Field measurement and correction of the very long in-vacuum X-ray undulator at the SPring-8

A 25 m long undulator has been constructed in order to be installed in one of the long straight sections at the Super Photon Ring-8GeV (SPring-8). An in-vacuum structure has been adopted to avoid phasing difficulties encountered in the case of an out-of-vacuum structure. The undulator is divided into five segments, each of which is similar to a standard in-vacuum undulator at the SPring-8. Field measurements and corrections have been carried out first for each segment and second for junctions between segments. After final corrections, it has been confirmed that both multipole components and phase errors are small at various gap values, showing that the long undulator can be operated without any problem.