Three-dimensional time-domain calculations for EMP simulators

We have used a three-dimensional finite-difference time-domain (FDTD) code for modeling an EMP simulator. This involves the discharge of a previously charged parallel-plate capacitor, through a switch, into a TEM horn-like structure with a matching resistive termination. The simulations yield the temporal behaviour of input current to the structure, and the 3-D electromagnetic field structure in the near-field. As expected, this shows the effects of standing wave structures within the horn. We have also compared the performance of the simulator with and without a resistive sheet termination. We observe a six nanosecond risetime of the electric field within the horn. This is comparable with the R-C time of the simulator. The main contribution to this field comes from frequencies below 150 MHz. For the assumed parameters of the switch and capacitor-to-horn connections, we see a small but still significant pre-pulse. Such studies could be generalized to optimize coupling to other kinds of pulsers. As a first step in this direction, we have calculated the input impedance of the structure over its ´useful´ range of frequencies. This can allow a study of the sensitivity of the EMP waveform to changes in pulser configuration.