  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 epsilon_r=16.66 and a loss tangent tan_delta=0.0000343. It consists of
a very compact dielectric disk with a width of 2.035 mm and a tilt angle of 60
degree, 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 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.
