PEP-II recently attained its own record beam luminosity, generating twice as many collisions per second as the machine was designed to deliver.

During owl shift on May 3, the Main Control Center (MCC) operators achieved a record peak luminosity of 6.1 x 10³³ cm^-2s^-1, double the machine’s design luminosity. The
best peak luminosity last year was 4.6 x 10³³ cm^-2s^-1.

Two beam pipes of the PEP-II Storage Ring at SLAC—the upper pipe carries positrons, the lower pipe carries electrons. (Photo by Peter Ginter)

"We really have excellent performance," said accelerator physicist Uli Wienands (AD), system manager for the PEP-II storage rings. "For BABAR, higher luminosity means more events, which translates into more accurate results and the ability to find physics effects they otherwise couldn’t see."

PEP-II collides a positron beam from one storage ring with an electron beam from the other ring. Luminosity is a measure of the concentration of these particle collisions, so that adding more particles to a given beam, or reducing beam size, increases luminosity. Picture a polka-dotted balloon: you can add more polka dots or let air out of the balloon—or do both—to increase the density of polka dots.

This remarkable improvement comes from several changes. During the downtime last summer and fall, the Accelerator Department added cooling systems in vulnerable spots. Before the upgrades, beam current was limited to prevent overheating and damaging parts of the vertex chamber. With new cooling systems in place, the beam current has increased 10 to 15 percent over last year, with further room to grow. More current equals more particles, which amounts to higher luminosity.

The huge challenge of adding a cooling system in the heart of the sensitive detector was met by a team of engineers, designers and technicians led by Stan Ecklund, Deputy Head of the Accelerator Department (AD), and Mike Sullivan, PEP-II Run Coordinator.

The other big change required only tiny adjustments. Accelerator physicists finely tuned the machine, slightly changing the number of betatron oscillations the particles make on each trip around the rings. With this ‘tune’ set just right, the interaction of the two electrically charged beams actually reduces—instead of increases—beam sizes at the collision point, thus increasing luminosity.

In tweaking the oscillations "to a very dangerous region, very close to a resonant value, you can actually use this ‘beam-beam interaction’ to reduce the beam size at the point where it matters," Wienands said. "But you lose the beam instantly if you hit the resonant value, so it’s a balancing act."

It took several attempts to achieve the delicate balance, using a new technique developed at SLAC by accelerator theorists John Irwin and Yiton Yan (both ARD-A), working with students in their group.

The team used ‘Model Independent Analysis’ to help analyze the machine settings, calculate new magnet settings and verify that the machine did what it was set to do. "It’s really the fruit of cooperation between the Accelerator Department—the PEP people—and the Accelerator Research Department," said Wienands.

A third factor in raising performance was the experimental discovery that adjusting the particles’ orbit positions very specifically in different locations (called ‘orbit bumps’) could increase luminosity. This work was done by Franz-Josef Decker (AD).

With changed machine settings and higher beam currents, Wienands said, "the operators are rapidly learning to work with this new, somewhat tricky machine. They are the ones who translate peak luminosity into delivered luminosity–the total number of events BABAR gets."

As the PEP team makes everyday performance as good as this new peak performance, the BABAR collaboration is looking forward to an abundance of data and even more striking experimental results.