Terahertz radiation from an ultra-relativistic charge exiting the open end of a waveguide with a dielectric layer

We analyze radiation produced by an ultrarelativistic charge as it exits the open end of a cylindrical waveguide with a dielectric lining. The end of the waveguide can be either orthogonal to the structure axis or skewed. To obtain terahertz radiation from waveguides with centimeter or millimeter radii, we consider high order TM(0m) modes driven by the beam. We obtain an integral representation which describes the radiation produced by a single waveguide mode in the Fraunhofer zone. We perform a series of numerical calculations for structures which look promising for generation of THz radiation. It is shown that for a mode with large mode number, the aperture of the vacuum channel gives the main contribution to the field if the skew angle of the waveguide aperture is not too small. Simple expressions for the angle of the main pattern lobe maximum are obtained.