by Mike Perricone

With recycled particles and recycled parts, the Recycler will change the way Fermilab conducts its antiproton business. Along the way, it may also show that some of the best new ideas can be old ideas that have been recycled, too.

The unique Recycler will be used to recover and store antiprotons, those rare and costly particles comprising one component for collider experiments in the Tevatron. Sharing the two-mile tunnel with the new Main Injector accelerator, the Recycler is also the world’s largest collection of permanent magnets, a key ingredient in its low-cost construction.

And it works.

A beam of protons moved through about one-third of the Recycler ring on January 12, 1999, before ending its run by being absorbed at a temporary steel target. Since protons and antiprotons have the same mass but the opposite charge, the protons moved in the opposite direction than the antiprotons will take when the Recycler is operating under "real" conditions.

But moving particles "backward" in the machine was barely worth noting, compared to the level of success demonstrated by the permanent magnets. The proton beam was navigated along a curved path some two-thirds of a mile long without needing any course adjustments from the corrector magnets, which have been recycled from the old Main Ring accelerator that has been dismantled, with its functions assumed and extended by the new Main Injector.

"We were confident it would work because of the magnetic field measurements that were done when the permanent magnets were built, but the beam always has the final word," said Cons Gattuso of the Main Injector Department, who is Operations Specialist for the Recycler.

"But it was also very surprising," he continued. "A lot of us thought we would need to have the corrector magnets on and running in order to get the beam all the way around. We have correctors throughout that region, but we didn’t have to use them."

The beam also carried three critical messages: first, that the magnets had been aligned correctly; second, that there were no obstructions in the beam path; and third, that the Recycler was a true 8-GeV machine, meaning everything had been done right the first time, and the machine’s energy level was a good "match" with the Main Injector.

"If it hadn’t worked, we would have had to remove all the magnets from the tunnel and rebuild them," Gattuso said. "With permanent magnets, we have no way of adjusting the magnetic field without taking the magnets out of the tunnel. In the Main Injector, we adjust the magnetic fields so that an 8-GeV beam is centered in the beam pipe. When we injected that beam into the Recycler, and it was running in the middle of the Recycler beam pipe, we knew the magnets had been designed and built correctly."

The last of the magnets are being installed in the coming weeks, along with the last sections of vacuum pipe and instrumentation for measuring the beam’s intensity, while the accelerator complex is shut down to allow construction access in the tunnels.

"It will be interesting to see what the remaining two-thirds of the ring has in store for us," Gattuso said.

When it’s completed, the Recycler might have an old idea recycled for its future: electron cooling, which has a history dating back to 1978 at Fermilab, though that history had a long hiatus between 1986 and 1995.

Electron cooling was first developed in Russia and first tested in the laboratory in Novosibirsk, in Siberia, where Sergei Nagaitsev received his university training. Nagaitsev now has an office along Fermilab’s Linac corridor, complete with a poster of the classic VW Beetle, another recycled idea. He is working on the project to adapt electron cooling for use in the Recycler, alongside the stochastic cooling process that has become the standard in maintaining high-energy particle beams.

"At one point, electron cooling was considered an effective way to cool antiprotons coming from the target," Nagaitsev related. "However, stochastic cooling was invented and it turned out to be superior to electron cooling when the antiproton beam comes away from the target very hot, at very large angles. Stochastic cooling was invented at CERN (the European particle accelerator laboratory), and it led to the discovery of the W and Z bosons and to a Nobel Prize for Simon van der Meer and Carlo Rubbia (in 1984)."

Stochastic cooling, which shifts individual particles within the beam, can only take the cooling process so far before getting bogged down. For the Recycler, electron cooling will take over after stochastic cooling has reached its limit, and then cool down the beam to a small enough cross section for reuse in Tevatron collisions.

A 200-MeV cooling ring had actually been built at Fermilab to test both electron and stochastic cooling, in an area that is now a parking lot alongside the Booster tower. The ring was decommissioned in 1983, and many of the parts were sent to be recycled at the Indiana University Cyclotron Facility–coincidentally, where Nagaitsev earned his doctorate. He’s been bringing the idea back to life at Fermilab since 1995, and if all goes well, the electron cooling system will be installed for use on the Recycler some time within the next four years.

In electron cooling, a segment of the Recycler up to 150 feet long will actually house two particle beams–an antiproton beam and an electron beam–racing side-by-side and bumping into each other. The antiproton has 2,000 times the mass of the electron; when they bump, the antiproton gets nudged while the electron absorbs most of the energy of the collision and flies off at an angle. Essentially, the electrons take away energy from the outer edge of the antiproton beam, reducing the size of its cross section.

For electron cooling to work, the particles must be traveling at the same velocity. For an 8-GeV antiproton beam, that translates into a 4-MeV electron beam with 700 milliamps of current, which could take huge amounts of power to generate–approximately 2 megawatts. Nagaitsev described electron cooling as an everyday tool for low-energy particle beams, but said that using it for high-energy beams (such as in the Recycler) has been impractical. Nagaitsev hopes to change that through the work underway in part of a fixed-target building at Fermilab, and at test facilities provided by National Electrostatics Corporation in Middleton, Wisconsin.

The system uses a conventional electron gun and a trademarked variation on the classic Van de Graaff electrostatic generator called the Pelletron, which uses a chain filled with metal pellets (hence, "Pelletron") instead of a belt to charge a high-voltage terminal.

"The components are mainly old technology, and there’s really nothing exciting about them," Nagaitsev said. "But the combination we’re putting together is new. What’s really exciting is that to generate the power we need, we’re forced to use a trick. That trick is called beam recirculation."

The electrons used to cool the antiproton beam are recollected, and their energy is recaptured and restored to the Pelletron. Nagaitsev said the system recycles about 99.999 percent of its beam energy. Once the system is pumped up and running, it can be maintained with an introduction of power equal to the small loss incurred in the system.

In one example, Nagaitsev said a level of 300 kilowatts (needed to run the various motors and power supplies for generating and focusing the beam) could be sustained by adding just three watts from a Pelletron charging power supply, with the rest recycled from the electron beam. Nagaitsev said the system under development has now generated as much as 700 milliamps of current, "and this is the level we need to achieve electron cooling in the Recycler."

It’s an idea whose time has come and gone and come back again, altogether fitting for a machine called the Recycler.