Fermilab TodayWednesday, November 30, 2005  
Crash Course in Luminosity, Key to Discovery at Fermilab
Cross sectional view of the stripline position detector. (Click image for larger version.)

Updating the "prescription" of Tevatron beam position monitors
The glasses of the Tevatron were dirty, broken and scratched with a prescription more than 20 years out-of-date. Last spring, a group of much-needed optometrists came in the form of engineers, computer professionals and technicians within the Accelerator and Computing Divisions who upgraded the electronics and software for the 240 beam position monitors (BPMs) placed within the Tevatron ring. The result is a view of the Tevatron's inner workings clearer than ever before. As stores of tightly packed protons and antiprotons travel through the beam pipe, BPMs measure their locations. The secret to higher luminosity is guiding the particles on well-controlled helices (See Fermilab Today, November 23, 2005) in an aperture less than 3 inches in diameter. The recent BPM improvements will help scientists do that.

The original BPM system was installed when the Tevatron was constructed in the early 1980s. "There was nothing intrinsically wrong with the system that was there," said Steve Wolbers, BPM project manager for the Computing Division. "The issue, though, is that 20-year-old electronics are probably not ideal in any situation." The electronics were replaced with modern signal-processing equipment, abandoned Main Ring BPM cables were reused for Tevatron antiproton signals and computer equipment was updated. The result is a more reliable and accurate system. "The glasses don't fall off your face every three hours, and you say, 'Oh, wow, this prescription works a lot better than the one I got 20 years ago,'" said Bob Webber, Head of the Instrumentation Department in the Accelerator Division.

The upgrades have resulted in a number of benefits to Tevatron operation. Before the new system was in place, beam position information was collected by running proton-only stores that provided no luminosity for the experiments. The new BPM system can provide accurate beam position information during normal collider operation, thus maximizing time for the experiments to collect data. The new system also is capable of measuring the positions of antiprotons, something not achievable in the past.

The increased accuracy and resolution of the new system allows scientists to make better measurements of the Tevatron lattice, the magnetic characteristics of the entire ring. With better measurements, scientists can decide how to change the magnetic settings to squeeze the beam down to smaller spot sizes at the detectors. By making the bunch sizes smaller, the probability of collisions is increased.

Temperature-sensitive motion and vibration of Tevatron components causes the proton and antiproton orbits to oscillate or slowly drift. This can lead to a reduction in luminosity and a decrease of the beam's lifetime. The new BPM system allows scientists to track small orbit drifts and pinpoint the locations in the ring that might be causing the problem. Fixes can then be made during a shutdown or overcome with real-time orbit stabilization feedback.

With these and other instrumentation upgrades, scientists already have seen an increase in both Tevatron luminosity and in the understanding of the beam lines. The next step is a similar BPM upgrade in the Main Injector.

"One thing that we were trying to accomplish was an upgrade that gave better capabilities for the people who run the accelerator," Wolbers said. "The upgrade was meant to provide good data, reliable data and I think that's what we ended up accomplishing."

-Kendra Snyder

Fermilab Today