  When a relativistic, femtosecond laser pulse enters a waveguide, the pulse
energy is coupled into waveguide optical modes. The longitudinal laser field
effectively accelerates electrons along the axis of the channel, while the
asymmetric transverse electromagnetic fields tend to expel fast electrons
radially outwards. At the exit of the waveguide, the $\sim$${\rm nC}$,
$\sim$$10\ {\rm MeV}$ electron bunch converts its energy to a $\sim$$10\ {\rm
mJ}$ terahertz (THz) laser pulse through coherent diffraction radiation. In
this paper, we present 3D particle-in-cell simulations and theoretical analyses
of the aforementioned interaction process. We investigate the process of
longitudinal acceleration and radial expulsion of fast electrons, as well as
the dependence of the properties of the resulting THz radiation on laser and
plasma parameters and the effects of the preplasma. The simulation results
indicate that the conversion efficiency of energy can be over $5\%$ if the
waveguide length is optimal and a high contrast pump laser is used. These
results guide the design of more intense and powerful THz sources.
