Making the best antiproton beams
The recipe for producing large numbers of collisions, or high luminosity, in the Tevatron seems rather simple: produce beams with a large number of protons and antiprotons; make sure that the beams are "nice and tight," and that few particles are lost as the beams pass through the chain of accelerators; send the beams into the Tevatron for final acceleration; and make the beams collide head-on at the center of the CDF and DZero detectors.
Although there have been numerous improvements in the Fermilab accelerator complex during the past twelve months (and Fermilab Today will highlight several of them in this series), the flurry of luminosity records in the last six months is closely related to two success stories: the full integration of the Recycler storage ring into the antiproton supply chain, and the start-up of a new method to provide "nice and tight" antiproton beams via electron cooling.
Located inside the Main Injector tunnel, the Recycler storage ring relies on hundreds of permanent magnets to steer antiprotons around a two-mile circle at a constant energy of 8 GeV (billion electron volts). After a rather slow startup, the commissioning of the Recycler has progressed rapidly in the last twelve months.
"In August 2003, the Recycler was basically on the ropes," said Cons Gattuso, the Recycler Deputy Department Head. "Now the machine can handle stashes of more than 3E12 antiprotons," or three million million antiprotons. The Recycler can store these antiprotons for hundreds of hours without significant losses, providing the desperately-needed relief for the 20-year-old Accumulator Ring of the Antiproton Source, which cannot handle so many antiprotons. (A second function of the Recycler, to recycle antiprotons at the end of a store, was dropped because the technical challenges outweighed the potential gains in luminosity.)
In January 2005, scientists for the first time used both the Recycler and the Accumulator to supply the Tevatron with antiprotons, a process known as "combined shots." Providing the Tevatron with more antiprotons than ever before, accelerator operators set records of peak luminosity, reaching 122 E30 cm-2sec-1on April 29. Since then, antiproton experts have worked on two goals to achieve even higher peak luminosity: further increasing the number of antiprotons injected into the Tevatron per store; and producing a denser antiproton beam, minimizing the chance that oncoming protons will slip past the antiprotons.
To produce a dense antiproton beam, scientists must reduce the beam emittance-the spreading of antiprotons in longitudinal and transverse direction. Ideally, all particles in the beam should travel in exactly the same direction and at the same speed. In reality, the particles have a tendency to drift apart, and some travel slower or faster than the optimal speed.
To decrease the effect, Fermilab scientists have designed two cooling systems that reduce the spreading of the beam: stochastic cooling, which decreases primarily the transverse emittance of the beam, and electron cooling, which decreases primarily the longitudinal emittance. While stochastic cooling, which was invented at CERN, is a well-understood technique, Fermilab is the first lab to use electron cooling at high energy. On July 9, only a few months into the commissioning of the electron cooling system, scientists observed the first evidence for electron cooling to work. Today, electron cooling plays a key role in setting new peak luminosity records.
"Since August, we have used electron cooling for every store," said Sergei Nagaitsev, who joined Fermilab in 1995 to develop the Recycler electron cooling system. "Now the electron cooling works so well that we can store more than 3E12 antiprotons in the Recycler, something we couldn't do with stochastic cooling alone."
Because the Recycler now can hold such large stashes, and because the Recycler produces lower emittances than the Accumulator, accelerator experts now only use the Recycler to supply the Tevatron with antiprotons. About every four-six hours, accelerator operators transfer the antiprotons collected by the Accumulator to the Recycler. When the Recycler has enough antiprotons, usually after four transfers, the antiprotons undergo final cooling and are then injected into the Tevatron. The Accelerator Division has carried out these "Recycler only" shots since the beginning of September , pushing the peak luminosity record to 164 E30 cm-2sec-1on October 31.
Eventually, transfers from the Accumulator to the Recycler will occur every hour, and the Recycler will be able to store, cool and transfer up to 6E12 antiprotons. "We are still improving e-cooling," said Nagaitsev. "The system is not optimal yet. The Accumulator has existed for 20 years, while the Recycler has been in operation for less than a year. We still have a learning curve ahead of us."
Ultimately, scientists expect the Recycler to improve the Tevatron integrated luminosity by 50 to 100 percent. The results for peak luminosity have been very encouraging.