Particle-in-cell simulations of low-pressure air plasma generated by pulsed x rays

Abstract

When a photon pulse illuminates a cavity, photoelectrons are emitted from the surface of the cavity, thereby producing the system-generated electromagnetic pulse (SGEMP). Previously, simulations on the SGEMP in a low-pressure environment using a swarm model showed poor applicability to describe the dynamics of photoelectrons and plasmas. In this work, a 3D electromagnetic particle-in-cell with the Monte Carlo collisions model was constructed to investigate the effect of low-pressure air (0–500 mTorr) on the cavity SGEMP response. To model air plasma created by high-energy (keV) photoelectron flows, six kinds of particles (electrons, N2, O2, N2+, O2+, and O2−), as well as the elastic, ionization, attachment, and excitation collisions, were included in the model. The results showed that the peak electric field was in the order of 106 V/m. The peak electric field decreased with increasing air pressure because of dissipation of the space charge barrier (SCB) owing to the generation of secondary electron-ions. The dissipation of the SCB allowed more axially moving photoelectrons, so the fraction of transmitted current and the peak magnetic field increased. The energy conversion between the charged particles and the electromagnetic fields is discussed. Approximately 9% of the photoelectron energy was converted to electromagnetic energy for the SGEMP in vacuum. When the gas pressure increased, the electromagnetic energy conversion rate decreased. The energy relaxation of secondary electrons played an important role in the evolution of the plasma density. To validate the established simulation model, the calculated current was compared with those in the benchmark experiments.