ANL, Argonne, Illinois, USA
LBNL, Berkeley, California, USA
ASCo, Clayton, Victoria, Australia
Dimtel, San Jose, USA
Stable operation of a modern high current storage rings requires attention to the sources of coupling impedance. Methods ranging from EM modeling to bench and beam measurements are routinely used to characterize accelerator components. The beam-based method presented here complements existing approaches, and allows precise measurements and identification of narrowband resonances in structures such as RF cavities or in-vacuum undulators. We combine the well-established approach of characterizing eigenvalues of coupled-bunch instabilities with modern high-throughput data acquisition tools and automatic analysis methods. By sequentially exciting low-amplitude oscillation of individual modes below the instability threshold and observing open-loop damping, we automatically map the eigenvalues of all even-fill eigenmodes. Repeating the measurements while changing operating conditions, such as cavity temperatures or undulator gaps, provides a detailed mapping of the impedance source. These measurements are used to extract impedance parameters as well as to optimize accelerator settings. Experimental results from the Advanced Photon Source and other accelerators illustrate the technique.
