Jorge Fernandez-Coppel, Richard Wirz, Jaime Marian, “Fully discrete model of kinetic ion-induced electron emission from metal surfaces,” Journal of Applied Physics 135(8) (2024); https://doi.org/10.1063/5.0188000

Ion-induced electron emission (IIEE) is an important process whereby ions impinging on a material surface lead to net emission of electrons into the vacuum. While relevant for multiple applications, IIEE is a critical process of electric thruster (ET) operation and testing for space propulsion, and, as such, it must be carefully quantified for safe and reliable ET performance. IIEE is a complex physical phenomenon, which involves a number of ion-material and ion-electron processes, and is a complex function of ion mass, energy, and angle, as well as host material properties, such as mass and electronic structure. In this paper, we develop a discrete model of kinetic IIEE to gain a more accurate picture of the electric thruster chamber and facility material degradation processes. The model is based on three main developments: (i) the use of modern electronic and nuclear stopping databases, (ii) the use of the stopping and range of ions in matter to track all ion and recoil trajectories inside the target material, and (iii) the use of a scattering Monte Carlo approach to track the trajectories of all mobilized electrons from the point of first energy transfer until full thermalization or escape. This represents a substantial advantage in terms of physical accuracy over existing semi-analytical models commonly used to calculate kinetic IIEE. We apply the model to Ar, Kr, and Xe irradiation of W and Fe surfaces and calculate excitation spectra as a function of ion depth, energy, and angle of incidence. We also obtain minimum threshold ion energies for net nonzero yield for each ion species in both Fe and W and calculate full IIEE yields as a function of ion energy and incidence angle. Our results can be used to assess the effect of kinetic electron emission in models of full ET facility testing and operation.

Yuchen Qian, Walter Gekelman, Patrick Pribyl, Tom Sketchley, Shreekrishna Tripathi, Zoltan Lucky, Marvin Drandell, Stephen Vincena, Thomas Look, Phil Travis, Troy Carter, Gary Wan, Mattia Cattelan, Graeme Sabiston, Angelica Ottaviano, Richard Wirz, “Design of the Lanthanum hexaboride based plasma source for the large plasma device at UCLA,” Review of Scientific Instruments 94(8) (2023); PDF, https://doi.org/10.1063/5.0152216

The Large Plasma Device (LAPD) at UCLA (University of California, Los Angeles) produces an 18 m long, magnetized, quiescent, and uniform plasma at a high repetition rate to enable studies of fundamental plasma physics. Here, we report on a major upgrade to the LAPD plasma source that allows for more robust operation and significant expansion of achievable plasma parameters. The original plasma source made use of a heated barium oxide (BaO) coated nickel sheet as an electron emitter. This source had a number of drawbacks, including a limited range of plasma density (≲ 4.0× 1012 cm− 3), a limited discharge duration (∼ 10 ms), and susceptibility to poisoning following oxygen exposure. The new plasma source utilizes a 38 cm diameter lanthanum hexaboride (LaB6) cathode, which has a significantly higher emissivity, allowing for a much larger discharge power density, and is robust to exposure to air …

McKenna J.D. Breddan, Richard E. Wirz, “Electrospray plume evolution: Influence of drag,” Journal of Aerosol Science, Volume 167 (2023); https://doi.org/10.1016/j.jaerosci.2022.106079

The University of California, Los Angeles (UCLA) Plasma, Energy, & Space Propulsion Laboratory (PESPL) presents the Discrete Electrospray Lagrangian Interaction (DELI) Model for simulating the evolution of electrospray plumes. This publication describes the DELI Model, verification of its Coulomb collision module, model validation with atmospheric plume data, and novel comparisons of simulated plumes evolved with different fractions of the drag force. DELI Model results reproduce experimentally-observed droplet clustering events that yield plume expansion as a result of Coulomb repulsion. Furthermore, the presented comparison of identical emitted species evolved to steady state with different fractions of applied drag force demonstrates decreased mean droplet velocity and increased plume expansion with increased drag force. Publication results serve as a useful tool in examining electrospray plume evolution in atmospheric and vacuum regimes. Keywords: Electrospray; Plume evolution; Coulomb expansion; Drag deceleration

