X-band photonic band-gap accelerator structure breakdown experiment

Abstract

In order to understand the performance of photonic band-gap (PBG) structures under realistic high gradient, high power, high repetition rate operation, a PBG accelerator structure was designed and tested at $X$ band (11.424 GHz). The structure consisted of a single test cell with matching cells before and after the structure. The design followed principles previously established in testing a series of conventional pillbox structures. The PBG structure was tested at an accelerating gradient of $65\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ yielding a breakdown rate of two breakdowns per hour at 60 Hz. An accelerating gradient above $110\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ was demonstrated at a higher breakdown rate. Significant pulsed heating occurred on the surface of the inner rods of the PBG structure, with a temperature rise of 85 K estimated when operating in 100 ns pulses at a gradient of $100\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ and a surface magnetic field of $890\text{ }\text{ }\mathrm{kA}/\mathrm{m}$. A temperature rise of up to 250 K was estimated for some shots. The iris surfaces, the location of peak electric field, surprisingly had no damage, but the inner rods, the location of the peak magnetic fields and a large temperature rise, had significant damage. Breakdown in accelerator structures is generally understood in terms of electric field effects. These PBG structure results highlight the unexpected role of magnetic fields in breakdown. The hypothesis is presented that the moderate level electric field on the inner rods, about $14\text{ }\text{ }\mathrm{MV}/\mathrm{m}$, is enhanced at small tips and projections caused by pulsed heating, leading to breakdown. Future PBG structures should be built to minimize pulsed surface heating and temperature rise.