Accelerators

The development of linear accelerators is limited by some obvious physical constraints. For example, the SLAC linac is so long that engineers had to take into consideration the Earth's curvature when they laid out the drift tube sequence. One way of avoiding the problems associated with the construction of a linac is to accelerate particles in a circle. Machines that operate on this principle are known, in general, as circular accelerators.

The earliest circular accelerator, the cyclotron, was invented by University of California professor of physics Ernest Orlando Lawrence in the early 1930s. Lawrence's cyclotron added to the design of the linac one new fundamental principle from physics: a charged particle that passes through a magnetic field travels in a curved path. The shape of the curved path depends on the velocity of the particle and the strength of the magnetic field.

The cyclotron consists of two hollow metal containers that look as if a tuna fish can had been cut in half vertically. Each half resembles a uppercase letter D, so the two parts of the cyclotron are known as dees. At any one time, one dee in the cyclotron is charged positively and the other negatively. But the dees are connected to a source of alternating current so that the signs on both dees change back and forth many times per second.

The second major component of a cyclotron is a large magnet that is situated above and below the dees. The presence of the magnet means that any charged particles moving within the dees will travel not in straight paths, but in curves.

Imagine that an electron is introduced into the narrow space between the two dees. The electron is accelerated into one of the dees, the one carrying a positive charge. As it moves, however, the electron travels toward the dee in a curved path.

After a fraction of a second, the current in the dees changes signs. The electron is then repelled by the dee toward which it first moved, reverses direction, and heads toward the opposite dee with an increased velocity. Again, the electron's return path is curved because of the magnetic field surrounding the dees.

Just as a particle in a linac passes through one drift tube after another, always gaining energy, so does a particle in a cyclotron. As the particle gains energy, it picks up speed and spirals outward from the center of the machine. Eventually, the particle reaches the outer circumference of the machine, passes out through a window, and strikes a target.

Lawrence's original cyclotron was a modest piece of equipment—only 4.5 in (11 cm) in diameter—capable of accelerating protons to an energy of 80,000 electron volts (80 kiloelectron volts). It was assembled from coffee cans, sealing wax, and leftover laboratory equipment. The largest accelerators of this design ever built were the 86 in (218 cm) and 87 in (225 cm) cyclotrons at the Oak Ridge National Laboratory and the Nobel Institute in Stockholm, Sweden, respectively.