DOE Center for Predictive Control of Plasma Kinetics: Multi-Phase And

DOE Center for Predictive Control of Plasma Kinetics: Multi‐Phase and Bounded Systems

10th Annual Meeting

May 30‐31, 2019 Edward St. John Learning & Teaching Center, University of Maryland College Park, MD

Participating Institutions

We gratefully acknowledge the funding from The U.S. Department of Energy Office of Science Fusion Energy Sciences Program Grant # DE‐SC0001939

Schedule

Thursday, May 30, 2019

7:45 – 8:00 am Registration

8:00 – 8:15 Mark J. Kushner (University of Michigan) Introduction to Annual Meeting 8:15 –10:45 am Session I. Low Pressure and Dusty Plasmas Moderator: Yangyang Fu Page 8:15 – 8:45 Vincent Donnelly (University of Houston) Ubiquitous Ignition Delays in Power-Modulated and Spatially Separated Electronegative Plasmas 9

8:45 – 9:15 Steven Girshick (University of Minnesota) Numerical Modeling of Nanodusty Plasmas 10

9:15 – 9:45 Uwe Kortshagen (University of Minnesota) Particle Dynamics in Pulsed Dusty RF Plasmas 11

9:45 – 10:15 Edward Thomas (Auburn University) Modification of Nanoparticle Formation in a Strongly Magnetized Plasma 12 10:15 – 10:45 Valery Godyak (University of Michigan) Volt-Ampere Characteristics of Capacitively Coupled Plasma 13

10:45 – 11:00 am Coffee break

Thursday, May 30, 2019

11:00 am – 1:00 Session II. Fundamental Properties of Plasma pm Diagnostics Moderator: Alexander Khrabrov Page 11:00 – 11:30 Igor Adamovich (Ohio State University) Laser Diagnostics for Measurements of Electric Field and Excited Metastable Species in Nonequilibrium Plasmas 14

11:30 – 12:00 Ed Barnat (Sandia National Labs) Advancing Diagnostics to Interrogate Dynamic and Structured Plasma 15

12:00 – 12:30 Peter Bruggeman (University of Minnesota) Ions and Reactive Species Measurements in Time- modulated RF Driven Atmospheric Pressure Plasma Jets by Molecular Beam Mass Spectrometry 16

12:30 – 1:00 Marien Simeni Simeni (University of Minnesota) Measurements of Electric Field in Ns Pulse Discharges in Helium by Stark Splitting Polarization Spectroscopy 17

1:00 – 2:00 pm Lunch

Thursday, May 30, 2019

2:00 – 4:00 pm Session III. Fundamental Properties of Plasma Modeling Moderator: Yashuang Zheng Page 2:00 – 2:30 Igor Kaganovich (Princeton Plasma Physics Laboratory) Update on PPPL Modeling Efforts 18-19 2:30 – 3:00 Vladimir Kolobov (CFDRC/University of Alabama at Huntsville) Electron Kinetics in Low Temperature Plasma 20

3:00 – 3:30 Michael Lieberman (University of California-Berkeley) Striations in Atmospheric Pressure He/2%H2O Plasma Discharges 21

3:30 – 4:00 Savio Poovathingal (University of Michigan) Characterization of Non-Equilibrium Flow in Inductively Coupled Plasma Torches 22

4:00 – 4:30 pm Group photo Coffee break Poster setup

4:30 – 5:15 pm Poster session I

5:15 – 6:00 pm Poster session II

Friday, May 31, 2019

8:00 – 10:30 am Session IV. Plasma Surface Interactions and Sources Moderator: Janis Lai Page 8:00 – 8:30 Yevgeny Raitses (Princeton Plasma Physics Laboratory) Generation of Non-thermal Plasmas at Moderate and Atmospheric Pressures 23

8:30 – 9:00 John Foster (University of Michigan) Self-Organization in 1 ATM DC Glows: Current Understanding and Potential Applications 24

9:00 – 9:30 Gottlieb Oehrlein (University of Maryland) Studies of Plasma Surface Interactions of Plasma 25 Catalysis

9:30 – 10:00 John Verboncoeur (Michigan State University) Controlling Micro-gap Breakdown with Engineered Surface Morphology 26

