The first cyclotron particle accelerator, invented in 1931 by Ernest O. Lawrence and M. Stanley Livingston, accelerated protons to energies of 80 000 electron volts. A few months later, the second achieved 1 million electron volts (1 MeV). The world's largest cyclotron, built and commissioned in 1974 at Canada's National Laboratory for Particle and Nuclear Physics, known as the TRIUMF Laboratory, in Vancouver, B.C., reached energies of 500 MeV. Thirty-six years later, it's still the world's largest and one of the crowning achievements of modern engineering—which is why in December it was designated an IEEE Milestone in Electrical Engineering and Computing.
Through subsequent modifications, the TRIUMF cyclotron is now producing three times the beam intensity it was designed for, according to IEEE Member Ewart Blackmore, who has worked on TRIUMF since its inception. Even so, it's considered an intermediate-energy accelerator, one of only three such high-intensity accelerators in the world. But cyclotrons—so named because they accelerate particles in a spiral path—are the accelerators of choice for high-intensity and intermediate-energy applications, says IEEE Senior Member Dave Michelson, associate professor of electrical and computer engineering at the University of British Columbia, also in Vancouver. (The highest energy accelerator in 1974, a synchrotron at Fermilab, operated at 300 GeV, but with far lower beam intensity. Today's highest-energy machine, the Large Hadron Collider, runs at 3 TeV.)
“The real milestone is that for more than three decades this cyclotron has remained at the cutting edge of science,” says physicist Tim Meyer, TRIUMF's head of strategic planning and communications. “What's kept it there is its flexible and powerful design. The magic of this device is that it can provide not just one beam of protons but multiple beams at different energy levels and intensities.”
Adds Blackmore, “This was a unique accelerator, the only one in its energy range that accelerated H- ions instead of protons.”
Since protons are a cyclotron's desired output, it would seem logical to accelerate them directly, and many cyclotrons did just that. “But then it's difficult to kick the orbiting protons out of the machine's large magnetic field so you can extract them from the cyclotron and use them; you get beam spill and losses,” Blackmore adds.
The way to overcome that, developed by professor Reginald Richardson at the University of California at Los Angeles, was to use H- ions. An H- ion is a proton plus two electrons, a relatively loosely bound structure. According to Blackmore, when you extract it, you can strip away the electrons by passing it through a thin foil. That lets you extract more than one beam (the TRIUMF machine can extract four), and by placing the foils at different radii of the particles' spiral path, you can extract beams of different energies. That's what's kept TRIUMF in the forefront for research and practical applications.
The huge cyclotron has a 4000-ton main magnet 18 meters (59 feet) in diameter and a main RF amplifier that delivers almost 1 million watts of power. According to Michelson, who is chair of the IEEE Vancouver Section, building a cyclotron that large required the development of revolutionary computer-assisted design, modeling, measurement, and tuning technologies, some of them still used today at TRIUMF and elsewhere.
USES AND ACCOMPLISHMENTS
Cyclotrons and other accelerators produce particle beams that alter the composition of the atoms they strike—hence they're popularly called “atom smashers”—for research or practical applications.
“Because the TRIUMF cyclotron can provide very intense beams of protons, it's been able to perform some of the most detailed science experiments in particle and nuclear physics, enabling researchers to examine hundreds or millions of reactions and look for deviations and extremely rare reactions,” Meyer says. “In a recent experiment, we were able to measure the decay properties of muons [elementary particles that each decay to an electron, a neutrino, and an antineutrino] nearly 10 times more precisely than ever before.”
Similarly, TRIUMF's cyclotron was used to understand the roles of the strong and weak nuclear forces inside the nucleus. The weak force is associated with radioactive decay, and the strong force is what holds an atom's protons and neutrons together. With a series of “parity violation” experiments, TRIUMF scientists confirmed how the two different forces work.
Isotopes created by proton-beam bombardment have been used in experiments that study the nuclear reactions happening inside stars, one step at a time. Those detailed studies play into the larger question of how and why the universe came to be.
“When we look at our models of the early universe, we would expect equal parts matter and antimatter, thus leading to complete annihilation and very little else—but the universe is very full of matter!” Meyer says. “So physicists are out to study how and why matter and antimatter are different and how they got to be that way.”
On the practical side, the TRIUMF cyclotron has been used to study high-temperature superconductors, and to test how well materials such as semiconductor wafers and chip sets will do in space by irradiating them with a flood of protons or neutrons.
“Instead of manufacturers and space agencies waiting 10 years to see how the satellite materials will withstand space radiation, we can determine that in a week or less,” Meyer says.
The TRIUMF cyclotron has long been used in medicine, too, using proton beams to treat cancer, and making medically useful isotopes—including some that have not been made before—for sale.
The IEEE Milestone ceremony this month honors the TRIUMF cyclotron with a plaque on a wall outside the main control room. The plaque reads:
First 500 MeV Proton Beam from TRIUMF Cyclotron, 1974.
At 3:30 p.m. on 15 December 1974, the first 500 MeV proton beam was extracted from the TRIUMF cyclotron. Since then TRIUMF has used proton beams from its cyclotron (and secondary beams of pions, muons, neutrons, and radioactive ions produced in its experimental halls) to conduct pioneering studies that have advanced nuclear physics, particle physics, molecular and materials science, and nuclear medicine.