Beams Document 8509-v1

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Submicropulse Electron-Beam Dynamics Correlated with Short-Range Wakefields in TESLA-type Superconducting rf Cavities

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Rob Ainsworth
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Rob Ainsworth
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24 Jun 2020, 16:44
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24 Jun 2020, 16:44
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24 Jun 2020, 16:44
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We report direct observations of sub-micropulse beam centroid shifts (head-tail kicks) correlated with short-range wake fields generated by off-axis electron beam steering in TESLA-type superconducting rf cavities. The experiments were performed at the Fermilab Accelerator Science and Technology (FAST) Facility using its unique configuration of a photocathode rf gun injecting beam into two separated nine-cell cavities. The cavities are in series with corrector magnets and beam position monitors (BPMs) located before, between, and after them. The off-axis steering in the cavity was guided by the rf BPM data and higher order mode (HOM) circuitry targeting the first and second dipole passbands. The centroid shifts of up to 300 µm from head to tail of the ~10-ps-long micropulses at 500 pC/b in a 3-MHz pulse train were measured via optical transition radiation at a downstream screen with a Hamamatsu C5680 synchroscan streak camera. We also showed we could compensate such kicks from the first cavity with the short range wakefields (SRW) in the second cavity, and we observed the dilution of the beam size in the tail of the pulses. A simple numerical model of the SRW effect in a single TESLA cavity is compared to the experiment successfully. In principle, these fundamental results may be scaled to cryomodule configurations of major free-electron laser (FEL) facilities such as the European XFEL, Linac Coherent Light Source or LCLS-II XFEL, and the conceptual international linear collider.
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