Empire PIC Simulations of an Intense Electron Beam Propagation in Low Pressure N₂

We use 3D particle-in-cell (PIC) simulations with detailed gas chemistry to model the propagation of an intense electron beam in N₂ and the resulting plasma response. Prior work used a reduced-order rigid-beam model [1] that showed good agreement with experiments at (1 to 10) Torr but failed at lower pressures (40 to 200) mTorr. To address this, we developed a fully kinetic PIC model using Empire [2] and tested it at both low (<200 mTorr) and high (>200 mTorr) pressures, comparing results with the simulations from the rigid-beam model and experimentally measured peak electron densities.

Simulations replicate experiments on the Naval Research Laboratory (NRL’s) modified Febetron accelerator (100 kV, 4.5 kA, 100 ns pulse), where the electron beam passes through a pair of thin foils into a cylindrical N₂-filled cell (10 cm long, 16 cm in diameter). We self-consistently model the diode, vacuum gap, and gas cell, and incorporate anisotropic scattering and foil material effects. Results track the plasma evolution, electric fields, current contours (2πrBθ), and vibrationally resolved population densities of ion and neutral excitations and are compared with both experimental data and rigid-beam predictions. The PIC model shows improved agreement at low pressures and highlights the role of non-equilibrium kinetic effects in rarefied gases.