Implementation of the Thin-Slot Formalism in the Finite-Difference EMP Code THREDII

This article derives the thin-slot formalism in a manner compatible with the 3D EMP time-domain finite-difference code THREDII. The thin-slot formalism permits inclusion of conducting plates with arbitrarily narrow slots or gaps without requiring any corresponding need to reduce the cell size to the gap width. The basic concept of this formalism is to increase the effective ε seen by the electric field across the gap and to reduce, in proportion, the μ seen by the magnetic field transverse to the gap. The conducting plate and gap may be imbedded in a lossy medium, and the material on the two sides of the gap may be dissimilar.