The EMP Tunnel Code DHTRANS

This article describes the mathematical formulation of the EMP code DHTRANS for coupling to conductors in a buried conduit. The formulation basically reduces the 3D coupling problem to a set of coupled transmissionline-like 1D equations in the direction of the tunnel axis and an approximate 2D solution in the transverse plane. These remaining 1D equations are then solved by finite-differencing, using matrix exponentiation for time stepping. Discontinuities along the tunnel axis are permitted, as are nonlinear phenomena such as arcing (this is a benefit of time-domain programming). The 1D nature of the finite-difference algorithm permits time steps to be limited only by the desired temporal resolution of the signal profile, not by dimensions of the tunnel cross-section. A concluding section on examples shows favorable comparison with a fully three-dimensional solution for one problem, and plausible results for another problem involving a generic high-altitude-burst excitation.