Ten Years After the Revolution

Ten years after the revolution

by Burton Richter

In his inaugural talk as Direc­ tor of the Stanford Linear Accelerator Center last No­ vember, Burton Richter chose the theme 'Where Are We Going at SLAC?'. The occa­ sion also coincided with the tenth anniversary of the 'No­ vember Revolution' — the simultaneous discovery by Sam Ting and his group at Brookhaven and by Richter and the Mark I collaboration working at the SPEAR elec­ tron-positron collider at SLAC of a new kind of particle (the J/psi) which did not fit in with the established ideas of the time. Here are some ex­ tracts from Richter's inaugural talk.

'In high energy physics an interplay of experiment/theory and technol­ ogy advances our understanding of nature. These three horses pull understanding of how our physical The recent inauguration of Burton Richter as Director of the Stanford Linear the chariot of science forward; universe works. These new models Accelerator Center coincided with the tenth sometimes one pulls harder than require new experiments, and the anniversary of the 'November revolution', another, but all three are neces­ accomplishment of those new ex­ the simultaneous discovery by the groups of Richter (left) at Stanford and Sam Ting sary. In deference to our visitors periments requires new tools, par­ (right) of the J/psi, a new kind of particle from Washington, I should say ticularly new accelerators, to give which at first didn't fit in... that there is a fourth element in­ us the ability to probe more deeply (Photos Stanford) volved; it is money, and it might into matter. This sounds very evo­ be likened to the harness that lutionary, and sometimes it is, but hitches those three horses to that sometimes progress in science I joined a group of scientists who elegant chariot. comes about from revolutionary believed as I did, but who back What might be termed the advances in theory or experiment. then at least were considered odd 'standard model' for the advance Stanford University and SLAC by most physicists who thought of science involves an interplay of have played a major role in both that experiments at proton accel­ experiment, theory, and technolo­ the evolutionary and revolutionary erators were 'the only way to go'. gy. The experimenters, guided by advance in high energy physics as When I first arrived at Stanford, what we know, test the present long as I have been here, 28 years. the Mark III linac had just recently theories and uncover new facts It is unusual for a scientist to stay begun operation. It was a bold that sometimes fit and sometimes in one place so long, but when I step in energy, moving linac tech­ don't fit into our existing world first came to Stanford in 1956 I nology from the tens of MeV that model. The theorists take the out­ believed that the electron beams were the maximum energies of put of the experiments, particularly available from the accelerators at existing machines to over 800 those things that don't quite fit, Stanford were the best tools with MeV. Mark III was a remarkably and use them to extend the theo­ which to gain a better under­ large machine — all of 300 feet retical model to get at a deeper standing of the structure of matter. long! It was a technological tour Construction begins in 1970 for the SPEAR electron-positron collider at Stanford. This modest machine provided (and continues to provide) a physics harvest out of all proportion to its size and showed the worth of colliding beam machines. de force in its time, and it was used for many important physics experiments. Robert Hofstadter used its beams to measure the shape of the proton, showing that it was not a point particle and de­ termining its size. The advances in accelerator technology pioneered at the Stan- ord High Energy Physics Labora­ tory (HEPL) were, in the long run, probably just as important as the experiments done with the ma­ chine. The first colliding beam stor­ age ring was built there both to pioneer a new technology and to carry out experiments at a new energy inaccessible without the new technology. All major accel­ erators under construction today are colliders of one type or anoth­ er, and they all owe much to those early Stanford efforts on colliding beams that showed the way to get much higher energy for a given cost. While all this.was going on, a ney Drell put it at the time, 'there SPEAR, the first paper on higher group of scientists led by 'Pief seemed to be seeds in the grapes.' energy electron-positron colliding anofsky were thinking about the It changed our view of the suba­ beam systems was written by fiext step in linear accelerators. In tomic world. John Rees. Out of that paper came 1956 the conceptual design began In parallel with many important the machine that is now PEP, of a new giant machine, then called linac experiments, work began on which began its experimental pro­ 'the Monster' because it was so colliding beams. In 1961 Dave Rit- gramme in 1980 with a new set very large. The first beam from son and I started the design of of experiments of even greater that accelerator, the 10 000-foot- what would be the SPEAR storage sophistication than what had gone long SLAC linac, was delivered in ring. Construction started in 1970, before. 1966. and the turnon was in 1972. Here, In 1978 work began here on a The SLAC linac was a huge ex­ too, together with the innovation new kind of colliding beam device trapolation in technology, taking in accelerators was innovation in — what is now called the SLC (for linacs from the then 300-foot ma­ experimental apparatus. The Mark Stanford Linear Collider). The need chine at HEPL to 10 000 feet at I magnetic detector was a powerful for a new technology in colliding SLAC. Together with the innova­ tool in its own right, and has been beam devices became apparent tions in linac technology came the forerunner of much more so­ to some of us when we took a innovations in experimental appa­ phisticated devices of the same hard look at the greatest of all the ratus. The first major experiment general type at accelerators all storage ring colliders, the LEP pro­ proved the worth of the entire over the world. The SPEAR stor­ ject being built at CERN. This ma­ effort, showing that the proton age ring is still going strong, and chine is 27 kilometres around and had a substructure — it was not we are making major improve­ will cost more than half a billion elementary, but seemed to be ments to increase its colliding dollars. The scaling laws for elec­ made up of still smaller entities beam intensity. tron storage rings are well known; very tightly bound inside. As Sid­ In 1970, before the turnon of size and cost go as the square of Aerial view of the Stanford Laboratory site showing the two-mile linac in the background. Shown dotted are the arcs for the new Stanford Linear Collider, a novel concept in colliding beam machines in which electron and positron beams from the linac will be led round to collide just once, rather than continuously circulating in a ring. The tunnels for the SLC arcs have been cut, and the project is scheduled for completion in 1986.

