Abstract
We report direct observations of submicropulse beam centroid shifts (head-tail kicks) correlated with short-range wakefields 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 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 that we could compensate such kicks from the first cavity with the short-range wakefields (SRWs) 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.
Highlights
An emittance growth of bright electron beams created by dipolar kicks of higher-order modes (HOMs) excited by beam offsets in the accelerating cavities represents a concern [1]
These long-range wakefields (LRWs) deflect portions of the pulse train or lead to submacropulse beam oscillations such as we previously reported in Tesla-type superconducting cavities [2]
A series of observations of short-range wakefield effects from a Tesla cavity was made for the first time using a synchroscan streak camera to obtain y-t images at the submicropulse timescale
Summary
An emittance growth of bright electron beams created by dipolar kicks of higher-order modes (HOMs) excited by beam offsets in the accelerating cavities represents a concern [1] These long-range wakefields (LRWs) deflect portions of the pulse train or lead to submacropulse beam oscillations such as we previously reported in Tesla-type superconducting cavities [2]. Such off-axis steering generates short-range wakefields (SRWs) as seen in normal conducting L-band accelerators [3] and in S-band accelerators [4]. Ellipsoidal beam, the distinguishing of the short-range wakefield centroid effect from the HOMs’ submacropulse effect, and a comparison to a numerical model for shortrange, transverse effects in a Tesla cavity
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