10:00 – 10:30 Mark Kushner (University of Michigan) What Have We Learned About Controlling Atmospheric Pressure Plasmas? 27

10:30 – 10:45 am Coffee break

10:45 am – noon Group Discussion. What Have We Accomplished - What Are Next Steps? Moderator: Mark J. Kushner

Poster Session I Page 1 Yangyang Fu (Michigan State University) Low Temperature Plasma Similarities from Low to High Ionization Regimes 28

2 Keegan Orr (Ohio State University) Electric Field Distribution in an Atmospheric Pressure, Ns Pulse, Helium Plasma Jet Measured by Ps Second Harmonic Generation 29

3 Sophia Gershman (Princeton Plasma Physics Laboratory) Electrical Discharge in Gas Bubbles in Gel 30

4 Marien Simeni Simeni (University of Minnesota) Electron Densities and Temperatures Measurements in Atmospheric Pressure Nanosecond Pulse Helium Discharges 31

5 Yudong Li (University of Maryland) Infrared Studies of Catalyst Surface During Plasma-Catalysis: CHn Groups 32 6 Juliusz Kruszelnicki (University of Michigan) Modeling Interactions Between Artificial Bone Scaffolding and Atmospheric Pressure Plasmas 33

7 Chenhui Qu (University of Michigan) Optimizing Power Delivery using Impedance Matching Networks with Set-Point and Frequency Tuning for Pulsed Inductively Coupled Plasmas 34

8 Toshisato Ono (University of Minnesota) Particle Decharging and Agglomeration in Pulsed, Particle Dense Dusty RF Plasmas 35

9 Yashuang Zheng (University of Minnesota) The Etching Probability of Polystyrene by H, OH and O Radicals in an RF Driven Atmospheric Pressure Plasma Jet 36

Poster Session II Page

1 Janez Krek (Michigan State University) Benchmark of EEDF Evaluations in Global Modeling of Low Temperature Plasmas 37

2 Jian Chen (Princeton Plasma Physics Laboratory)

Two Dimensional Simulations of Carbon Arc 38 3 Alexander Khrabrov (Princeton Plasma Physics Laboratory)

Convenient Analytical Solution for Vibrational Spectrum of H2 39 4 Demetre Economou (University of Houston)

In-Plasma Photo-Assisted Etching of Silicon 40

5 Shiqiang Zhang (University of Maryland) DRIFTS Study of the Enhancement of Catalytic Partial Oxidation of Methane by Cold Atmospheric Plasma 41

6 Janis Lai (University of Michigan) Mapping of 2-D Plasma-induced Fluid Flow Using Particle Image Velocimetry 42

7 Sai Ranjeet Narayanan (University of Minnesota) Hybrid Method of Moments to Predict Nanoparticle Nucleation, Growth and Charging in Dusty Plasmas 43

8 Yuanfu Yue (University of Minnesota) Atomic Hydrogen Generation in the Ionizing Plasma Region and Effluent of a Helium-Water Atmospheric Pressure Plasma Jet by Two- Photon Absorption Laser Induced Fluorescence (TALIF) 44

Abstracts - Oral Presentations Ubiquitous Ignition Delays in Power-Modulated and Spatially Separated Electronegative Plasmas Vincent M. Donnelly and Demetre J. Economou University of Houston ([email protected]) This talk will summarize our studies a) of pulsed inductively-coupled plasmas in 12 electronegative gases, and will focus on 10 28 Power ON Cl2. Pulsed plasmas offer added control of Power 24

) 11 OFF average electron energy and number -3 10 20 density, and can achieve some level of 16 selectivity in the production of radical 1010 species. Pulsed plasma can also allow 12 more control of ion energy distributions 9 8 (IEDs). To achieve low energy and nearly 10 4 monoenergetic IEDs in pulsed ICPs it is (cm Number Density T n+ ne e (eV) Temperature Electron advantageous to suppress capacitive 108 0 0 200 400 600 800 1000 coupling. Time (s) Similar, anomalous ignition delays were found in three very different pulsed b)

