Annular Hall Thrusters Research Articles

Hybrid Modeling of Miniaturized 50 W Annular Hall Thruster

A 50-W-class annular Hall thruster is studied with a hybrid axial–radial two-dimensional model. Ions are described by a kinetic approach, whereas fluid conservation equations are solved for electrons. In such models, additional (anomalous) contributions must be added to the momentum-transfer electron collision frequency to obtain realistic values of the cross-field electron mobility. First, a parametric study is performed, where anomalous transport is described with a simple two-region model based on constant empirical parameters. The simulated global performance is subsequently compared with experimental measurements. Then, laser-induced fluorescence ion velocity measurements are employed to infer a continuous profile of the anomalous electron collision frequency along the channel centerline. The model reproduces the performance, the acceleration structure, the current oscillations, and the doubly charged ion fraction of the laboratory thruster. Measurements of the ion velocity distribution function highlight the presence of a slow ion population in the near plume. The production of the slow ions and their growth for increasing distances from the thruster channel exit is qualitatively reproduced by the model. The results obtained suggest that the generation and dynamics of the observed slow ions can be attributed to the presence of energetic electrons in the plume.

Generation of multiply charged ions in accordance with geometry and magnetic field in Hall thruster plasmas

The production of multiply charged ions in Hall effect plasmas has been investigated. The factors that significantly affect plasma characteristics, such as geometry, magnetic field configuration, and magnetic mirror ratio, were analyzed. Among the co-current and counter-current configurations of a cylindrical Hall thruster (CHT) and an annular Hall thruster (AHT), the co-current configuration of the CHT shows the largest fractions of multiply charged ions. In particular, for the CHT, the region with high magnetic flux density had a significant influence on the fraction of multiply charged ions, but the fraction of multiply charged ions was not significantly different depending on magnetic mirror ratios. In addition, the configuration having a similar magnetic field in the CHT demonstrated comparable fractions of multiply charged ions and thrust even with different channel lengths. Hence, the broad region with the high magnetic flux density could enhance the probability of further ionization inside the discharge channel.

Determination of the ionization region in Hall thruster plasmas with low perturbation

Information on the ionization of propellant gases is invaluable for Hall thruster plasma studies because the ion beam is the source of thrust generation. This study proposes a new method that identifies the ionization location of Xe+ ions in Hall thruster plasmas by combining the laser-induced fluorescence spectroscopy and floating emissive probe measurements in annular and cylindrical Hall thrusters. In an annular Hall thruster, this method indicated that ionization became the largest at a few millimeters upstream of the maximum radial magnetic field, whose result agreed with a one-dimensional hybrid fluid/particle-in-cell (PIC) simulation result. On the other hand, the ionization region in a cylindrical Hall thruster was located deeper inside the discharge cavity than that of the annular thruster. In addition, a large upstream shift of the ionization region was observed with the decreasing fraction of the radial magnetic field in the cylindrical Hall thruster.

Ion acceleration in a wall-less Hall thruster

In wall-less Hall thrusters, the ionization of the propellant and the acceleration of the ions occur outside the thruster [S. Mazouffre, S. Tsikata, and J. Vaudolon, in 50th AIAA/ASME/SAE/ASEE Joint Propulsion Conference (American Institute of Aeronautics and Astronautics, Cleveland, OH, 2014)]. This reduces interactions between the plasma and the thruster parts as compared to conventional annular and cylindrical Hall thrusters and promises a longer thruster lifetime. With a much simpler design, these non-conventional thrusters are also easier to miniaturize for operation at low power levels of a few hundred watts and lower. In this work, experiments demonstrate that a miniaturized (3 cm diameter) 200 W wall-less thruster is also able to achieve similar voltage utilization, propellant utilization, and current utilization efficiencies as conventional Hall thrusters. Yet, thruster performance of the wall-less thruster is generally lower due to a much larger plume divergence than that in conventional Hall thrusters. This plume divergence is a consequence of ion acceleration in the fringing magnetic field. Thrust and plasma measurements suggest that the thrust generated by the wall-less thruster is due to two components: ion acceleration by the JxB force in the region of the fringing magnetic field radially away from the thruster and by plasma expansion in the diverging magnetic field near the thruster axis.

