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

Relativistic charged-particle beams that generate intense longitudinal fields in accelerating structures also inherently couple to transverse modes. The effects of this coupling may lead to beam breakup 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-quadrupolelike 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.

Figure 1

Example of field lines associated with $F_{\perp }(x,y)$ for the case of a two-particle beam (solid line with circles) injected parallel (a) or with negative (b) and positive (c) tilt with respect to the horizontal axis. The dashed lines indicate the directions of the force experienced by the center of the beam. The color map indicates the peak-normalized transverse force amplitude.

Figure 2

(a) Layout of experiment in the vicinity of the structure, where the double-sided arrows indicate the direction of motions of remotely controllable devices, and (b) closeup view of the dielectric slab structure and flat beam with incoming tilt angle $\theta$. The two sides are independently controllable to vary the gap and allow for single-slab studies.

Figure 3

(a)–(c) Transverse beam images for injection angle of $\theta \approx 2°$ measured at (a) center of structure, YAG2, (b) downstream of structure without presence of slab at YAG3, and (c) downstream in presence of slab at YAG3. (d)–(f) Transverse beam distributions for injection angle of $\theta \approx 7°$ measured at (d) center of structure, YAG2, (e) downstream of structure without presence of slab at YAG3, and (f) downstream in presence of slab at YAG3. All images use the same color map normalized to the peak value in image (a).

Figure 4

Simulations of transverse beam distribution at YAG2 (a),(d) and YAG3 (b),(c),(e),(f) with parameter correspondence to the data presented in Fig. 3. The color map represents charge density in arbitrary units.

Figure 5

(a) Beam transverse profile in absence of slab structure. [Charge density (arb. units) color-scale bar is constant between images.] (b) Reconstructed vector kick map. The unshaded region contains 90% of the beam charge. (c) Ground truth beam distribution after interaction with slab from experiment. (d) Implied final beam distribution after interaction from reconstructed wakefield kick map.

Figure 6

Transverse field map fitted from particle momentum kicks in simulation for the same parameters as in the experiment. The black arrows are the field map and the superimposed density plot is the beam distribution with the color map representing the number of binned particles.

Figure 7

Experimental transverse distributions of the flat beam with $\theta \approx 7°$ (a) without the structure present and (b) after interaction through the two-sided structure.