Ottaviano, A., & Wirz, R. E., “Secondary electron emission of reticulated foam materials,” Journal of Applied Physics, 133(10) (2023); https://doi.org/10.1063/5.0133253

Complex material surfaces can reduce secondary electron emission (SEE) and sputtering via geometric trapping. In this work, the SEE yields for a range of open-cell reticulated carbon foam geometries are characterized using scanning electron microscopy. The total reduction in the SEE yield from carbon foams with a 3% volume fill density and 10–100 pores per inch (PPI) is shown to be between 23.5% and 35.0%. Contributions of a foam backplate are assessed by experimentally and analytically defining the critical parameter, transparency. The transparency of a foam is quantified and is shown to affect the primary electron angular dependence on the SEE yield. For the same thickness of 6 mm, it is found that higher PPI decreases foam transparency from 32% to 0% and reduces the SEE yield. The SEE yield from carbon foams is also shown to have weaker dependence on the morphology of the surface compared with fuzzes and velvets and less variation across individual sample surfaces due to the rigidity of their ligament structures and isotropic geometries.

NM Uchizono, RE Wirz, AL Collins, C Marrese-Reading, SM Arestie, JK Ziemer, “A diagnostic for quantifying secondary species emission from electrospray devices,” Review of Scientific Instruments, 92(2) (2023); pdf

Measuring the polydisperse beam of charged species emitted by an electrospray device requires accurate measurements of current. Secondary species emission (SSE) caused by high velocity nanodroplet or molecular ion impacts on surfaces contributes to substantial uncertainty in current measurements. SSE consists of both positive and negative species, so mitigating measurement uncertainty requires different considerations than plasma diagnostic techniques. The probe and analysis methods described herein distinguish between current contributions from positive SSE, negative SSE, and primary species. Separating each contribution provides positive and negative SSE yield measurements, and corrected current measurements that reflect the true primary current. Sources of measurement uncertainty in probe design are discussed, along with appropriate mitigation methods. The probe and analysis technique are demonstrated on an ionic liquid electrospray operating in droplet emission mode to obtain an angular distribution of positive and negative SSE yields for an ionic liquid electrospray.

Mary F Konopliv, Vernon H Chaplin, Lee K Johnson, Richard E Wirz, “Accuracy of using metastable state measurements in laser-induced fluorescence diagnostics of xenon ion velocity in Hall thrusters,” Plasma Sources Science and Technology, Volume 32 (2023); pdf

Laser-induced fluorescence measurements of singly-charged xenon ion velocities in Hall thrusters typically target metastable states due to lack of available laser technology for exciting the ground state. The measured velocity distribution of these metastable ions are assumed to reflect the ground state ion behavior. However, this assumption has not been experimentally verified. To investigate the accuracy of this assumption, a recently developed xenon ion (Xe II) collisional-radiative model is combined with a 1D fluid model for ions, using plasma parameters from higher fidelity simulations of each thruster, to calculate the metastable and ground state ion velocities as a function of position along the channel centerline. For the HERMeS and SPT-100 thruster channel centerlines, differences up to 0.5 km s−1 were observed between the metastable and ground state ion velocities. For the HERMeS thruster, the difference between the metastable and ground state velocities is less than 150 m s−1 within one channel length of the channel exit, but increases thereafter due to charge exchange (CEX) that reduces the mean velocity of the ground state ions. While both the ground state ions and metastable state ions experience the same acceleration by the electric field, these small velocity differences arise because ionization and CEX directly into these states from the slower neutral ground state can reduce their mean velocities by different amounts. Therefore, the velocity discrepancy may be larger for thrusters with lower propellant utilization efficiency and higher neutral density. For example, differences up to 1.7 km s−1 were calculated on the HET-P70 thruster channel centerline. Note that although the creation of slow ions can influence the mean velocity, the most probable velocity should be unaffected by these processes. Keywords: metastable, diagnostics, electric prop

Uchizono, N. M., Marrese-Reading, C., Arestie, S. M., Collins, A. L., Ziemer, J. K., & Wirz, R. E., “Positive and negative secondary species emission behavior for an ionic liquid electrospray,” Applied Physics Letters, 121(7) (2022); https://doi.org/10.1063/5.0102592

