Wirz Research Lab - Publications

Journal Publications

2024

Brian Na, Blake Haist, Shilp R. Shah, Graeme Sabiston, Steven J. Jonas, Jeremie Vitte, Richard E. Wirz, Marco Giovannini “Cold Atmospheric Plasma Induces Growth Arrest and Apoptosis in Neurofibromatosis Type 1-Associated Peripheral Nerve Sheath Tumor Cells,” Biomedicines (2024); https://doi.org/10.3390/biomedicines12091986, PDF

Neurofibromatosis type 1 (NF1) is an autosomal dominant disorder resulting from mutations in the NF1 gene. Patients harboring these mutations are predisposed to a spectrum of peripheral nerve sheath tumors (PNSTs) originating from Schwann cells, of which malignant peripheral nerve sheath tumors (MPNSTs) are the deadliest, with limited treatment options. Therefore, an unmet need still exists for more effective therapies directed at these aggressive malignancies. Cold atmospheric plasma (CAP) is a reactive oxygen species (ROS) and reactive nitrogen species (RNS) generating ionized gas that has been proposed to be a potential therapeutic modality for cancer. In this study, we sought to determine the effects of CAP on NF1-associated PNSTs. Utilizing established mouse and human cell lines to interrogate the effects of CAP in both in vitro and in vivo settings, we found that NF1-associated PNSTs were highly sensitive to CAP exposure, resulting in cell death. To our knowledge, this is the first application of CAP to NF1-associated PNSTs and provides a unique opportunity to study the complex biology of NF1-associated tumors.

Peter L. Wright, Richard E. Wirz, “Transient Flow in Porous Electrosprays,” Transp Porous Med (2024); https://doi.org/10.1007/s11242-024-02113-9, PDF

Porous ionic electrospray emitters have received significant interest for space propulsion due to their performance and operational simplicity. We have developed a diffusion equation for describing the transient flow response in a porous electrospray emitter, which allows for the prediction of the settling time for flow in the porous emitter. This equation accounts for both the change in liquid storage at exposed pores on the emitter with pressure and viscous diffusion through Darcy’s law. Transient flow solutions are provided for the most common emitter topologies: pillar, cone, and wedge. Transient flow solutions describe the settling time and magnitude of current overshoot from porous electrosprays, while providing useful guidelines for reducing transient response time through emitter design. Comparing diffusion of pressure to the onset delay model for electrospray emission shows that diffusion is most relevant at higher voltages and when a porous reservoir is used. Accounting for multiple emission sites on the wedge geometry shows that emission sites settle in proportion to emission site spacing to the power − 1.74.

Graeme Sabiston, Richard E Wirz “Ion–surface interactions in plasma-facing material design,” Journal of Applied Physics 135 (2024); https://doi.org/10.1063/5.0201758

A multi-scale simulation framework for ion–solid interactions in plasma-exposed materials provides crucial insight into advancing fusion energy and space electric propulsion. Leveraging binary-collision approximation (BCA) simulations, the framework uniquely predicts sputter yields and analyzes material transport within volumetrically complex materials. This approach, grounded in the validated BCA code TRI3DYN, addresses key limitations in existing models by accurately capturing ion–solid interaction physics. A case study is presented, highlighting the framework’s ability to replicate experimental sputter yield results, underscoring its reliability and potential for designing durable materials in harsh plasma environments. Insights into sputtering transport phenomenology mark a significant advancement in material optimization for improved resilience in plasma-facing applications.

McKenna JD Breddan, Richard E Wirz “Machine learning electrospray plume dynamics,” Engineering Applications of Artificial Intelligence 133 (2024); https://doi.org/10.1016/j.engappai.2024.108095

Machine learning models are applied to simulated electrospray particle data to investigate plume dynamics from emission to final particle properties. A limited set of final particle properties are successfully regressed exclusively from emission property inputs. Random Forest model feature rankings for final plume angle reveal that particle charge has dominant influence when emission velocity is strictly axial, while lateral emission velocity has dominant influence when particles are emitted with an off-axis velocity component. In addition to providing correlations between initial and final particle properties, the machine learning models also identify correlations between different final particle properties. These correlations reveal opportunities for experimental approaches and diagnostic design by determining experimental measurements that offer insight into desired final particle properties.

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.

2023

Shehan M Parmar, Daniel D Depew, Richard E Wirz, Ghanshyam L Vaghjiani, “Structural Properties of HEHN-and HAN-Based Ionic Liquid Mixtures: A Polarizable Molecular Dynamics Study,” The Journal of Physical Chemistry B 170(40) (2023); https://doi.org/10.1021/acs.jpcb.3c02649

Molecular dynamics simulations of binary mixtures comprising 2-hydroxyethylhydrazinium nitrate (HEHN) and hydroxylammonium nitrate (HAN) were conducted using the polarizable APPLE&P force field to investigate fundamental properties of multimode propulsion (MMP) propellants. Calculated densities as a function of temperature were in good agreement with experiments and similar simulations. The structural properties of neat HEHN and HAN–HEHN provided insights into their inherent, protic nature. Radial distribution functions (RDFs) identified key hydrogen bonding sites located at N–H···O and O–H···O within a first solvation shell of approximately 2 Å. Angular distribution functions further affirmed the relatively strong nature of the hydrogen bonds with nearly linear directionality. The increased hydroxylammonium cation (HA+) mole fraction shows the influence of competitively strong hydrogen bonds on the overall hydrogen bond network. Dominant spatial motifs via three-dimensional distribution functions along with nearly nanosecond-long hydrogen bond lifetimes highlight the local bonding environment that may precede proton transfer reactions.

