Abstract
Transverse beam coupling impedance is a source of beam instabilities that limits the machine performance in circular accelerators. Several beam based techniques have been used to measure the transverse impedance of an accelerator, usually based on the optics distortion produced by the impedance source itself. Beam position monitor turn-by-turn analysis for impedance characterization has been usually employed in large circumference machines, while synchrotron light sources have mainly used slow orbit based techniques. Instead, the work presented in this paper uses for the first time turn-by-turn data at ALBA to advance the measurement technique into the range of the typically small impedance values of modern light sources. We have measured local impedance contributions through the observation of phase advance versus bunch charge using the betatron oscillations excited with a fast dipole kicker. The ALBA beam position monitor system and the precision of the turn-by-turn analysis allowed to characterize the main sources of transverse impedance, in good agreement with the model values, including the impedance of an in-vacuum undulator.
Highlights
Transverse coupling impedance plays a steadily growing role in the landscape of synchrotron light sources, where new designs require small sized beam pipes, multilayer materials, and complex vacuum chamber structures
Global impedance measurements are a valid tool to evaluate local impedance source using a differential approach provided that the mechanical settings of a particular element in the machine can be changed between two global measurements as in the case of movable devices such as scrapers or in-vacuum undulators (IVU)
Adding up the tune shift induced by all the elements presented in Table II except for the IVUs, which where opened to the maximum gap, the beam pipe and the injection section, already considered by the fit, we obtain an overall tune shift ΔQnot-fit 1⁄4 −0.46 × 10−3, reducing the discrepancy to 2%
Summary
Transverse coupling impedance plays a steadily growing role in the landscape of synchrotron light sources, where new designs require small sized beam pipes, multilayer materials, and complex vacuum chamber structures All such elements contribute to the machine impedance budget, whose evaluation is a key task in the design stage of a new machine since the maximum current that can be stored in an accelerator depends on the ring impedance. We show how the results of the turn-by-turn measurements agree with the transverse impedance model based on computer simulations: for all elements analyzed in this paper, the discrepancies are within the 10% experimental error, except on one case which shows a 20% deviation This stresses the good performance of the measurement technique, and the impedance model itself. We summarize the results and discuss possible improvements of the presented technique
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