ICP plasma configurations. In Fig. 1(a), a 1012 8 High Power Low Power Faraday shielded, purely inductive pulsed ) -3 7

11 cm Cl2 ICP with solenoidal coil was “seeded” ( 10 6 with a tandem plasma that continuously + ni 5 supplied a low density background 10 ne 4 plasma. [1] Rather than promptly igniting, 10 T 3 delays of 10s to 100s of s were found e 9 (yellow shaded region). In a flat coil ICP 10 2 Number Densities Number Densities system, power was modulated between a 1 Electron Temperature (eV) Temperature Electron high and low level. [2] Long ignition 108 0 0 500 1000 1500 2000 2500 delays could be produced (such as the Time s) yellow shaded region of Fig. 1b), depending on subtle details of the Figure 1 - Positive ion and electron densities and electron impedance matching. Both these cases are temperature as a function of time in pulsed 13 MHz Cl2 ICPs. P = 5 mTorr. a) Tandem configuration with CW similar to previous studies in a pulsed flat seed ICP and pulsed downstream Faraday-shielded ICP, coil ICP, with continuous bias power separated by a grid. b) Power-modulated, unshielded flat supplied to a substrate electrode in the coil ICP. plasma. [3] In all cases, ne drops rapidly as power is reduced or turned off, due to dissociative attachment by Cl2, while (+) and (-) ion densities decay at a much slower rate. It is not until the ion number density decays to a critical level that re- ignition is possible. The ignition delays can be explained by power balance arguments. Delays will occur if power gain and loss curves do not initially cross when dropping to the low power state, or at the beginning of the power-on state. References [1] L. Liu, S. Sridhar, V. M. Donnelly and D. J. Economou, J. Phys. D: Appl. Phys. 48 (48), 485201 (2015). [2] T. List, T. Ma, P. Arora, V. M. Donnelly, and S. Shannon, Plasma Sources Sci. Technol. 28, 025005 (2019). [3] M. Malyshev and V. Donnelly, Plasma Sources Sci. Technol. 9, 353 (2000). 9

Numerical Modeling of Nanodusty Plasmas Steven L. Girshick Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN ([email protected]) Numerical models of nanodusty plasmas have been developed under Center support. These models provide numerical simulations of particle nucleation, surface growth, coagulation, charging and transport, self-consistently coupled to plasma behavior. Both 1-D [1-2] and 2-D [3] models have been developed, with a focus on formation of silicon particles in silane-containing RF plasmas. Additionally we have conducted numerical simulations of pulsed RF plasmas for producing controlled fluxes of nanoparticles to a substrate [4], have developed an analytical expression for particle charge distributions that accounts for single-particle charge limits [5], and have conducted Monte Carlo simulations to explore the combined effects of electron tunneling from nanoparticles and departures from orbital-motion-limited theory caused by charge-exchange collisions that occur close to the particle [6]. In recent work, we are developing a nanodusty plasma model based on the hybrid method of moments, which while being more approximate than sectional methods used in [1-4] will afford considerable reductions in computational expense.

Figure 1 – Left: particle size distribution and average particle charge (white lines) using a 1-D numerical model for a parallel-plate geometry [1]. Right: spatial profile of gas temperature and temperature of 2-nm- diameter silicon particles, using a 2-D numerical model for plasma flowing through a circular tube [3].

References [1] P. Agarwal and S. L. Girshick, Plasma Source. Sci. Technol. 21, 055023 (2012). [2] P. Agarwal. S. L. Girshick, Plasma Chem. Plasma Process. 34, 489 (2014). [3] R. Le Picard, A. H. Markosyan, D. H. Porter, S. L. Girshick and M. J. Kushner, Plasma Chem. Plasma Process. 36, 941 (2016). [4] C. Larriba-Andaluz and S. L. Girshick, Plasma Chem. Plasma Process. 37, 43 (2017). [5] R. Le Picard and S. L. Girshick, J. Phys. D 49, 095201 (2016). [6] M. Mamunuru, R. Le Picard, Y. Sakiyama and S. L. Girshick, Plasma Chem. Plasma Process. 37, 701 (2017). 10

