Measurements of magnetic field of variable period undulator and correction of field errors

A new variable period undulator of unique design was developed and built recently at Budker INP. It will replace the electromagnetic undulator in use now on the second FEL of the Novosibirsk FEL facility. As a result, the FEL tunability range will be substantially extended. In this paper, we present the results of measurements of the undulator magnetic field for different periods and discuss ways to reduce the field errors, which include sorting of magnets, weakening of undulator edge poles, and using of steering coils.