3D particle-in-cell study of the electron drift instability in a Hall Thruster using unstructured grids

Abstract

In a Hall Thruster, the [Formula: see text] current generates a strong electron drift with respect to the ions, which can trigger plasma instabilities, such as the electron drift instability (EDI), also called the electron cyclotron drift instability. The EDI has drawn a great deal of attention in the [Formula: see text] community as previous one- and two-dimensional Particle-In-Cell (PIC) studies suggest it plays a major role in the anomalous transport of electrons across the magnetic barrier. However, experiments showed that the EDI has an inherent three-dimensional nature, which cannot be accurately described in a 1D or 2D configuration. Unfortunately, due to their prohibitive computational cost, needed 3D PIC simulations remained inaccessible without scaling plasma parameters, which inevitably modified the physics in a Hall Thruster. Thanks to recent computational developments, this paper presents a 3D fully kinetic investigation of the EDI in a typical Hall Thruster channel and near plume region. The study was conducted using an unstructured grid, thus demonstrating the feasibility to model more complicated geometries in the future. The growth and development of the EDI are described along with other plasma fluctuations, possibly the signature of another plasma instability, the modified-two-stream-instability. 3D effects on the anomalous transport are also assessed and found to be lower than in an analogous 2D simulation. This is due to lower losses in 2D, which lead to the saturation of the EDI occurring at higher energy levels.

Effect of plasma initialization on 3D PIC simulation of Hall thruster azimuthal instability

“…The simulation domain is a rectangular 3D box with uniform Cartesian grids, as illustrated in figure 1. The curvature in the azimuthal direction is ignored as applied in many previous numerical studies [7,10,17]. The x, y, z directions represent the radial (r), azimuthal (θ), and axial directions in a Hall thruster, respectively.…”

“…We follow the commonly applied magnetic field profile along z [7,17], which has a Gaussian shape and only the radial component,…”

“…We apply the mostly used source term as described similarly in [7,17] to approximate the ionization. The ionization profile varies radially and axially, and is uniform in the azimuthal direction, according to equation (3),…”

“…However, scaling down the geometry unavoidably increases the area to volume ratio, which may enhance the plasma-wall interaction; and other important physical parameters have to be scaled too, such as the electron cyclotron frequency and the ion convection time, which may overestimate the growth rate of ECDI and modify the saturation mechanism. Excitingly, without applying any of the above numerical tricks, Villafana et al in 2023 first conduct a massive 3D PIC simulation successfully on the Hall thruster azimuthal instability [17]. The simulation case includes the discharge channel and a near plume region in the real centimeter scale with about 13.5 × 10 6 unstructured tetrahedral cells, and utilizes 1920 cores for a total cost of 1.4 × 10 6 CPU core hours to simulate 10μs.…”

Growth mechanism and characteristics of electron drift instability in Hall thruster with different propellant types

“…The EDI is considered to be one of the main reasons for anomalous axial electron transport. [24][25][26][27][28][29][30] Lazurenko et al [31] demonstrated experimentally that the EDI significantly increases the axial electron mobility of the thruster. Lafluer et al [5] established a modified model of the electron axial mobility under the influence of EDI through theoretical derivation, and the modified electron mobility is in the same order of magnitude as the actual value.…”