Tuesday, February 20, 2018
We report our study on the internal dark current generation by multipactor inside a 17 GHz single cell standing wave disk-loaded waveguide accelerator structure. The multipactor takes place on the side wall of the central cell, driven by the local rf electric and magnetic field. In the range of the acceleration gradient that can be achieved in the high power testing of the structure, our theories indicated that there were two different multipactor modes that could appear at different gradient levels, respectively. We designed our accelerator structure with two thin slits opened on the side wall of the central cell to directly extract the internal dark current from the high field regions. The dark current measurement in the high power testing of the structure confirmed our theoretical prediction of the two multipactor modes. To further study the effect of the central cell side wall surface properties on the structure performance, we did the high power testing of the same structure but with the central cell side wall coated with a layer of diamond-like carbon and titanium nitride. The comparison of the results showed that the coating enhanced the structure performance considerably.