Alejandro Lopez Ortega

Facility Effect Characterization Test of NASA's HERMeS Hall Thruster

A test to characterize the effect of varying background pressure on NASAs 12.5-kW Hall Effect Rocket with Magnetic Shielding had being completed. This thruster is the baseline propulsion system for the Solar Electric Propulsion Technology Demonstration Mission (SEP TDM). Potential differences in thruster performance and oscillation characteristics when in ground facilities versus on-orbit are considered a primary risk for the propulsion system of the Asteroid Redirect Robotic Mission, which is a candidate for SEP TDM. The first primary objective of this test was to demonstrate that the tools being developed to predict the zero-background-pressure behavior of the thruster can provide self-consistent results. The second primary objective of this test was to provide data for refining a physics-based model of the thruster plume that will be used in spacecraft interaction studies. Diagnostics deployed included a thrust stand, Faraday probe, Langmuir probe, retarding potential analyzer, Wien filter spectrometer, and high-speed camera. From the data, a physics-based plume model was refined. Comparisons of empirical data to modeling results are shown.

Assessment of Pole Erosion in a Magnetically Shielded Hall Thruster

Numerical simulations of a 6-kW laboratory Hall thruster called H6 have been performed to quantify the erosion rate at the inner pole. The assessments have been made in two versions of the thruster, namely the unshielded (H6US) and magnetically shielded (H6MS) configurations. The simulations have been performed with the 2-D axisymmetric code Hall2De which employs a new multi-fluid ion algorithm to capture the presence of low-energy ions in the vicinity of the poles. It is found that the maximum computed erosion rate at the inner pole of the H6MS exceeds the measured rate of back-sputtered deposits by ~4.5 times. This explains only part of the surface roughening that was observed after a 150-h wear test, which covered most of the pole area exposed to the plasma. For the majority of the pole surface the computed erosion rates are found to be below the back-sputter rate and comparable to those in the H6US which exhibited little to no sputtering in previous tests. Possible explanations for the discrepancy are discussed.

The importance of the cathode plume and its interactions with the ion beam in numerical simulations of Hall thrusters

Numerical simulations with a 2-D axisymmetric multi-fluid plasma code illustrate the significance of the near-plume interactions in investigations of the anomalous electron transport in Hall thrusters. In our simulations, the transport of electrons is modeled using an anomalous collision frequency, νanom, yielding νanom ≈ ωce (i.e., the electron cyclotron frequency) in the near-plume region. We first show that restricting the anomalous collision frequency in this region to only within the ion beam, where the current density of ions is large, does not alter the plasma discharge in the Hall thruster as long as the interaction between the beam and the cathode plume is captured properly. These simulations suggest that electron transport occurs largely inside the beam. A second finding is on the significance of accounting for the ion acoustic turbulence (IAT), now known to occur in the vicinity of the cathode exit. We have included in our simulations a model of the IAT-driven anomalous collision frequency base...

Self-Consistent Model of a High-Power Hall Thruster Plume

A new model of the plasma plume from Hall effect thrusters (HETs) has been developed for the purpose of more accurately predicting the interactions between future high-power thrusters and large high-voltage solar arrays, such as those being developed under NASAs Game Changing Technology awards by ATK and Deployable Space Systems. The HET plume mainly consists of two types of ions. The first are the energetic main beam ions produced upstream of the thruster acceleration zone. These are the dominant ion species along the thrust axis. The other group of ions have lower kinetic energy and are generated downstream of the acceleration zone from neutral xenon gas atoms by charge exchange (CEX) with main beam ions and by electron impact ionization. The neutral gas is due to neutral propellant atoms leaving the thruster and the hollow cathode without being ionized, and, in the case of laboratory testing, background neutrals present in the vacuum chamber. The new model uses the 2-D axisymmetric Hall thruster code, Hall2De, to self-consistently calculate the three major components of the plume in the vicinity of the thruster, which are the neutral gas atoms, high-energy beam ions, and low-energy ions. From the boundary computational region in the near plume, the Hall2De results are propagated to distances of tens of meters using a continuum hydrodynamic algorithm. This approach offers important advantages with respect to prior models of Hall thruster plumes, such as the NASA Electric Propulsion Interactions Code (EPIC), which uses an analytical fit to laboratory data from a single thruster for the main beam velocity boundary conditions at the channel exit. EPIC assumes that the neutral gas density emanates uniformly and isotropically from the channel exit. Low-energy ions are generated only by CEX; low-energy ions generated by electron impact are not included. The results for the far-field plume of a conceptual high-power thruster (H6) show important differences between EPIC and simulations using the new plume model. High-energy ions undergo less expansion in the azimuthal direction than in EPIC. This can be attributed to magnetic focusing of the beam. We also observe that the peak in low-energy ion density at approximately 90° from the thrust axis predicted by EPIC is moved downstream to angles from 70° to 80° when the new plume model is employed.