Ionic liquid electrosprays can emit a polydisperse population of charged droplets, clusters, and molecular ions at high velocity. Secondary species emission (SSE) is a term that encompasses the many concurrent impact and emission phenomena that occur when electrosprayed primary species strike a surface, resulting in a diverse population of secondary electrons, ions, clusters, and droplets. This letter examines the spatial dependency of SSE behavior across an [EMI]Im electrospray beam using microscopy of the target surface, and experimental quantification of SSE yields as a function of plume angle. Microscopy of the beam target confirms our prediction of shock-induced desorption when operating at elevated beam voltages. SSE yield measurements show that, upon impact with a surface, incident primary species that consist of entirely positive charge will produce both positive and negative SSE. Furthermore, results show that the SSE yields for an ionic liquid electrospray have strong spatial and energy dependencies. These findings have significant implications for understanding and predicting ionic liquid electrospray thruster lifetime and performance, and focused ion beam applications.

Uchizono N.M., Collins A.L, Marrese-Reading C., Arestie S.M., Ziemer J.K., Wirz R.E., “The Role of Secondary Species Emission in Vacuum Facility Effects for Electrospray Thrusters,” J. Applied Physics 130, 143301 (2021); https://doi.org/10.1063/5.0063476

Theoretical, analytical, and experimental investigations of electrospray operation in vacuum facilities show that secondary species emission (SSE) plays a significant role in the behavior of electrospray thrusters during ground testing. A review of SSE mechanisms, along with an analysis of onset thresholds for electrospray thruster conditions, indicates that secondary species (e.g., electrons, anions, cations, etc.) must be carefully considered for accurate measurements and determination of performance and life. Presented models and experiments show that SSE-induced thruster-to-facility coupling can lead to considerable measurement uncertainty but can be effectively mitigated with an appropriate beam target design. The Electrospray SSE Control-volume Analysis for Resolving Ground Operation of Thrusters model is applied to experimental data to analyze SSE behavior. A heat and mass flux analysis of the Air Force Electrospray Thruster Series 2 (AFET-2) shows that SSE-induced Ohmic dissipation can cause performance limitations in ionic liquid ion source thrusters. The presented analytical models show that backstreaming current density contributing to less than 0.1% of measured emitter current density can cause substantial variation in propellant properties. Additionally, backstreaming current density contributing to less than 3% of emitted current can cause the 0.86ugs^-1 neutral loss rate estimated during AFET-2 testing. Arguments are presented to support the notion that glow discharges observed in electrospray thrusters during vacuum operation are a consequence of secondary species backstreaming to the emission site, rather than a process intrinsically caused by ion evaporation. Recommendations for general best practices to minimize the effects of SSE on electrospray thruster operation are provided.

Ottaviano A., Thuppul A., Hayes J., Dodson C., Li G., Chen Z., and Wirz, R.E., “In-situ Microscopy of Ion-Inducted Sputter Erosion of a Featured Surface,” Review of Scientific Instruments, 92, 073701 (2021); https://doi.org/10.1063/5.0043002

A novel method for the in situ visualization and profilometry of a plasma-facing surface is demonstrated using a long-distance microscope. The technique provides valuable in situ monitoring of the microscopic temporal and morphological evolution of a material surface subject to plasma–surface interactions, such as ion-induced sputter erosion. Focus variation of image stacks enables height surface profilometry, which allows a depth of field beyond the limits associated with high magnification. As a demonstration of this capability, the erosion of a volumetrically featured aluminum foam is quantified during ion-bombardment in a low-temperature argon plasma where the electron temperature is ∼7 eV and the plasma is biased relative to the target surface such that ions impinge at ∼300 eV. Three-dimensional height maps are reconstructed from the images captured with a long-distance microscope with an x–y resolution of 3 × 3 μm² and a focus-variation resolution based on the motor step-size of 20 μm. The time-resolved height maps show a total surface recession of 730 μm and significant ligament thinning over the course of 330 min of plasma exposure. This technique can be used for developing plasma-facing components for a wide range of plasma devices for applications such as propulsion, manufacturing, hypersonics, and fusion.

