Characterization of plasma in a Hall thruster operated at high discharge voltage (original) (raw)
Plasma measurements in a 100 W cylindrical Hall thruster
Journal of Applied Physics, 2004
Conventional annular Hall thrusters become inefficient when scaled to low power. Their lifetime decreases significantly due to the channel wall erosion. Cylindrical Hall thrusters, which have lower surface-to-volume ratio and, thus, seem to be more promising for scaling down, exhibit performance comparable with conventional annular Hall thrusters of the similar size. Plasma potential, ion density, and electron temperature profiles were measured inside the 2.6 cm cylindrical Hall thruster with the use of stationary and slow movable emissive and biased Langmuir probes. Potential drop in the 2.6 cm cylindrical Hall thruster is localized mainly in the cylindrical part of the channel and in the plume, which suggests that the thruster should suffer lower erosion of the channel walls due to fast ion bombardment. Plasma density has a maximum of about (2.6-3.8)ϫ10 12 cm Ϫ3 at the thruster axis. At the discharge voltage of 300 V, the maximum electron temperature is about 21 eV, which is not enough to produce multiple ionization in the accelerated flux of Xe ϩ ions.
Plasma properties downstream of a low-power Hall thruster
Physics of Plasmas, 2005
Triple Langmuir probes and emissive probes were used to measure the electron number density, electron temperature, and plasma potential downstream of a low-power Hall thruster. The results show a polytropic relation between electron temperature and electron number density throughout the sampled region. Over a large fraction of the plume, the plasma potential obeys the predictions of ambipolar expansion. Near the thruster centerline, however, observations show larger gradients of plasma potential than can be accounted for by this means. Radial profiles of plasma potential in the very-near-field plume are shown to contain large gradients that correspond in location to the boundaries of a visually intense plasma region.
Effects of Cathode Electron Emission of Hall Thruster Discharge
44th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, 2008
Low power cylindrical and annular geometry Hall thrusters are operated in a non-selfsustained regime with different thermionic cathode-neutralizers. The enhancement of the electron emission with a keeper current for the hollow cathode and with a wire heating for the filament cathode leads to a significant (up to 30%) narrowing of the plasma plume and increase of the energetic ion fraction. For the cylindrical Hall thruster, the observed variations of the plasma potential, electron temperature, and plasma density with the keeper current suggest that the electron emission from the cathode can affect the electron cross-field transport and the ionization in the thruster channel.
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.
Preliminary Results of Plasma Flow Measurements in a 2 KW Segmented Hall Thruster
2003
A 2 kW Hall thruster was developed, built and operated in an upgraded vacuum facility. The thruster performance and parameters of the plasma flow were measured by new diagnostics for plume measurements and plasma measurements inside the thruster channel. The thruster demonstrated efficient operation in terms of propellant and current utilization efficiencies in the input power range of 0.5-3.5 kW. Preliminary measurements of the ion energy spectra from the thruster axis region and the distribution of plasma parameters in the vicinity of the thruster exit are reported.
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.
Measurements of Plasma Potential Distribution in Segmented Electrode Hall Thruster
2001
Use of a segmented electrode placed at the Hall thruster exit can substantially reduce the voltage potential drop in the fringing magnetic field outside the thruster channel. In this paper we investigate the dependence of this effect on thruster operating conditions and segmented electrode configuration. A fast movable emissive probe is used to measure plasma potential in a 1 kW laboratory Hall thruster with segmented electrodes made of a graphite material. Relatively small probe-induced perturbations of the thruster discharge in the vicinity of the thruster exit allow a reasonable comparison of the measured results for different thruster configurations. It is shown that the plasma potential distribution is almost not sensitive to changes of the electrode potential, but depends on the magnetic field distribution and the electrode placement.
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.