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 …

Angelopoulos, V., Zhang, XJ., Artemyev, A.V. et al. “Energetic Electron Precipitation Driven by Electromagnetic Ion Cyclotron Waves from ELFIN’s Low Altitude Perspective,” Space Sci Rev 219, 37 (2023); https://doi.org/10.1007/s11214-023-00984-w

We review comprehensive observations of electromagnetic ion cyclotron (EMIC) wave-driven energetic electron precipitation using data collected by the energetic electron detector on the Electron Losses and Fields InvestigatioN (ELFIN) mission, two polar-orbiting low-altitude spinning CubeSats, measuring 50-5000 keV electrons with good pitch-angle and energy resolution. EMIC wave-driven precipitation exhibits a distinct signature in energy-spectrograms of the precipitating-to-trapped flux ratio: peaks at >0.5 MeV which are abrupt (bursty) (lasting ∼17 s, or ) with significant substructure (occasionally down to sub-second timescale). We attribute the bursty nature of the precipitation to the spatial extent and structuredness of the wave field at the equator. Multiple ELFIN passes over the same MLT sector allow us to study the spatial and temporal evolution of the EMIC wave - electron interaction region. Case studies employing conjugate ground-based or equatorial observations of the EMIC waves reveal that the energy of moderate and strong precipitation at ELFIN approximately agrees with theoretical expectations for cyclotron resonant interactions in a cold plasma. Using multiple years of ELFIN data uniformly distributed in local time, we assemble a statistical database of ∼50 events of strong EMIC wave-driven precipitation. Most reside at at dusk, while a smaller subset exists at at post-midnight. The energies of the peak-precipitation ratio and of the half-peak precipitation ratio (our proxy for the minimum resonance energy) exhibit an -shell dependence in good agreement with theoretical estimates based on prior statistical observations of EMIC wave power spectra. The precipitation ratio’s spectral shape for the most intense events has an exponential falloff away from the peak (i.e., on either side of MeV). It too agrees well with quasi-linear diffusion theory based on prior statistics of wave spectra. It should be noted though that this diffusive treatment likely includes effects from nonlinear resonant interactions (especially at high energies) and nonresonant effects from sharp wave packet edges (at low energies). Sub-MeV electron precipitation observed concurrently with strong EMIC wave-driven >1 MeV precipitation has a spectral shape that is consistent with efficient pitch-angle scattering down to ∼ 200-300 keV by much less intense higher frequency EMIC waves at dusk (where such waves are most frequent). At ∼100 keV, whistler-mode chorus may be implicated in concurrent precipitation. These results confirm the critical role of EMIC waves in driving relativistic electron losses. Nonlinear effects may abound and require further investigation.

Zhitong Chen, Richard Obenchain, Richard E Wirz, “Cold plasma treatment for biomedical applications: using aluminum foam to reduce risk while increasing efficacy,” arXiv preprint arXiv:2305.18349 (2023); pdf

Plasma medicine is an emerging and innovative interdisciplinary research field combining biology, chemistry, physics, engineering, and medicine. However, the safe clinical application of cold atmospheric plasma (CAP) technology is still a challenge. Here, we examine the use of aluminum (Al) foam with three pores-per-inch (PPI) ratings in clinical plasma applications. Al foams can filter sparks to avoid damage from high voltage discharge during surgery and efficiently deliver reactive species generated in CAP to the target. The sparks appear and plasma intensity increases at the foam/discharge interface, which just slightly increases the interface temperature without changing the interface microstructure during a 30-minute treatment. After CAP penetrated the Al foams, N2, N2+, *OH, O, and He emission peaks were characterized, and the highest values appeared using Al foams with 10 PPI. CAP with and without Al foam intermediating was used to treat deionized water, and the results indicate CAP in combination with 10 PPI Al foam led to much higher ROS concentration than CAP alone. For melanoma cell experiments, CAP with and without Al foam had a similar effect on cell viability after 30-second treatment, while CAP with the 10-PPI Al foam had much higher killing efficiency than CAP alone after 60-second treatment. In summary, 10-PPI Al foam can not only prevent damage to tissues resulting from high discharge voltage during clinical surgery but also increase the delivery efficiency of reactive species generated in plasma for biomedical applications.

