Observation of Skewed Electromagnetic Wakefields in an Asymmetric Structure Driven by Flat Electron Bunches

TL;DR

This paper reports the experimental observation of skewed electromagnetic wakefields caused by flat electron beams passing near a dielectric structure, revealing complex transverse interactions that impact beam stability.

Contribution

It introduces the first experimental characterization of skew-quadrupole-like wakefields in flat-beam interactions with dielectric structures, combining measurements, modeling, and simulations.

Findings

Discovered skew-quadrupole-like wakefields in flat-beam experiments.

Developed a multipole field fitting algorithm for wakefield reconstruction.

Used particle-in-cell simulations to analyze realistic beam effects.

Abstract

Relativistic charged-particle beams which generate intense longitudinal fields in accelerating structures also inherently couple to transverse modes. The effects of this coupling may lead to beam break-up instability, and thus must be countered to preserve beam quality in applications such as linear colliders. Beams with highly asymmetric transverse sizes (flat-beams) have been shown to suppress the initial instability in slab-symmetric structures. However, as the coupling to transverse modes remains, this solution serves only to delay instability. In order to understand the hazards of transverse coupling in such a case, we describe here an experiment characterizing the transverse effects on a flat-beam, traversing near a planar dielectric lined structure. The measurements reveal the emergence of a previously unobserved skew-quadrupole-like interaction when the beam is canted transversely, which is not present when the flat-beam travels parallel to the dielectric surface. We deploy a multipole field fitting algorithm to reconstruct the projected transverse wakefields from the data. We generate the effective kick vector map using a simple two-particle theoretical model, with particle-in-cell simulations used to provide further insight for realistic particle distributions.