| The Advanced Photon Source is in the design phase of a major upgrade that will increase the x-ray brightness by two to three orders of magnitude. Stably storing such an intense beam requires very strong magnets that demand a narrow gap vacuum chamber. These narrow gap chambers and the associated small aperture vacuum components must be designed with minimal coupling impedance so as to minimize potential rf-heating and to avoid deleterious collective instabilities of the electron beam. This thesis focuses on impedance measurements, simulations, and analysis of critical vacuum chamber components for the APS Upgrade (APS-U), using both the traditional coaxial wire method and the novel Goubau line (G-line) method.;Impedance measurements of accelerator components have traditionally been done with the coaxial wire method, which is based on the fact that the TEM mode of the coaxial cable can mimic the Coulomb field of a particle beam. We describe how a similar field profile can be exploited using the fundamental TM mode to measure impedance with a G-line, which is essentially a transmission line designed to propagate Sommerfeld-like surface waves. We describe in detail the measurement procedure that we have developed for the G-line, including the measurement setup and proper definition of a reference, measurement procedure and advantages, and our experience regarding how to reduce systematic experimental error that we learned over the course of the measurements. After describing the measurement and simulation techniques, we then turn to our results. Since there has been some controversy regarding the impedance cost of NEG, we first present impedance measurements of APS-U NEG-coated copper chambers based on the traditional coaxial wire method. We then discuss the impedance analysis of APS-U vacuum chamber components using the G-line, starting with our initial suite of measurements and simulations designed to benchmark and validate the novel G-line based measurement technique. We then present the measured results for the beam position monitor (BPM)-bellows assembly, gate valve liner, rf-flanges, pumping cross etc., along with some simulated results and associated analysis.;The measured results of the NEG-coated chamber show that the effect of impedance due to the 1.5-micron thick NEG coating on copper is mostly negligible up to 21 GHz, as predicted by simulations. In the 28--29 GHz range, the NEG-coated chamber exhibited a slightly higher (≤ to 1 dB) transmission, which is unexplained but of little impact to APS-U design. In addition, the measured results of the APS-U BPM-bellows assembly, gate valve liner, and the pumping cross have been properly designed and manufactured to specifications, with no observable resonance peaks. On the other hand, impedance evaluations of several flange designs have displayed resonances that we subsequently attributed to improper machining and/or poor tolerance control, and we have worked to ensure future designs can be made to specifications. Finally, we show that the G-line is relatively simple and in our opinion better way to measure the impedance over a broad frequency range. |