Li G., Wirz R.E., “Persistent Sputtering Yield Reduction in Plasma-Infused Foams,” Physical Review Letters, 126 (3), 035001 (2021); https://doi.org/10.1103/PhysRevLett.126.035001

Aluminum microfoams are found to exhibit persistent sputtering yield reductions of 40%–80% compared to a flat aluminum surface under 100 to 300 eV argon plasma bombardment. An analytical model reveals a strong dependency of the yield on the foam geometry and plasma sheath. For foam pore sizes near or larger than the sheath thickness, the plasma infuses the foam and transitions the plasma-surface interactions from superficial to volumetric phenomena. By defining a plasma infusion parameter, the sputtering behavior of foams is shown to be separated into the plasma-facing and plasma-infused regimes. While plasma infusion leads to a larger effective sputtering area, geometric recapture of ejected particles facilitates an overall reduction in yield. For a given level of plasma infusion, the reductions in normalized yield are more pronounced at lower ion energies since angular sputtering effects enable more effective geometric recapture of sputterants.

Magnusson J.M., Collins A.L., Wirz R.E., “Polyatomic Ion-Induced Electron Emission (IIEE) in Electrospray Thrusters”, Aerospace, Special Issue: Electric Propulsion, 2020, 7(11), 153, https://doi.org/10.3390/aerospace7110153

To better characterize the lifetime and performance of electrospray thrusters, electron emission due to electrode impingement by the propellant cation 1-ethyl-3-methylimidazolium (EMI+) has been evaluated with semi-empirical modeling techniques. Results demonstrate that electron emission due to grid impingement by EMI+ cations becomes significant once EMI+ attains a threshold velocity of ∼9×10⁵ cm s⁻¹. The mean secondary electron yield, 𝛾, exhibits strong linearity with respect to EMI+ velocity for typical electrospray operating regimes, and we present a simple linear fit equation corresponding to thruster potentials greater than 1 kV. The model chosen for our analysis was shown to be the most appropriate for molecular ion bombardments and is a useful tool in estimating IIEE yields in electrospray devices for molecular ion masses less than ∼1000 u and velocities greater than ∼10⁶ cm s⁻¹. Droplet-induced electron emission (DIEE) in electrospray thrusters was considered by treating a droplet as a macro-ion, with low charge-to-mass ratio, impacting a solid surface. This approach appears to oversimplify back-spray phenomena, meaning a more complex analysis is required. While semi-empirical models of IIEE, and the decades of solid state theory they are based upon, represent an invaluable advance in understanding secondary electron emission in electrospray devices, further progress would be gained by investigating the complex surfaces the electrodes acquire over their lifetimes and considering other possible emission processes.

Uchizono N.M., Samples S.A., Wirz R.E., "Tunable Reflectionless Absorption of Electromagnetic Waves in a Plasma–Metamaterial Composite Structure", Plasma Sources Sci. Technol., 2020, 29, 085009, https://doi.org/10.1088/1361-6595/aba489, pdf

We present the first experimental demonstration of a tunable reflectionless absorption resonance in a metamaterial integrated with a plasma discharge. A one-dimensional metamaterial structure excites transverse magnetic slow-wave modes known as 'spoof' surface plasmon polaritons. When interfaced with an argon plasma discharge, the metamaterial-induced 'spoof' plasmon mode is converted to a plasmon polariton mode confined to the plasma/dielectric interface. The reflectionless absorption band that manifests in the metamaterial's spectral response exhibits a dependency on the plasma's electron density that agrees well with theory.

Huerta C.E., Wirz R.E., “Ion-induced electron emission reduction via complex surface trapping,” AIP Advances, Vol. 9, Issue 12, (Editor’s Pick, Featured on Cover) 2019, https://doi.org/10.1063/1.5120519

abstract

Patino M.I., Wirz R.E., Raitses Y., Koel B.E., "Angular, Temperature, and Impurity Effects on Secondary Electron Emission from Ni(110)", Journal of Applied Physics, Editor’s Pick, 2018, https://doi.org/10.1063/1.5025344

