  A new CAD program, PolyCAD, designed for X-ray photon tracing in
polycapillary optics is described. To understand the PolyCAD code and its
results, the theoretical basis of X-ray transmission by a single cylindrical
channel (monocapillary) is discussed first. Then the simplest cases of
cylindrically and conically shaped polycapillary optics are examined. PolyCAD
allows any type of X-ray source to be used: an X-ray tube of finite beam
dimensions or an astrophysical object can be simulated in combination with the
polycapillary optics. The radiation distribution images formed on a screen
located at various focal distances are discussed. The good agreement of some of
the PolyCAD results with those reported in earlier papers validate the code.
This is the first paper of a series dedicated to the development of an
exhaustive CAD program, work is in progress to develop the code to include
other polycapillary-optics shapes, such as semi-lenses and full-lenses.
