  We consider a semi-infinite open-ended cylindrical waveguide with uniform
dielectric filling placed into collinear infinite vacuum waveguide with larger
radius. Electromagnetic field produced by a point charge or Gaussian bunch
moving along structure's axis from the dielectric waveguide into the vacuum one
is investigated. We utilize the modified residue-calculus technique and obtain
rigorous analytical solution of the problem by determining coefficients of mode
excitation in each subarea of the structure. Numerical simulations in CST
Particle Studio are also performed and an excellent agreement between
analytical and simulated results is shown. The main attention is paid to
analysis of Cherenkov radiation generated in the inner dielectric waveguide and
penetrated into vacuum regions of the outer waveguide. The discussed structure
can be used for generation of Terahertz radiation by modulated bunches (bunch
trains) by means of high-order Cherenkov modes. In this case, numerical
simulations becomes difficult while the developed analytical technique allows
for efficient calculation of the radiation characteristics.
