Rectifying unexpected injection issues due to an elliptically polarizing undulator

Top-up operation requires high efficiency injection with insertion devices at small gaps. We present a case study of one elliptically polarizing undulator at the Canadian Light Source that unexpectedly began having a detrimental impact on injection efficiency after operating for many years without issue. By performing beam -based measurements in the storage ring and magnetic measurements in our magnetic measurement laboratory, we built a model that provided an explanation and a remedy. An unrelated change in the vertical limiting physical aperture coupled to an octupole resonance caused a loss of dynamic aperture that explained the decreased injection efficiency. The remedy was to change the on-axis tunes of the storage ring, avoiding the resonance. We hope this detailed and thorough case study may help other laboratories to maintain high injection efficiency while requiring a minimum of restrictions on insertion device operating ranges.