Going along for the Ride

Abstract

For 70 years or so physicists studying the microcosm have used accelerated particles to probe structures too small to see and to bring out information about their architecture. The method started with molecules and atoms, progressed to atomic nuclei, and is now being used to elucidate the structure and behavior of subnuclear particles, such as protons and neutrons. At first physicists used naturally accelerated particles, the alpha, beta and gamma rays that come from naturally radioactive substances. But as the need to study smaller and smaller objects grew, the energy needed by the probe particle went up and artificial means of acceleration had to be developed. First were linear accelerators that moved electrically charged particles by attracting them from one end of a long vacuum tube to an electrode at the other end and letting them fly out the end when they were up to speed. Then came cyclotrons, which both extended the power and compressed the size of linear accelerators by effectively wrapping them into spirals. Now there are synchrotrons, which use radio waves rather than static electric fields to accelerate particles. But as energies have gone up, the accelerators have gotten bigger and more expensive, until now the construction of a sizable one is an undertaking equal to the establishment of a fairly large industrial enterprise. The latest United States project, the 200-400billion-electron-volt (GeV) National Accelerator Laboratory now under construction, will cost about $250 million. And more is yet to come. Russian physicists have laid plans for a 1,000GeV synchrotron (SN: 1/ 18, p. 63). They don't say how much this would cost, but they do say that it would need a ring-shaped vacuum tube something over five kilometers in diameter and a lake or reservoir for cooling water. In addition to being cumbersome, these machines are major items in national and international budgets, and many physicists see the budgetary handwriting on the wall: There may be no more money for any accelerators bigger than those now being built. Thus there is a good deal of interest among physicists in a method of accelerating particles that, if it works, might provide a 1,000-GeV accelerator in a space 1.5 kilometers long. This is only a tenth of the length of the Russian plan, which would have a circumference of 15 kilometers. A 70-GeV machine, about equal to the most energetic synchrotron now operating, could be about 500 meters long and might be built for around $20 million. The current rule of thumb for synchrotrons, $1 million per GeV, would give $70 million or more for a synchrotron that size. This new type of accelerator would be called an electron ring accelerator (ERA) and is based on an idea that has been mentioned from time to time over the past 20 years, but until now never really pursued. The idea is basically this: Light particles, such as electrons, can be accelerated to high speeds fairly easily and in short distances. If one accelerated a large cluster of electrons, and if this cluster had trapped within it a few heavy particles, protons or ions, the heavy particles would go along for the ride. At the end of the trip the heavy particles would have the same velocity as the electrons, but since they are many times heavier than electrons, each of them would have many times the energy of a single electron. A proton, says the current calculation, would have