Mode transitions in nonlinear evolution of the electron drift instability in a 2D annular E × B system

Nonlinear development of electron drift instability is studied using 2D3V azimuthal-radial Particle-in-Cell simulations of an annular Hall thruster channel of 10 cm diameter. The full 2π azimuthal domain of the annular cross section is simulated with reflecting boundary conditions at the radial boundaries. It is shown that the instability, which starts as a short length scale linear instability, undergoes a sequence of nonlinear transitions into longer wavelength modes. The transitions in the mode wavelengths are accompanied by related transitions of the magnitude of anomalous axial current. In the nonlinear stages, there is evidence of azimuthal trapping and dragging of ions by the propagating wave resulting in saturation of instability. It is demonstrated that the size of the azimuthal domain influences mode dynamics and, thereby, the anomalous cross field electron transport.

Comparative Study of Annular–Cylindrical Combined Channel With Annular Channel Hall Thruster

To improve the performance of thrusters by using two permanent ring-shaped magnets to push a magnetic field downward, we propose an annular–cylindrical combined-channel Hall thruster and numerically study its performance. The results indicate that, compared with simple annular channels, the annular–cylindrical combined channel can effectively increase the neutral gas density contained within it and increase the degree of ionization, efficiency, thrust, and specific impulse. Moreover, the sputtering zone of discharge channel is in front of the permanent magnets, i.e., wall erosion occurs upon ion bombardment during discharge. However, ions cannot bombard the permanent magnetic rings, thus guaranteeing a constant magnetic field and long discharge lifetime.

A 2-D Hybrid Hall Thruster Simulation That Resolves the <inline-formula> <tex-math notation="LaTeX">$\boldsymbol{E}\times \boldsymbol{B}$ </tex-math></inline-formula> Electron Drift Direction

A 2-D axial–azimuthal model of an annular Hall thruster discharge is developed. We use a hybrid fluid–particle-in-cell (PIC) treatment to model a Xe-propellant Hall thruster. Using a PIC approach, the positive Xe+ ions are modeled as collisionless, nonmagnetized, discrete super-particles, whereas the electrons are treated as a magnetized 2-D fluid. The model includes a continuously replenished Xe neutral background, with an imposed radial magnetic field and an applied axial electric potential. Motivated by experimental evidence of anomalously high electron mobility across the magnetic field that has been attributed to quasi-coherent fluctuations in the plasma properties, we use the numerical model to resolve azimuthal electron dynamics and study the impact of fluctuations on electron transport. Representative low-voltage (100 V) operating condition simulations predict localized plasma properties similar to those observed in experiments. The simulations predict coherent azimuthally propagating disturbances which appear to contribute to enhanced electron transport beyond that due to classical scattering.

Effect of multiply charged ions on the performance and beam characteristics in annular and cylindrical type Hall thruster plasmas

Plasma plume and thruster performance characteristics associated with multiply charged ions in a cylindrical type Hall thruster (CHT) and an annular type Hall thruster are compared under identical conditions such as channel diameter, channel depth, propellant mass flow rate. A high propellant utilization in a CHT is caused by a high ionization rate, which brings about large multiply charged ions. Ion currents and utilizations are much different due to the presence of multiply charged ions. A high multiply charged ion fraction and a high ionization rate in the CHT result in a higher specific impulse, thrust, and discharge current.

A comparison of emissive probe techniques for electric potential measurements in a complex plasma

The major emissive probe techniques are compared to better understand the floating potential of an electron emitting surface in a plasma. An overview of the separation point technique, floating point technique, and inflection point in the limit of zero emission technique is given, addressing how each method works as well as the theoretical basis and limitations of each. It is shown that while the floating point method is the most popular, it is expected to yield a value ∼1.5Te/e below the plasma potential due to a virtual cathode forming around the probe. The theoretical predictions were checked with experiments performed in a 2 kW annular Hall thruster plasma (ne ∼ 109−1010 cm−3and Te ∼ 10−50 eV). The authors find that the floating point method gives a value around 2Te/e below the inflection point method, which is shown to be a more accurate emissive probe technique than other techniques used in this work for measurements of the plasma potential.

