@article{lietz_groenewald_scherpelz_hopkins_2023, title={Kinetic simulations of ignited mode cesium vapor thermionic converters}, url={https://doi.org/10.1063/5.0117599}, DOI={10.1063/5.0117599}, abstractNote={Cesium vapor thermionic converters are an attractive method of converting high-temperature heat directly to electricity, but theoretical descriptions of the systems have been difficult due to the multi-step ionization of Cs through inelastic electron–neutral collisions. This work presents particle-in-cell simulations of these converters, using a direct simulation Monte Carlo collision model to track 52 excited states of Cs. These simulations show the dominant role of multi-step ionization, which also varies significantly based on both the applied voltage bias and pressure. The electron energy distribution functions are shown to be highly non-Maxwellian in the cases analyzed here. A comparison with previous approaches is presented, and large differences are found in ionization rates due especially to the fact that previous approaches have assumed Maxwellian electron distributions. Finally, an open question regarding the nature of the plasma sheaths in the obstructed regime is discussed. The one-dimensional simulations did not produce stable obstructed regime operation and thereby do not support the double-sheath hypothesis.}, journal={Journal of Applied Physics}, author={Lietz, A. M. and Groenewald, R. E. and Scherpelz, P. and Hopkins, M. M.}, year={2023}, month={Jan} }
 @article{lietz_yee_musk ii_moffat_wiemann_settecerri_fergenson_omana_hopkins_2022, title={Laser-driven ionization mechanisms of aluminum for single particle aerosol mass spectrometry}, volume={197}, ISSN={["1873-3565"]}, DOI={10.1016/j.sab.2022.106543}, abstractNote={Single particle aerosol mass spectrometry (SPAMS), an analytical technique for measuring the size and composition of individual micron-scale particles, is capable of analyzing atmospheric pollutants and bioaerosols much more efficiently and with more detail than conventional methods which require the collection of particles onto filters for analysis in the laboratory. Despite SPAMS’ demonstrated capabilities, the primary mechanisms of ionization are not fully understood, which creates challenges in optimizing and interpreting SPAMS signals. In this paper, we present a well-stirred reactor model for the reactions involved with the laser-induced vaporization and ionization of an individual particle. The SPAMS conditions modeled in this paper include a 248 nm laser which is pulsed for 8 ns to vaporize and ionize each particle in vacuum. The ionization of 1 μm, spherical Al particles was studied by approximating them with a 0-dimensional plasma chemistry model. The primary mechanism of absorption of the 248 nm photons was pressure-broadened direct photoexcitation to Al(y2D). Atoms in this highly excited state then undergo superelastic collisions with electrons, heating the electrons and populating the lower energy excited states. We found that the primary ionization mechanism is electron impact ionization of various excited state Al atoms, especially Al(y2D). Because the gas expands rapidly into vacuum, its temperature decreases rapidly. The rate of three-body recombination (e− + e− + Al+ → Al + e−) increases at low temperature, and most of the electrons and ions produced recombine within several μs of the laser pulse. The importance of the direct photoexcitation indicates that the relative peak heights of different elements in SPAMS mass spectra may be sensitive to the available photoexcitation transitions. The effects of laser intensity, particle diameter, and expansion dynamics are also discussed.}, journal={SPECTROCHIMICA ACTA PART B-ATOMIC SPECTROSCOPY}, author={Lietz, Amanda M. and Yee, Benjamin T. and Musk II, Jeffrey and Moffat, Harry K. and Wiemann, Dora K. and Settecerri, Taylor and Fergenson, David and Omana, Michael A. and Hopkins, Matthew M.}, year={2022}, month={Nov} }
 @article{parsey_lietz_kushner_2021, title={Guided plasma jets directed onto wet surfaces: angular dependence and control}, volume={54}, url={https://doi.org/10.1088/1361-6463/abbf1a}, DOI={10.1088/1361-6463/abbf1a}, abstractNote={Abstract}, number={4}, journal={Journal of Physics D: Applied Physics}, publisher={IOP Publishing}, author={Parsey, Guy and Lietz, Amanda M and Kushner, Mark J}, year={2021}, month={Jan}, pages={045206} }
