Removal of axial twist in RHIC insertion quadrupole magnets

The focusing triplets located on either side of the six interaction points of RHIC each consist of three 13 cm aperture quadrupoles with magnetic lengths of 1.44 m (Q1), 3.40 m (42), and 2.10 m (43). The field quality and alignment of these magnets are critical to the performance of the accelerator. The maximum allowable axial twist of the cold mass, defined as the standard deviation in the quadrupole roll angle, is 0.5 mrad. This requirement has occasionally exceeded the capabilities of the assembly fixturing and the procedures used to complete the axial welding of the shell halves around the cold mass yoke. A corrective shell welding technique has been successfully employed to remove excessive axial twist of the 13 cm quadrupoles. This “custom straightening” method is described along with the mechanical inspection data before and after straightening. The magnetic results which confirm the untwisting procedure are also discussed