Observation of Wakefield Suppression in a Photonic-Band-Gap Accelerator Structure

We report experimental observation of higher order mode (HOM) wakefield suppression in a room-temperature traveling-wave photonic-band-gap (PBG) accelerating structure at 11.700 GHz. It has been long recognized that PBG structures have the potential for reducing long-range wakefields in accelerators. The first ever demonstration of acceleration in a room-temperature PBG structure was conducted in 2005. Since then, the importance of PBG accelerator research has been recognized by many institutions. However, the full experimental characterization of the wakefield spectrum and demonstration of wakefield suppression when the accelerating structure is excited by an electron beam has not been performed to date. We conducted an experiment at the Argonne Wakefield Accelerator test facility and observed wakefields excited by a single high charge electron bunch when it passes through a PBG accelerator structure. Excellent HOM suppression properties of the PBG accelerator were demonstrated in the beam test.

Figure 1

A conceptual schematic of the photonic-band-gap accelerator structure.

Figure 2

Transmission ($S_{21}$) and reflection ($S_{11}$) in the tuned 16-cell PBG accelerator structure.

Figure 3

(a) Photograph of the electroformed 11.700 GHz PBG accelerator structure installed on the ten inch stainless steel flange. (b) Photograph of the PBG structure with six SiC slabs for attenuating wakefields and pickup loop antennas. (c) Photograph of the PBG structure with no SiC attenuators, wrapped in foil and installed in the vacuum chamber.

Figure 4

The experimental setup on the beam line at Argonne Wakefield Accelerator.

Figure 5

The rf signal in the downstream coupling waveguide of the PBG structure: (a) time domain; (b) Fourier transform. The transmitted charge is approximately 0.4 nC.

Figure 6

Measured power in the downstream coupling waveguide of the PBG structure plotted as a function of transmitted charge. Also shown is the gradient in the accelerating structure computed from the measured power.

Figure 7

The rf spectrum of the signal picked up by a loop antenna at the periphery of the PBG structure: (a) the structure with six SiC absorber slabs; (b) the structure wrapped in foil. The transmitted charge is approximately 3 nC.