The secondary electron emission from a temperature-controlled Ni(110) sample was examined for 50–1500 eV electrons impacting at 0°–35°, 50°, and 78°. Measurements showed a non-cosine dependence on an electron incidence angle: the yield has a maximum at 0°, minima at ±12°, and increases at larger angles up to 35°. This trend in angular dependence is characteristic of single crystal materials and is due to increased secondary electron generation when primary electrons are directed along a close-packed direction. For example, compared to polycrystalline nickel, the yield for Ni(110) from primary electrons at 0° (i.e., along the [110] direction) is up to 36% larger. Additionally, secondary electron yields are highly sensitive to incident electron energy (most notably between 0 and 500 eV) and to the presence of adsorbed carbon monoxide [with an up to 25% decrease compared to clean Ni(110)]. However, yields are independent of sample temperature between 300 and 600 K and of exposure to deuterium ions leading to formation of subsurface hydrogen. These results reaffirm the unique secondary electron emission properties of single crystals materials and highlight the importance of crystal orientation. Results are important for plasma-enhanced chemistry applications that utilize Ni(110) catalysts, since larger secondary electron emission may facilitate reactions of adsorbed species.

Patino M.I., Wirz R.E., "Characterization of Xenon Ion and Neutral Interactions in a Well-Characterized Experiment", Physics of Plasma, Vol. 25, 2018, 062108, https://doi.org/10.1063/1.5030464

Interactions between fast ions and slow neutral atoms are commonly dominated by charge-exchange and momentum-exchange collisions, which are important to understanding and simulating the performance and behavior of many plasma devices. To investigate these interactions, this work developed a simple, well-characterized experiment that accurately measures the behavior of high energy xenon ions incident on a background of xenon neutral atoms. By using well-defined operating conditions and a simple geometry, these results serve as canonical data for the development and validation of plasma models and models of neutral beam sources that need to ensure accurate treatment of angular scattering distributions of charge-exchange and momentum-exchange ions and neutrals. The energies used in this study are relevant for electric propulsion devices ∼1.5 keV and can be used to improve models of ion-neutral interactions in the plume. By comparing these results to both analytical and computational models of ion-neutral interactions, we discovered the importance of (1) accurately treating the differential cross-sections for momentum-exchange and charge-exchange collisions over a large range of neutral background pressures and (2) properly considering commonly overlooked interactions, such as ion-induced electron emission from nearby surfaces and neutral-neutral ionization collisions.

Li G.Z., Matlock T.S., Goebel D.M., Dodson C.A., Matthes C.S., Ghoniem N.M., Wirz R.E., "In situ plasma sputtering and angular distribution measurements for structured molybdenum surfaces", Plasma Sources Sci. Technol., Vol. 26, 065002, April 2017, https://doi.org/10.1088/1361-6595/aa6a7d

We present in situ sputtering yield measurements of the time-dependent erosion of flat and micro-architectured molybdenum samples in a plasma environment. The measurements are performed using the plasma interactions (Pi) Facility at UCLA, which focuses a magnetized hollow cathode plasma to a material target with an exposure diameter of approximately 1.5 cm. During plasma exposure, a scanning quartz crystal microbalance (QCM) provides angular sputtering profiles that are integrated to estimate the total sputtering yield. This technique is validated to within the scatter of previous experimental data for a planar molybdenum target exposed to argon ion energies from 100 to 300 eV. The QCM is then used to obtain in situ measurements during a 17 h exposure of a micro-architectured-surface molybdenum sample to 300 eV incident argon ions. The time-dependent angular sputtering profile is shown to deviate from classical planar profiles, demonstrating the unique temporal and spatial sputtering effects of micro-architectured materials. Notably, the sputtering yield for the micro-architectured sample is initially much less than that for planar molybdenum, but then gradually asymptotes to the value for planar molybdenum after approximately 10 h as the surface features are eroded away.

Matthes C.S., Ghoniem N.M., Li G.Z., Matlock T.S., Goebel D.M., Dodson C.A., Wirz R.E., "Fluence-Dependent Sputtering Yield of Micro-architectured Materials", Applied Surface Science, Vol. 407, pp. 223-235, June 2017, https://doi.org/10.1016/j.apsusc.2017.02.140