 @article{lietz_barnat_nail_roberds_fierro_yee_moore_clem_hopkins_2021, title={High-fidelity modeling of breakdown in helium: initiation processes and secondary electron emission}, volume={54}, url={https://doi.org/10.1088/1361-6463/ac0461}, DOI={10.1088/1361-6463/ac0461}, abstractNote={Abstract}, number={33}, journal={Journal of Physics D: Applied Physics}, publisher={IOP Publishing}, author={Lietz, Amanda M and Barnat, Edward V and Nail, George R and Roberds, Nicholas A and Fierro, Andrew S and Yee, Benjamin T and Moore, Chris H and Clem, Paul G and Hopkins, Matthew M}, year={2021}, month={Aug}, pages={334005} }
 @article{mohades_lietz_kushner_2020, title={Generation of reactive species in water film dielectric barrier discharges sustained in argon, helium, air, oxygen and nitrogen}, volume={7}, url={https://doi.org/10.1088/1361-6463/aba21a}, DOI={10.1088/1361-6463/aba21a}, abstractNote={Activation of liquids with atmospheric pressure plasmas is being investigated for environmental and biomedical applications. When activating the liquid using gas plasma produced species (as opposed to plasmas sustained in the liquid), a rate limiting step is transport of these species into the liquid. To first order, the efficiency of activating the liquid is improved by increasing the ratio of the surface area of the water in contact with the plasma compared to its volume—often called the surface-to-volume ratio (SVR). Maximizing the SVR then motivates the plasma treatment of thin films of liquids. In this paper, results are discussed from a computational investigation using a global model of atmospheric pressure plasma treatment of thin water films by a dielectric barrier discharge (DBD) sustained in different gases (Ar, He, air, N2, O2). The densities of reactive species in the plasma activated water (PAW) are evaluated. The residence time of the water in contact with the plasma is increased by recirculating the PAW in plasma reactor. Longer lived species such as H2O2aq and NO3−aq accumulate over time (aq denotes an aqueous species). DBDs sustained in Ar and He are the most efficient at producing H2O2aq, DBDs sustained in argon produces the largest density of NO3−aq with the lowest pH, and discharges sustained in O2 and air produce the highest densities of O3aq. Comparisons to experiments by others show agreement in the trends in densities in PAW including O3aq, OHaq, H2O2aq and NO3−aq, and highlight the importance of controlling desolvation of species from the activated water.}, journal={Journal of Physics D: Applied Physics}, publisher={IOP Publishing}, author={Mohades, Soheila and Lietz, Amanda M and Kushner, Mark J}, year={2020}, month={Oct} }
 @article{mohades_lietz_kruszelnicki_kushner_2020, title={Helium plasma jet interactions with water in well plates}, url={https://doi.org/10.1002/ppap.201900179}, DOI={10.1002/ppap.201900179}, abstractNote={Abstract}, journal={Plasma Processes and Polymers}, author={Mohades, Soheila and Lietz, Amanda M. and Kruszelnicki, Juliusz and Kushner, Mark J.}, year={2020}, month={Mar} }
 @article{lietz_barnat_foster_kushner_2020, title={Ionization wave propagation in a He plasma jet in a controlled gas environment}, url={https://doi.org/10.1063/5.0020264}, DOI={10.1063/5.0020264}, abstractNote={Characterizing ionization wave propagation in low temperature plasma jets is critical to predicting production of reactive species and plasma–surface interactions for biomedical applications and surface functionalization. In this paper, results from optical emission and laser induced fluorescence measurements of the ionization wave in a He plasma jet operating in a controlled gas environment are discussed and used for comparison with numerical modeling. The ionization wave was observed using ICCD (Intensified Charge Coupled Device) imaging and characterized by time and spatially resolved electron density measurements using laser-collision-induced fluorescence. The plasma jet was initially characterized using pure He (nominally at 200 Torr), while varying pressure and voltage. When operating in pure He, the ionization wave broadly expands exiting the plasma tube. Increasing the operating pressure reduces the speed and isotropic expansion of the ionization wave. The jet operated with a humid He shroud was also studied. The humid He shroud results in the electron density increasing and having an annular profile due to the lower ionization potential of H2O compared to He and localized photoionization in the mixing region. Numerical modeling highlighted the importance of resonance radiation emitted by excited states of He, photoelectron emission from the quartz tube, and the kinetic behavior of the electrons produced by photoionization ahead of the ionization front.