Hall Discharge Research Articles

Low-frequency model of breathing oscillations in Hall discharges

A paradigm for Hall discharge modeling is presented whereby only the time scale of the lowest-frequency mode is explicitly resolved. The ability of such a low-frequency model to reproduce with excellent accuracy the breathing mode is demonstrated through comparisons with a fully time-dependent numerical model. Based on this formalism, an approximate linearized model is derived which essentially constitutes a one-dimensional generalization of the classical zero-dimensional predator-prey model. The model highlights the interaction of standing plasma waves with the transport of neutral species, which involves standing and convective waves of similar magnitude. It predicts a frequency which is in close agreement with the frequency of the small perturbation modes observed in simulations. Finally, it is shown that unstable modes are in general strongly nonlinear and characterized by frequencies obeying a scaling law different from that of linear modes.

Kinetic simulations of a plasma thruster

The modelling of the Hall thruster SPT-100 is a very important issue in view of the increasing importance of such propulsion devices in space applications. Only kinetic models can investigate the rich variety of physical mechanisms involved in the Hall discharge and in the plume emitted from the thruster. This paper collects a number of different particle-in-cell/Monte Carlo collision models which have been able to reveal different phenomena related to the peculiar physics of Hall thruster, such as sheath instability, azimuthal fluctuations and plume backflow.

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

A 2-D radial-axial (r-z) hybrid fluid/particle-in-cell (PIC) model has been developed to model energetic particle-induced 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-thruster-configuration 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.

Plasma sheaths in Hall discharge

The sheath region of a Hall discharge is studied in a four-dimensional phase space which consists of one spatial (radial in cylindrical metrics) and three velocity dimensions by means of a particle-in-cell∕Monte Carlo model coupled with a probabilistic method for the secondary electron emission. Different axial regions (anode, ionization, and acceleration zones) of the channel have been investigated using the local field approximation and distinguishing between inner and outer walls. The presheath and sheath structures are different in the three regions simulated showing a charge saturated regime in the acceleration region. Small differences in behavior for the external and internal walls of the channel are detected. Further, trapped ions are found near the walls in the acceleration region which could have an important effect on the wall recombination enhancing the axial electron current. The results could be used to obtain boundary conditions and lateral wall losses which are suitable for incorporation into one-axial and two-dimensional macroscopic models which simulate the bulk neutral plasma in Hall discharge.

Arcjet Plasma Neutralization of Hall Thrusters, Part 2: Experimental Demonstration

Arcjet Plasma Neutralization of Hall Thrusters, Part 2: Experimental Demonstration

An investigation of spatial distribution of characteristics of a plasma stream formed by a hall discharge in a channel with metal walls

The effect of the system of delivery of the working medium on the characteristics of a plasma stream in a Hall accelerator with closed azimuthal current is experimentally investigated. The gas delivery system utilizes a porous diaphragm from a carbon–carbon composite material whose pore size is comparable with the Debye radius of the electrons of plasma formed in a discharge channel. The calorimetric method is used to determine the degree of azimuthal and radial nonuniformity of distribution of the energy of plasma stream developed by the accelerator, namely, the distribution of temperature of a titanium target mounted perpendicularly to the incident stream is taken with the aid of an infrared imager. The probe method is used to measure the spatial distribution of temperature and the concentration of charge-exchange plasma at the accelerator channel exit. It is shown that the ratio of the kinetic energy of the flow of heavy particles to the expended electric energy (energy efficiency) exceeds 0.5.

Kinetic study of wall collisions in a coaxial Hall discharge.

