Measurement of internal dark current in a 17 GHz, high gradient accelerator structure

We report the measurement of the internal dark current in a 17 GHz, high gradient accelerator cavity and its comparison with theory. The cavities were fabricated from copper and had a sidewall that was either uncoated or coated with diamondlike carbon or TiN. The dark current was monitored by a downstream detector and by detectors behind two small slits made in the cavity sidewall. With an increasing gradient, the downstream current increased monotonically, as expected for field emission. The variation of the internal, side dark current was not monotonic but showed the onset of peaks at gradients near 45 and $65\text{ }\text{ }\mathrm{MV}/\mathrm{m}$. These were identified as the $N=2$ and $N=1$ single point multipactor resonances. The total internal dark current was estimated at $\ensuremath{\sim}15--30\text{ }\text{ }\mathrm{A}$. The magnitude of the internal dark current and its dependence on the gradient were in good agreement with simulations using the cst code as well as an in-house code. Processing to a higher gradient, $\ensuremath{\sim}90\text{ }\text{ }\mathrm{MV}/\mathrm{m}$, eliminated the $N=2$ mode, but the $N=1$ mode persisted. The coated sidewall cavities showed the same multipactor resonances as the uncoated structure. However, at the highest gradient achieved in testing, the coated structures showed a modest reduction in the internal dark current.