Impedance evaluation of in-vacuum undulator at KEK Photon Factory

The estimate of impedance and kick factors of the recently installed at KEK Photon Factory (PF) four In-Vacuum Undulators (IVU) is currently a very important issue, because they could be considerable contributors to the total impedance of PF. Moreover, the coupling impedance of the IVUs could lead to the beam energy loss, changes in the bunch shape, betatron tune shifts and, finally, to the various beam instabilities. Using the simulation tool (CST Particle Studio), longitudinal and transverse impedances of the IVUs were evaluated and compared to analytical formulas and measurement results. The study provides a basis for the subsequent mitigation of unwanted impedance, for the accurate estimate of its effects on the beam quality and beam instabilities and also for the impedance budget of a newly designed next-generation machine which has many IVUs and small-aperture beam pipes.