Improved Hall Thruster Plume Simulation by Including Magnetic Field Effects

Hall-effect thrusters (HETs) are affordable and efficient electric propulsion devices for space exploration, with higher specific impulse than conventional chemical propulsion and higher thrust at a given power compared to ion thrusters. A detailed understanding and an accurate characterization of the physical processes occurring in HET plume are critical from both the thruster performance and spacecraft integration perspectives. Therefore, a new electron model that includes full 2-D axisymmetric magnetic field effects is developed and incorporated within the framework of a 2-D axisymmetric hybrid particle-fluid code. The governing equation of this new electron model consists of an electron mobility coefficient tensor. The new electron model can simulate any shape magnetic fields. The accuracy of the model is first assessed using the method of manufactured solutions and a Hall thruster test case to confirm 2nd order accuracy. Then, the simulation results of a 6-kW laboratory Hall thruster are directly compared with experimental measurements to validate the model. By including the magnetic field, modeling of the anomalous electron mobility is required. Since the anomalous electron mobility is still not yet well-understood, it is modeled using the Bohm coefficient. A parametric study of the Bohm coefficient is performed to examine its effect on plasma properties. Due to the concave shape of magnetic field lines, the plasma potential in the plume does not show a linear trend with the anomalous collision frequency. Comparisons with experimental data show that the new model with the magnetic field captures the detailed physics than without the magnetic field. In particular, the plasma potential profile agrees well with data by accurately capturing the strong negative gradient near the discharge channel exit of the thruster. In order to extend the capability of the plume simulation, a sputter model is also implemented. The sputter model is applied to simulate the sputtering process of xenon propellants bombarding the surface of the "keeper" for the cathode, which can be an important failure mechanism in Hall thrusters. The steady-state mean erosion rate suggests that keeper erosion is as low as the erosion rate of the discharge channel walls in magnetically-shielded Hall thrusters.PhDAerospace EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/133320/1/mclang_1.pd

A Numerical Study of Wall Erosion and Material Transport in Hall Thrusters

This dissertation describes the development and application of numerical models for the analysis of channel wall erosion and the transport of erosion products in Hall thrusters. The models build on previous work to characterize the sputtering process that underlies wall erosion using the molecular dynamics (MD) technique, and then implements the results of the sputtering model into a hybrid fluid--particle-in-cell (hybrid-PIC) model of a Hall thruster to predict the motion of the erosion products inside the thruster and in the near-field plume. The MD model is applied to characterize the sputtering of hexagonal boron nitride (h-BN) under xenon ion bombardment. It is demonstrated that h-BN tends to sputter in the form of atomic boron and diatomic nitrogen, with heavy molecules becoming somewhat more common at higher incident ion energies and angles of incidence. The calculated sputter yields are shown to agree well with experimental measurements over the range of ion energies (20 eV{300 eV) and incidence angles (0{75) investigated. The behavior of sputtered boron atoms is also analyzed, and these atoms are shown to obey the Sigmund-Thompson velocity distribution function (VDF) predicted by sputtering theory in the direction normal to the h-BN surface. The calculated surface binding energy for boron is shown to agree to within 6% of experimental measurements for ion energies of 100 eV and greater. The calculated sputter yields and boron VDFs are implemented within the established hybrid-PIC model HPHall, and the model is then applied to simulate the NASA HiVHAc Hall thruster at operating conditions that were investigated experimentally using cavity ring-down spectroscopy (CRDS). The effects of ionization and excitation of boron are assessed, and it is found that the majority of sputtered boron atoms do not undergo ionization or excitation in the bulk plasma. These results provide partial validation of CRDS as an in situ diagnostic for measurement of erosion in Hall thrusters. The computed ground-state boron number density in the near-field plume is found to be more uniform than measured using CRDS, but otherwise shows reasonably good agreement with the measurements given the numerous assumptions and sources of uncertainty associated with the models.PhDAerospace EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/116633/1/bradenis_1.pd

Self consistent kinetic simulations of SPT and HEMP thrusters including the near-field plume region