Particle Dynamics in Pulsed Dusty RF Plasmas

Toshisato Ono, Christopher J. Hogan and Uwe R. Kortshagen Dept. of Mechanical Engineering, University of Minnesota ([email protected], [email protected] and [email protected])

The vast majority of research in the field of nanodusty plasmas has focused on conditions in which the plasma is in steady state. Much less work has addressed the situation of particle dynamics in the plasma afterglow. However, the discharge afterglow is of exceeding importance, as particles will quickly lose their negative charge and are no longer confined by space charge electric fields. In semiconductor processing, this can cause particle deposition on wafers after the turning off of the plasma, which continues to be a problem even after decades of research into particle formation. Another issue that is often overlooked is the nature of the nanoparticle material. In studies of the charging of nanoparticles, quite often particles are considered to be electron and ion absorbing surfaces, without regard for the actual physical mechanisms of the electron trapping or detrapping to neutralize an ion. While some mechanisms, such as thermionic emission, consider the work function or electron affinity of a material, shallow surface trap states for electrons often remain unconsidered. However, due to the relatively low activation energy out of these trap states, electron desorption can be a significant process. In this presentation, we discuss results of a simple global model that explores the afterglow dynamics in a nanodusty plasma. In general, after the power source is removed, the electron temperature decreases rapidly on a time scale of the energy relaxation time, typically on the scale of microseconds. The electron density is also expected to decrease rapidly via recombination and/or diffusion losses to the boundaries. However, if electron detachment from the particles is a significant process, it can contribute to electron generation in the afterglow. In this study, the evolution of the electron desorption and electron-ion recombination on particle surfaces during the afterglow was investigated through modeling. The electron binding energy to the charged particle surface and the electron desorption rate were evaluated using quantum mechanical calculations by Bronold et al. [Physical Review Letters 101, 175002 (2008)]. The model suggests that electron detachment from particles can lead to a momentary increase of the electron density in the afterglow of pulsed dusty plasmas. A strong dependence on materials properties, in particular, the materials dielectric constant is observed. We also present experimental results from studies in a pulsed capacitive RF plasma. In these studies, laser light scattering is used to track the motion of injected micron-sized particles. Interestingly, we observe for microsized particles a similar trend as that predicted by our model, namely a strongly pronounced dependence of the particle dynamics on the dielectric constant of the particles used. While particles of low dielectric constant coagulate quickly in the afterglow of a plasma, particles of high dielectric constant remain individually dispersed. This suggests that particles of high dielectric constant remain negatively charged for relatively long periods of time in the afterglow of the pulsed plasma, which reduces particle agglomeration. This work was supported by the US Dept. of Energy Office of Fusion Energy Science (DE- SC0001939). Partial support is also acknowledged from Applied Materials.

Modification of Nanoparticle Formation in a Strongly Magnetized Plasma Edward Thomas, Jr.(a), Surabhi Jaiswal(a), N. Ivan Arnold(a), Mohamad Menati(a), Lenaic Couëdel(b), Mark J. Kushner(c) and Vijaya Rangari(d) (a) Auburn University ([email protected]) (b) University of Saskatchewan ([email protected]) (c) University of Michigan ([email protected]) (d) Tuskegee University ([email protected]) The formation of nanoparticles from the gas phase of plasmas has been a topic of interest to both basic and applied plasma physics research for some time. In space environments, these growth processes ultimately lead to the formation of the dust grains that will be the seed particles for planet formation. In industrial processes, the controlled formation of nanoparticles can be used to achieve desirable surface or optical properties. For over a decade, it has been postulated that the addition of a magnetic field can have a profound influence on the properties of a complex/dusty plasma. A number of experimental devices have been built around the world to explore the physics of dusty plasmas in strongly magnetized plasmas. Just over four years ago, the Magnetized Dusty Plasma Experiment (MDPX) device at Auburn University became the most recent facility to study dusty plasmas in strongly magnetized plasmas [1]. The MDPX device is a flexible, high magnetic field research instrument with a mission to serve as an open access, multi-user facility for the dusty plasma and basic plasma research communities. In a high magnetic field, B ≥ 1 T, the transport of ions and electrons in the plasma will be modified. Recent experiments have shown that this modification of the plasma at high magnetic field can have a strong influence on the properties of the plasma – through the generation of filamentary structures – and, in turn, on the growth of nanoparticles in the magnetized plasmas. In this presentation, the results of recent studies of particle growth in mixed Figure 1 – Photograph of the growth of nano- and argon-acetylene plasmas is reported [2]. microparticles in an argon-acetylene plasmas the MDPX The presentation will discuss the device. The vertical stripes that appear in the dust cloud are experimental observations of the aligned with filamentary plasma structures that are present in particle growth process, a comparison the plasma. The insert shows SEM images that illustrate differences in the particle morphology at different magnetic of particles grown with and without a fields. magnetic field, and the possible role of the plasma filaments in influencing the particle growth process.

