Modeling of an EMP Simulator Using a 3-D FDTD Code

We have performed self-consistent, three-dimensional (3-D), time-domain calculations for a bounded-wave electromagnetic pulse simulator. The simulator consists of a constantimpedance transverse electromagnetic structure driven by a charged capacitor, discharging through a fast closing switch. These simulations yield the detailed 3-D electromagnetic field structure in the vicinity of the simulator. The prepulse seen in these simulations can be explained quantitatively in terms of capacitive coupling across the switch and the known charging waveform across the capacitor. Placement of a test object within the simulator significantly modifies the electric fields within the test volume, in terms of field strength as well as the frequency spectrum. This means that, for a given simulator, larger objects would be subjected to somewhat lower frequencies. The E-field waveform experienced by a small test object is reasonably close to that for free-space illumination, but the mismatch increases with object size. The use of a resistive sheet as a matching termination significantly reduces radiation leakage as compared to two parallel resistive rods. For a given termination, larger test objects marginally reduce leakage. A physical interpretation of these conclusions is also included. This work is a first step toward full-fledged optimization of such simulators using 3-D modeling.