Nicolas Gascon

Comparison of hybrid Hall thruster model to experimental measurements

** A two -dimensional hybrid particle -in -cell numerical model has been constructed in the radial -axial plane with the intent of examining the physics governing Hall thruster operation. The electrons are treated as a magnetized quasi -one -dimensi onal fluid and the ions are treated as collisionless, nonmagnetized discrete particles. The anomalously high electron conductivity experimentally observed in Hall thrusters is accounted for using experimental measurements in the Stanford Hall thruster. An evaluation is made of differing treatments of electron mobility, background gas, neutral wall interactions, and charge exchange collisions. The results are compared to experimental measurements of ion and neutral number densities and velocities, electron t emperature, and electric potential.

Growth of resistive instabilities in E×B plasma discharge simulations

Two-dimensional hybrid numerical simulations of E×B discharges used in Hall thruster propulsion point to the presence of strong fluctuations attributable to resistive instabilities in the frequency range of f≈0.1–10MHz and the wavenumber range of λ−1≈10–500m−1. Analytical analyses confirm that these resistive modes are of the convective type, become increasingly unstable at low electron mobility, and are particularly intense at high voltage. The simulations, which model cross-field electron flow via an experimentally measured mobility, exhibit large fluctuation power in a region corresponding to a strong electron transport barrier. The analysis gives an electron mobility (μe) -dependent growth rate (γ) scaling as γ∼μe−1∕2. The predicted phase velocity of these waves is close to the ion velocity, somewhat lower than that seen in the simulations. Including the electron pressure contribution lowers the growth rate at high frequencies, and introduces a phase velocity that is shifted by ± the ion acoustic spee...

Performance and Evolution of Stationary Plasma Thruster Electric Propulsion for Large Communications Satellites

Space Systems/Loral (SS/L) has extensive experience with electric propulsion dating back to the early 1990s when an agreement was made to develop the newly available Russian manufactured Stationary Plasma Thruster (SPT-100) for use on western communications satellites. The western qualification and integration of the SPT-100 subsystem onto SS/L spacecraft was completed in 2001 with the first flight in 2004. SS/L has now launched six spacecraft with SPT-100 electric propulsion subsystems, with ten more satellites under construction. The SPT-100 subsystem provides impulse for on-orbit inclination management (north-south stationkeeping), eccentricity control, and momentum wheel unloads, as well as orbit raising capability when desired. It enables a large reduction of on-orbit propellant mass and thereby significant increases in communications payload mass and capability. The SPT subsystem now has more than thirteen years of cumulative on orbit experience, with a single thruster accumulating over 6 years of near-daily operation in orbit. This paper summarizes SS/L’s experience from the western qualification of the SPT-100 subsystem through successful deployment and operation on orbit. The evolution of the subsystem building on this experience is described, including an already flight-proven universal thruster module and the qualification of the higherthrust SPT-140 subsystem. These advancements will further capitalize on the benefits of electric propulsion, including significant electric orbit raising as well as orbit control of very large spacecraft to support ever more complex and capable communications payloads.

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.

Shear-Based Model for Electron Transport in Hybrid Hall Thruster Simulations

An electron cross-field transport model based on instantaneous simulated plasma properties is incorporated into a radial-axial hybrid simulation of a Hall plasma thruster. The model is used to capture the reduction of fluctuation-based anomalous transport that is seen experimentally in the region of high axial shear in the electron fluid. Similar transport barriers are observed by the magnetic confinement fusion community due to shear suppression of plasma turbulence through an increase in the decorrelation rate of plasma eddies. The model assumes that the effective Hall parameter can be computed as the sum of the classical term, a near-wall conductivity term, and a fluctuation-based term that includes the effect of shear. A comparison is made between shear-based, experimental, and Bohm-type models for cross-field transport. Although the shear-based model predicts a wider transport barrier than experimentally observed, overall, it better predicts measured plasma properties than the Bohm model, particularly in the case of electron temperature and electric potential. The shear-based transport model also better predicts the breathing-mode oscillations and time-averaged discharge current than both the Bohm and experimental mobility models. The plasma property that is most sensitive to adjustment of the fitting parameters used in the shear-based model is the plasma density. Applications of these fitting parameters in other operating conditions and thruster geometries are examined in order to determine the robustness and portability of the model. Without changing the fitting parameters, the simulation was able to reproduce macroscopic properties, such as thrust and efficiency, of an SPT-100-type thruster within 30% and match qualitative expectations for a bismuth-fueled Hall thruster.

