NEW PARTICLE The next generation of electron-positron colliders Andrew M. Sessler In 1932 John Cockcroft and Ernest Walton developed an electrostatic accelerator at the Cavendish Laboratory, thus starting the modern age of particle accelerators. Since then, our capabilities have increased tremendously, as may be seen in figure 1, which shows the energy of accelerators through the years. There has been an exponential increase in energy (the "Livingston curve")-but it is the envelope that displays this exponential behavior, not any particular technique. One can conclude that if we are to stay anywhere near the Livingston curve, new techniques need to be developed. I shall describe some of the new techniques that will be employed as we build accelerators even more powerful than the colliders just completed (the Tevatron collider at Fermilab, Tristan at KEK in Japan and the Stanford Linear Collider), those under construction (LEP at CERN, HERA in Hamburg and UNK at Serpukhov) and those under design (SSC in the US and the Large Hadron Collider in the LEP tunnel). Circular accelerators are already remarkably effective. In fact, no one has any ideas for improving them, other than in an evolutionary manner. For example, new superconductors may provide stronger bending magnets, or less demanding cryogenic requirements. (If the new high-temperature superconductors are to be useful for accelerators, they will require high critical current density and the material properties that will permit the formation of wire.) Needless to say, such evolutionary changes can be very significant. For protons, even at beam energies very much greater than that of the 20-TeV SSC, circular accelerators are Andrew Sessier is a physicist ar, and former director of, the Lawrence Berkeley Laboratory. He works in close collaboration with the Beam Research Program ar Livermore. quite adequate. Not so for electrons, where synchrotron radiation forces one to use linear machines. At 1 TeV, for example, an electron in the 50-mile-circumference SSC tunnel would radiate away more than half its energy in just one turn! Because center-of-mass energy increases only as the square root of beam energy in a fixed-target relativistic accelerator, all modern accelerators are collidingbeam devices. Thus, all of the research efforts on new acceleration techniques for high-energy physics are focused on linear electron colliders, and here I will consider only such schemes. The Stanford Linear Collider will soon be providing experimenters with 100-GeV ete collisions. The twomile-long linear accelerator that speeds these particles up to 50 GeV provides an accelerating gradient of 17 megavolts per meter-more or less the present state of the art for rf linacs. If we are to have TeV linear e*ecolliders of reasonable length, we will need an order of magnitude increase in accelerating gradients. We do not yet have such a capability in hand. Intense efforts are now underway at CERN, SLAC, KEK and Serpukhov to develop the techniques that would allow these labs to build TeV linear colliders early in the 1990s. We will discuss only one aspect of such colliders-the acceleration process itself. The reader interested in other important aspects of linear colliders, such as damping rings, focusing systems and the disruptive interaction of the beams at collision, should consult the literature.' A number of recent conferences have been devoted to the subject of novel acceleration schemes, and the reader seeking more detail than can be included here may wish to consult their proceedings.2 2-6 Categorizing accelerators Particle accelerators do their work by means of the electromagnetic force. Because Maxwell's equations are 26 PHYSICS TODAY JANUARY 1988 1988 American Institute of Physics |