| 1. |
- Brynes, A. D., et al.
(författare)
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Mitigation of the microbunching instability through transverse Landau damping
- 2024
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Ingår i: Physical Review Accelerators and Beams. - : American Physical Society. - 2469-9888. ; 27:7
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Tidskriftsartikel (refereegranskat)abstract
- The microbunching instability has been a long-standing issue for high-brightness free-electron lasers (FELs) and is a significant showstopper to achieve full longitudinal coherence in the x-ray regime. This paper reports the first experimental demonstration of microbunching instability mitigation through transverse Landau damping, based on linear optics control in a dispersive region. Analytical predictions for the microbunching content are supported by numerical calculations of the instability gain. The effect is confirmed through the experimental characterization of the spectral brightness of the FERMI FEL under different transverse optics configurations of the transfer line between the linear accelerator and the FEL.
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| 2. |
- 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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| 3. |
- Khan, Shaukat, et al.
(författare)
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Evolution of density-modulated electron beams in drift sections
- 2024
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Ingår i: Physical Review Accelerators and Beams. - : American Physical Society. - 2469-9888. ; 27:4
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Tidskriftsartikel (refereegranskat)abstract
- Initiating the amplification process in a short-wavelength free-electron laser (FEL) by external seed laser pulses results in radiation with a high degree of longitudinal and transverse coherence. The basic layout in seeded harmonic generation involves a periodic electron energy modulation by laser-electron interaction in a short undulator (the "modulator"), which is converted into a density modulation in a dispersive section immediately followed by a long FEL undulator (the "radiator") tuned to a harmonic of the seed laser wavelength. With the advent of more complex seeding schemes, density-modulated beams may need to be transported in drift sections before entering the radiator. Long FEL undulators may also contain several drift spaces to accommodate focusing elements and diagnostics. Therefore, it is of general interest to study the evolution of density-modulated electron beams in drift sections under the influence of repulsive Coulomb forces. At FERMI, a seeded FEL user facility in Trieste, Italy, systematic studies of the impact of varying drift length on coherent harmonic emission were undertaken. In order to make the underlying physics transparent, the emphasis of this paper is on reproducing the experimental findings with analytical estimates and a simple one-dimensional numerical model. Furthermore, the Coulomb forces in a drift section may be employed to enhance the laser-induced energy modulation and yield an improved density modulation before entering the FEL radiator.
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| 4. |
- Mansten, Erik, et al.
(författare)
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Cancellation of klystron-induced energy and arrival-time variations in linear accelerators with arc-type bunch compressors
- 2024
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Ingår i: Physical Review Accelerators and Beams. - 2469-9888. ; 27:4
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Tidskriftsartikel (refereegranskat)abstract
- We explain how the accelerating field amplitude and phase vary with modulator voltage in pulsed radio frequency high-power amplifiers based on klystron tubes. Changes in modulator voltage give rise to correlated changes of amplitude and phase, affecting the properties of the accelerated beam, in particular energy, arrival time, and bunch duration. We show, both theoretically and experimentally, that there exists a postcrest acceleration phase (the magic angle) where the changes of beam energy due to phase and amplitude shifts caused by modulator-voltage variations cancel out. When accelerating at the magic angle, the klystron modulator voltage jitter no longer contributes to energy and arrival-time jitter in the accelerator. Off-crest operation at the magic angle can be implemented for bunch compression schemes in accelerators with arc-type bunch compressors, which have positive momentum compaction. The experimental results, obtained at the MAX IV laboratory, show the benefit of operating close to the magic angle in arc-type bunch compressors. In a direct measurement of normalized electron-energy jitter, the energy jitter was reduced by a factor of 1.8 down to 8.2×10-5 when operating at the magic angle.
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| 5. |
- Ziemann, Volker
(författare)
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New method to measure the unloaded quality factor of superconducting cavities
- 2024
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Ingår i: Physical Review Accelerators and Beams. - : American Physical Society. - 2469-9888. ; 27:3
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Tidskriftsartikel (refereegranskat)abstract
- We describe a method that measures the unloaded quality factor Q0, the external quality factor QE, and the cavity detuning Delta omega with a recursive least -squares algorithm. It combines a large number of consecutive measurements to successively improve an estimate of fit parameters that asymptotically converges to the "real" values. Exploiting the large amount of data acquired by a digital low-level radio frequency system permits us to reach this asymptotic regime in a moderate time frame of seconds to minutes. Simulations show that the method works both for critically coupled and overcoupled cavities. A new calibration method addresses very tight tolerances of the method on system parameters.
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