Simulation Study of Air Effects on SGEMP Based on Swarm Model

A system-generated electromagnetic pulse (SGEMP) occurs when the system is exposed to an X-ray or
$\gamma $
-ray beam. And the presence of air will greatly alter the fields of SGEMP. In this article, 1-D simulations are performed with a newly developed relativistic swarm model to investigate the air effects on SGEMP. For comparison, the equilibrium model and the non-relativistic swarm model are also used in simulations. The simulation results are divided into different pressure regions for discussion, that is, the high-pressure region (I and II), the transition region, and the intermediate pressure region. In the high-pressure region, the air plasma will dramatically reduce the electric field generated by the primary current. And the electric field increases with decreasing air density in high-pressure region I, due to the decreases of the ionization source of primary electrons interacting with air molecules. Then the electric field decreases with decreasing air density in high-pressure region II, mainly because of cascading ionization begins to dominate the generation of secondary electrons. In the transition region, the electric field is reduced to a minimal level, as the cascading rate saturates with the increase of electron energy. As air density continues to decrease, the electric field rises again and begins to oscillate in the intermediate region, where the relativistic effect becomes important. This investigation will be helpful in understanding future results of both experiment and simulation.