully acknowledges the helpfulcomments of W. Kirk and V. Kistiakowsky in prepar-ing this talk for publication.)

SLAC BEAM LINE, x2979, Mail Bin 94

The Beam Line is a publication of the StanfordLinear Accelerator Center (SLAC), containingtechnical news and features for the staff andusers of the laboratory. This issue was edited byBill Ash.

Stanford University operates SLAC under contractwith the US Department of Energy.