}, journal={Journal of Applied Physics}, author={Lietz, Amanda M. and Barnat, Edward V. and Foster, John E. and Kushner, Mark J.}, year={2020}, month={Aug} }
 @article{kruszelnicki_lietz_kushner_2019, title={Atmospheric pressure plasma activation of water droplets}, volume={5}, url={https://doi.org/10.1088/1361-6463/ab25dc}, DOI={10.1088/1361-6463/ab25dc}, abstractNote={Low temperature plasma treatment of water is being investigated due to its use in pollution abatement, wound treatment and agriculture. Plasma produced reactive oxygen and nitrogen species (RONS) are formed in the gas phase and solvate into the liquid. Activation of the liquid is often limited by transport of these RONS to the liquid surface. Micrometer scale droplets immersed in the plasma have a large surface to volume ratio, which increases the interaction area for a given volume of water, and can increase the rates of transport from the gas to liquid. In this paper, results from 0 and 2D modeling of air-plasma activation of water micro-droplets are discussed. The solvation dynamics are sensitive to the Henry’s law constant (h) of each species, which describes its hydrophobicity (low h) or hydrophilicity (high h). The liquid densities of stable species with high h values (e.g. H2O2, HNOx) are sensitive to droplet diameter. For large droplets, hydrophilic species may deplete the gas-phase inventory of RONS before liquid-phase saturation is reached, limiting the total in-liquid density for species with high h. For smaller droplets, higher average in-droplet densities of these species can be produced. Liquid concentrations of stable species with low h (e.g. O3, N2O, H2) had a weak dependence on droplet size as droplets are quickly saturated and solvation does not deplete the gas phase. An analysis of this behavior is discussed using the well-stirred reactor (0D) approximation. Spatial non-uniformity of the plasma also has an impact on the solvation rates and kinetics. Gas phase depletion of high-h species leads to a decrease in solvation rates. Low-h species that saturate the surface of the droplets during plasma-on periods can quickly de-solvate in the afterglow.}, journal={Journal of Physics D: Applied Physics}, publisher={IOP Publishing}, author={Kruszelnicki, Juliusz and Lietz, Amanda M and Kushner, Mark J}, year={2019}, month={Aug} }
 @article{norberg_parsey_lietz_johnsen_kushner_2019, title={Atmospheric pressure plasma jets onto a reactive water layer over tissue: pulse repetition rate as a control mechanism}, volume={9}, url={https://doi.org/10.1088/1361-6463/aae41e}, DOI={10.1088/1361-6463/aae41e}, abstractNote={The use of plasma jets to treat tissue in the context of plasma medicine often involves a thin intervening liquid layer on top of the tissue. Plasma activated species first transport through and react in the liquid layer prior to reaching the tissue. Of the many parameters that can be used to control this process, pulse repetition frequency (PRF) stands out. Results from a computational investigation of multiple pulses at varying PRF from an atmospheric pressure plasma jet (APPJ) onto a reactive liquid layer are discussed, and three key trends are made clear. First, a high PRF (short time between pulses) enables the gaseous species produced during the previous pulse to remain in the vicinity of the plasma at the onset of the next pulse, thereby increasing the inventory of (H)NxOy and O3 in the gas phase. These species then solvate into the liquid, water in this case, and produce higher densities of aqueous ozone, nitrate, and peroxynitrite. With a lower PRF, reactants produced on a previous pulse are convected away prior to the next discharge pulse with more spatial separation of reactants both above and within the