Further Development of a Micro Hall Thruster

The operation and performance characterization of a micro Hall thruster is presented. The thruster is co-axial in design, with a 0.5 mm channel width and 4 mm outer diameter. The magnetic circuit includes a SmCo permanent magnet generating approximately 0.7 T at the exit plane and 1 T in the channel. Operation with a hollow cathode neutralizer is achieved in the 10-40 W power range with an anode flow rate of 0.12-0.20 mg/s Xe. The thrust is measured to be in the range of 0.6-1.6 mN for an anode flow rate of 0.17-0.20 mg/s and an applied voltage of 110-275V. Thrust efficiency and the specific impulse are in the range of 10-15 % and 300-850 s, respectively, for the same conditions. Relatively broad ion energy distributions and large beam divergence are observed from an analysis of the plume using a retarding potential analyzer and ion current probe. The thruster exhibits the characteristic “breathing mode” instability in the 35-70 kHz frequency range.

A 90 GHz Phase-Bridge Interferometer for Plasma Density Measurements in the Near Field of a Hall Thruster

Abstract : Preliminary measurements are described of electron number density obtained in the near field of a 200W Busek Hall thruster. The approach taken is ultra-high high frequency (3.3 mm wavelength, 90 GHz) microwave interferometry that affords high spatial resolution suitable for studying the near exit region, where the plasma density can be as high as 10 (11)cm-3. A 90 GHz beam can be focused down to a waist of approximately 7 mm, and over a 50 mm plasma path length, a l0(11)cm-3 plasma density gives rise to an easily measurable 30 phase shift. The system is of a phase-bridge design, utilizing two signal anns split from a fixed frequency source (one passing through the plasma) that recombine at two balanced nuxers. A line-integrated electron density is obtained by comparing the two signals from the mixers, This interferometer is suitable for measuring time-dependent plasma density fluctuations offering unprecedented information about plasma turbulence in the near exit region, and the opportunity to study turbulence-enhanced electron transport in regions where the plasma is presumably collisionless and free of interactions with the channel wall.

A Comparison of 2-D Hybrid Hall Thruster Model to Experimental Measurements

A two-dimensional hybrid particle-in-cell numerical model has been constructed in the radial-axial plane with the intent of examining the physics governing Hall thruster operation. The electrons are treated as a magnetized quasi-one-dimensional fluid and the ions are treated as collisionless, nonmagnetized discrete particles. The anomalously high electron conductivity experimentally observed in Hall thrusters is accounted for using experimental measurements in the Stanford Hall thruster. An evaluation is made of differing treatments of electron mobility, background gas, neutral wall interactions, and charge exchange collisions. The results are compared to experimental measurements of ion and neutral number densities and velocities, electron temperature, and electric potential.

The Deflagration - Detonation Transition in Gas-Fed Pulsed Plasma Accelerators

A hydromagnetic Rankine-Hugoniot model for propellant acceleration in gas-fed pulsed plasma thrusters is presented and verified against available experimental data. Building on early work by Cheng et al [Nuclear Fusion 10, pp. 305-317, 1970], the snowplow mode is compared to a detonation process and it is shown here that a competing “second” mode, which was observed independently in gas-fed pulsed plasma thrusters by Ziemer et al [Ph.D. Dissertation, Princeton University, 2001], can be explained as a plasma deflagration process. Based on the presented model, a single equation is derived for these two modes, which gives the exhaust velocity of gas-fed pulsed plasma thrusters as a function of controllable parameters. It is found that the developed model is consistent with experimentally measured exhaust velocities for varying mass bit sizes.

Introduction of Physical Transport Mechanisms into 2D Hybrid Hall Thruster Simulations

A limitation of many currently existing Hall thruster models is the need for an ad-hoc or experimentally-based electron cross fleld mobility. In this work, the potential ∞uctuations simulated using a radial-axial hybrid Hall thruster model are used to calculate a new electron mobility based only on simulated properties. A small amplitude perturbation model is used to compute number density and electron velocity ∞uctuations and to develop a linearized dispersion relation based on a two-stream instability. The simplifled dispersion relation is used to infer azimuthal wave behavior from the simulated axial properties. The correlation between the relative phase of the plasma density and electron velocity are used to compute anomalous contributions to axial and azimuthal current density. The computed Hall parameter based on this approach leads to improved agreement between simulation and experimental measurements of plasma properties. However, for the speciflc thruster modelled, the simulated perturbation quantities are not small as originally assumed. While this decreases the validity of the computed mobility, it shows that ∞uctuations likely play a signiflcant role in transport and emphasizes the need for a self-consistent non-linear model.