The Particle-in-Cell (PIC) method was used to study two different ion thruster concepts - Stationary Plasma Thrusters (SPT) and High Efficiency Multistage Plasma Thrusters (HEMP-T), in particular the plasma properties in the discharge chamber due to the different magnetic field configurations. Special attention was paid to the simulation of plasma particle fluxes on the thrusters channel surfaces. In both cases, PIC proved itself as a powerful tool, delivering important insight into the basic physics of the different thruster concepts. The simulations demonstrated that the new HEMP thruster concept allows for a high thermal efficiency due to both minimal energy dissipation and high acceleration efficiency. In the HEMP thruster the plasma contact to the wall is limited only to very small areas of the magnetic field cusps, which results in much smaller ion energy flux to the thruster channel surface as compared to SPT. The erosion yields for dielectric discharge channel walls of SPT and HEMP thrusters were calculated with the binary collision code SDTrimSP. For SPT, an erosion rate on the level of 1 mm of sputtered material per hour was observed. For HEMP, thruster simulations have shown that there is no erosion inside the dielectric discharge channel.Comment: 14 pages, 11 figures This work was presented at 21st International Conference on Numerical Simulation of Plasmas (ICNSP'09

Review of the EP activities of US academia

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76897/1/AIAA-2001-3227-398.pd

Hall Thruster Erosion

Hall thruster (HT) is one of the thrusters that are systematically applied in space. If to compare HT with plasma ion thrusters, it has lower lifetime and specific impulse. HT has a set of advantages, and that is why interest to this plasma thruster is high. It has relatively simple design and technology of production. HT does not require a complex power supply unit, and it is very important for spacecraft. Propulsion system on the base of HT has lower mass, simpler technology, and less time of production. One of the main HT characteristics that require improvement is the lifetime of thruster. As it is known, one of the main factors that decrease thruster lifetime is the wear of discharge chamber (DCh). With the analysis of demands to HT, it is understandable that the required lifetime is more than 10 years. So the question about lifetime of the HT is still open. This chapter presents the overview of the thruster elements lifetimes and the overview of methods of thruster erosion investigation. It shows advantages and disadvantages of optical methods of DCh erosion rate investigation. Chapter presents modified method of optical investigation. The results of HT research under various modes of operation and results of tests with different ceramic are presented

Modeling a Two-Stage High-Power Anode Layer Thruster and its Plume

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76625/1/AIAA-22185-240.pd

Operation of a low-power Hall thruster: comparison between magnetically unshielded and shielded configuration

International audienc

Hybrid-Particle-In-Cell Simulation of Backsputtered Carbon Transport in the Near-Field Plume of a Hall Thruster

Magnetic shielding has eliminated boron nitride erosion as the life limiting mechanism in a Hall thruster but has resulted in erosion of the front magnetic field pole pieces. Recent experiments show that the erosion of graphite pole covers, which are added to protect the magnetic field pole pieces, causes carbon to redeposit on other surfaces, such as boron nitride discharge channel and cathode keeper surfaces. As a part of the risk-reduction activities for Advanced Electric Propulsion System thruster development, this study models transport of backsputtered carbon from the graphite front pole covers and vacuum facility walls. Fluxes, energy distributions, and redeposition rates of backsputtered carbon on the anode, discharge channel, and graphite cathode keeper surfaces are predicted

Use of emission spectroscopy for real-time assessment of relative wall erosion rate of BHT-200 hall thruster for various regimes of operation

Radiation emission due to Boron atoms sputtered from the Boron-Nitride ceramic walls of a BHT-200 Hall thruster was measured as a diagnostic for real time assessment of thruster wall erosion and to determine the e ects of various operation conditions on thruster lifetime. Boron neutral 249.677 and 249.773nm lines were measured using a high resolution spectrometer. Spectral measurement results and the accompanying analysis and discussion are presented in this study. From the spectral measurements it was observed that the Boron emission intensity significantly increases for increased discharge voltage pointing to a large increase in the thruster wall erosion rate. Additionally, the measurements show that for the nominal discharge voltage and the applied magnetic field intensity, there is an optimum propellant flow rate for minimum Boron emission, thus minimum wall erosion rate. The variation in the current to the magnet coils showed that the Boron emission intensity increases for increased magnetic field and the Boron emission intensity shows similar behavior to that of the Xenon single ion emission line intensity at 248.911nm. The findings of the study show that emission spectroscopy can be used in determining the optimum operational parameters for minimum wall erosion for SPT type Hall thrusters

An Investigation of Factors Involved in Hall Thruster Wall Erosion Modeling

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76234/1/AIAA-2006-4657-608.pd