  An electron beam passing through a tube which is lined on the inside with a
dielectric layer will radiate energy in the THz range due to the interaction
with the boundary. The resonant enhancement of certain frequencies is
conditioned by structure parameters as tube radius and permittivity of the
dielectric layer. In low loss structures narrow-band radiation is generated
which can be coupled out by suitable antennas. For higher frequencies the
coupling to the resistive outer metal layer becomes increasingly important. The
losses in the outer layer prohibit to reach high frequencies with narrow-band
conditions. Instead short broad-band pulses can be generated with still
attractive power levels. In the first section of the paper a general theory of
the impedance of a two-layer structure is presented and the coupling to the
outer resistive layer is discussed. Approximate relations for the radiated
energy, power and pulse length for a set of structure parameters are derived
and compared to numerical results in the following section. Finally first
numerical result of the out-coupling of the radiation by means of a Vlasov
antenna and estimates of the achieved beam quality are presented.