Experimental studies of anode sheath phenomena in a Hall thruster discharge
Journal of Applied Physics, 2005
Both electron-repelling and electron-attracting anode sheaths in a Hall thruster were characterized by measuring the plasma potential with biased and emissive probes ͓L. Dorf, Y. Raitses, V. Semenov, and N. J. Fisch, Appl. Phys. Lett. 84, 1070 ͑2004͔͒. In the present work, two-dimensional structures of the plasma potential, electron temperature, and plasma density in the near-anode region of a Hall thruster with clean and dielectrically coated anodes are identified. Possible mechanisms of anode sheath formation in a Hall thruster are analyzed. The path for current closure to the anode appears to be the determining factor in the anode sheath formation process. The main conclusion of this work is that the anode sheath formation in Hall thrusters differs essentially from that in the other gas discharge devices, such as a glow discharge or a hollow anode, because the Hall thruster utilizes long electron residence times to ionize rather than high neutral pressures.
Investigation of Low Discharge Voltage Hall Thruster Operating Modes and Ionization Processes
2009
A study of low discharge voltage Hall thruster operation revealed the existence of two operating regimes. Small variations in cathode flow fraction and magnetic field resulted in sharp changes in discharge current with constant thrust, corresponding to a visible change in the jet-mode plume structure. The phenomenon is characterized with a systematic map of thruster operation for discharge voltages ranging from 100-120 V, anode flow rates from 10-20 mg/s, and cathode flow rates from 7% to 25% of the anode flow. Far-field plume measurements show electron current to the anode is decreased during this transition. The ion energy distributions exhibit significant differences between the highcurrent and low-current operating modes. These differences are indicative of a significant increase in the fraction of low-energy multiply charged ions and a longer region of primary ion acceleration in the high-current mode. DISTRIBUTION STATEMENT A. Approved for public release; distribution is unlimited.
Modeling the Effect of Plasma-Wall Interaction in a Hall Thruster
41st Aerospace Sciences Meeting and Exhibit, 2003
Among many reasons limiting the efficiency and lifetime of a Hall thruster, the most critical is the wear of the surface layer of the ceramic walls due to the plasma-wall interactions. The plasma-wall interaction is a function of wall potential, which in turn is determined by the secondary electron emission and sputtering yield. In this paper, we document the numerical result of the Hall thruster dynamics in the presence of plasma-wall interaction in one and twodimensional framework. A comparison is made with the two dimensional simulation. The changes in the plasma density, the potential and the azimuthal electron velocity due to the sputter yield are significant in the acceleration region. The plasma number density, temperature, velocity and potential decrease in the presence of the SEE and the sputter yield. However, the neutral number density and the velocity do not exhibit any significant change. The neutral velocity, which decreases initially, starts increasing toward the exit consistent with the computed neutral density profile. Numerical potential distribution shows a good agreement with experimental data reported in the literature.
Effects of enhanced cathode electron emission on Hall thruster operation
Physics of Plasmas, 2009
Interesting discharge phenomena are observed that have to do with the interaction between the magnetized Hall thruster plasma and the neutralizing cathode. The steady-state parameters of a highly ionized thruster discharge are strongly influenced by the electron supply from the cathode. The enhancement of the cathode electron emission above its self-sustained level affects the discharge current and leads to a dramatic reduction in the plasma divergence and a suppression of large amplitude, low frequency discharge current oscillations usually related to an ionization instability. These effects correlate strongly with the reduction in the voltage drop in the region with the fringing magnetic field between the thruster channel and the cathode. The measured changes in the plasma properties suggest that the electron emission affects the electron cross-field transport in the thruster discharge. These trends are generalized for Hall thrusters of various configurations.
