Orbit Distortions and Bumps in the PEP-II LER Ring

The PEP-II B-factory has already achieved twice the design daily-integrated luminosity. This is even more remarkable when looking at the beam orbits, especially in the low energy ring (LER). Orbit oscillations with an amplitude of 7 mm have grown over the years. Four attempts to steer them down resulted in a much lower luminosity and were therefore backed out. Finally, in August of 2001, the main part of the ring (5/6) could be steered flat, while keeping the sixth where the interaction region (IR) is located untouched. This resulted in a lower global dispersion and more luminosity. Since then the orbit is flat to 0.5 mm (slowly increasing to 2-3 mm), except in the IR where about four oscillations on each side of the interaction point reach peaks of 7-9 mm. This area is highly coupled by skew quadrupoles, which are compensating the effects of the BaBar solenoid field. The region also has strong sextupoles. In order to attack the remaining orbit excursions one at a time, an orbit bump program has been extended to handle coupled regions, by closing the oscillations in the other plane. Due to the big oscillations the beam is often close to the walls generating beam loss and reducing the lifetime. An interesting observation occurred a few times when the high- current LER beam was pulled away from the wall. The size of the LER or its tune appeared to have changed since the colliding beam started to blow up the high energy beam implying a decrease in the size of the LER beam. This was in a linear region so the effect has to come from unknown field errors, or more likely it is an effect of the high-current beam with its surrounding either electron cloud or wakefields.