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.