Fabrication of a high-field short-period superconducting undulator

The visible free electron laser (FEL) experiment at the accelerator test facility (ATF) at Brookhaven National Laboratory is designed to utilize the high brightness beam from a laser-photocathode rf gun injected into a 50 MeV S-band linac in conjunction with a 8.8 mm period undulator. The undulator is designed to provide a 0.5 T field on axis which is reached at a current 15% below the quench current. It consists of 3 contiguous sections. We discuss the field errors by evaluating the trajectory wander and the optical phase error. Whereas undulator sections with low errors can be built as machined without any shimming or trimming, special care has to be taken during the assembly of a full length device.