Stretchforming vacuum chambers for the PEP-II B-factory high energy storage ring

Dipole vacuum chambers for the PEP-II HER, fabricated from copper extrusions, must follow the arc of the electron beam in order to minimize impedance losses. The 165 m bend radius requires that the chambers have a sagitta of 25 mm over each 5.84 m length. Stretchforming provides a relatively smooth continuous bend radius and results in low overall residual stresses. Structural analyses of the chamber during the forming process are discussed. These analyses are used to estimate the residual stresses in the stretchformed chambers. The impact of residual stresses on actual chamber operation are discussed. The stretchforming process and apparatus utilized during prototype testing is described. Permanent deflections of the chambers during prototype manufacture are presented and compared with predictions