| The losses for TE and TM waves in infrared WG metallic waveguides are calculated. Maxwell's equations are set up first in spherical coordinates and then in oblate spheroidal coordinates, and are solved to find the leading-order term of all six field components. These are used to express the boundary conditions at the finite-conductivity wall, and to calculate the fields propagating into the metal. From this, the propagation losses in the waveguides are obtained, showing the explicit dependence on both curvatures.; The allowed angles of incidence are calculated from the caustic of the rays associated with particular WG modes. It is found that in many practical situations the pseudo-Brewster angles for metals are large compared to the allowed ray angles, so that usual assumption about TM loss calculation must be reconsidered. It is also found that the TM losses are much lower than generally assumed, and that the TM and TE loss ratio is independent of the wall material.; Dielectric coating techniques for improving the transmission of metallic WG waveguides are studied by means of the local plane-wave model. The general expressions of power loss ratio between coated and bare metal guides are derived. Numerical results for an optimized two-layer coating for improving either TE or TM transmission for one wavelength are presented. Numerical optimization also shows that losses below one percent per turn can in principle be obtained with a four-layer structure over entire 1 to 10 {dollar}mu{dollar}m region of interest for free-electron lasers.; Temperature distributions in metallic WG waveguides are also briefly discussed. |