This work is supported with funding from the NSF-DOE Partnership in Basic Plasma Science and Engineering (PHY-1613087 / DE - SC0016330), the NSF-EPSCoR program (OIA-1655280), and the Department of Energy Plasma Science Facilities (DE-SC0019176) programs. The MDPX device was originally designed and built with funding from the NSF-MRI program (NSF-1126067).

References [1] E. Thomas, et. al, J. Plasma Phys. 81, 345810206 (2015). [2] L Couëdel, et. al, Plasma Res. Express, 1, 015012 (2019). 12

Volt-Ampere Characteristics of Capacitively Coupled Plasma

Valery Godyak

University of Michigan ([email protected])

Volt-Ampere characteristics of gas discharges and scaling laws are valuable for understanding discharge physics and for plasma system design. In spite of highly nonlinear processes in plasma and in the electrode rf sheaths, the dynamic I/V characteristic of CCP, I(V), is quite linear during an rf period. That is not the case for the CCP static I/V characteristic shown in Fig.1, from Ref. [1].

Ar 0.03 Torr 13.56 MHz ∝ I2

∝ I discharge power, W discharge voltage, V

discharge current, A

Figure 1- Generalized CCP I/V Figure 2 - Experimental CCP V(I) and 2 characteristic for different eff = 0.1; W(I) characteristics. W ∝ I corresponds

1.0; and 10. VRp is Ohmic plasma to the ion acceleration power.

2 2 In practice, for CCPs with the discharge voltage exceeding the plasma voltage, V >> Vp , CCPs I/V characteristics are defined by the capacitive impedance of the electrode rf sheaths. 2 2 Experimental studies of static CCP I/V characteristics I(V), at V >> Vp for wide ranges of discharge power (10-2 – 102) W, frequency (3-100) MHz, and gas pressure (2∙10-4 – 10) Torr for Hg- He- and Ar-gas, have demonstrated close to linear CCP I/V characteristics (shown in Fig. 2, from Ref. [2]) contradicting to the widely accepted Lieberman rf sheath model, [3]. An explanation of this discrepancy and comparison of rf sheath models will be presented at the meeting.

References:

[1] V. Godyak, Sov. J. Plasma Phys. 2, 78 (1976). [2] V. Godyak et al, IEEE Trans. Plasma Sci. 19, 660 (1991). [3] M. Lieberman, IEEE Trans. Plasma Sci.16, 638 (1988).

Laser Diagnostics for Measurements of Electric Field and Excited Metastable Species in Nonequilibrium Plasmas E. Jans, K. Orr, Y. Tang, M. Simeni, and I.V. Adamovich Ohio State University ([email protected])

