THREDS: A Finite-Difference Time-Domain EMP Code in 3d Spherical Coordinates

This article illustrates the three-dimensional time-domain finite differencing of Maxwell's equations in spherical coordinates when azimuthal asymmetry is present. Problems occurring on the coordinate system axis are discussed, and the ease of implementing a radiating outer boundary in spherical coordinates is demonstrated. Sample problems show that the radiating outer boundary is usually better applied on H- than on E-, and that a linearly interpolating outer boundary may be as effective as the more complicated parabolically interpolating one.