water. As a result, more of the hydroxyl anion (), ozone anion () and nitric oxide (NOaq) reach the tissue beneath the water. The second trend is that the production of H2O2aq and its fluence to the underlying tissue are relatively independent of the PRF. The precursors for H2O2aq are primarily produced by the surface ionization wave (SIW) on the top of the liquid, which then directly solvate into the liquid. Lastly, when the plasma plume touches the liquid, the SIW on the water layer increases the production of all aqueous species compared to configurations where the plasma plume does not touch the liquid. These trends are true for all PRF.}, journal={Journal of Physics D: Applied Physics}, publisher={IOP Publishing}, author={Norberg, Seth A and Parsey, Guy M and Lietz, Amanda M and Johnsen, Eric and Kushner, Mark J}, year={2019}, month={Jan} }
 @article{lietz_damany_robert_pouvesle_kushner_2019, title={Ionization wave propagation in an atmospheric pressure plasma multi-jet}, volume={10}, url={https://doi.org/10.1088/1361-6595/ab4ab0}, DOI={10.1088/1361-6595/ab4ab0}, abstractNote={The atmospheric pressure multi-plasma jet produces an array of individual plasma jets which originate from the branching of a single ionization wave (IW). The use of arrays of such plasma jets could enable treatment of larger surface areas than is possible with a single plasma jet. In this paper, we discuss results from a combined experimental and two-dimensional modeling investigation of the behavior of IWs in an atmospheric pressure plasma multi-jet device. In this multi-jet, a rare gas is flowed through a tube having a line of holes, producing gas jets into the ambient from each of the holes. A primary ionization wave (PIW) propagates through the tube which launches a series of secondary ionization waves (SIWs) propagating out each hole through the plumes of the individual gas jets. The propagation of the SIWs is more intense using a positive polarity voltage pulse due to the higher electric field at the ionization front. The diameter of the holes determines the delay of the SIW after passage of the PIW past the hole, with smaller holes resulting in larger delays. The larger delay results from a smaller view angle for photoionization outside the tube from photons originating in the PIW. Higher helium flow rates result in a greater tendency for SIW propagation because the air concentrations in the individual gas jets outside the tube are lower and so the electron temperature is higher. The interaction between SIWs is primarily electrostatic, and is a sensitive function of geometric parameters including proximity of ground planes and the spacing between the holes through which these SIWs emerge.}, journal={Plasma Sources Science and Technology}, publisher={IOP Publishing}, author={Lietz, Amanda M and Damany, Xavier and Robert, Eric and Pouvesle, Jean-Michel and Kushner, Mark J}, year={2019}, month={Dec} }
 @article{luo_lietz_yatom_kushner_bruggeman_2019, title={Plasma kinetics in a nanosecond pulsed filamentary discharge sustained in Ar–H2O and H2O}, volume={10}, url={https://doi.org/10.1088/1361-6463/aaeb14}, DOI={10.1088/1361-6463/aaeb14}, abstractNote={The plasma kinetics of Ar–H2O and H2O at atmospheric pressure are of interest for applications in biotechnology where rare-gas plasma jets treat liquid surfaces and in water treatment where discharges are generated in bubbles or directly in liquid water. Due to evaporation resulting from heat transfer to the liquid, for many conditions the mole fraction of water in the plasma can be large—approaching nearly pure water. In this paper, results are discussed from a combined experimental and computational investigation of the chemical kinetics in a high electron density plasma filament sustained in Ar–H2O at atmospheric pressure. The chemical kinetics were simulated using a 0D global model, validated by measurements of the absolute OH and H densities by laser induced fluorescence (LIF) and two-photon absorption LIF. The primary sources of H and OH during the discharge pulse are dissociative excitation transfer from metastable Ar atoms and Ar dimer excimers at low water concentration and electron impact dissociation of H2O at high water concentration. In spite of their similar sources, the density of OH was