Coaxial Hall discharges (also known as Hall thrusters, stationary plasma thrusters, and closed-drift accelerators) are cross-field plasma sources under development for space propulsion applications. The importance of the electron-wall interaction to the Hall discharge operation is studied the through analysis of experimental data and simulation of the electron energy distribution function (EEDF) inside the discharge channel. Experimental time-average plasma property data from a laboratory Hall discharge are used to calculate the electron conductivity and to estimate the rate of wall-loss collisions. The electron Boltzmann equation is then solved in the local field limit, using the experimental results as inputs. The equation takes into account ionization and wall collisions, including secondary electrons produced at the wall. Local electron balances are used to calculate the sheath potential at the insulator walls. Results show an EEDF depleted at high energy due to electron loss to the walls. The calculated EEDFs agree well with experimental electron temperature data when the experimentally determined effective collision frequency is used for electron momentum transport. The electron wall-loss and wall-return frequencies are extremely low compared to those predicted by a Maxwellian of equal average energy. The very low frequency of wall collisions suggests that secondary electrons do not contribute to cross-field transport. This conclusion holds despite significant experimental uncertainty.

Ion beam dynamics in a linear Hall discharge

Ion current oscillations in a linear geometry Hall discharge with an open electron drift are studied using a planar Langmuir probe. The bulk discharge behavior is reconstructed from time-resolved measurements of the local ion current density. These ion beam images reveal the same fluctuating breathing behavior found in closed-drift Hall thrusters.

Laser-induced fluorescence measurements of velocity within a Hall discharge

The results of a study of laser-induced fluorescence velocimetry of neutral and singly ionized xenon in the plume and interior portions of the acceleration channel of a Hall thruster plasma discharge operating at powers ranging from 250 to 725 W are described. Axial ion and neutral velocity profiles for four discharge voltage conditions (100 V, 160 V, 200 V, 250 V) are measured as are radial ion velocity profiles in the near-field plume. Ion velocity measurements of axial velocity both inside and outside the thruster as well as radial velocity measurements outside the thruster are performed using laser-induced fluorescence with nonresonant signal detection on the xenon ion 5d[4]7/2–6p[3]5/2 excitation transition while monitoring the signal from the 6s[2]3/2–6p[3]5/2transition. Neutral axial velocity measurements are similarly performed in the interior of the Hall thruster using the 6s[3/2]0 2–6p[3/2]2transition with resonance fluorescence collection. Optical access to the interior of the Hall thruster is provided by a 1-mm-wide axial slot in the insulator outer wall. While the majority of the ion velocity measurements used partially saturated fluorescence to improve the signal-to-noise ratio, one radial trace of the ion transition was taken in the linear fluorescence region and yields a xenon ion translational temperature between 400 and 800 K at a location 13 mm into the plume.

Stability of a magnetized Hall plasma discharge

Using recent experimental data on the time-averaged, spatially varying plasma properties within a Hall discharge plasma, we present in this article, a theoretical study of the response of this plasma to small (linear) perturbations in its properties. As a starting point for this analysis, we assume a two-dimensional fluid description that includes a simplified equation for the electron energy, and constrain the azimuthal wave vector such that we excite only the dominant (m=1) azimuthal modes. The growth rate and frequencies of predominantly axial and azimuthally propagating plasma disturbances are obtained by numerical solution of the resulting eigenvalue problem under a quasiuniform plasma condition, along the entire discharge channel. The results identify the persistence of a low frequency instability that is associated with the ionization process, concentrated largely in the vicinity of the exit plane, where the magnetic field is at its maximum value, consistent with experimental observations for the relatively low operating voltages (∼100 V) considered in this study.

Anomalous electron mobility in a coaxial Hall discharge plasma.

A comprehensive analysis of measurements supporting the presence of anomalous cross-field electron mobility in Hall plasma accelerators is presented. Nonintrusive laser-induced fluorescence measurements of neutral xenon and ionized xenon velocities, and various electrostatic probe diagnostic measurements are used to locally determine the effective electron Hall parameter inside the accelerator channel. These values are then compared to the classical (collision-driven) Hall parameters expected for a quiescent magnetized plasma. The results indicate that in the vicinity of the anode, where there are fewer plasma instabilities, the electron-transport mechanism is likely elastic collisions with the background neutral xenon. However, we find that in the vicinity of the discharge channel exit, where the magnetic field is the strongest and where there are intense fluctuations in the plasma properties, the inferred Hall parameter departs from the classical value, and is close to the Bohm value of (omega(ce)tau)(eff) approximately 16. These results are used to support a simple model for the Hall parameter that is based on the scalar addition of the electron collision frequencies (elastic collision induced plus fluctuation induced), as proposed by Boeuf and Garrigues [J. Appl. Phys. 84, 3541 (1998)]. The results also draw attention to the possible role of fluctuations in enhancing electron transport in regions where the electrons are highly magnetized.

