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- Cullinan, F. J., et al.
(författare)
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Experimental observation of a mode-1 instability driven by Landau cavities in a storage ring
- 2024
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Ingår i: Physical Review Accelerators and Beams. - 2469-9888. ; 27:4
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Tidskriftsartikel (refereegranskat)abstract
- Landau cavities used to lengthen the bunches in storage rings necessarily constitute a significant impedance. Because of the particular phase of the field required for bunch lengthening, they are often detuned quite considerably from resonance, more so than the main cavities. As a result, their impedance can excite the first coupled-bunch mode such that it becomes unstable. This phenomenon has previously been predicted [M. Venturini, Phys. Rev. Accel. Beams 21, 114404 (2018)PRABCJ2469-988810.1103/PhysRevAccelBeams.21.114404] and characterized in simulations [T. He, Phys. Rev. Accel. Beams 25, 024401 (2022)PRABCJ2469-988810.1103/PhysRevAccelBeams.25.024401] but experimental observation is yet to be documented. In this paper, the experimental observation of coupled-bunch modes-±1 excited by the Landau and main cavities in a fourth-generation light-source storage ring is presented. Features of the instability such as amplitude and coherent frequency at saturation have been measured and its dependency on the main rf voltage has been explored. The impact of a parked main cavity has also been investigated.
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- Cullinan, F. J., et al.
(författare)
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Harmonic-cavity stabilization of longitudinal coupled-bunch instabilities with a nonuniform fill
- 2020
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Ingår i: Physical Review Accelerators and Beams. - 2469-9888. ; 23:7
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Tidskriftsartikel (refereegranskat)abstract
- In synchrotrons, nonuniform fill patterns, which give rise to beam phase transients and a spread in synchrotron tune between bunches, have been observed to damp longitudinal coupled-bunch instabilities driven by higher-order modes in rf cavities. The transients are especially large in the presence of Landau cavities, which are used commonly in storage-ring light sources and particularly in the new generation of diffraction-limited storage rings. A method has recently been devised to predict the beam transient including complex form factors for the different bunches. This has now been extended to accurately predict the growth-rates and oscillation frequencies of coupled-bunch modes for arbitrary fill patterns, taking the individual complex form factors and equilibrium phases of the different bunches into account. In this paper, the extended method is presented and the theory is outlined. For a case with significant transient beam loading, predictions of the resulting beam transient and bunch profiles are compared to measurements. Predictions of coupled-bunch mode behavior are then benchmarked against results from the macroparticle tracking code mbtrack with good agreement. Finally, the method is used to predict the behavior of coupled-bunch modes as a function of the fields in passive Landau cavities.
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- Cullinan, F. J., et al.
(författare)
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Quadrupole stability in nonuniform fill patterns
- 2022
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Ingår i: Physical Review Accelerators and Beams. - 2469-9888. ; 25:4
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Tidskriftsartikel (refereegranskat)abstract
- In previous publications, the effect of inhomogeneous beam loading of Landau cavities due to a nonuniform fill pattern in a storage ring has been studied with the aim of predicting the resulting longitudinal profiles of the bunches and their time offsets relative to the main rf [T. Olsson et al., Phys. Rev. Accel. Beams 21, 120701 (2018)PRABCJ2469-988810.1103/PhysRevAccelBeams.21.120701]. This work was extended to treat dipolar coupled-bunch modes driven by beam-excited higher-order modes in the rf cavities [F. J. Cullinan et al., Phys. Rev. Accel. Beams 23, 074402 (2020)PRABCJ2469-988810.1103/PhysRevAccelBeams.23.074402]. These coupled-bunch modes are of interest because they can become unstable, leading to an increase in the energy spread. The theory has now been extended once again to cover the case of coupled-bunch quadrupole modes where it is the lengths of the bunches that are oscillating, not their centroids. The theory is outlined and its predictions are thoroughly benchmarked against predictions from macroparticle tracking. Observations of the effects of nonuniform fill patterns on coupled-bunch quadrupole instabilities are made and interpreted. Results of measurements at the MAX IV 3 GeV ring, interpreted using theoretical calculations, are then presented to continue the investigation.
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