We present an experimental examination of the relationship between the surface morphology of Mo and its instantaneous sputtering rate as function of low-energy plasma ion fluence. We quantify the dynamic evolution of nano/micro features of surfaces with built-in architecture, and the corresponding variation in the sputtering yield. Ballistic deposition of sputtered atoms as a result of geometric re-trapping is observed, and re-growth of surface layers is confirmed. This provides a self-healing mechanism of micro-architectured surfaces during plasma exposure. A variety of material characterization techniques are used to show that the sputtering yield is not a fundamental property, but that it is quantitatively related to the initial surface architecture and to its subsequent evolution. The sputtering yield of textured molybdenum samples exposed to 300 eV Ar plasma is roughly 1/2 of the corresponding value for flat samples, and increases with ion fluence. Mo samples exhibited a sputtering yield initially as low as 0.22 ± 5%, converging to 0.4 ± 5% at high fluence. The sputtering yield exhibits a transient behavior as function of the integrated ion fluence, reaching a steady-state value that is independent of initial surface conditions. A phenomenological model is proposed to explain the observed transient sputtering phenomenon, and to show that the saturation fluence is solely determined by the initial surface roughness.

Rivera D., Wirz R.E., Ghoniem N.M., "Experimental measurements of surface damage and residual stresses in micro-engineered plasma facing materials", Journal of Nuclear Materials, Vol. 486, pp. 111-121, April 2017, http://dx.doi.org/10.1016%2Fj.jnucmat.2016.12.035

The thermomechanical damage and residual stresses in plasma-facing materials operating at high heat flux are experimentally investigated. Materials with micro-surfaces are found to be more resilient, when exposed to cyclic high heat flux generated by an arc-jet plasma. An experimental facility, dedicated to High Energy Flux Testing (HEFTY), is developed for testing cyclic heat flux in excess of 10 MW/m2. We show that plastic deformation and subsequent fracture of the surface can be controlled by sample cooling. We demonstrate that W surfaces with micro-pillar type surface architecture have significantly reduced residual thermal stresses after plasma exposure, as compared to those with flat surfaces. X-ray diffraction (XRD) spectra of the W-(110) peak reveal that broadening of the Full Width at Half Maximum (FWHM) for micro-engineered samples is substantially smaller than corresponding flat surfaces. Spectral shifts of XRD signals indicate that residual stresses due to plasma exposure of micro-engineered surfaces build up in the first few cycles of exposure. Subsequent cyclic plasma heat loading is shown to anneal out most of the built-up residual stresses in micro-engineered surfaces. These findings are consistent with relaxation of residual thermal stresses in surfaces with micro-engineered features. The initial residual stress state of highly polished flat W samples is compressive (-1.3 GPa). After exposure to 50 plasma cycles, the surface stress relaxes to −1.0 GPa. Micro-engineered samples exposed to the same thermal cycling show that the initial residual stress state is compressive at (-250 MPa), and remains largely unchanged after plasma exposure.

Huerta C.E., Matlock T.S., Wirz R.E., "View Factor Modeling of Sputter-Deposition on Micron-Scale-Architectured Surfaces Exposed to Plasma", Journal of Applied Physics, Vol. 119, 113303, March 2016, https://aip.scitation.org/doi/10.1063/1.4944035

The sputter-deposition on surfaces exposed to plasma plays an important role in the erosion behavior and overall performance of a wide range of plasma devices. Plasma models in the low density, low energy plasma regime typically neglect micron-scale surface feature effects on the net sputter yield and erosion rate. The model discussed in this paper captures such surface architecture effects via a computationally efficient view factor model. The model compares well with experimental measurements of argon ion sputter yield from a nickel surface with a triangle wave geometry with peak heights in the hundreds of microns range. Further analysis with the model shows that increasing the surface pitch angle beyond about 45° can lead to significant decreases in the normalized net sputter yield for all simulated ion incident energies (i.e., 75, 100, 200, and 400 eV) for both smooth and roughened surfaces. At higher incident energies, smooth triangular surfaces exhibit a nonmonotonic trend in the normalized net sputter yield with surface pitch angle with a maximum yield above unity over a range of intermediate angles. The resulting increased erosion rate occurs because increased sputter yield due to the local ion incidence angle outweighs increased deposition due to the sputterant angular distribution. The model also compares well with experimentally observed radial expansion of protuberances (measuring tens of microns) in a nano-rod field exposed to an argon beam. The model captures the coalescence of sputterants at the protuberance sites and accurately illustrates the structure's expansion due to deposition from surrounding sputtering surfaces; these capabilities will be used for future studies into more complex surface architectures.

