Finite-difference electrodynamic analysis of a tapering coaxial waveguide incorporating a dielectric wedge-shaped lens for concentrating Transverse Electromagnetic (TEM) waves

The three-dimensional, finite-difference, electrodynamic computer code SOS models a portion of a coaxial, azimuthally symmetric, tapering waveguide. A transverse electromagnetic (TEM) wave is launched at the inlet section in an oil medium. The wave is reflected from a Lucite section, from a wedge-shaped water dielectric, from a vacuum section, and from the tapering conductors. The edge is a lens that aids concentrating the energy into the tapering section. Voltage and current traces are examined at several locations within the waveguide. They determine the 40 pF wedge capacitance that increases the pulse risetime from the input 0.5 ns to 1.5 ns at the outlet. Alterations are discussed to include more effects within the waveguide, to reduce the outlet-signal risetime, and to reduce the probability of arcing. Appendixes describe a dielectric thin-wedge subgrid model and a treatment for dielectric-interface boundary conditions that were developed and implemented into SOS so the code would be applicable.