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

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

2022

Collins, A.L., Wright, P.L., Uchizono, N.M. et al., “High angle mass flux of an electrospray plume,” J Electr Propuls 1, 32 (2022); https://doi.org/10.1007/s44205-022-00031-w

High-resolution mass flux measurements of an electrospray plume are reported at high (>30°) angles, which are relevant to direct impingement of downstream electrodes. Interrogation of the plume edge greatly reduces uncertainty in electrospray device lifetime estimation related to mass flux to electrode surfaces. An angularly-actuated Thermoelectric Quartz Crystal Microbalance (TQCM) provides resolution down to 2 pg cm² s⁻¹, allowing the highest resolution mass flux measurements of an electrospray plume to be reported herein. In-situ microscopy of the electrospray meniscus revealed changes to the electrode lines-of-sight of approximately 2°–3° due to the increasing meniscus tip height with beam current. Using the TQCM measurements and previous QCM results, a data-driven model is proposed for estimating electrode impingement as a function of beam current and aperture line-of-sight, which quantitatively captures the rapid increase in mass flux at higher beam currents or at low angles. The results show it is possible to guarantee negligible electrode impingement within a specified range of throttle levels.

Huh, H., & Wirz, R. E., “Simulation of electrospray emission processes for low to moderate conductivity liquids,” Physics of Fluids, 34(11) (2022); https://doi.org/10.1063/5.0120737

An electrohydrodynamic numerical model is used to explore the electrospray emission behavior of both moderate and high electrical conductivity liquids under electrospray conditions. The Volume-of-Fluid method, incorporating a leaky-dielectric model with a charge relaxation consideration, is used to conserve charge to accurately model cone-jet formation and droplet breakup. The model is validated against experiments and agrees well with both droplet diameters and charge-to-mass ratio of emitted progeny droplets. The model examines operating conditions such as flow rate and voltage, with fluid properties also considered, such as surface tension, electrical conductivity, and viscosity for both moderate and high conductivity. For high conductivity and surface tension, the results show that high charge concentration along with the meniscus and convex cone shape results in a higher charge-to-mass ratio of the emitted droplets while lower conductivity and surface tension tend towards concave cone shapes and lower charge-to-mass droplets. Recirculation flows inside the bulk liquid are investigated across a range of non-dimensional flow rates, δ, and electric Reynolds numbers, ReE. For high conductivity liquid emission at the minimum stable flow rate, additional recirculation cells develop near the cone tip suggesting the onset of the axisymmetric instability.

Wright, P., Wirz, R., “Transient Flow in Porous Electrosprays,” (2022); https://doi.org/10.21203/rs.3.rs-1865748/v1

Porous ionic electrospray emitters have received significant interest for space propulsion due to their performance and operational simplicity. We have developed a diffusion equation for describing the transient flow response in a porous electrospray emitter, which allows for the prediction of the settling time for flow in the porous emitter. This equation accounts for both the change in liquid storage at exposed pores on the emitter with pressure, and viscous diffusion through Darcy’s law. Transient flow solutions are provided for the most common emitter topologies: pillar, cone, and wedge. Transient flow solutions describe the settling time and magnitude of current overshoot from porous electrosprays. Comparing diffusion of pressure to the onset delay model for electrospray emission shows that diffusion is most relevant at higher voltages and when a porous reservoir is used. Accounting for multiple emission sites on the wedge geometry shows that emission sites settle in proportion to emission site spacing to the power -1.74. Applying the diffusion equation to published results shows good agreement between analytical predictions and experimental data.

Uchizono, N., Wright, P., Collins, A., et al., “Emission spectra of glows produced by ionic liquid ion sources,” Appl. Phys. Lett. 121, 154101 (2022); https://doi.org/10.1063/5.0096595

Electrospray devices, such as ionic liquid ion sources, often exhibit glows during operation in high vacuum facilities. The lack of electron excitation mechanisms during electrospray operation prompts the question: “What causes glow in an electrospray device?” Our optical emission spectroscopy results show that electrospray glow exhibits a broad spectral response between 350 and 800 nm with emission lines corresponding to atomic metal constituents of impinged surfaces, neutral and ionized atomic constituents of the ionic liquid propellant, and molecular line shapes that may also be dissociation products of the ionic liquid. We have previously defined secondary species emission to describe the many complex interfacial phenomena that occur when electrosprayed species impact surfaces downstream of the emitter. Our analysis of the optical emission spectra shows that these glows are only possible in the presence of secondary species emission. Therefore, the answer to the proposed question: high-velocity impacts that generate secondary species are the root cause of glow for electrosprays in high vacuum facilities.

Enomoto, T., Parmar, S.M., Yamada, R. et al., “Molecular Dynamics Simulations of Ion Extraction from Nanodroplets for Ionic Liquid Electrospray Thrusters,” J Electr Propuls 1, 13 (2022); https://doi.org/10.1007/s44205-022-00010-1

Molecular dynamics (MD) simulations were performed for ion extraction from electrospray thrusters to investigate relevant extraction processes numerically. To approximate the electrospray jet tip, a simulation domain consisting of 4-5 nm-sized ionic liquid droplets was used. The extracted ion angles and kinetic energies from EMI–BF4 (1-ethyl-3-methylimidazolium tetrafluoroborate) and EMI–Im (1-ethyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide) droplets were quantified by applying uniform electric fields of 1.3–1.7 V nm−1. The MD simulations are in great agreement with simulations presented in the literature and consistently show a greater preference for monomer emission than reported experimentally. At field strengths above 1.5 V nm−1, apparent droplet fracturing and breakup lead to an increase in ion angular velocity distributions. Greater mobility of EMI–BF4 ions than EMI–Im was also observed, indicative of the crucial role of cation-anion hydrogen bond strengths in ion extraction and beam composition between different propellants.