Non-intrusive laser diagnostic measurements of temporal and spatial distributions of electric field and the number densities of excited metastable species in nonequilibrium plasmas are essential for development of engineering applications such as plasma flow control, plasma-assisted combustion, plasma materials processing, and plasma medicine. This talk presents an overview of recent electric field and species measurements in ns pulse discharge plasmas, by ps Four-Wave Mixing (FWM), ps Electric Field Induced Second Harmonic (EFISH) generation, Cavity Ring Down Spectroscopy (CRDS), and Tunable Diode Laser Absorption Spectroscopy (TDLAS). Picosecond FWM and EFISH have been used to measure electric field in dielectric barrier discharge plasma flow actuators, atmospheric pressure flames enhanced by transient plasmas, ionization waves and streamers, and atmospheric pressure plasma jets. Both techniques provide sub-ns time resolution. Electric field vector components are isolated by monitoring signals with different polarizations, and absolute calibration is done by measuring a known Laplacian field. The main advantage of EFISH over FWM is that it is considerably more sensitive and species independent, such that it can be used in any high-pressure plasma. Absolute time-resolved populations of 3 + N2(A Σu ) excited electronic state, which is a major precursor of O atoms and NO in air plasmas, as well as H atoms and other radical species in fuel-air plasmas, are measured in a repetitive ns pulse discharge and the 3 + afterglow in nitrogen. N2(A Σu ) is also a likely precursor of UV radiation (NO γ bands) behind strong shock waves. Two complementary techniques are used for these measurements, CRDS and single- pass TDLAS. The results demonstrate considerable potential of laser diagnostic techniques for characterization of high- pressure nonequilibrium plasmas, where they provide quantitative insight into kinetics of ionization, charge transport, molecular energy transfer, energy thermalization rate, and plasma chemical 3 Figure 1 – N2(A Σu,v=0,1) populations during a 5-pulse ns reactions. discharge burst in nitrogen and in the afterglow, at P=130 Torr. References [1] E.R. Jans, K. Frederickson , T.A. Miller , and I.V. Adamovich, AIAA Paper 2019-0193, 2019 AIAA Aerospace Sciences Meeting (SciTech 2019), 7-11 January 2019, San Diego, CA [2] M. Simeni Simeni, Y. Tang, K. Frederickson, and I.V. Adamovich, Plasma Sources Sci. Technol. 27, 104001 (2018)

Advancing Diagnostics to Interrogate Dynamic and Structured Plasma E. V. Barnat(a), A. A. Hubble(b), J. E. Foster(b), A. M. Lietz(b), M. J. Kushner(b), M. S. Simeni(c), P. J Bruggeman(c) and V. I. Kolobov(d) (a) Sandia National Laboratories ([email protected]) (b) University of Michigan ([email protected], [email protected], [email protected]) (c) University of Minnesota ([email protected], [email protected]) (d) University of Alabama and CFD Research Corporation ([email protected])

Diagnostics play a key role in assessing our understanding of processes that occur in low- temperature plasma discharge environments. Not only do these diagnostics provide quantitative information that can be used to benchmark predictive simulations, but they often provide new insight into otherwise unappreciated phenomenon. As this is the final year of the plasma science center, and overview of advancements made in diagnostics will be presented as well as description of current efforts underway. Emphasis will be placed on the collaborative application of these diagnostics to study plasma systems of interest to the plasma science community. Early efforts centered on the development and application of laser-collision induced fluorescence (LCIF) technique enabled the measurement of spatial and temporal evolution of electron densities over a range of operating conditions spanning low pressure (mTorr) [1] to atmospheric pressure environments [2]. After successful benchmarking of the diagnostic, we have participated in collaborative endeavors with members of the plasma science center to utilize the LCIF method to interrogate various plasma systems of interest (Figure 1). Electric fields play a key role in governing the formation of plasma discharge and controlling when, where, and how energy gets deposited in the plasma environment. Therefore, recent efforts have focused on Figure 1 – Examples of plasma systems extending both second harmonic generation (SHG) and interrogated with the LCIF method. The upper set pump-probe based laser induced fluorescence-dip of images illustrate low-pressure magnetized spectroscopy (LIF-Dip) [3] to interrogate spatial and discharges (<100 mTorr), the middle sequence temporal evolution of electric fields at higher pressure illustrates modest pressure (10 Torr) striated and ultimately atmospheric pressure discharges. positive column and the lower set of images Preliminary results will be presented. illustrate higher presure (200 Torr) helium jet.

