Broadband impedance modeling and single bunch instabilities estimations of the advanced light source upgrade project

• Published in: Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment Vol. 1031
• Main Authors: Wang, Dan, Bane, Karl, Li, Derun, Luo, Tianhuan, Omolayo, Olusola, Penn, Gregory, De Santis, Stefano, Steier, Christoph, Venturini, Marco
• Format: Journal Article
• Language: English
• Published: United States Elsevier 04.03.2022
• Subjects: 07 ISOTOPE AND RADIATION SOURCES; ALS-U project; broadband impedance; impedance budget; impedance modeling; pseudo-green function; single bunch instability
• ISSN: 0168-9002; 1872-9576

We report the Advanced Light Source Upgrade (ALS-U) is a 4th generation diffraction-limited soft x-ray radiation source, consisting of a new accumulator ring (AR) and a new storage ring (SR). In both rings coupling-impedance driven instabilities need careful evaluation to ensure meeting the machine’s high-performance goals. This paper presents the workflow followed in building the impedance models and the beam-stability analysis based on those models. We follow the commonly accepted approach of separating the resistive-wall and the geometric parts of the impedance; the former is obtained by analytical formulas, the latter by numerical electro-magnetic codes (primarily CST Studio software) with perfectly-conducting boundary conditions. Impedance budgets are established and pseudo-Green functions calculated to be used in beam dynamics studies. We also present various ways to cross-check simulation results for reliable impedance modeling. Finally, the crucial single-bunch instability current thresholds for various operation modes are determined and discussed.