Crandall, P., Wirz, R.E., “Air-breathing electric propulsion: mission characterization and design analysis.,” J Electr Propuls 1, 12 (2022); https://doi.org/10.1007/s44205-022-00009-8

Air breathing electric propulsion (atmosphere-breathing electric propulsion) (ABEP) has attracted significant interest as an enabling technology for long duration space missions in very low Earth orbit (VLEO) altitudes below about 300 km. The ABEP spacecraft and mission analysis model developed allows parametric characterization of key spacecraft geometry and thruster performance parameters such as spacecraft length-to-diameter, the ratio of solar array span to spacecraft diameter, thrust-to-power, effective exhaust velocity, and inlet efficiency. For the missions analyzed ABEP generally outperforms conventional electric propulsion (EP) below 250 km altitude. Using a 6U spacecraft architecture the model shows that below 220 km ABEP is the only viable propulsion option for desirable mission lifetimes. Parametric evaluations of key spacecraft and ABEP characteristics show that the most significant technological improvements to ABEP spacecraft performance and range of applicability for VLEO missions will come from advancements in inlet efficiency, low drag materials, solar array efficiency, and thrust-to-power.

Chen, Z., Bai, F., Jonas, S. J., & Wirz, R. E.,“Cold atmospheric plasma for addressing the COVID‐19 pandemic,” Plasma Processes and Polymers, 19(9) (2022); https://doi.org/10.1002/ppap.202200012

The coronavirus disease 2019 (COVID‐19) pandemic has greatly stressed the global community, exposing vulnerabilities in the supply chains for disinfection materials, personal protective equipment, and medical resources worldwide. Disinfection methods based on cold atmospheric plasma (CAP) technologies offer an intriguing solution to many of these challenges because they are easily deployable and do not require resource‐constrained consumables or reagents needed for conventional decontamination practices. CAP technologies have shown great promise for a wide range of medical applications from wound healing and cancer treatment to sterilization methods to mitigate airborne and fomite transfer of viruses. This review engages the broader community of scientists and engineers that wish to help the medical community with the ongoing COVID‐19 pandemic by establishing methods to utilize broadly applicable CAP technologies.

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.

Chen, Z., Chen, G., Obenchain, R., Zhang, R., Bai, F., Fang, T., Wang, H., Lu, Y., Wirz, R. E., & Gu, Z., “Cold atmospheric plasma delivery for biomedical applications,” Materials Today, 54, 153–188 (2022); https://doi.org/10.1016/j.mattod.2022.03.001

As the fourth state of matter, plasma’s unique properties and interactions with other states of matter offer many promising opportunities for investigation and discovery. In particular, cold atmospheric plasma (CAP), operating at atmospheric pressure and room temperature, has remarkable potential for biomedical applications through various delivery methods. These biomedical applications include sterilization, wound healing, blood coagulation, oral/dental diseases treatment, cancer therapy, and immunotherapy. Effective delivery of plasma constituents is critical to its efficacy for these applications. Therefore, this review presents the key research activities related to CAP delivery (including direct CAP delivery, delivery of plasma-activated media, biomedical device-assisted plasma delivery, and CAP delivery with other therapeutics) and needs for future research. This review will be of great interest for understanding the current state-of-the-art of biomedical applications of plasma medicine while also giving researchers from a broad range of communities insight into research efforts that would benefit from their contributions. Such communities include biomedicine, physics, biochemistry, material science, nanotechnology, and medical device manufacturing.

2021

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.

Chen G., Chen Z., Wang Z., Obenchain R., Wen D., Li H., Wirz R.E., Gu Z., “Portable air-fed cold atmospheric plasma device for postsurgical cancer treatment ,” Science Advances, Vol 7, Issue 36 (2021); https://www.science.org/doi/10.1126/sciadv.abg5686

Surgery represents the major option for treating most solid tumors. Despite continuous improvements in surgical techniques, cancer recurrence after surgical resection remains the most common cause of treatment failure. Here, we report cold atmospheric plasma (CAP)–mediated postsurgical cancer treatment, using a portable air-fed CAP (aCAP) device. The aCAP device we developed uses the local ambient air as the source gas to generate cold plasma discharge with only joule energy level electrical input, thus providing a device that is simple and highly tunable for a wide range of biomedical applications. We demonstrate that local aCAP treatment on residual tumor cells at the surgical cavities effectively induces cancer immunogenic cell death in situ and evokes strong T cell–mediated immune responses to combat the residual tumor cells. In both 4T1 breast tumor and B16F10 melanoma models, aCAP treatment after incomplete tumor resection contributes to inhibiting tumor growth and prolonging survival.