Model of the plasma discharge in a Hall thruster with heat conduction
Physics of Plasmas, 2002
The inclusion of heat conduction into a one-dimensional, macroscopic model of the plasma inside a Hall thruster and in the near plume is found to smooth the temperature profile of previous solutions with a nonconductive model. The spatial structure still consists of reverse-flow, ionization, and acceleration regions. Conductive energy flow, being of the same order of convective flow, has significant effects on the rear part of the channel where it can make impossible the establishment of a steady anode sheath. As a result, there is an upper bound on the plasma reverse flow for the existence of stationary solutions. The analysis of inertial effects on the electron dynamics concludes that the main contribution is the azimuthal electron motion, which can produce extra collisionality, mainly in the near plume. The different contributions to the effective axial diffusion of electrons and the ion temperature are evaluated. A parametric investigation yields the basic scaling laws of the thruster stationary performance.
Effects of Enhanced Eathode Electron Emission on Hall Thruster Operation
2009
Interesting discharge phenomena are observed that have to do with the interaction between the magnetized Hall thruster plasma and the neutralizing cathode. The steadystate parameters of a highly ionized thruster discharge are strongly influenced by the electron supply from the cathode. The enhancement of the cathode electron emission above its self-sustained level affects the discharge current and leads to a dramatic reduction of the plasma divergence and a suppression of large amplitude, low frequency discharge current oscillations usually related to an ionization instability. These effects correlate strongly with the reduction of the voltage drop in the region with the fringing magnetic field between the thruster channel and the cathode. The measured changes of the plasma properties suggest that the electron emission affects the electron cross-field transport in the thruster discharge. These trends are generalized for Hall thrusters of various configurations.
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...
Space charge saturated sheath regime and electron temperature saturation in Hall thrusters
Physics of Plasmas, 2005
Existing electron-wall interaction models predict that secondary electron emission in Hall thrusters is significant and that the near-wall sheaths are space charge saturated. The experimental electron-wall collision frequency is computed using plasma parameters measured in a laboratory Hall thruster. In spite of qualitative similarities between the measured and predicted dependencies of the maximum electron temperature on the discharge voltage, the deduced electron-wall collision frequency for high discharge voltages is much lower than the theoretical value obtained for space charge saturated sheath regime, but larger than the wall recombination frequency. The observed electron temperature saturation appears to be directly associated with a decrease of the Joule heating rather than with the enhancement of the electron energy loss at the walls due to a strong secondary electron emission. Another interesting experimental result is related to the near-field plasma plume, where electron energy balance appears to be independent on the magnetic field.
Effects of segmented electrode in Hall current plasma thrusters
Journal of Applied Physics, 2002
Segmented electrodes placed along a ceramic channel in a Hall thruster are shown to influence significantly the plasma potential distribution. Both the radial potential and the axial acceleration region are sensitive to the location of the segmented electrodes. The measured and theoretical potential profiles appear to be affected in detail by the electrode material ͑graphite͒ having lower secondary electron emission than the ceramic channel walls. The measured plasma potential profile is shown as well to correlate with the observed and desirable narrowing of the plasma plume emanating from the thruster.
Electron energy distribution function in a Hall discharge plasma
37th Joint Propulsion Conference and Exhibit, 2001
The role of inelastic collisions in Hall thruster operation is studied through simulation of the electron energy distribution function (EEDF) inside the thruster channel. The electron Boltzmann equation is solved using the Lorentz approximation (two-term expansion) and the "local-field" approximation. The resultant zero-dimensional Boltzmann equation takes into account inelastic losses due to ionizing collisions and wall-collisions. Secondary electrons from ionization and wall-collisions are also included in the model. Electron continuity is used to calculate the sheath potential at the insulator walls. Results show an EEDF cut off at high energy due to electron loss to the walls. Secondary and scattered electrons from ionization provide a large population of low-energy electrons. The calculated EEDFs agree well with experimental electron temperature data when an experimentally-determined effective collision frequency is used for electron momentum transport. Predicted values for the wall-sheath potential agree with results from a charge-balance model, except where said model predicts sheath collapse.
A numerical study of low-frequency discharge oscillations in Hall thrusters
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A 2D numerical model has been constructed for use in modeling Hall thruster plasma dynamics. An important feature is a detailed electron-insulator interaction model which has yielded improved predictions of electron temperature. The complete simulation is used here as a ...