High-Gradient Photonic Bandgap (PBG) Accelerator Structure Breakdown Testing

Photonic bandagp (PBG) accelerator structures provide unique insight into the surface physics involved in vacuum breakdown in accelerator structures because of their unique surface E and H fields. Peak operating gradients in accelerator structures are limited by surface breakdowns. These breakdowns occur at regions of high surface E and H fields. The maximum surface H field can easily be varied relative to the peak surface E field and accelerating gradient in PBG structures, allowing new regimes of parameter space to be explored. A standing wave PBG structure utilizing elliptical rods at the highest field locations has been tested at SLAC to determine the breakdown properties of the structure under high-gradient operation. The structure achieved a maximum gradient of approximately $125$ MV/m at a breakdown probability of $3.6 *10^{-3}$ per pulse per meter for 150 ns pulses. This performance is comparable to conventional disc-loaded waveguide accelerator structures, but in a structure that provides intrinsic damping of wakefields.