Patino M.I., Raitses Y., Koel B.E., Wirz R.E., "Analysis of Secondary Electron Emission for Conducting Materials Using 4-grid LEED/AES Optics", Journal of Physics D: Applied Physics, Vol. 48, 195204, April 2015, https://doi.org/10.1088/0022-3727/48/19/195204

A facility utilizing 4-grid optics for LEED/AES (low energy electron diffraction/Auger electron spectroscopy) was developed to measure the total secondary electron yield and secondary electron energy distribution function for conducting materials. The facility and experimental procedure were validated with measurements of 50–500 eV primary electrons impacting graphite. The total yield was calculated from measurements of the secondary electron current (i) from the sample and (ii) from the collection assembly, by biasing each surface. Secondary electron yield results from both methods agreed well with each other and were within the spread of previous results for the total yield from graphite. Additionally, measurements of the energy distribution function of secondary electrons from graphite are provided for a wider range of incident electron energies. These results can be used in modeling plasma-wall interactions in plasmas bounded by graphite walls, such as are found in plasma thrusters, and divertors and limiters of magnetic fusion devices.

Matlock T.S., Goebel D.M., Conversano R., Wirz R.E., "A DC Plasma Source for Plasma–Material Interaction Experiments", Plasma Sources Science and Technology, Vol. 23, 025014, March 2014, https://doi.org/10.1088/0963-0252/23/2/025014

A new device has been constructed for the investigation of interactions between engineered materials and a plasma in regimes relevant to electric propulsion and pulsed power devices. A linear plasma source, consisting of a hollow cathode, cylindrical anode, and axial magnetic field, delivers a 3 cm diameter beam to a biased target 70 cm away. The ion energy impacting the surface is controlled by biasing the sample from 0 to 500 V below the local plasma potential. This paper discusses the major aspects of the plasma source design and presents measurements of the plasma parameters achieved to date on argon and xenon. Experiments show that splitting the gas injection between the hollow cathode and the anode region provides control of the discharge voltage to minimize cathode sputtering while providing ion fluxes to the target in excess of 1021 m−2 s−1. Sputtering rate measurements on a non-textured molybdenum sample show close agreement with those established in the literature.

Araki S.J., Wirz R.E., "Ion–Neutral Collision Modeling Using Classical Scattering with Spin-Orbit Free Interaction Potential", IEEE Transactions on Plasma Science, Vol. 41(3), pp. 470-480, Feb. 2013, https://doi.org/10.1109/TPS.2013.2241457

A particle-in-cell Monte Carlo collision model is developed to explore dominant collisional effects on high-velocity xenon ions incident to a quiescent xenon gas at low neutral pressures. The range of neutral pressure and collisionality examined are applicable for electric propulsion as well as plasma processing devices; therefore, the computational technique described herein can be applied to more complex simulations of those devices. Momentum and resonant charge-exchange collisions between ions and background neutrals are implemented using two different models, classical scattering with spin-orbit free potential and variable-hard-sphere model, depending on the incident particle energy. The primary and charge-exchange ions are tracked separately, and their trajectories within a well-defined “Test Cell” domain are determined. Predicted electrode currents as a function of the Test Cell pressure are compared with electrode currents measured in an ion gun experiment. The simulation results agree well with the experiment up to a Test Cell pressure corresponding to a mean free path of the Test Cell length and then start to deviate with increasing collisionality at higher pressures. This discrepancy at higher pressures is likely due to the increasing influence of secondary electrons emitted from electrodes due to the high-velocity impacts of heavy species (i.e., beam ions and fast neutrals created by charge-exchange interaction) at the electrode surfaces.