Plasma Plume of Annular and Cylindrical Hall Thrusters

Hall thrusters hold considerable advantage over chemical thrusters and other kinds of electrical propulsion devices, except that the plume of the Hall thruster tends to be wide. An ongoing objective in Hall-thruster research is to narrow this plume. The plume is sensitive both to the specific geometry of the magnetic field as well as to the voltage potential induced within the plasma. Annular-geometry Hall thrusters tend to have narrower plumes. However, the cylindrical-geometry thruster has been shown to be suited particularly to low-power operation. New techniques have been advanced to narrow the Hall-thruster plumes. In this paper, images of the plasma plume in the two geometries of the Hall thruster are presented.

Performance of a Low-Power Cylindrical Hall Thruster

Recent mission studies have shown that a Hall thruster which operates at relatively constant thrust efficiency (45-55%) over a broad power range (300W - 3kW) is enabling for deep space science missions when compared with slate-of-the-art ion thrusters. While conventional (annular) Hall thrusters can operate at high thrust efficiency at kW power levels, it is difficult to construct one that operates over a broad power envelope down to 0 (100 W) while maintaining relatively high efficiency. In this note we report the measured performance (I(sub sp), thrust and efficiency) of a cylindrical Hall thruster operating at 0 (100 W) input power.

Enhanced performance of cylindrical Hall thrusters

The cylindrical thruster differs significantly in its underlying physical mechanisms from the conventional annular Hall thruster. It features high ionization efficiency, quiet operation, ion acceleration in a large volume-to-surface ratio channel, and performance comparable with the state-of-the-art conventional Hall thrusters. Very significant plume narrowing, accompanied by the increase of the energetic ion fraction and improvement of ion focusing, led to 50%–60% increase of the thruster anode efficiency. These improvements were achieved by overrunning the discharge current in the magnetized thruster plasma.

Kinetic effects in a Hall thruster discharge

Recent analytical studies and particle-in-cell simulations suggested that the electron velocity distribution function in E×B discharge of annular geometry Hall thrusters is non-Maxwellian and anisotropic. The average kinetic energy of electron motion in the direction parallel to the thruster channel walls (across the magnetic field) is several times larger than that in the direction normal to the walls. Electrons are stratified into several groups depending on their origin (e.g., plasma or channel walls) and confinement (e.g., lost on the walls or trapped in the plasma). Practical analytical formulas are derived for the plasma flux to the wall, secondary electron fluxes, plasma potential, and electron cross-field conductivity. Calculations based on these formulas fairly agree with the results of numerical simulations. The self-consistent analysis demonstrates that the elastic electron scattering in collisions with atoms and ions plays a key role in formation of the electron velocity distribution function and the plasma potential with respect to the walls. It is shown that the secondary electron emission from the walls may significantly enhance the electron conductivity across the magnetic field but only weakly affects the insulating properties of the near-wall sheath. Such self-consistent decoupling between the secondary electron emission effects on the electron energy losses and the electron cross-field transport is currently not captured by the existing fluid and hybrid models of Hall thrusters.

Electron cross-field transport in a miniaturized cylindrical Hall thruster

Conventional annular Hall thrusters become inefficient when scaled to low power. Cylindrical Hall thrusters, which have lower surface-to-volume ratio, are more promising for scaling down. They presently exhibit performance comparable with conventional annular Hall thrusters. The present paper gives a review of the experimental and numerical investigations of electron cross-field transport in the 2.6-cm miniaturized cylindrical Hall thruster (100-W power level). We show that, in order to explain the discharge current observed for the typical operating conditions, the electron anomalous collision frequency /spl nu//sub B/ has to be on the order of the Bohm value, /spl nu//sub B/ /spl ap/ /spl omega//sub c//16. The contribution of electron-wall collisions to cross-field transport is found to be insignificant. The optimal regimes of thruster operation at low background pressure (below 10/sup -5/ torr) in the vacuum tank appear to be different from those at higher pressure (/spl sim/10/sup -4/ torr).