A New Cell-Centered Implicit Numerical Scheme for Ions in the 2-D Axisymmetric Code Hall2de

We present a new algorithm in the Hall2De code to simulate the ion hydrodynamics in the acceleration channel and near plume regions of Hall-effect thrusters. This implementation constitutes an upgrade of the capabilities built in the Hall2De code. The equations of mass conservation and momentum for unmagnetized ions are solved using a conservative, finite-volume, cell-centered scheme on a magnetic-field-aligned grid. Major computational savings are achieved by making use of an implicit predictor/multi-corrector algorithm for time evolution. Inaccuracies in the prediction of the motion of low-energy ions in the near plume in hydrodynamics approaches are addressed by implementing a multi-fluid algorithm that tracks ions of different energies separately. A wide range of comparisons with measurements are performed to validate the new ion algorithms. Several numerical experiments with the location and value of the anomalous collision frequency are also presented. Differences in the plasma properties in the near-plume between the single fluid and multi-fluid approaches are discussed. We complete our validation by comparing predicted erosion rates at the channel walls of the thruster with measurements. Erosion rates predicted by the plasma properties obtained from simulations replicate accurately measured rates of erosion within the uncertainty range of the sputtering models employed.

Algebraic Flux Correction and Geometric Conservation in ALE Computations

In this chapter, we describe the important role played by the so-called Geometric Conservation Law (GCL) in the design of Flux-Corrected Transport (FCT) methods for Arbitrary Lagrangian-Eulerian (ALE) applications. We propose a conservative synchronized remap algorithm applicable to arbitrary Lagrangian-Eulerian computations with nodal finite elements. Unique to the proposed method is the direct incorporation of the geometric conservation law (GCL) in the resulting numerical scheme. We show how the geometric conservation law allows the proposed method to inherit the positivity preserving and local extrema diminishing (LED) properties typical of FCT schemes for pure transport problems. The extension to systems of equations which typically arise in meteorological and compressible flow computations is performed by means of a synchronized strategy. The proposed approach also complements and extends the work of the first author on nodal-based methods for shock hydrodynamics, delivering a fully integrated suite of Lagrangian/remap algorithms for computations of compressible materials under extreme load conditions. Numerical tests in multiple dimensions show that the method is robust and accurate in typical computational scenarios.

Numerical simulations of the XR-5 Hall thruster for life assessment at different operating conditions

NASA’s Jet Propulsion Laboratory has been investigating the applicability of Aerojet Rocketdyne’s XR-5 thruster, a 4.5 kW class Hall thruster, for deep-space missions. Major considerations for qualifying the XR-5 for deep-space missions are demonstration of a wide throttling envelope and a usable life capability in excess of 10,000 h. Numerical simulations with the 2-D axisymmetric code Hall2De are employed to inform the qualification process by assessing erosion rates at the thruster surfaces in a wide range of throttling conditions without the need for conducting costly endurance testing. In previous work at JPL by Jorns et al., the anomalous collision frequency distribution for 11 different throttling conditions of the XR-5 spanning 0.3-4.5 kW were identified based on probe measurements of the electron temperature in the near plume region. In this paper, we provide estimates for the erosion rates at the channel walls and pole covers for the same 11 conditions. Uncertainties in the plasma measurements and in the anomalous collision frequency distribution are addressed by determining upper and lower bounds of the erosion rates. Results suggest that erosion of the walls only occurs in the last 5% of the acceleration channel and the rate of such erosion decreases as the geometry of the thruster changes in time due to magnetic shielding. A quasi-zero-erosion state is eventually achieved in all the examined throttling conditions. Examination of the results for pole surface erosion and estimated cathode life indicates that the XR-5 propellant throughput capability will exceed 700 kg, which provides 50% margin over the usable throughput capability of 466 kg as already demonstrated in wear testing

Hollow Cathode Simulations with a First-Principles Model of Ion-Acoustic Anomalous Resistivity

A mathematical model of the ion–acoustic turbulence that is known to develop in the plume of hollow cathodes is presented. The model takes the form of a partial differential equation for the ion–ac...

Simulation of a hall effect thruster with krypton propellant

The multi-fluid framework Hall2De is updated to perform simulations of Hall thrusters operating with krypton propellant. A series of eight simulations of the NASA-300M thruster are performed with both xenon and krypton and the implementation is validated by comparison to experimental measurements from NASA Glenn Research Center. For all operating points the computed thrust matches experimental values to within 10%. Krypton gas generates a specific impulse value between 10 and 16 percent higher than xenon propellant, while producing thrust values between 15 and 25 percent lower than in the xenon simulations. These results are consistent with a back-of-the-envelope theoretical calculation. Next, an unshielded H6 thruster, which has not yet been operated experimentally with a krypton propellant, is also simulated. A 28% lower mass flow rate of krypton gas is required to operate the thruster at the same 6 kW power level as with xenon propellant. The performance trends observed are consistent with the NASA-300M thruster simulations. Further, the singly charged ion current fraction for xenon is lower than for krypton, while for the higher charge species the trend is reversed. This behavior is explained by the relative differences in cross-sections between the different ionization processes for the same gas.