Wirz R.E., Anderson J., Katz I., "Time-Dependent Erosion of Ion Optics", Journal of Propulsion and Power, Vol. 27(1), pp. 211-217, Feb. 2011, https://arc.aiaa.org/doi/pdf/10.2514/1.46845

The accurate prediction of ion thruster life requires time-dependent erosion estimates for the ion optics assembly. Such information is critical to end-of-life mechanisms such as electron backstreaming. A two-dimensional ion optics code, CEX2D, was recently modified to handle time-dependent erosion, double ions, and multiple throttle conditions in a single run. The modified code is called CEX2D-T. Comparisons of CEX2D-T results with the NASA solar electric propulsion technology application readiness (NSTAR) thruster life demonstration test and extended life test results show good agreement for both screen and accelerator grid erosion, including important erosion features such as chamfering of the downstream end of the accelerator grid and reduced rate of accelerator grid aperture enlargement with time. The influence of double ions on grid erosion proved to be important for simulating the erosion observed during the NSTAR life demonstration test and extended life test.

Wirz R.E., Katz I., Goebel D., Anderson J., "Electron Backstreaming Determination for Ion Thrusters", Journal of Propulsion and Power, Vol. 27(1), pp. 206-210, Feb. 2011, https://arc.aiaa.org/doi/pdf/10.2514/1.46844

Electron backstreaming in ion thrusters is caused by the random flux of beam electrons past a potential barrier established by the accelerator grid. A technique that integrates this flux over the radial extent of the barrier reveals important aspects of electron backstreaming phenomena for individual beamlets, across the thruster beam, and throughout thruster life. For individual beamlets it was found that over 99% of the electron backstreaming occurs in a small area at the center of the beamlet that is less than 20% the area of the beamlet at the potential barrier established by the accelerator grid. For the thruster beam it was found that over 99% of the backstreaming current occurs inside of r = 6 cm for the over 28 cm diameter NSTAR grid. Initial validation against extended life test data for the NSTAR thruster shows that the technique provides the correct behavior and magnitude of electron backstreaming limit, 𝑉 𝑏 𝑠 V bs ​ . From the sensitivity analyses it is apparent that accelerator grid chamfering due to sputter erosion contributes significantly to the sharp rise in electron backstreaming limit observed in the extended life test, but does not explain the rise in grid ion transparency. Reduction of the grid gap over the life of the thruster also contributes to increases in electron backstreaming limit and increases in ion transparency. Screen grid erosion contributes generally to rises in 𝑉 𝑏 𝑠 V bs ​ and grid ion transparency, but for the assumptions used herein, it appears to not have as much of an effect as chamfering or grid gap change. Overall, it is apparent that accelerator grid chamfering, grid gap change, and screen grid erosion are important to the increase in electron backstreaming observed during the NSTAR extended life test.

Wirz R.E., Anderson J.R., Goebel D.M., Katz I., "Decel Grid Effects on Ion Thruster Grid Erosion", IEEE Transactions on Plasma Science, Vol. 36(5), pp. 2122-2129, Oct. 2008, https://doi.org/10.1109/TPS.2008.2001041

Jet Propulsion Laboratory (JPL) is currently assessing the applicability of the 25-cm Xenon Ion Propulsion System (XIPS) as part of an effort to infuse low-cost technically mature commercial ion thruster systems into NASA deep space missions. Since these mission require extremely long thruster lifetimes to attain the required mission DeltaV, this paper is focused on understanding the dominate wear mechanisms that effect the life of the XIPS three-grid system. Analysis of the XIPS three-grid configuration with JPL's CEX3D grid erosion model shows that the third ldquodecelrdquo grid effectively protects the accel grid from pits and grooves erosion that is commonly seen with two-grid ion thruster grid systems. For a three-grid system, many of the charge-exchange ions created downstream of the grid plane will impact the decel grid at relatively low energies ( ~25 V), instead of impacting the accel grid at high energies ( ~200 V), thus reducing overall erosion. JPL's CEX3D accurately predicts the erosion patterns for the accel grid, although it appears to overpredict the pits and grooves erosion rates due, mainly, to uncertainties in incident energies and angles for sputtering ions and since it does not account for local redeposition of sputtered material. Since the model accurately simulates the erosion pattern but tends to overpredict the erosion rates for both the two- and three-grid sets, this comparative analysis clearly shows how the decel grid significantly suppresses the erosion of the downstream surface of the accel grid as observed in experimental tests. The results also show that the decel grid has a relatively small effect on barrel erosion (erosion of the aperture wall) and erosion of the upstream surface of the accel grid. Decreasing the accel grid voltage of the XIPS can reduce barrel (and total) erosion of the accel grid and should be considered for high-DeltaV missions.