Shift Dependent Skew Quadrupole in Advanced Light Source Elliptically Polarizing Undulators, Cause and Corrections

Three elliptically polarizing undulators (EPU) are installed and operational at the Advanced Light Source (ALS); the most recent was installed in April '05. Operational experience has shown a variation in electron beam size which correlates with the EPU's magnetic quadrant shifts used to vary polarization. Storage ring electron dynamics studies pointed to the existence of a shift dependent skew quadrupole (SQ) component generated within the EPU's. Detailed magnetic and mechanical measurements demonstrated that the field errors were the result of systematic individual magnetic block displacements which vary with quadrant shift. This paper will discuss the results of electron dynamics studies, magnetic and mechanical measurements, design modifications planned for future EPU's to eliminate the SQ source, and the design and implementation of SQ compensation coils