A characterization of plasma fluctuations within a Hall discharge

Experimental results are presented for studies of low (2-20 kHz) and intermediate-frequency (20-100 kHz) oscillations in crossed-field closed electron-drift Hall discharges. Conditional sampling using two electrostatic probes is used to identify and extract properties of coherent structures associated with the propagation of azimuthal and longitudinal instabilities within the discharge channel. The azimuthal component phase velocities are determined for a wide range of wave frequencies and over characteristic regimes of operation of these devices. A variety of propagation modes are observed and analyzed, including the appearance of an induced mode due to the presence of the probes themselves. This later result is believed to be the first direct evidence of how fluctuations can be influenced in these Hall discharges using relatively simple actuation methods.

A low-power, linear-geometry Hall plasma source with an open electron-drift

This paper presents a discussion of the physics of modern Hall plasma thrusters and its impact on the design of new plasma thrusters of varying geometry and power. A particular emphasis is placed on the design and development of a linear-geometry (non- coaxial) source with an open electron-drift current. The operating characteristics of a linear-geometry Hall discharge scaled to operate in the 50 to 100 W power range are presented. Two thruster acceleration channels were fabricated—one of alumina and one of boron nitride. Differences in operation with the two channel materials are attributable to differences in the secondary electron emission properties. In either case, however, operation is achieved despite the lack of a closed electron current drift in the Hall direction, suggesting that there is an anomalous axial electron mobility, due to either plasma fluctuations or collisions with the channel wall. Strong low-frequency oscillations in the discharge current, associated with the depletion of propellant within the discharge, are seen to appear and vary with changes in the applied magnetic field strength. The frequency of this oscillatory mode is higher than that seen in larger (and higher power) discharges, due to the decreased residence time of the propellant within the channel.

Coherent structures in crossed-field closed-drift Hall discharges

The structure of a magnetized, collisionless crossed-field coaxial Hall discharge is investigated using low-impedance, negatively biased Langmuir probes. We present images depicting frequency-position renderings and cross-correlation plots, which identify coherent and random fluctuations in plasma density. These fluctuations are found to depend on the discharge operating conditions, and are believed to greatly affect cross-field electron transport.

Laser-induced fluorescence study of a xenon Hall thruster

0→6p[3/2]2(3P2-1D2) transition at 823.2 nm and the xenon-ion 5d[3]7/2→6p[2]5/2 0(4D7/2-4P5/2) transition is used to measure plasma parameters in the plume of a laboratory-model xenon Hall thruster. The Hall discharge operates nominally at 62 V, 4.2 A, and 3.2 mg s-1 xenon flow, with an overall thruster power of 320 W. A tunable semiconductor diode laser and an Ar+-pumped dye laser are used to probe the respective excited-state transitions. Axial velocity measurements are made at a number of axial and radial locations up to 4.5 cm downstream of the thruster-exit plane and under a variety of thruster operating conditions. Neutral velocities from 100 m s-1 to 400 m s-1 and ion velocities as high as 12 km s-1 are calculated from measured Doppler shifts. The charge-exchange phenomenon evidently does not significantly affect the xenon neutrals. The spectral-line shapes of the ion indicate a spread in ion energies through a non-Maxwellian distribution of axial velocities. Neutral kinetic temperatures of 500 (±200) K are observed under standard operating conditions. Zeeman and Stark effects on the spectral-line shapes, from the thruster’s magnetic and electric fields, are not substantial. The measured line center of the ion transition is 16521.23 (±0.02) cm-1.