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 TM₀₁-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 TM₀₁-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  MV/m, in excellent agreement with theory.