Implementation and Optimization of the Thin-Strut Formalism in Threde

This article describes the implementation of the thin-strut formalism in the 3D EMP time-domain finite-difference code THREDE. The thin-strut formalism permits inclusion of arbitrarily fine wires in THREDE without imposing any corresponding demand to reduce the mesh size to the wire size. The keystone of this technique is the concept of in-cell inductance - the inductance per unit length a thin wire would have with respect to an enclosing conductor half a mesh removed. THREDE results using this formalism are compared with analytic EMP solutions for a linear dipole antenna. Errors are around 7%; this error seems to be fundamental to the basic THREDE approximations.