Thuppul A., Collins A.L., Wright P.L., Uchizono N.M., Wirz R.E., “Mass Flux and Current Distributions of Electrospray Plumes,” J. Applied Physics 130, 103301 (2021); https://doi.org/10.1063/5.0056761

Performance and lifetime analysis of electrospray thrusters requires accurate knowledge of the mass and charge distributions of the plume. Mass flux and current density distributions were measured for a single capillary electrospray emitter using EMI-Im and found to be substantially different across a wide range of flow rates and emission voltages. Mass flux measurements yield an n~3 super-Gaussian profile across all flow rates and voltages, while current density measurements change shape from n~1.5-2.5 –super-Gaussian profiles monotonically with decreasing flow rate, where n=1 is Gaussian and higher n values correspond to increasingly more flat-top “super-Gaussian” profiles with steeper drop-off toward higher angles. For increasing flow rate, the mass flux profile grows while maintaining its shape, whereas the current density profile exhibits higher kurtosis, i.e., plumes that distribute proportionately more charge to higher angles. Additionally, higher extraction voltages exhibited tilted emission that led to highly off-axis plumes, ~10 degrees, for both mass flux and current density. Lifetime and performance assessments of electrospray thrusters must consider that mass flux and current density in the plume display different distribution shapes and trends in shape across changes in extraction voltage and flow rate.

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.

Chen Z., Obenchain R., Wirz R.E., “Tiny cold atmospheric plasma jet for biomedical applications,” Featured Article Processes, 9(2), 249 (2021); https://doi.org/10.3390/pr9020249

Conventional plasma jets for biomedical applications tend to have several drawbacks, such as high voltages, high gas delivery, large plasma probe volume, and the formation of discharge within the organ. Therefore, it is challenging to employ these jets inside a living organism’s body. Thus, we developed a single-electrode tiny plasma jet and evaluated its use for clinical biomedical applications. We investigated the effect of voltage input and flow rate on the jet length and studied the physical parameters of the plasma jet, including discharge voltage, average gas and subject temperature, and optical emissions via spectroscopy (OES). The interactions between the tiny plasma jet and five subjects (de-ionized (DI) water, metal, cardboard, pork belly, and pork muscle) were studied at distances of 10 mm and 15 mm from the jet nozzle. The results showed that the tiny plasma jet caused no damage or burning of tissues, and the ROS/RNS (reactive oxygen/nitrogen species) intensity increased when the distance was lowered from 15 mm to 10 mm. These initial observations establish the tiny plasma jet device as a potentially useful tool in clinical biomedical applications.

Wright P.L., Wirz R.E., “Multiplexed Electrospray Emission on a Porous Wedge,” Physics of Fluids 33, 012003 (2021); https://doi.org/10.1063/5.0030031

Linear porous wedge electrospray emitters exhibit a discrete number of emission sites that naturally form during operation. An analytical model is developed to examine the behavior and spacing of these emission sites via the pressure variation in the porous fluid flow associated with the flow focusing on each emission site, which is coupled with the local electric field. The solution for site spacing and current is informed by empirical results with support from electric field modeling and investigation of porous media parameters. Emission site currents of up to 500 nA and site spacings of roughly 50 µm–300 µm are predicted. Results from the model match well with experimental trends and provide further insights into the current and spacing of the discrete emission sites. These insights include the following: (1) for the investigated geometry, the total current can be estimated without taking into account the effects local to each emission site, (2) the wedge hydraulic resistance shows how the emitter output scales with emitter geometry and propellant properties, and (3) the emitted charge to mass ratio increases with the applied electric field. Last, we present a physical description of how specific charge increases with the restorative pressure from the reservoir.

Samples S.A., Wirz R.E., “Parametric Analysis of High Delta-V CubeSat Missions with a Miniature Ion Thruster,” Journal of Spacecraft and Rockets, Vol. 58, No. 3, 2021; https://doi.org/10.2514/1.A34827

The increasing capabilities of the CubeSat platform have led to growing interest in performing more complex commercial and science missions with these miniature spacecraft. In particular, electric propulsion provides unprecedented mission Δ𝑉 and enables ambitious but low-cost Earth missions as well as lunar, asteroid, and interplanetary exploration. As a case study, a 6 U (where U represents a 10×10×10cm “unit”) CubeSat using the Miniature Xenon Ion Thruster was designed for a notional 3000m/s Δ𝑉 mission with a 2 kg, 1.6 U payload, resulting in a spacecraft wet mass of 11.9 kg and a burn duration of 15 months. This spacecraft is capable of up to 5.8km/s Δ𝑉 for a 0.5 U payload, as well as 2.1km/s for a 2 U payload. Parametric analyses with generalized electric thruster properties show that mission performance is sensitive to thruster 𝐼𝑠𝑝 and total efficiency 𝜂𝑇, with a 10% increase in efficiency resulting in a 16% decrease in burn time. This decrease in burn time is also possible by decreasing 𝐼𝑠𝑝, but it incurs mass and payload volume penalties. Parametric studies of neutralizer cathode properties show that neutralizer cathode selection is critical, and that such cathodes should be designed to require low power, low to zero flow rate, and long life for high-Δ𝑉 missions.

