Collisions in the Right Place
Usually, when we think of luminosity, we think of increasing collisions. But it's Ron Moore's job to prevent collisions in the Tevatron--at least until the particle bunches reach the detectors and head-on collisions have been initiated at CDF and DZero. Now, with rising pbar numbers , Moore, the head of the Tevatron department, has a big job on his hands. "The recycler and the pbar source have recently delivered record pbar intensities to the Tevatron," said Moore. "They're really pushing the envelope, so now we have to deal with that." Moore and his colleagues worry about how to make the most of the protons and antiprotons that are injected into the Tevatron. Among other things, that means preventing premature collisions that waste antiprotons and reduce luminosity at the detectors.
"The hard part is that we have 36 bunches of protons and 36 bunches of antiprotons orbiting inside the same pipe," said Yuri Alexahin, a physicist in the Accelerator Physics department. "It is very difficult to separate them enough to avoid unwanted collisions." As the 36 bunches of protons and antiprotons circle the Tevatron in opposite directions, they snake around each other. "We call it the helix," said Moore. "If we didn't steer the bunches in separate orbits, each proton and pbar bunch would see 72 head-on collisions per revolution rather than just two." With a total of 138 possible collision points and over a trillion particles per bunch, there are plenty of opportunities for beam-beam interference. For this reason, the helical orbits are essential for preventing parasitic collisions.
The helical orbits are achieved using stainless steel electrostatic separators, which create an electric field that pulls the protons in one direction and the antiprotons in the other direction. Electrostatic separators have been in operation since Run II started, but they have been adjusted and improved over the last few years. In 2001, when Run II began, parasitic collisions claimed 30 to 35 percent of the antiprotons in the beams. But in the last couple of years, thanks largely to installation of additional separators and fine-tuning by Alexahin, the number of antiprotons lost to parasitic collisions has fallen to less than 3 percent. Now that there are more particles in each beam, Alexahin, Moore and the whole division will face new challenges.
"It's really a balancing act," said Moore. According to Moore, the biggest problems occur at places called the "nearest parasitic crossings"--two points, about 59 meters on either side of each detector, where the beams come closest together before they actually collide head-on. "We'd like to maximize the beam separation at the nearest parasitic crossings," said Moore. "And the best way to do that is to increase the voltage on the separators-we want the highest possible voltage without creating a spark." High voltages are needed to keep the beams far apart, but large electric fields can create a spark, and a single spark can destroy a Tevatron store. Moore says this has happened before: "We've lost more than a few stores to separator sparks, so we backed off. We have to stay within a certain limit."
To go beyond this limit, the Tevatron department will add two new stainless-steel separators to the current 24, replace three others and electropolish them during the upcoming March shutdown. Electropolising will allow the separators to handle higher voltages because it will smooth out rough surfaces that usually start the spark. "The Technical Division really helped us with these improvements," said Moore. "We could not have done this without their help." The three improvements will allow the Tevatron department to increase separation at the nearest parasitic crossings, thus reducing the detrimental beam-beam effects.
But if beam-beam separation is needed to maintain luminosity, it is only part of the equation. "Luminosity is proportional to the product of number of the antiprotons and protons divided by the area of the beam in the collision point," said Alexander Valishev, of the Tevatron department. "So the other part of the story is shrinking the beam." Decreasing the cross-section of the beam, which is determined by the "optics" of the collider, packs the antiprotons tighter together so they will be more likely to collide with oncoming protons (and vice versa). Valishev and his colleague, Valery Lebedev, have made vast improvements in tightening the beams just before they collide. "We haven't made any hardware changes," said Lebdev. "Mostly just changes of currents in the focusing magnets near the collision points." Rather than fiddling with the beam itself, Lebedev says that the physicists use complex computer programs that model the effects of changing parameters on the beams. "A couple of years ago, Valery designed a code that helped us nail the optics," said Valishev. The new model allowed them to reduce beam size by 20 percent, which led to a 28-30 percent increase in luminosity in 2004, and, more recently, a 10 percent increase in peak luminosity this September. But there is still work to be done.
With more particles packed closer together, interactions between particles in the beam and between the colliding beams will become a more important consideration in luminosity. Upgrades in the Beam Position Monitoring system, which records position of the beam inside the vacuum chamber, should help the Tevatron department make even larger improvements. "The new BPM system will make even higher luminosity possible," said Valishev. "It will really be the future tool for tuning the optics, and help with luminosity in the Tevatron."
The next luminosity story will detail the upgrades in the BPM system.