Electron cross-field transport in a low power cylindrical Hall thruster

Conventional annular Hall thrusters become inefficient when scaled to low power. Cylindrical Hall thrusters, which have lower surface-to-volume ratio, are therefore more promising for scaling down. They presently exhibit performance comparable with conventional annular Hall thrusters. Electron cross-field transport in a 2.6 cm miniaturized cylindrical Hall thruster (100 W power level) has been studied through the analysis of experimental data and Monte Carlo simulations of electron dynamics in the thruster channel. The numerical model takes into account elastic and inelastic electron collisions with atoms, electron-wall collisions, including secondary electron emission, and Bohm diffusion. It is shown that in order to explain the observed discharge current, the electron anomalous collision frequency νB has to be on the order of the Bohm value, νB≈ωc/16. The contribution of electron-wall collisions to cross-field transport is found to be insignificant.

Plasma measurements in a 100 W cylindrical Hall thruster

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)×1012 cm−3 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+ ions.

Enhanced ionization in the cylindrical Hall thruster

Conventional annular Hall thrusters do not scale efficiently to low power. An alternative approach, a cylindrical Hall thruster with a cusp-type magnetic field distribution, has been investigated. A relatively large 9-cm-diam version of a cylindrical thruster, operated in 300–1000 W power range, and the 2.6 cm miniaturized cylindrical Hall thruster, operated in the power range 50–300 W, exhibited performance comparable with conventional annular Hall thrusters of the similar size. The cylindrical thrusters have unusually high propellant utilization, compared to conventional Hall thrusters. Numerical simulations, performed within the framework of a quasi-one-dimensional stationary thruster model, show that the increase in the propellant utilization does not appear to be quantitatively explained by a reduction of plasma wall losses. A more complete theoretical model, likely including kinetic effects, will be necessary to explain the observed propellant utilization effect.

Parametric investigation of miniaturized cylindrical and annular Hall thrusters

Conventional annular Hall thrusters become inefficient when scaled to low power. An alternative approach, a 2.6 cm miniaturized cylindrical Hall thruster with a cusp-type magnetic field distribution, was developed and studied. Its performance was compared to that of a conventional annular thruster of the same dimensions. The cylindrical thruster exhibits discharge characteristics similar to those of the annular thruster, but it has a much higher propellant ionization efficiency. Significantly, a large fraction of multicharged xenon ions might be present in the outgoing ion flux generated by the cylindrical thruster. The operation of the cylindrical thruster is quieter than that of the annular thruster. The characteristic peak in the discharge current fluctuation spectrum at 50–60 kHz appears to be due to ionization instabilities. In the power range 50–300 W, the cylindrical and annular thrusters have comparable efficiencies (15%–32%) and thrusts (2.5–12 mN). For the annular configuration, a voltage less than 200 V was not sufficient to sustain the discharge at low propellant flow rates. The cylindrical thruster can operate at voltages lower than 200 V, which suggests that a cylindrical thruster can be designed to operate at even smaller power.

Radial macroscopic model of a plasma flowing along annular dielectric walls

The classical model of the transverse interaction of a collisionless, partially ionized plasma with two insulator walls confining it, is here generalized to plasmas flowing parallel to the walls. A macroscopic formulation is proposed, where the contributions of the axial flow are included into the radial equations through source terms for particle production, ion friction, and ion heating. These contributions compete with those due to ionization, and add three new parameters to the classical model. Zero-ionization cases are presented to show that the net production (and loss) of plasma can be independent of ionization. Friction effects on the radial ion acceleration is found to reduce plasma fluxes into the walls. The ion temperature is determined from the competition between heating due to ionization and cooling due to radial rarefaction; isothermal and isentropic behaviors are recovered only for particular situations. Planar and annular geometries are studied. For the latter one, the differences in the inner and outer plasma fluxes are evaluated. The application of the model to the plasma discharge in an annular Hall thruster is commented.