Understanding In-Chamber Plasma Behavior Using a Dimensionally Scaled Gridded Ion Thruster in Three-Dimensional Kinetic Particle-in-Cell Simulations

TL;DR

This work addresses facility-related distortions in gridded ion thruster plumes by employing a three-dimensional, fully kinetic PIC–MCC framework informed by a DSMC neutral background, using a dimensionally scaled NSTAR–Little Little Brother geometry. The approach solves Poisson’s equation $\nabla^2 \phi = -\rho/\varepsilon_0$ with $\mathbf{E} = -\nabla \phi$ to capture beam neutralization, wall interactions, and sheath formation under nonuniform neutral conditions, across elastic and inelastic collision channels. Key findings show that including inelastic electron–neutral collisions is essential for a physically consistent, neutralized beam; higher background pressure increases collisionality, lowers ion/electron energies, slightly broadens the beam, and enhances sidewall losses, while electron dynamics reveal a neutralization cloud sustained by low-energy post-collision electrons. The results highlight the need to couple plasma kinetics with chamber-scale geometry when interpreting ground-test measurements and guide future full-scale simulations, boundary-condition refinements, and direct experimental comparisons to quantify facility effects on plume performance and wall coupling.

Abstract

We investigate facility effects on a reduced-scale gridded ion thruster plume using a fully kinetic, three-dimensional Particle-in-Cell/Monte Carlo Collision (PIC-MCC) solver coupled with a Direct Simulation Monte Carlo (DSMC) neutral background. This approach enables detailed examination of key plasma processes governing beam neutralization and wall interactions under ground-test conditions. We find that inelastic electron cooling is essential for achieving a physically consistent, neutralized beam. Increasing the background pressure enhances ion-neutral collisions, leading to more charge- and momentum-exchange events that reduce ion mean energies, broaden the beam, and increase sidewall losses. Including inelastic processes flattens the potential, sustains quasi-neutrality, and preserves beam collimation farther downstream. Single-particle trajectory analyses show that primary electrons undergo mixed escape and temporary trapping, while low energy post-inelastic electrons remain confined, sustaining the neutralization cloud. Sheath diagnostics reveal that at the beam dump, classical Child-Langmuir and Hutchinson models underpredict the sheath length due to residual electrons, while near the sidewall, the sheath is truncated by beam-sheath interference within the compact domain. Current-flow analysis indicates that higher background pressure conditions yield lower beam energies and increased sidewall currents.

Figures (17)

• Figure 1: Electron–xenon collision cross sections (Biagi v7.1) from LXCat LXCat.
• Figure 2: Simulation schematic of the NSTAR–Little Little Brother setup: $yz$-plane cross-section (left) and $xy$-plane cross-section (right). Key dimensions are shown in centimeters. $r_i$ and $r_e$ refer to the radii of the ion beam thruster and neutralizer, respectively. Figures are not to scale.
• Figure 3: Xenon neutral distributions in the $yz$--plane in the $x=0$ plane. Top: neutral number density $n_g$ (m$^{-3}$). Bottom: neutral temperature $T_g$ (K). The dark gray and light gray areas indicate the plasma screen and neutralizer, respectively.
• Figure 4: Xenon neutral pressure ($\mu\mathrm{Torr}$). Top: yz-plane at $x=0\,\mathrm{m}$ from the DSMC solution, which is applied to case 0A and 1A. Bottom: the same plane with a uniform number-density offset applied so that $p=(n+\Delta n)k_\mathrm{B}T$ matches the experimental pressure. This corrected background neutral field is used for case 1B. The light-gray structure is the plasma screen, and the dark-gray structure is the neutralizer.
• Figure 5: Left: Electric potential ($\phi$) contours in the $x=0$ plane for cases 0A (top) and 1A (bottom). Right: Axial number density profiles of ions (red) and electrons (blue) for cases 0A (dashed) and 1A (solid). Data were post-processed using Tecplot smoothing to reduce numerical fluctuations.