Slip-stacking to bombard pbar target
Ioanis Kourbanis judges his upgrade contributions by the state of the antiproton target. He's looking for damage to the target, and damage is what he hopes to find.
Kourbanis, of the Accelerator Division's Main Injector Department, has been coordinating the introduction of the technique called "slip-stacking" for the transfer of protons from the Booster to the Main Injector, with the final goal of increasing the number of protons on the antiproton target-and thus, increasing antiproton (pbar) production. The slip-stacking scheme was proposed by Chuck Ankenbrandt of the Accelerator Division in 1981 and was first studied in the Main Ring accelerator in the late '90s. The scheme was incorporated into Run II upgrades with a design goal of achieving 8E12 protons per pulse on target. In December 2004, the antiproton target was being hit by about 4.5E12 protons per pulse; since slip-stacking has been introduced, the antiproton target has seen pulses in excess of 8.0e12 protons. Now Kourbanis has to start considering the issue of damage to the antiproton target resulting from the increased intensity. He doesn't mind. "That means we've been successful," he says.
In Tevatron luminosity, all paths seem to lead back to pbars. Increasing the number of collisions in the Tevatron means increasing the number of antiprotons. The only way to increase antiproton production is to fire more protons from the Main Injector at the antiproton target. But the Main Injector gets its protons from the Booster, and therein lies a challenge. The Booster accelerates protons from an initial energy of 400 MeV to a final energy of 8 GeV in 0.033 seconds, the largest proportional increment in the lab's accelerator chain. But this workhorse accelerator is just 475 meters in circumference, compared to 3.3 kilometers for the Main Injector. It takes seven "Booster-fuls," or batches, to fill the Main Injector. Since building a bigger Booster wasn't in the cards, the question was how to get significantly more protons into the Main Injector, to send more protons to the antiproton target. Complicating the question: only one MI batch per pulse can be sent to the antiproton target. So how could it be done?
The answer: By putting two Booster batches in the same place at the same time, in orbit around the Main Injector.
By putting one Booster batch into the Main Injector, then manipulating a second Booster batch to "slip" it into orbit alongside the first batch, the result is "stacking" twice as many protons into each "bucket" that goes toward filling the Main Injector ring. In effect, slip-stacking means filling the Main Injector with 14 batches from the Booster, instead of seven. A simplified explanation involves five steps:
1. The first batch from the Booster is injected into a central orbit in the Main Injector using RF system A.
2. The first Booster batch is slightly decelerated in the Main Injector using RF system A.
3. A second Booster batch is injected into a central orbit in the Main Injector using RF system B, following the path of the first Booster batch.
4. Both batches are symmetrically accelerated around the central orbit.
5. When the first batch overtakes the second batch, and both batches line up, snap on RF system C while turning off systems A and B. The two batches are now combined into one.
Easier said than done, of course. "You think you understand it," Kourbanis said, "and then new problems appear."
In order to provide the small longitudinal emittance that a high- intensity beam required for slip stacking, the Booster had to contend with new beam instabilities and other problems. A Rapid Response Team headed by Jim Steimel of AD's Tevatron Department started up last June and addressed beam instabilities, instrumentation improvements and fine-tuning operations procedures. The overall Rapid Response Team effort for the accelerator upgrades and operational improvements, headed by Dave McGinnis, will be covered in a future Fermilab Today installment of the luminosity series.
The Main Injector is not a silent partner in slip-stacking; it required significant revamping to its RF systems to enable acceptance of the new injection technique. To handle the new beam intensity, dampers were installed to help control beam emittance. Higher intensity comes with higher emittance. A damper measures the position of the beam and how far away it strays from the ideal, then delivers a kick to get it back into alignment.
There was also a significant upgrade of the MI's beam position monitors and beam loss monitors. "With the beam manipulations introduced by slip- stacking, and the increased beam intensities, understanding the positions of each Booster batch and the beam loss pattern in Main Injector becomes more critical," Kourbanis said.
The number of the solid state amplifiers at each RF station was doubled, with four more amplifiers added to the rack of four existing at each RF station. The amp upgrades were done gradually, three or four stations at a time, at the level upstairs from the tunnel; machine downtime was used to run cables in the tunnel. This RF upgrade was critical in order to compensate the beam loading during the slip stacking process.
With the increased proton intensity, Kourbanis is shifting focus to the antiproton target, and part of the solution involves shifting the spot that the pulses hit. "Pulses of very high intensity can damage the target," he said. "We have to optimize spot sizes on the target. We have to be concerned about the peak intensity of the beam. We do have a beam sweeping system, with the magnets upstream of the target installed and the downstream magnets ready for installation."
Even beam sweeping isn't the final step. Next comes an analysis of beam losses in the Main Injector, an important ingredient because the Main Injector operates in two modes in the same cycle: one to send beam to the antiproton source, the other to supply beam for NuMI. The possibility of slip-stacking for the NuMI beam will be studied over the next year. And it's highly likely that Kourbanis will have more opportunities to say: "You think you understand it, and then new problems appear."