Impedance measurements on a test bench model of the ILC crab cavity

Abstract

In order to verify detailed impedance simulations, the resonant modes in an aluminium model of the ILC crab cavity were investigated using a bead-pulling technique as well as a stretched-wire frequency domain measurement. The combination of these techniques allow for a comprehensive study of the modes of interest. For the wire measurement, a transverse alignment system was fabricated and RF components were carefully designed to minimize any potential impedance mismatches. The measurements are compared with direct simulations of the stretched-wire experiments using numerical electromagnetic field codes. High impedance modes of particular relevance to the ILC crab cavity are identified and characterized. EXPERIMENTAL APPARATUSThe baseline ILC design calls for two superconducting nine-cell 3.9GHz dipole mode cavities [1] in order to rotate the bunches at the interaction point and preserve luminosity. A numerical study carried out on the cavity identified a number of modes that have significant loss factors [2] and therefore would require significant damping. In order to verify the impedances calculated by the numerical simulations, a modular aluminium model of the cavity was constructed.The model is composed of modular pairs of half-cells, connected at the equator. These can be arranged to form a cavity having anything from one to thirteen cells. The cell profiles are based on the C15 shape of the CKM 3.9GHz cavity [3]. Figure 1: Picture of the experimental model.The stretched-wire measurements were taken using matching sections at both ends of the cavity beam-pipe in order to minimise reflections and mode conversion. The matching sections were comprised of a tapered cone, and a quarter-wave transformer which allowed a reasonably good pass-band. It was possible to use different quarterwave transformers to study different frequency ranges.The wire position was controllable in the X and Y directions on both ends of the cone, which allowed measurements at various offsets and allowed control of the skew. The translation plates as well as tensioning mechanisms are shown in Figure 2. Figure 2: Schematic of the stretched-wire apparatus. STRETCHED-WIRE THEORYThe field pattern generated by a current passing through a wire in a coaxial structure is a TM mode that can be equated to the wake-fields left by a bunch passing through a structure.From simple transmission line theory, it is possible to derive a relationship between the coupling impedance of the device under test (DUT) and the ratio of transmission parameters, S 21 , of the DUT and a reference vessel [4]. The ratio can be expressed as follows:where θ is the TEM modes wavenumber multiplied by the length of the DUT and || 0 1 iZ Z ζ = − θHence if we can measure S 21 of a DUT and a reference vessel of known characteristic impedance Z 0 we can solve equation (1) to obtain the longitudinal coupling impedance Z || .

X radiation sources based on accelerators

“…The difficulties for such a scheme lie on the emittance rise due to non-linearities and the chromaticity correction [59]. Technological developments are launched on CRAB cavities in a CERN/Karlsruhe collaboration (2.8 GHz), at Cornell (1.5 GHz), at KEK (500 MHz), at Fermilab and at APS (2.8 GHz) [60].…”