Getting more pbars out of the Antiproton Source
In the quest for higher luminosity in the Tevatron, scientists are focusing their attention on the proton's negatively charged counterpart - the antiproton. Protons are plentiful, but antiprotons are a different story. For every million protons that crash into the Antiproton Source target, only about 15 antiprotons (pbars) are collected. One of the central strategies for the Run II luminosity upgrades is to increase this production rate and the corresponding antiproton stacking rate while decreasing the time it takes to cool the particles and set up their transfer to the Recycler storage ring.
At Fermilab, antiprotons are made by aiming a 120 GeV proton beam from the Main Injector onto a metallic target at the Antiproton Source. When the protons smash into the disc, antiprotons, along with many other particles, spray out with different energies and at different angles. A lithium lens collects negatively charged particles that have a certain energy.
But that's just the beginning. "Particles coming off the target and off the lens are like a hot gas," said Elvin Harms, head of Antiproton Source Department. "You've got myriad particles and they're bouncing all over the place. Well, that's not good for collecting antimatter."
To change the "hot gas" to a cold one, the antiprotons collected by the lens are transported down a beam transfer line, called AP2, to the Debuncher ring. There, stochastic cooling compresses the antiproton beam. The particles are then transferred every 2-4 seconds to the Accumulator ring, which stacks individual pulses of antiprotons. When a certain stack size is reached in the Accumulator, usually after 2-4 hours, the antiprotons are transferred to the Recycler for additional cooling and stacking (See Fermilab Today, November 11, 2005).
The goal is to provide the Tevatron with a densely packed beam with a large number of antiprotons. To do that, scientists are trying to improve several components of the Antiproton Source complex. One such component is the collection lens. The number of antiprotons collected after protons collide with the target increases with the magnetic-field gradient in the collection lens. A new lens under development would increase the magnetic field gradient, giving a 10-15 percent increase in acceptance of initial antiprotons.
After collection, antiprotons are transferred through beam pipes made of steel. If the spread of antiprotons is larger than the size of the pipes, the particles are scraped off and lost. "We can actually tell that the beam is zigzagging down the pipe, and now we're trying to straighten that out," said Keith Gollwitzer, of the Antiproton Source. The challenge is to differentiate the beam's antiprotons from other particles produced at the target that decay along the beam line. To check whether the antiprotons are traveling close to the center of the beam pipes, scientists determine their trajectory by switching off the antiproton production and sending protons in the opposite direction through the beam line. Adjusting the magnetic fields along the beam line, scientists are able to straighten the trajectory.
The Antiproton Source Department also is adjusting the systems for cooling antiprotons, mostly because of an increase in the number of protons sent to the antiproton target. (see Fermilab Today, November 16, 2005) "We have twice as many particles coming in, but the same amount of time to do the cooling on them," said Paul Derwent, Deputy Head of the Antiproton Source Department. "We've had to make the cooling systems work twice as fast." And as part of that race against time, antiproton stacking also has changed. Previously, antiprotons were stacked in the Accumulator to 250E10 particles before being transferred to the Recycler. Now, they are stacked to only 60-80E10 antiprotons. By keeping the number of particles in the Accumulator lower, cooling is easier and precious time is saved, Derwent said. Time also is a concern in the transfer of the antiproton stacks to the Recycler. Every time a transfer is made, scientists have to turn off stacking and reorient and test the trajectory with reverse protons. "We want to get rid of most of those steps," Harms said. Presently, each transfer takes about 45 minutes. Scientists hope to cut that time down to just minutes by upgrading and simplifying a computer application that largely automates the transfer process. Time also can be shaved by using upgraded beam position monitors in the transport lines to tell if the antiprotons are on the right path and ramping the power supplies used for the AP1 beam line magnets.
The ultimate goal is to increase the number of antiprotons collected for every 1 million protons that hit the target to about 30. With these upgrades and improved instrumentation that would permit a more stable operation with a larger number of diagnostic devices to study the beam, the scientists are well on their way. "Right now, what most limits the Tevatron is the availability of antiprotons," said Bill Ashmanskas, who has worked on Antiproton Source instrumentation with Dave Peterson and engineers in the Particle Physics Division. "So anything we can do to improve the stacking rate of the Antiproton Source contributes to increased Tevatron luminosity."