2020

Chen Z., Garcia G., Arumugaswami V., Wirz R.E., “Cold atmospheric plasma for SARS-CoV-2 inactivation,” Featured Article Physics of Fluids 32, 111702 (2020); https://doi.org/10.1063/5.0031332

Syndrome coronavirus 2 (SARS-CoV-2) infectious virions are viable on various surfaces (e.g., plastic, metals, and cardboard) for several hours. This presents a transmission cycle for human infection that can be broken by developing new inactivation approaches. We employed an efficient cold atmospheric plasma (CAP) with argon feed gas to inactivate SARS-CoV-2 on various surfaces including plastic, metal, cardboard, basketball composite leather, football leather, and baseball leather. These results demonstrate the great potential of CAP as a safe and effective means to prevent virus transmission and infections for a wide range of surfaces that experience frequent human contact. Since this is the first-ever demonstration of cold plasma inactivation of SARS-CoV-2, it is a significant milestone in the prevention and treatment of coronavirus disease 2019 (COVID-19) and presents a new opportunity for the scientific, engineering, and medical communities.

Chen G., Chen Z., Wen, D., Wang, Z., Li, H., Zeng., Y., Dotti, G., Wirz, R., Gu, Z., “Transdermal cold atmospheric plasma-mediated immune checkpoint blockade therapy,” Proceedings of teh National Academy of Sciences (2020); https://doi.org/10.1063/5.0031332

Despite the promise of immune checkpoint blockade (ICB) therapy against cancer, challenges associated with low objective response rates and severe systemic side effects still remain and limit its clinical applications. Here, we described a cold atmospheric plasma (CAP)-mediated ICB therapy integrated with microneedles (MN) for the transdermal delivery of ICB. We found that a hollow-structured MN (hMN) patch facilitates the transportation of CAP through the skin, causing tumor cell death. The release of tumor-associated antigens then promotes the maturation of dendritic cells in the tumor-draining lymph nodes, subsequently initiating T cell-mediated immune response. Anti-programmed death-ligand 1 antibody (aPDL1), an immune checkpoint inhibitor, released from the MN patch further augments the antitumor immunity. Our findings indicate that the proposed transdermal combined CAP and ICB therapy can inhibit the tumor growth of both primary tumors and distant tumors, prolonging the survival of tumor-bearing mice.

Peter L. Wright , Stephen A. Samples , Nolan M. Uchizono , and Richard E. Wirz, “Comment on “Jet propulsion by microwave air plasma in the atmosphere” [AIP Adv. 10, 055002 (2020)]”, AIP Advances 10, 099101 (2020); https://doi.org/10.1063/5.0013575

In this Comment, we analyze the performance of a microwave plasma device presented by Ye et al. [AIP Adv. 10, 055002 (2020)]. The efficiency analysis, using conservation of energy, shows that the methods used by the original authors predict up to 8000% device efficiency. Our analytical model is based on a control volume analysis of the original authors’ experimental setup and conditions, indicating that blocking the exit of the device yields stagnation pressure rather than jet pressure. The results from this analysis are consistent with the reported experimental data, demonstrating that the measured pressure using this method is internal chamber pressure and cannot be used to estimate thrust.

Angelopoulos,…, R.E. Wirz, et al., “The ELFIN Mission,” Space Science Reviews (2020) 216:103 https://doi.org/10.1007/s11214-020-00721-7

