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Recycling Antimatter Becomes Reality by Kurt Riesselmann
Antimatter is arguably the rarest stuff Mother Nature provides here on earth. Created in high-energy particle collisions, antiparticles quickly disappear by reacting with ordinary matter. Using powerful accelerators, physicists have learned to produce and control tiny amounts of antimatter. The yield, however, is less than a microgram per year, and antiparticles remain a precious commodity.
Scientists at Fermilab are now taking a new approach to satisfy the ever-increasing demand for antimatter: they will recycle antiprotons. To maximize the use that experimenters can get out of the antimatter created at the lab, scientists have built a storage ring, the Recycler, in which they can inject antiprotons that have survived many hours of collision experiments. When the experiments receive the subsequent batch of antiparticles, the beam will contain recycled antiprotons – assuming scientists manage to keep the antiprotons “alive” inside the Recycler for a long enough time.
“As of now, we’ve achieved a lifetime of greater than forty hours,” said Shekhar Mishra, who heads the Main Injector Department and Recycler Commissioning. “And plans are already in place to improve the lifetime by at least a factor of three.”
Things looked less promising as few as three months ago, when the lifetime was a mere nine hours. The innovative and energy-saving design of the Recycler relies on a two-mile-long ring of permanent magnets to steer antiprotons through a two-inch-wide beam pipe. Imperfections in the alignment and the absence of adjustable electromagnets have made it difficult to keep antiprotons on track as they fly close to the speed of light through the vacuum-filled Recycler beam pipe, about 100,000 times per second. Too many antiprotons were lost due to contact with the pipe.
“The Recycler is only a success if the beam goes around many billions of times,” said Fermilab physicist Bill Foster, who worked on a smaller beam line with permanent magnets in the past. “People didn’t realize how hard it would be to align the beam pipe between the permanent magnets so that the beam can go through. It is a tremendous amount of work to put things together the way it was meant.”
International help
In Spring 2001, Mishra and other Recycler experts began to make plans for major improvements to the antiproton storage ring. Clearly, the magnets and the beam pipe needed to be aligned with greater precision. In addition, the group wanted to install corrector magnets, spares that Fermilab had received from CERN, the European particle physics laboratory, which was disassembling its LEP accelerator. Finding enough people to carry out this work proved to be the critical factor.
“We knew what we needed to do,” said Mishra. “But we could only have five to six weeks of access to the tunnel that hosts the Recycler. We had to look for additional people to get the work done during shutdown time.”
A call for help went to all Fermilab divisions as well as particle physics institutions around the world, and the response was overwhelming.
“We received offers from the University of Pennsylvania and the University of Glasgow, Scotland,” recalled Dave Augustine, who coordinated the work of all technicians during the shutdown. “We also had offers from Germany, Russia and other countries. I had more volunteers than I could use. Many department heads at Fermilab also pared down their to-do lists, making their personnel available for the shutdown. We trimmed it to roughly 90 people, about 40 people beyond our own group. We got help from top-notch, willing individuals. They were fun people to be around. I miss them.”
“We planned very carefully,” said Mishra. “If a screw needed to be ordered, it was there on time. All the jobs on my list got done. It couldn’t have happened without the help of these people. They knew how important this was. And they were all very professional. We had no accidents.”
Lifetime increases dramatically
The upgrades had the desired impact on the performance of the Recycler. All magnets and beam pipe sections are now aligned to one tenth of a millimeter, ten times better than before the shutdown. And the correctors give scientists additional control of the beam to minimize beam losses.
“The alignment problems and the lack of correctors was a double whammy,” Foster pointed out. “Now the Recycler is in good shape. We now have both belts and suspenders.”
Despite the progress, Mishra and Foster are not yet content with the Recycler.
“We need a lifetime of more than 100 hours,” said Mishra. “If we improve the vacuum by a factor of three, it’s do-able. We are going to have another shutdown, probably in July, to upgrade the Recycler even more.”
At present, the vacuum inside the beam pipe is not as good as hoped for. There are still gases like argon and water vapor inside the pipe. They reduce the lifetime of the antiproton beam as antiprotons collide with the gas particles. Adding special heavy-ion vacuum pumps to the Recycler, scientists can eliminate the argon, which is a residue from welding the beam pipe. To “bake out” the traces of water vapor, technicians surrounded the beam pipe with special heating tape. During the next shutdown, scientists will heat the beam pipe to more than 240°F. Additionally, crews are looking for tiny air leaks, but they haven’t found any yet.
This fall, Mishra’s group expects to integrate the Recycler into daily Fermilab accelerator operations. Initially, scientists will use the Recycler to store new antiprotons. Actually recycling antiprotons will be the second phase. It will increase the number of proton-antiproton collisions inside the Tevatron, the world’s most powerful particle accelerator, by a factor of two to three, significantly improving the chance that experi-mentalists may find the elusive Higgs boson or some new building block of nature in the next several years.
“The Recycler is an integral part of achieving the Run II goals that the lab has set for itself in collaboration with the Department of Energy,” said Ron Lutha, a manager at the local DOE office, who had supervised the civil construction of the Recycler. “We have a long way to go. But the more antiprotons you have out there to smash together the better.”
Thanks to all the people helping during the recent Recycler upgrade, the day is near that no antiproton will go to waste.
Beams Division
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The technicians worked at various locations inside the two-mile tunnel that hosts the Recycler. To complete the alignment work on time, Augustine enlisted the help of every surveyor of the Fermilab survey group, organizing their work in two shifts. Cons Gattuso coordinated the technical aspects of the tunnel work, consulting with other scientists and beam experts. And the preparations paid off.
