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Frontiers of Particle Beams: Intensity Limitations pp 80–109Cite as

Bench methods for bean-coupling impedance measurement

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Part of the book series: Lecture Notes in Physics ((LNP,volume 400))

Abstract

The coaxial wire method is a useful and efficient tool for (simulated) beam-coupling impedance and loss-factor measurements. Usually one obtains reliable results for a single, localized impedance with Z < ZL. This does not mean that in other cases (several discontinuities Z ⩾ ZL) the results are incorrect, but caution and cross checking are required. It should be kept in mind that the wire permits coupling (= energy exchange) between two or more discontinuities, which would not be coupled in the case of an empty beam pipe. In general, transmission rather than reflection measurements are better. The effect of multiple reflections due to mismatch in adapter pieces can be eliminated by time filtering (= gating).

As a second choice de-embedding (calibration) procedures may be used, as they are more difficult to handle than gating, but sometimes also more effective in rejecting undesired signals. In all cases a high electrical and mechanical repeatability is required.

The beam-coupling impedance and the loss factor can be measured with the same instrumentation using the synthetic-pulse technique. Synthetic pulses are more powerful in terms of stability and signal-to-noise ratio than real pulses but the set-up may be more expensive. Caution should be exercised for frequencies beyond waveguide cut-off,and the effect of waveguide modes must be carefully considered. Impedance measurements without a coaxial wire relying on TM modes appear to be possible within certain limitations. Band-limited Time Domain Reflectometry (TDR) using TM or TE modes can return useful information on beam pipe discontinuities. In order to simulate transverse fields of very slow beams, an array of probe and loop antennas gave promising results.

Bead-pull measurements on cavity-like objects are nowadays used rather for tuning purposes (multiple cell, RFQ) than for R/Q determination. With the availability of powerful computer codes for field calculation, often only the Q measurement is still required.

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Authors and Affiliations

  1. CERN, Geneva, Switzerland

    F. Caspers

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  1. F. Caspers
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M. Dienes M. Month S. Turner

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© 1992 Springer-Verlag

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Caspers, F. (1992). Bench methods for bean-coupling impedance measurement. In: Dienes, M., Month, M., Turner, S. (eds) Frontiers of Particle Beams: Intensity Limitations. Lecture Notes in Physics, vol 400. Springer, Berlin, Heidelberg. https://doi.org/10.1007/3-540-55250-2_27

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  • DOI: https://doi.org/10.1007/3-540-55250-2_27

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  • Print ISBN: 978-3-540-55250-5

  • Online ISBN: 978-3-540-46797-7

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