the energy. Given this scaling law, lifetime. The big SLAC linac itself exploited. to go up a factor of 10 in energy — the reason for building this Lab­ What now? With the completion would require a machine of some oratory — is no longer being used of SLC in 1986, our linac will be 2700 kilometres in circumference for frontier high energy physics back as a forefront facility. It is costing about 50 billion dollars. A experiments, but serves as an in­ the heart of the linear collider, but new technique was needed to con­ jector for our present generation it will have undergone considerable tinue at a price that our real mas­ of storage rings. However the Lab­ improvement. Beginning with ters, the taxpayers, would consider oratory is still doing frontier re­ 1986, a new era of experiments to be reasonable, and the SLC search, thanks to innovations in will start at SLAC that we all ex­ seems to be that technology. accelerators and technology. Work pect to contribute important new No Laboratory is static. No tool on those innovations proceeds information to our evolving view for physics research has an infinite while the 'old' facilities are being of the structure of the physical universe. We can expect some­ their studies already. The first is accelerator gradient, or more than thing like 10 or 15 years of pro­ that much more efficient radiofre- six times what is now used in the ductive experiments from the SLC. quency power sources than our SLAC linac. Since this has been Indeed, the first SLC improvement present klystrons strongly impact demonstrated in a section about project is already under design the design of the large linear col­ 1 foot long, I have issued a chal­ (polarized beams) even though the lider and can sharply reduce the lenge to the Technical Division: machine is not yet complete. We cost for a given energy. Necessity deliver a 1 GeV accelerator less have a clear idea of what we will is indeed the mother of invention, than 30 feet long for less than one be doing until the second half of and, in response to this need, an million dollars' I expect they will •*he 1990s. r.f. power source, which uses a do it! What will we do for an encore? laser to produce the electron beam As to the future of SLAC, I have The answer really depends on the in the tube, has been analysed on already told you some of the things imagination and the initiative of paper and looks to be considerably that I see in the next 10-15 years, the SLAC staff. I can tell you what more efficient than our present but my real message is perhaps we're thinking about now, but if klystrons. A combined group of best summarized in the old axiom someone has a sufficiently bright people from the Technical Division 'the Lord helps those who help idea, we may be thinking about, and the Research Division has done themselves'. Over the entire time and working on, something quite a thorough analysis, including com­ I have been at Stanford, we have different a few years from now. puter simulations, and has started continued to develop high energy At present we are striving to on the construction of a proof-of- physics here by helping ourselves develop a new technique for col­ principle device which we hope to through the invention of new the­ liding beams to allow affordable have ready in about two years. ories, new accelerators and new machines of considerably higher This power source is supposed to techniques of experiments. How energy to be built. The SLAC linac have a peak power output of about far we will go beyond the next 10- is not optimized for the accelera­ 100 MW — about twice the power 15 years depends on all of you. tion of the kind of beams required output of the new klystron we are for a very high energy linear collid­ developing for the SLC, — have a er. The linac was aimed at deliver­ pulse length of one microsecond, ing long bursts of particles with and be more than 70 per cent effi­ -as uniform an energy spectrum as cient. When we achieve it, this possible. Linear colliders deliver will be the most efficient high pow­ very few bursts within a single er pulsed high frequency r.f. source pulse, and in that mode their op­ in existence. If it performs as ex­ timization is different. The SLC is pected we will know that we can already teaching us a great deal reasonably expect to go on to about beam dynamics in linear col­ power sources of greater than 90 liders, and we will learn much more per cent efficiency. when the project is in routine oper­ The second thing to come out ation. of the very early stages of the There is a group now working work of the Big Collider Study at SLAC called the Big Collider Group is that much higher acceler­ Study Group — composed of the­ ating gradients than we use in the oretical physicists, experimental­ SLAC linac look to be desirable to ists, and accelerator physicists. make cost-effective machines. In This group is studying linear collid­ response to that need, we are try­ er optimization with a view toward ing to find out just how large an understanding what technical de­ accelerating gradient can be ob­ velopments are important in mak­ tained in a structure similar to the ing very large machines both prac­ SLAC accelerating structure. It has tical and affordable. already been shown that we can Two things have come out of get more than 100 MeV per metre