Suppression of Deflecting Forces in Planar-Symmetric Dielectric Wakefield Accelerating Structures with Elliptical Bunches

Wakefield based accelerators capable of accelerating gradients 2 orders of magnitude higher than present accelerators offer a path to compact high energy physics instruments and light sources. However, for high gradient accelerators, beam instabilities driven by commensurately high transverse wakefields limit beam quality. Previously, it has been theoretically shown that transverse wakefields can be reduced by elliptically shaping the transverse sizes of beams in dielectric structures with planar symmetry. Here, we report experimental measurements that demonstrate reduced transverse wakefields for elliptical beams in planar symmetric structures which are consistent with theoretical models. These results may enable the design of gigavolt-per-meter gradient wakefield based accelerators that produce and stably accelerate high quality beams.

Figure 1

(a) Isometric and (b) top-down view of the layout of the experiment. Beams propagate from left to right, in the $\widehat{z}$ direction. An initial displacement in the structure, $\mathrm{\Delta }x$, is added by moving the structure perpendicular to the direction of beam propagation, along the $\widehat{x}$ direction. The average offset of the beam in the $\widehat{x}$ direction, $\overline{x}$, is measured 6.64 m downstream of the structure using a beam position monitor. The black dot indicates the undisturbed beam propagation direction, when the beam is on axis in the structure.

Figure 2

(a) Spectral content of an annular DWA when an electron beam travels down the symmetry axis (blue, dashed) and $60\mu \mathrm{m}$ offset from the symmetry axis (red). When traveling off axis, the electron beam couples to HEM modes that result in transverse forces on the beam. (b) The measured average offset of the beam on the BPM, $\overline{x}$, as a function of the initial displacement in the structure, $\mathrm{\Delta }x$ is shown. The red line represents a fit to the data and is used to calculate the strength of the linear forces on the beam.

Figure 3

The measured average final offset of the beam as observed downstream of the planar structures, $\overline{x}$, as a function of $\mathrm{\Delta }x$. The colors represent different beam sizes normalized to the vacuum half gap, $\chi ={\sigma }_y/a$. For each value of $\chi$, a linear fit is performed and used to calculate the integrated transverse forces on the beam.

Figure 4

Panel (a): formula (1) derived in Ref. [11] and normalized experimental values for the average kick ${\mathrm{SiO}}_2$ (blue) and ZTA (black). Panels (b),(c): The average force per $\mu \mathrm{m}$ offset in the $x$ direction as a function of the normalized beam size, $\chi ={\sigma }_y/a$, for (b) ${\mathrm{SiO}}_2$ slabs and (c) ZTA slabs. Parameters for each are contained in Table 1. The red diamonds represent theoretical calculations using the methods of Ref. [10] with the parameters described in the text and tables. The blue points represent the measured average force on the beam and the error bars are the 95% confidence intervals.