The Electron Loss and Fields Investigation with a Spatio-Temporal Ambiguity-Resolving option (ELFIN-STAR, or heretoforth simply: ELFIN) mission comprises two identical 3-Unit (3U) CubeSats on a polar (∼93∘ inclination), nearly circular, low-Earth (∼450 km altitude) orbit. Launched on September 15, 2018, ELFIN is expected to have a >2.5 year lifetime. Its primary science objective is to resolve the mechanism of storm-time relativistic electron precipitation, for which electromagnetic ion cyclotron (EMIC) waves are a prime candidate. From its ionospheric vantage point, ELFIN uses its unique pitch-angle-resolving capability to determine whether measured relativistic electron pitch-angle and energy spectra within the loss cone bear the characteristic signatures of scattering by EMIC waves or whether such scattering may be due to other processes. Pairing identical ELFIN satellites with slowly-variable along-track separation allows disambiguation of spatial and temporal evolution of the precipitation over minutes-to-tens-of-minutes timescales, faster than the orbit period of a single low-altitude satellite (Torbit ∼ 90 min). Each satellite carries an energetic particle detector for electrons (EPDE) that measures 50 keV to 5 MeV electrons with E/E < 40% and a fluxgate magnetometer (FGM) on a ∼72 cm boom that measures magnetic field waves (e.g., EMIC waves) in the range from DC to 5 Hz Nyquist (nominally) with <0.3 nT/sqrt(Hz) noise at 1 Hz. The spinning satellites (Tspin 3 s) are equipped with magnetorquers (air coils) that permit spin-up or -down and reorientation maneuvers. Using those, the spin axis is placed normal to the orbit plane (nominally), allowing full pitch-angle resolution twice per spin. An energetic particle detector for ions (EPDI) measures 250 keV – 5 MeV ions, addressing secondary science. Funded initially by CalSpace and the University Nanosat Program, ELFIN was selected for flight with joint support from NSF and NASA between 2014 and 2018 and launched by the ELaNa XVIII program on a Delta II rocket (with IceSatII as the primary). Mission operations are currently funded by NASA. Working under experienced UCLA mentors, with advice from The Aerospace Corporation and NASA personnel, more than 250 undergraduates have matured the ELFIN implementation strategy; developed the instruments, satellite, and ground systems and operate the two satellites. ELFIN’s already high potential for cutting-edge science return is compounded by concurrent equatorial Heliophysics missions (THEMIS, Arase, Van Allen Probes, MMS) and ground stations. ELFIN’s integrated data analysis approach, rapid dissemination strategies via the SPace Environment Data Analysis System (SPEDAS), and data coordination with the Heliophysics/Geospace System Observatory (H/GSO) optimize science yield, enabling the widest community benefits. Several storm-time events have already been captured and are presented herein to demonstrate ELFIN’s data analysis methods and potential. These form the basis of on-going studies to resolve the primary mission science objective. Broad energy precipitation events, precipitation bands, and microbursts, clearly seen both at dawn and dusk, extend from tens of keV to >1 MeV. This broad energy range of precipitation indicates that multiple waves are providing scattering concurrently. Many observed events show significant backscattered fluxes, which in the past were hard to resolve by equatorial spacecraft or non-pitch-angle-resolving ionospheric missions. These observations suggest that the ionosphere plays a significant role in modifying magnetospheric electron fluxes and wave-particle interactions. Routine data captures starting in February 2020 and lasting for at least another year, approximately the remainder of the mission lifetime, are expected to provide a very rich dataset to address questions even beyond the primary mission science objective.

Uchizono N.M., Collins A.L., Thuppul A., Wright P.L., Eckhardt D.Q., Ziemer J.K., Wirz R.E., “Emission modes in electrospray thrusters operating with high conductivity ionic liquids,” Aerospace, Special Issue: Electric Propulsion, 2020, 7(10), 141; https://doi.org/10.3390/aerospace7100141

Electrospray thruster life and mission performance are strongly influenced by grid impingement, the extent of which can be correlated with emission modes that occur at steady-state extraction voltages, and thruster command transients. Most notably, we experimentally observed skewed cone-jet emission during steady-state electrospray thruster operation, which leads to the definition of an additional grid impingement mechanism that we termed “tilted emission”. Long distance microscopy was used in conjunction with high speed videography to observe the emission site of an electrospray thruster operating with an ionic liquid propellant (EMI-Im). During steady-state thruster operation, no unsteady electrohydrodynamic emission modes were observed, though the conical meniscus exhibited steady off-axis tilt of up to 15°. Cone tilt angle was independent over a wide range of flow rates but proved strongly dependent on extraction voltage. For the geometry and propellant used, the optimal extraction voltage was near 1.6 kV. A second experiment characterized transient emission behavior by observing startup and shutdown of the thruster via flow or voltage. Three of the four possible startup and shutdown procedures transition to quiescence within ∼475 μs, with no observed unsteady modes. However, during voltage-induced thruster startup, unsteady electrohydrodynamic modes were observed.

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.

Thuppul A., Wright P.L., Collins A.L., Ziemer J.K., Wirz R.E., "Lifetime Considerations for Electrospray Thrusters", Aerospace, Special Issue: Electric Propulsion, 2020, 7(8), 108, https://doi.org/10.3390/aerospace7080108

Ionic liquid electrospray thrusters are capable of producing microNewton precision thrust at a high thrust–power ratio but have yet to demonstrate lifetimes that are suitable for most missions. Accumulation of propellant on the extractor and accelerator grids is thought to be the most significant life-limiting mechanism. In this study, we developed a life model to examine the effects of design features, operating conditions, and emission properties on the porous accelerator grid saturation time of a thruster operating in droplet emission mode. Characterizing a range of geometries and operating conditions revealed that modifying grid aperture radius and grid spacing by 3–7% can significantly improve thruster lifetime by 200–400%, though a need for explicit mass flux measurement was highlighted. Tolerance analysis showed that misalignment can result in 20–50% lifetime reduction. In addition, examining the impact of electron backstreaming showed that increasing aperture radius produces a significant increase in backstreaming current compared to changing grid spacing. A study of accelerator grid bias voltages revealed that applying a reasonably strong accelerator grid potential (in the order of a kV) can minimize backstreaming current to negligible levels for a range of geometries.