measured to be two orders of magnitude smaller than that of H at power densities on the order of 105 Jm−3. This disparity is due to electron impact dissociation of OH during the discharge pulse and rapid reactions of OH in the presence of high H and O densities in the afterglow. It is often assumed that OH is the dominant non-selective reactive species in water-containing plasmas. These results reinforce the importance of atomic species such as H and O in water containing high energy density plasmas. A numerical parametric study revealed that the lowest energy cost for H2O2 production is achieved at low energy densities in pure water. The high concentration of atomic radicals, which rapidly recombine, results in an overall lower energy efficiency of reactive species production. In particular, the selectivity of H2O2 production decreases with increasing power density which instead favors H2 and O2 production.}, journal={Journal of Physics D: Applied Physics}, publisher={IOP Publishing}, author={Luo, Yuchen and Lietz, Amanda M and Yatom, Shurik and Kushner, Mark J and Bruggeman, Peter J}, year={2019}, month={Jan} }
 @article{lietz_kushner_2018, title={Electrode configurations in atmospheric pressure plasma jets: production of reactive species}, volume={9}, url={https://doi.org/10.1088/1361-6595/aadf5b}, DOI={10.1088/1361-6595/aadf5b}, abstractNote={Atmospheric pressure plasma jets (APPJs) are a preferred plasma source for many biomedical applications. These jets typically consist of a rare gas flowing through a dielectric tube, possibly with an O2 or H2O admixture, and flowing into the ambient. They are typically powered by pulsed or sinusoidal voltage waveforms. However, in most other aspects APPJ designs differ greatly. In this paper, APPJ design parameters and their consequences on ionization wave (IW) propagation and reactive oxygen and nitrogen species (RONS) production are discussed using results from a two-dimensional plasma hydrodynamics model. The base case is an APPJ with a single powered ring electrode wrapped around a dielectric tube. This configuration was varied by adding a grounded ring electrode, changing the powered and grounded electrode positions, and moving the powered electrode to the inside of the tube. Placing the powered electrode closer to the outlet of the tube increased the RONS production by increasing the energy deposition outside the tube. Adding a grounded ring increased the IW intensity inside the tube while slightly increasing the power deposition outside of the tube. An inner powered electrode increased the IW intensity and propagation velocity, and the resulting RONS production. Co-axial ground planes within 5 cm of the APPJ significantly affected the IW behavior, increasing its intensity and increasing RONS production. The consequences of voltage rise time and dielectric constant of the tube are also discussed. The systematic trends from this investigation may facilitate more informed APPJ design choices that may be tailored to the goals of a specific application.}, journal={Plasma Sources Science and Technology}, publisher={IOP Publishing}, author={Lietz, Amanda M and Kushner, Mark J}, year={2018}, month={Oct} }
 @article{lietz_kushner_2018, title={Molecular admixtures and impurities in atmospheric pressure plasma jets}, volume={124}, url={https://doi.org/10.1063/1.5049430}, DOI={10.1063/1.5049430}, abstractNote={A more complete understanding of reactive chemistry generated by atmospheric pressure plasma jets (APPJs) is critical to many emerging medical, agricultural, and water treatment applications. Adding molecular gases to the noble working gas which flows through the jet is a common method to tailor the resulting production of reactive oxygen and nitrogen species (RONS). In this paper, results are discussed from a computational investigation of the consequences of H2O and O2 admixtures on the reactive chemistry of He APPJs flowing into humid air. This investigation, performed with a 2-dimensional plasma hydrodynamics model, addresses the RONS that are initially produced and the evolution of that chemistry on longer time scales. Without an admixture, the impurities in 99.999% pure helium are a major source of RONS. The addition of H2O decreases the production of reactive nitrogen species (RNS) and