Dynamic Internal Electromagnetic Pulse Calculations in Three Spatial Dimensions

When a photon pulse impinges on a cavity, it causes electrons to be emitted from the cavity walls, thereby producing electromagnetic fields in the cavity. This constitutes what is known as Internal Electromagnetic Pulse (IEMP). The produced fields for a given incident photon pulse and spectrum (in the case where transient space charge effects are important) is calculated by means of a particle-in-cell numerical simulation. The calculations are three-dimensional and fully electromagnetic. The calculated fields for pressures of 0.4¿ and 720¿ show good quantitative agreement with recent underground test results. Both the calculations and the experimental data show that the presence of background gas in the cavity greatly affects the magnitude of the fields in the cavity. At 0.4¿ the emitted current is heavily space charge limited and the magnetic fields are comparatively low. At 720¿, the emitted currents ionize the background gas and the secondary currents neutralize the space charge barrier. Then, almost the full emission current traverses the cavity causing much higher magnetic fields. The space charge neutralization process causes the risetime of the fields to be shorter than that of the emitted currents. System implications of these phenomena are discussed.