Modeling the Effect of Plasma-Wall Interaction in a Hall Thruster (original) (raw)

Plasma–wall interaction inside a Hall thruster

Journal of Plasma Physics, 2002

The dynamics of a Hall thruster is investigated numerically in the presence of a plasma–wall interaction. The plasma–wall interaction is a function of the wall potential, which in turn is determined by the secondary electron emission and sputtering yield. In the present work, the effect of secondary electron emission and sputter yield have been considered simultaneously. Owing to disparate temporal scales, ions and neutrals have been described by a set of time-dependent equations while electrons are considered in a steady state. Based on the experimental observations, a third-order polynomial in electron temperature is used to calculate the ionization rate. The changes in the plasma density, potential and azimuthal electron velocity due to the sputter yield are significant in the acceleration region. The change in ion and electron velocity and temperature is small. The neutral velocity, which decreases initially, starts increasing towards the exit consistent with the computed neutra...

Parametric study of the radial plasma-wall interaction in a Hall thruster

Journal of Physics D: Applied Physics, 2019

An investigation on the influence of relevant parameters on an annular Hall effect thruster plasma discharge is performed using a radial particle-in-cell simulation code with secondary electron emission from the walls and prescribed axial electric and radial magnetic fields. A simulation with true-secondary electrons only is taken as reference. First, the near-wall conductivity effects on the magnetized secondary electrons are illustrated by doubling the E × B, allowing a further code validation. Second, when secondary backscattered electrons are included, the enhanced secondary emission yields lower sheath potential drops and primary electron temperature. Moreover, the dominant backscattered electrons increase the average secondary electrons emission energy, greatly affecting its temperature anisotropy ratio and increasing the replenishment level of the wall collectable tails of the primary electrons velocity distribution function. Third, the effect of the truesecondary electrons emission energy on the potential profile is shown to be negligible, the latter being mainly set by the dominant magnetic mirror effect. Finally, a planar case featuring symmetric plasma profiles permits to confirm the validity of the large cylindrical asymmetries present in the reference case, induced by the combined effects of the geometric expansion, the magnetic mirror and the centrifugal force (due to the E × B drift). A smaller deviation of the primary electron momentum equation from the Boltzmann relation along the magnetic lines is still found in the planar case, induced by the parallel temperature non-uniformity.

Simulating Plasma-Induced Hall Thruster Wall Erosion With a Two-Dimensional Hybrid Model

IEEE Transactions on Plasma Science, 2000

A 2-D radial-axial (r-z) hybrid Fluid/Particle-in-Cell (PIC) model has been developed to model energetic particleinduced channel-wall erosion in coaxial Hall discharge plasma thrusters. The discharge model geometry corresponds to that of a so-called stationary plasma thruster with an extended dielectric channel, and the computational domain extends from the anode at the base of this channel through the channel interior and into the near-field plume region. A model of the wall-erosion process has been added to the simulation in order to assess thruster degradation due to ion and energetic-neutral-induced sputtering of the channel walls. The effect of ion-neutral collisions, including momentum and charge-exchange collisions, on the erosion process is examined. These models are used to simulate the long-term wall-erosion history. For the specific Hall-thrusterconfiguration modeled, collisions were found to have less than a 10% effect on wall erosion. The erosion rate is seen to decrease with time, in good agreement with experimental measurements of long-term erosion in similar thrusters, resulting in a wall recession of as much as 2 mm after 4000 h of simulated operation.

Development of a Finite Element-Based Hall-Thruster Model

Journal of Propulsion and Power, 2003

Modeling the Hall thruster for sputter yield prediction is of considerable interest to the electric propulsion community. This paper documents the status of a finite element based computational development for modeling unsteady plasma flow in the acceleration channel of a stationary plasma thruster (SPT). The results are validated with the available test data and compared with the reported results of particle-in-cell (PIC) method in the literature. Computational challenges are discussed. The lifetime issues also are considered.

Electron-wall interaction in Hall thrusters

Physics of Plasmas, 2005

Electron-wall interaction effects in Hall thrusters are studied through measurements of the plasma response to variations of the thruster channel width and the discharge voltage. The discharge voltage threshold is shown to separate two thruster regimes. Below this threshold, the electron energy gain is constant in the acceleration region and therefore, secondary electron emission ͑SEE͒ from the channel walls is insufficient to enhance electron energy losses at the channel walls. Above this voltage threshold, the maximum electron temperature saturates. This result seemingly agrees with predictions of the temperature saturation, which recent Hall thruster models explain as a transition to space-charge saturated regime of the near-wall sheath. However, in the experiment, the maximum saturation temperature exceeds by almost three times the critical value estimated under the assumption of a Maxwellian electron energy distribution function. The channel narrowing, which should also enhance electron-wall collisions, causes unexpectedly larger changes of the plasma potential distribution than does the increase of the electron temperature with the discharge voltage. An enhanced anomalous crossed-field mobility ͑near wall or Bohm-type͒ is suggested by a hydrodynamic model as an explanation to the reduced electric field measured inside a narrow channel. We found, however, no experimental evidence of a coupling between the maximum electron temperature and the location of the accelerating voltage drop, which might have been expected due to the SEE-induced near-wall conductivity.

