Report on Modifications to the BX12 and BX13 BC1 Dipoles

Emittance growth seen during the last commissioning run in the bunch compressor optics section, BC1, was blamed on inadequate dipole field quality. The significant linear and nonlinear field non-uniformities generated large horizontal dispersion errors beyond BC1. The linear dispersion after BC1 was corrected using two small 'corrector' quadrupoles placed in BC1 for this purpose, but the remaining nonlinear field caused growth of the normalized horizontal emittance of 40% or more. At best {gamma}{epsilon}{sub x} went from 1.2 {micro}m before BC1 up to 1.7 {micro}m after BC1. The problem was magnified by the larger-than-design energy spread in BC1 due to a long initial bunch length. To improve the field quality we decided to modify the two 'inner dipoles', BX12 and BX13, of the four magnet chicane during the four month down time in the Fall of 2007. Only the two inner dipoles were chosen because of the limited time available and the fact that the beam is particularly sensitive to field quality of the inner dipoles due to its very large transverse size when going through them. The modifications were completed in November and included new poles and a new pinning scheme. The outer dipoles were left unchanged.