Design, fabrication, and high-gradient testing of X-band choke-mode damped structures

Choke-mode accelerating structures are higher-order-mode (HOM) damped structures that are relatively simple to fabricate and have low surface magnetic fields. C-band choke-mode accelerating structures have been successfully applied in free electron lasers. However, studies on X-band choke-mode structures for the main linac of the Compact Linear Collider (CLIC) currently remain at the theoretical design stage, and the high-gradient performance of a choke remains unknown. In this study, we designed and fabricated five different single-cell choke-mode accelerating structures and subjected them to high-gradient tests to study related radio frequency (rf) breakdown characteristics. High electrical field and small choke gap caused serious breakdowns in the choke, which mainly limited high-gradient performance. The choke-mode accelerating structure reached 121 MV/m at a breakdown rate of 8.30×10^{-4} per pulse per meter and a flat top pulse width of 250 ns. A new quantity proposed to give the high-gradient performance limitation of choke-mode accelerating structures due to rf breakdown. The new quantity was obtained from the summary of the high-gradient experiments and can be used as reference for high-gradient choke-mode accelerating structure design.