Numerical investigation of a Hall thruster plasma

Physics of Plasmas, 2002

The dynamics of the Hall thruster is investigated numerically in the framework of a one-dimensional, multifluid macroscopic description of a partially ionized xenon plasma using finite element formulation. The model includes neutral dynamics, inelastic processes, and plasma–wall interaction. Owing to disparate temporal scales, ions and neutrals have been described by set of time-dependent equations, while electrons are considered in steady state. Based on the experimental observations, a third order polynomial in electron temperature is used to calculate ionization rate. The results show that in the acceleration channel the increase in the ion number density is related to the decrease in the neutral number density. The electron and ion velocity profiles are consistent with the imposed electric field. The electron temperature remains uniform for nearly two-thirds of the channel; then sharply increases to a peak before dropping slightly at the exit. This is consistent with the predict...

Influence of the magnetic field configuration on the plasma flow in Hall thrusters

In Hall propulsion, the thrust is provided by the acceleration of ions in a plasma generated in a cross-field configuration. Standard thruster configurations have annular channels with an almost radial magnetic field at the channel exit. A potential difference is imposed in the axial direction and the intensity of the magnetic field is calibrated in order to hinder the electron motion, while leaving the ions non-magnetised. Magnetic field lines can be assumed, as a first approximation, as lines of constant electron temperature and of thermalized potential. In typical thruster configurations, the discharge occurs inside a ceramic channel and, due to plasma-wall interactions, the electron temperature is typically low, less than few tens of eV. Hence, the magnetic field lines can be effectively used to tailor the distribution of the electrostatic potential. However, the erosion of the ceramic walls caused by the ion bombardment represents the main limiting factor of the thruster lifetime and new thruster configurations are currently under development. For these configurations, classical first order models of the plasma dynamics fail to grasp the influence of the magnetic topology on the plasma flow. In the present paper, a novel approach to investigate the correlation between magnetic field topology and thruster performance is presented. Due to the anisotropy induced by the magnetic field, the gradients of the plasma properties are assumed to be mainly in the direction orthogonal to the local magnetic field, thus enabling a quasi-one-dimensional description in magnetic coordinates. Theoretical and experimental investigations performed on a 5 kW class Hall thruster with different magnetic field configurations are then presented and discussed.

Surface-Driven Asymmetry and Instability in the Acceleration Region of Hall Thruster

Contributions to Plasma Physics, 2008

It has been shown experimentally that the channel wall material has a substantial effect on the behaviour of Hall discharges. For this reason, the radial profile inside the Hall thruster SPT-100 is investigated in detail. This is done by a one-dimensional fully kinetic self-consistent Particle-in-Cell model between the two walls in the acceleration region of the channel. A detailed Monte Carlo probabilistic model for secondary electron emission is implemented as boundary module. Using the local field approximation, two different operative conditions (axial electric field Ez=100 V/cm and 300 V/cm) have been simulated. For high discharge voltage case, a strong radial asymmetry and a stream instability propagating all along the radial domain are detected, while in the low voltage case a stable classical situation is recovered. The critical parameters for triggering this unstable regime are the electron azimuthal drift energy and the induced secondary electron emission, while the saturation mechanism is the increasing of the temperature of the initially cold secondary-electrons.

Characterization of plasma in a Hall thruster operated at high discharge voltage

2005

The electron-wall interaction and its dependence on the discharge voltage and channel width are studied through measurements of the electron temperature, plasma potential and density in a 2 kW Hall thruster. Experimental results are compared with theoretical predictions for different thruster configurations and operating conditions. The channel width is shown to have a more significant effect on the axial distribution of the plasma potential than the discharge voltage.

Influence of design and operation parameters on Hall thruster performances

Journal of Applied Physics, 2004

Plasmas 10, 3397 ͑2003͔͒ is used to carry out parametric investigations on the effects of ͑i͒ the discharge voltage, ͑ii͒ the gas flow rate, ͑iii͒ the axial gradient of the magnetic field, and ͑iv͒ the chamber length on the Hall thruster performances and the axial structure of the plasma discharge. The high-thrust and high-specific-impulse modes for dual-mode thrusters are compared too. The results of the simulations agree well with the main tendencies observed in different experiments. The interaction among the several physical phenomena is discussed and useful scaling laws are proposed. Special attention is paid to understand ͑i͒ the adjustment of the magnetic field strength with the discharge voltage for optimum operation, ͑ii͒ the effect of the magnetic field shape, ͑iii͒ the dimensions of the different regions of the discharge, and ͑iv͒ the parameter trends needed to increment the propulsive and ionization efficiencies ͑the product of which determines the thrust efficiency͒.