Significant lifetime and background improvements in pep-ii by reducing the 3rd order chromaticity in ler with orbit bumps

Orbit bumps in sextupoles are routinely used for tuning the luminosity in the PEP-II B-Factory. Anti-symmetric bumps at a pair of identical sextupoles separated by -I section generate the net dispersion, while symmetric horizontal bumps induce a tune shift and beta beat. By combining two of these symmetric bumps with opposite signs, where the second pair is 90deg away, the tune shift cancels and the beta beat doubles. In the low energy ring (LER), there are four -I sextupole pairs per arc, located one after another 90deg apart, where pairs 1 and 3 are at the same phase and pairs 2 and 4 are 90deg away. By making two symmetric bumps with opposite sign in pairs 1 and 3, the tune shift and beta beat outside this region cancel, but there is a local change of phase and beta in the 2n sextupole pair located in the middle. By using this bump knob, the LER lifetime improved by a factor of 3, losses by a factor of 5, and the beam-beam background in the drift chamber of the BaBar detector by 20%. Optics analysis showed that the local phase change at the 2^nd sextupole pair can compensate the 3^rd order chromaticity.