Abstract
The Advanced Photon Source plans to upgrade to a multibend achromat (MBA) lattice that will dramatically decrease the electron beam emittance, thereby enhancing the x-ray brightness by two to three orders of magnitude. Electron beam focusing in the MBA requires small-aperture vacuum components that must also have a small impedance so as to minimize rf-heating and collective instabilities. As part of this effort, this paper focuses on coupling impedance measurements and analysis of certain critical Advanced Photon Source Upgrade vacuum components. Impedance measurements of accelerator components have traditionally been done with the coaxial wire method, which is based on the fact that the Transverse Electro-Magnetic (TEM) mode of the coaxial cable can mimic the Coulomb field of a particle beam; however this measurement technique has various limitations. This paper describes our approach to measure the coupling impedance using a Goubau line (G-line), which is essentially a single wire transmission line designed to propagate Sommerfeld-like surface waves whose fundamental Transverse Magnetic (TM) mode mimics the Coulomb field of a relativistic particle beam. 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. Starting with our initial suite of measurements and simulations designed to benchmark and validate the novel G-line based measurement technique, we present the measured results for several Advanced Photon Source Upgrade vacuum components, including those of two rf-gasket designs and the beam position monitor-bellows assembly.
Highlights
Ultralow emittance storage rings based on multibend achromat lattices can provide a generational leap in x-ray performance by increasing the brightness and coherent flux by two to three orders in magnitude [1,2,3]
In this paper we describe our experimental efforts to measure the longitudinal impedance of a variety of Advanced Photon Source Upgrade (APS-U) vacuum components
Some of the challenges associated with these higher order modes (HOMs) can be reduced by, for example, using an appropriate combination of resistive matching outside and rf-absorbing foam inside the device under test (DUT) [11,12], but this is very challenging if the DUT is a closed system
Summary
Ultralow emittance storage rings based on multibend achromat lattices can provide a generational leap in x-ray performance by increasing the brightness and coherent flux by two to three orders in magnitude [1,2,3]. Some of the challenges associated with these HOMs can be reduced by, for example, using an appropriate combination of resistive matching outside and rf-absorbing foam inside the device under test (DUT) [11,12], but this is very challenging if the DUT is a closed system Another issue with the traditional method is that it typically requires a relatively large-diameter central conductor to obtain a suitable characteristic impedance of the DUT. The Goubau line [14,15,16,17] is a dielectric coated single wire transmission line in which the fundamental TM mode of the surface wave has an electric field that mimics the properties of the Coulomb field of a relativistic particle beam. The latter experimental setup differs from ours in that Ref. [18] replaced the receiver cone in Fig. 1 with rf-absorbers, and determined the impedance from the S11 reflection coefficient; typically, measurements based on the S11 is not as accurate as those that employ the S21 transmission coefficient [8]
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