Barde A., Nithyanandam K., Shinn M., Wirz R.E., "Sulfur Heat Transfer Behavior for Uniform and Non-uniform Thermal Charging of Horizontally-oriented Isochoric Thermal Energy Storage Systems", Intl. J. Heat Mass Transfer, 2020, 153, 119556, https://doi.org/10.1016/j.ijheatmasstransfer.2020.119556

Elemental sulfur is a low-cost, chemically stable thermal storage medium suitable for many medium to high temperature applications. In this study, we investigate the heat transfer behavior of sulfur, isochorically stored in a horizontally-oriented thermal storage element (steel tube) using experimental, analytical, and computational methods. The sulfur container was uniformly and non-uniformly heated along its axis from 50 to 600 °C to simulate the potential operating conditions for the full-scale thermal energy storage systems. The results of the study reveal distinct sulfur heat transfer mechanisms based on the temperature range and mode of thermal charging. For temperatures from 50 to 200 °C, the sulfur heat transfer behavior is governed by two primary mechanisms; 1) solid–liquid phase change, and 2) sulfur viscosity that varies strongly with temperature. From 200 to 600 °C, the buoyancy-driven natural convection is the dominant heat transfer mechanism and facilitates significantly high thermal charge rates. For axially non-uniform thermal charging, the axial temperature gradient induces natural convection along the axis that rapidly redistributes the thermal energy within the sulfur mass. Such axial convection has a strong impact on the thermal characteristics, including thermal charge/discharge rate and exergetic efficiency of the thermal storage systems. These observations and the high-fidelity computational model used in this study provide important means to identify the design parameters and operating conditions for which sulfur-based thermal energy storage (SulfurTES) systems will provide desirable thermal performance at a low thermal storage cost.

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.

Samples S.A., Wirz R.E., "Development of the MiXI Thruster with the ARCH Discharge", Plasma Res. Express, 2020, 2, 025008, https://doi.org/10.1088/2516-1067/ab906d

The Miniature Xenon Ion (MiXI) thruster with the Axial Ring-Cusp Hybrid 'MiXI(ARCH)' discharge was developed and operated with beam extraction at 1 kV. The thruster achieved 59% cathode-free total discharge efficiency at 23.7 mA xenon beam current with filament cathodes and low temperature operation, corresponding to a discharge loss of 226 W/A and propellant utilization of 72%. Thruster efficiency was observed to increase with increasing flow rate and decrease with increasing temperature up to thermal steady state. At thermal steady state, the thruster anode reached ∼320 °C due to the thermal isolation of the thruster head. Reducing the discharge chamber aspect ratio from 0.5 to 0.4 increased thermal steady state efficiency from 46% to 57% but required slow ramping of beam voltage and was limited to stable operation to above 0.5 sccm discharge propellant flow. In contrast to the 3-ring cusp configuration, MiXI(3-Ring), the performance is generally higher but is not able to achieve lower thrust levels and requires more complex start-up for stable operation. An analytical single-cell model was developed and applied to investigate internal processes of the MiXI(ARCH) discharge. The model emulated the effect of increasing flow on performance, indicating that the dominant loss mechanism is plasma electron current to the anode, in contrast to the 3-Ring geometry, which is dominated by primary electron losses. This model also matched trends reported in previous works of strongly increasing electron temperature and primary density with propellant utilization. Through this effort, the MiXI thruster's highest achievable total efficiency has been increased, and several mechanisms for further improved efficiency have been identified.

Jin K., Wirz R.E., "Sulfur Heat Transfer Behavior in a Vertically-Oriented and Nonuniformly-Heated Isochoric Thermal Energy Storage System", Applied Energy, 2020, Vol. 260, 114287, https://doi.org/10.1016/j.apenergy.2019.114287

Elemental sulfur thermal energy storage (SulfurTES) is a promising low-cost solution for many medium to high temperature (300–1200 °C) TES applications. Demonstrations of SulfurTES have shown that the heat transfer behavior of sulfur in isochoric tubes is critical to system thermal performance. Previous studies have elucidated and quantified the sulfur heat transfer rate for idealized uniform charge and discharge; however, nonuniform conditions are more likely to be encountered in practice and need to be understood. This paper uses experimental and computational efforts to investigate sulfur heat transfer as well as exergy and energy performance in vertically-oriented tubes for two nonuniform thermal charge scenarios: top-heating and bottom-heating. In comparison with uniform thermal charge, the top-heating causes significant thermal stratification of sulfur that helps the SulfurTES system achieve superior exergetic performance. In contrast, the bottom-heating causes rapid mixing between hot and cold sulfur resulting in high charge rates. Both nonuniform charge strategies could be utilized during the operation of the SulfurTES system to improve system performance as well as provide operational flexibility. Using the computational results, this article originally develops two simplified analytical procedures to estimate the energy and exergy performance of sulfur in tubes of different sizes under top- and bottom-heating. The current study provides significant qualitative and quantitative heat transfer descriptions and design bases for SulfurTES systems and encourages further investigations into the complicated thermal performance for other thermal storage applications.