increases the production of reactive oxygen species (ROS). The addition of O2 significantly decreases the production of RNS, as well as hydrogen-containing ROS, but increases the production of ROS without hydrogen. This selectivity comes from the lower ionization energy of O2 compared to N2 and H2O, which then allows for charge exchange reactions. These charge exchange reactions change the RONS which are produced in the afterglow by dissociative recombination. The consequences of impurities were also examined. Humid air impurities as low as 10 ppm in the helium can account for 79%-98% of the production of most RONS in the absence of an intentional admixture. The degree to which the impurities affect the RONS production depends on the electrode configuration and can be reduced by molecular admixtures.}, number={15}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Lietz, Amanda M. and Kushner, Mark J.}, year={2018}, month={Oct}, pages={153303} }
 @article{lietz_johnsen_kushner_2017, title={Plasma-induced flow instabilities in atmospheric pressure plasma jets}, volume={111}, url={http://dx.doi.org/10.1063/1.4996192}, DOI={10.1063/1.4996192}, abstractNote={Pulsed plasma excitation of rare gases flowing into air has been shown to impact the stability of the flow in non-equilibrium atmospheric pressure plasma jets (APPJs). In this paper, the results from a numerical modeling investigation of the stability of a round He APPJ with a powered electrode exposed to the gas flow are discussed. Localized gas heating at the powered electrode occurs on the time scale of the voltage pulse, tens to 100 ns, which is short compared to the fluid timescales. An acoustic wave propagates from this heated, expanding gas and exits the jet. The wave disturbs the shear layer between the He and surrounding humid air, exciting a shear instability which grows downstream with the flow and increases the mixing of the humid air into the He. The effects of the eddy-dominated flow on ionization wave (IW) propagation in an APPJ were investigated. The IW followed the regions of the highest helium concentration, resulting in an increased production of NO, HO2, and NO2.}, number={11}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Lietz, Amanda M. and Johnsen, Eric and Kushner, Mark J.}, year={2017}, month={Sep}, pages={114101} }
 @article{lietz_kushner_2016, title={Air plasma treatment of liquid covered tissue: long timescale chemistry}, volume={49}, url={http://dx.doi.org/10.1088/0022-3727/49/42/425204}, DOI={10.1088/0022-3727/49/42/425204}, abstractNote={Atmospheric pressure plasmas have shown great promise for the treatment of wounds and cancerous tumors. In these applications, the sample is usually covered by a thin layer of a biological liquid. The reactive oxygen and nitrogen species (RONS) generated by the plasma activate and are processed by the liquid before the plasma produced activation reaches the tissue. The synergy between the plasma and the liquid, including evaporation and the solvation of ions and neutrals, is critical to understanding the outcome of plasma treatment. The atmospheric pressure plasma sources used in these procedures are typically repetitively pulsed. The processes activated by the plasma sources have multiple timescales—from a few ns during the discharge pulse to many minutes for reactions in the liquid. In this paper we discuss results from a computational investigation of plasma–liquid interactions and liquid phase chemistry using a global model with the goal of addressing this large dynamic range in timescales. In modeling air plasmas produced by a dielectric barrier discharge over liquid covered tissue, 5000 voltage pulses were simulated, followed by 5 min of afterglow. Due to the accumulation of long-lived species such as ozone and NxOy, the gas phase dynamics of the 5000th discharge pulse are different from those of the first pulse, particularly with regards to the negative ions. The consequences of applied voltage, gas flow, pulse repetition frequency, and the presence of organic molecules in the liquid on the gas and liquid reactive species are discussed.}, number={42}, journal={Journal of Physics D: Applied Physics}, publisher={IOP Publishing}, author={Lietz, Amanda M and Kushner, Mark J}, year={2016}, month={Oct}, pages={425204} }