PHYSICISTS have probed deeply into the basic structure of matter using the ever more powerful particle accelerators that have evolved in this century. But if they are to continue their work in the next century, they need a whole new class of machines. The design principles embodied in today's accelerators have gone about as far as they can go. But ``there are some nice concepts out there'' for radically new types of machines, says Robert Siemann, a physics professor at Cornell University in Ithaca, N.Y.
They include such exotic notions as waves in high-temperature gases pushing bunches of electrons along, like surfers in the sea, or using a large electron cluster to drag along a smaller cluster, like a water skier riding the waves in the wake of a boat.
Dr. Siemann reviewed the prospects for such concepts last month at a session of the annual meeting of the American Association for the Advancement of Science in New Orleans. Commenting on that talk recently, he noted: ``Most of these advance concepts are in some stage between idea and small-scale experiment. A working machine is still several steps away.'' However, he added, ``It's important to know that those ideas are out there.''
In saying this, Siemann reflects a consensus among high-energy physicists that their field is at a major crossroads. They have to learn how to design the next generation of accelerators even while they try to make the most of what they already have.
Two new electron accelerators came on line in Europe and the United States last year. Both accelerate electrons and positrons (positively charged electrons) to unprecedented energies and then smash them together.
These energies are expressed in terms of the electron volt or eV. An electron gains 1 eV when it is accelerated by a voltage difference of 1 volt. Modern accelerators speed particles to many billion electron volts or GeV. The ``G'' stands for ``giga,'' the metric prefix for billion. An energy of 100 GeV is equivalent to particle temperatures of a million billion degrees - temperatures that prevailed very early in our universe's history. In striving for higher energies, physicists not only want to probe more deeply into matter itself, they also want to explore the evolution of the cosmos.
The new collider at the Stanford Linear Accelerator Center (SLAC) near San Francisco speeds the particles down a two-mile-long pipe, boosting them to 50 GeV before separating the two types of electron into individual beams that collide head on. At the European Center for High Energy Physics (CERN) at Geneva, electrons and positrons are accelerated as counter-rotating beams in a circular tube nearly 17 miles in circumference. The particles gain 45.5 GeV before smashing together.
Meanwhile, American physicists are designing their ultimate proton accelerator - the Superconducting Supercollider - to be built near Dallas.
These powerful new accelerators will keep high-energy physicists occupied for the next 10 to 15 years. Yet they represent the end of the road for such devices. As John M. Dawson, one of the fathers of some of the futuristic concepts, has explained, it isn't just prohibitively high cost that makes such designs obsolete. Fundamental physical effects are also involved.
These machines use the strongest magnetic fields obtainable to guide particles while strong electric fields accelerate them. Dawson explained in a Scientific American article a year ago that traditional accelerator design has reached the point where ``the forces from magnetic fields are becoming greater than the structural forces that hold a magnetic material together.'' Furthermore, he added, the energy needed for the accelerating electric fields ``is reaching the energies that bind electrons to atoms; it would tear electrons from nuclei in the accelerator's support structures.''
Thus, not only can physicists no longer afford to design machines in the conventional way, any such machine much more powerful than the latest versions probably would tear itself apart.
Future accelerators will still use magnetic fields to guide particles and electric fields to accelerate them. But they will do this in ways that eliminate the potential for self-destruction and probably will keep costs down.
These machines will likely concentrate on beams of electrons and positrons accelerated in a straight line, Robert Siemann says. Electron-positron collisions give clean products that are relatively simple to study. Also, the new concepts are easier to apply to straight-line accelerators.
New design concepts
One such general concept involves accelerating electrons with waves in a type of high-temperature gas called a plasma. It's a mixture of free electrons and positively charged atoms.
If waves are set up in a plasma-filled tube and bunches of electrons are injected with the right timing, these bunches can ride the waves and gain energy like surfers. Siemann explains that, in one of the currently most promising devices, a laser pulse excites a plasma wave that can speed electrons to enormous energies.
Siemann notes that such a device promises to accelerate electrons by 2.5 GeV for every meter of its length. In fact, three such devices staged together could boost electrons to a trillion electron volts in only a kilometer. That's 20 times the energy achieved by the new SLAC collider in a device less than half the collider's length. ``That's pretty exciting,'' Siemann says, ``But it's only a paper idea.''
James Simpson heads a team at the Argonne National Laboratory that has already made small-scale tests of another promising technique (see diagram). Siemann calls this ``a solid idea'' that could be practical in 10 to 15 years.''
Right now, Siemann explains, accelerator physicists are at the stage ``where we really need to understand the basic ideas to move forward.'' But, he adds, as they look to the next century, ``it's not a situation where people have no ideas to work with.''