  Highly overmoded iris-line structures carry desirable features for the
efficient transportation of THz radiation over long distances. Previous studies
have analyzed the iris line, modeled approximately as a long open-resonator
structure with thin screens, using methods such as Vainstein's impedance
boundary condition or perturbative mode matching. The aim in those methods was
to seek the eigensolution that represents the dominant (least lossy)
propagation mode in a long iris-line structure at steady state. In this paper,
a forward-traveling wave analysis is presented, wherein a short THz pulse
paraxially traverses the oversized structure cell by cell, including the
transient regime. The iris line's periodic discontinuities are analyzed in
terms of forward-wave orthogonal mode decompositions, which are then used to
build a 'forward-scatter' model (matrix) for each cell. The presented approach
predicts a diffraction loss behavior that agrees with that of the eigensolution
found by the previous analytical methods at the steady state for a long
waveguide, but adds the ability of analyzing waveguides of finite lengths,
showing transients as they evolve and settle to the dominant mode down the
line, and offers numerical implementation speeds that are typically an
order-of-magnitude faster than those previously reported for the mode-matching
method. This approach may also be easily extended to analyze iris lines with
mechanical imperfections (e.g. misalignments or aperiodic defects), allowing
for investigations of performance sensitivity against manufacturing tolerances.
