Session DT1: Hall Thrusters

Effects of electron-drift instability in 1D fluid simulations of Hall Effect Thrusters

A key problem of Hall Effect Thruster(HET) is the understanding of the anomalous electron transport. Recent studies have suggested as a possible cause the electron-drift instability (EDI), driven by the relative motion of electrons respects to the ions in the $\mathbf{E}\times\mathbf{B}$ direction of the thruster. Although the recent success in describing the instability and its consequences on the electron transport, few attempts have been performed to include such effects in a fluid simulation of HET, in which the anomalous transport is still artificially enhanced in order to fit experimental results. We propose in this work a comparison between the macroscopic effects induced by the EDI and the corresponding ones obtained by the ad-hoc transport. The HET is modeled through a 1D fluid simulation reproducing the axial direction of the thruster. An expression for the instability-induced friction force and power loss can be obtained from quasi-linear theory, providing the corrections to the transport equations induced by the instability. The results show a good agreement between the transport induced by the instability and that obtained from the artificial mobility, further suggesting that the relevance of the EDI in causing the anomalous electron transport.   

3D particle-in-cell simulation of a Thruster Anode Layer

Hall thrusters are becoming increasingly popular for the propulsion of satellites. These devices efficiently take leverage of charged particles speed to reduce propellant use, and subsequently cutting costs dramatically. Although such systems have been extensively studied, the detailed physics of the magnetized plasma in these thrusters is very complex and several plasma processes that have direct influence on the thruster performances and lifetime are still poorly understood. Among remaining issues, the anomalous transport of electrons in the near exhaust region needs to be addressed. Particle-in-cell (PIC) models have the capability to describe this phenomenon but their use in 3D real geometries is very challenging. We present here the AVIP-PIC code coupled with a Poisson solver to model the plasma dynamics in industrial 3D geometries using unstructured grids and parallel computing. An AVIP-fluid version also exists which is presented in a separate contribution. In this pap er, a thruster anode layer (TAL), that can be viewed as a compact Hall thruster is studied in real dimensions and compared to experiment. After a presentation of the code and simulation setup, results about the TAL are analyzed to demonstrate the capabilities of the 3D AVIP-PIC solver.   

2D (axial-azimuthal) Particle-In-Cell simulations of Hall Effect Thrusters

Even though Hall-Effect Thrusters(HET) have been intensively studied for the past few decades, electron transport across the magnetic field is still not well understood. Recent studies have shown that Electron Cyclotron Drift Instabilities(ECDI) could be a main cause of the anomalous transport observed. To get more insights on this phenomena, we modified a highly-parallelized 2D Particle-In-Cell Monte Carlo Collision (PIC MCC) model, called LPPic, that was previously used to simulate the radial-azimuthal plane of an HET, in order to study the axial-azimuthal one. Comparisons with the theory developed by Lafleur et al. have been conducted. In a HET, electrons are injected at the cathode to ionize the neutral gas coming from the anode and this injection needs to be properly modeled in an axial-azimuthal simulation. Two emission conditions have been previously investigated in an axial-radial code by Szabo et al. while using an "artificial permittivity" to speed-up the code, which is not needed in LPPic. Another one was recently proposed by Boeuf et al. with a simplified simulation case that was used to get a 2D PIC benchmark of ExB discharges. Through the analysis of this case, we were able to better understand the impact of this cathodic emission model on the discharge behavior.   

Plasma sheath in presence of secondary electron emission in Hall Effect Thrusters

The plasma sheath is a recurrent problem in the study of plasma discharges. Indeed, it controls the particle and heat fluxes to the wall, which control in return the plasma density and temperature. Even though it has been studied since Langmuir in the 1900's, it is not well understood yet. In addition, the role of the wall material on plasma discharges is known to be important in Hall Effect Thrusters (HET) [Goebel-08]. The main phenomena proposed to explain the role of the wall in HET is the secondary electron emission (SEE).\\ In this study, we use a bi-dimensional particle in cell simulation code to investigate the plasma sheath in presence of SEE. We perform a parametric study on the SEE rate in order to better understand the sheath behaviour, using the SEE model of Barral (2003). The mean SEE rate mainly depends on the electron energy flux to the wall, and we show that this energy flux is not correctly described by an isothermal sheath model. New insights from the simulations are presented, and a new description of an non-isothermal sheath is proposed.