Experimental high gradient testing of a 17.1 GHz photonic band-gap accelerator structure

We report the design, fabrication, and high gradient testing of a 17.1 GHz photonic band-gap (PBG) accelerator structure. Photonic band-gap (PBG) structures are promising candidates for electron accelerators capable of high-gradient operation because they have the inherent damping of high order modes required to avoid beam breakup instabilities. The 17.1 GHz PBG structure tested was a single cell structure composed of a triangular array of round copper rods of radius 1.45 mm spaced by 8.05 mm. The test assembly consisted of the test PBG cell located between conventional (pillbox) input and output cells, with input power of up to 4 MW from a klystron supplied via a ${\mathrm{TM}}_{01}$ mode launcher. Breakdown at high gradient was observed by diagnostics including reflected power, downstream and upstream current monitors and visible light emission. The testing procedure was first benchmarked with a conventional disc-loaded waveguide structure, which reached a gradient of $87\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ at a breakdown probability of $1.19\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}1}$ per pulse per meter. The PBG structure was tested with 100 ns pulses at gradient levels of less than $90\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ in order to limit the surface temperature rise to 120 K. The PBG structure reached up to $89\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ at a breakdown probability of $1.09\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}1}$ per pulse per meter. These test results show that a PBG structure can simultaneously operate at high gradients and low breakdown probability, while also providing wakefield damping.