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Superconducting radio-frequency (SRF) accelerating structures allow high-gradient operation in continuous-wave mode. These machines can be limited by beam-breakup instability at high currents because higher-order modes with very high Q factors are easily excited by the beam. Photonic band gap (PBG) structures provide a way to strongly damp higher-order modes without compromising the performance of the structure in the fundamental mode. We first address the design of the structure and issues that arise from incorporating a complex PBG cell into an SRF module. In particular, the module was tuned to have uneven accelerating gradient profile in order to provide equal peak surface magnetic field in every cell. We then cover the fabrication steps and surface treatment of the five-cell niobium structure and report results of the high gradient tests at temperatures of 4 K and 2 K.

High Gradient Testing of the Five-cell Superconducting RF Module with a PBG Coupler Cell