  An elegant and convenient rigorous approach for solving circular open-ended
dielectric-loaded waveguide diffraction problems is presented. It uses the
solution of corresponding Wiener-Hopf-Fock equation and leads to an infinite
linear system for reflection coefficients (S-parameters) of the waveguide, the
latter can be efficiently solved numerically using the reducing technique. As a
specific example directly applicable to beam-driven radiation sources based on
dielectric-lined capillaries, diffraction of a slow TM symmetrical mode at the
open end of a circular waveguide with uniform dielectric filling is considered.
A series of such modes forms the wakefield (Cherenkov radiation field)
generated by a charged particle bunch during its passage along the waveguide
axis. Calculated S-parameters were compared with those obtained from COMSOL
simulation and an excellent agreement is shown. This method is expected to be
very convenient for analytical investigation of various electromagnetic
interactions of Terahertz (THz) waves (both free and guided) and charged
particle bunches with slow-wave structures prospective in context of modern
beam-driven THz emitters, THz accererators and THz-based bunch manipulation and
bunch diagnostic systems.
