Demonstration of a 17-GHz, High-Gradient Accelerator with a Photonic-Band-Gap Structure

We report the testing of a high gradient electron accelerator with a photonic-band-gap (PBG) structure. The photonic-band-gap structure confines a fundamental ${\mathrm{TM}}_{01}$-like accelerating mode, but does not support higher-order modes (HOM). The absence of HOM is a major advantage of the PBG accelerator, since it suppresses dangerous beam instabilities caused by wakefields. The PBG structure was designed as a triangular lattice of metal rods with a missing central rod forming a defect confining the ${\mathrm{TM}}_{01}$-like mode and allowing the electron beam to propagate along the axis. The design frequency of the six-cell structure was 17.14 GHz. The PBG structure was excited by 2 MW, 100 ns pulses. A 16.5 MeV electron beam was transmitted through the PBG accelerator. The observed electron beam energy gain of 1.4 MeV corresponds to an accelerating gradient of $35\mathrm{MV}/\mathrm{m}$, in excellent agreement with theory.

Figure 1

(a) Schematics of the disk-loaded PBG waveguide. (b) Cut away drawing of a six-cell PBG accelerator.

Figure 2

Electric field patterns in the ${\mathrm{TM}}_{01}$-like mode and the ${\mathrm{TM}}_{11}$-like mode in a PBG waveguide: (a) the ${\mathrm{TM}}_{11}$-like mode is not supported by the PBG structure when $a/b<0.2$; (b) the ${\mathrm{TM}}_{11}$-like mode is confined by the PBG structure when $a/b>0.2$ (see 19). The calculations have been made using the HFSS code [20].

Figure 3

Comparison between the computed and the final measured ${\mathrm{S}}_{11}$ (reflection) and ${\mathrm{S}}_{21}$ (transmission) coupling curves for the PBG accelerator structure.

Figure 4

Schematics (not to scale) showing the experimental linac, klystron, PBG structure, and spectrometer.

Figure 5

Measured energy of the electron beam vs square root of the PBG accelerator input power, $P$.