Commissioning the fast luminosity dither for PEP-II

To maximize luminosity, a feedback system adjusts the relative transverse (x, y) position and vertical angle (y) of the electron and positron beams at the interaction point (IP) of PEP-II. The original system sequentially moved ("dithered") the electrons in four steps per coordinate. Communication with DC corrector magnets and field penetration through copper vacuum chambers led to a 9-s cycle time. Machine tuning can shift the beams at the IP, and so must be slowed to wait for the feedback. The new system simultaneously applies a small sinusoidal dither to all three coordinates at three frequencies. Air-core coils around stainless-steel chambers give rapid field penetration. A lock-in amplifier at each frequency detects the magnitude and phase of the luminosity\´s response. Corrections for all coordinates are applied by the same DC correctors used previously, but with only one adjustment per cycle for an expected nine-fold increase in speed. The commissioning of this system uncovered a sinusoidal vibration of the support for the IP that caused relative motion of the two beams and masked the y dither. Correcting this gave an immediate luminosity gain, and allowed for successful feedback commissioning.