A compact, low field, broadband matching section for externally-powered X-band dielectric-loaded accelerating structures

It has been technically challenging to efficiently couple external radiofrequency (RF) power to cylindrical dielectric-loaded accelerating (DLA) structures, especially when the DLA structure has a high dielectric constant. This paper presents a novel design of matching section for coupling the RF power from a circular waveguide to an X-band DLA structure with a dielectric constant $\varepsilon$$_{r}$ = 16.66 and a loss tangent tan$\delta$ = 3.43 x 10$^{-5}$. It consists of a very compact dielectric disk with a width of 2.035 mm and a tilt angle of 60$^{0}$, resulting in a broadband coupling at a low RF field which has the potential to survive in the high-power environment. To prevent a sharp dielectric corner break, a 45$^{0}$ degree chamfer is also added. A microscale vacuum gap, caused by metallic clamping between the thin coating and the outer thick copper jacket, is also studied in detail. Through optimizations, more than 99% of RF power is coupled into the DLA structure, with the maximum electromagnetic fields located at a DLA structure. Tolerance studies on the geometrical parameters and mechanical design of the full-assembly structure are also carried out as a reference for realistic fabrication