References [1] E. V. Barnat and K. Frederickson, “Two-dimensional mapping of electron densities and temperatures using laser-collisional induced fluorescence”, Plasma Sources Sci. Technol. 19, 055015 (2010). [2] E. V. Barnat and A. Fierro, J. Phys. D: Appl. Phys., 50 14LT01 (2017). [3] E. V. Barnat “Multi-Dimensional Optical and Laser Based Diagnostics of Low Temperature Ionized Plasma Discharges”, Plasma Sourc. Sci. Technol. 20 053001 (2011). [https://doi.org/10.1088/0963- 0252/20/5/053001] 15

Ions and Reactive Species Measurements in Time-modulated RF Driven Atmospheric Pressure Plasma Jets by Molecular Beam Mass Spectrometry Jingkai Jiang, Yolanda Aranda Gonzalvo and Peter J. Bruggeman University of Minnesota, Minneapolis, MN ([email protected]) Non-equilibrium atmospheric pressure plasmas are intensively used to interact with materials ranging from polymers and tissues to catalytic surfaces for surface modification, activation or disinfection. The detailed underlying mechanisms of the impact of plasma on materials are not well understood. This is mainly due to the extreme complexity in accurately measuring reactive species fluxes at the plasma-material interface. Such measurements are particularly challenging as species responsible for the plasma-material interactions are often present at ppm and sub-ppm concentrations. Plasma-material interactions strongly impact plasma properties. Nonetheless many of these interactions are included with reaction probabilities in models with huge uncertainties. In this work, a molecular beam mass spectrometer (MBMS) was designed and implemented to measure fluxes of neutral and ionic species from atmospheric pressure plasma at a substrate. MBMS is widely used as a diagnostic method in plasma processing with the advantage to be able to detect various species simultaneously including ions. We measured both neutral and ionic species from a time- modulated RF driven atmospheric pressure plasma jet in Ar+1% O2 to show the capability of the system. The average absolute densities of O and O3 as a function of nozzle-substrate distance in the afterglow of the plasma jet in the open air were measured, as well as the air mixing Figure 1 - Relative positive ion flux as a function concentration. Time-resolved measurements of of the nozzle-substrate distance in the afterglow of O, O2 and O3 during the modulated period an Ar+1%O2 APPJ in an open air environment. showed that the temporal increases of O3 densities were correlated with the increases in O2 due to transient vortexes at the plasma ignition and extension. This result quantitatively shows a direct impact of plasma-flow interaction on the generation of reactive oxygen species. Both positive and negative ion measurements were performed for a time-modulated RF driven APPJs in an open-air environment with different feed gases including Ar, Ar+1%O2, Ar+1% Air and Ar+0.244% water. An example of selected dominant ionic species is shown in Figure 1 as a function + of the nozzle-substrate distance for the Ar+1%O2 case. The figure shows a rapid transition from Ar2 , + + + + O2 and O4 , the dominant ions in the ionizing plasma, to NO and NO (H2O)n, in the early afterglow. + This transition coincides with the emergence of H3O (H2O)n, the dominant positive ions in the far + - afterglow. All gas compositions showed that water clusters of H3O and NO3 were found to be the dominant positive and negative ions in the late afterglow region. This is consistent with the fact that these ions are the most stable ions (lowest ionization energy and highest electron affinity). We have installed a vacuum chamber in front of the MBMS that enables us to measure in a controlled environment. The next step is to extend our measurements to Ar/CH4/O2 mixtures to identify the variations of species fluxes in the afterglow of APPJ and correlate these findings with the characterization plasma activated catalytic surfaces by the group of Oehrlein at the University of Maryland.

Measurements of Electric Field in Ns Pulse Discharges in Helium by Stark Splitting Polarization Spectroscopy Marien Simeni Simeni(a), Chuan-Jie Chen(a),(b), Mahsa Mirzae(a), Edward V. Barnat(c), Peter J. Bruggeman(a)

(a) University of Minnesota ([email protected]) (b) Dalian University of Technology ([email protected]) (c) Sandia National Laboratories ([email protected])

Stark splitting polarization spectroscopy of the helium line at 492.19 nm is used to measure spatially and temporally resolved electric fields in a nanosecond pin-pin discharge sustained in helium at atmospheric pressure and in a nanosecond atmospheric pressure helium plasma jet impinging on an ITO glass substrate in room air. Measurements of the electric field were performed by imaging the plasma onto the entrance slit of a high resolution monochromator with a spectral resolution of 40 pm backed by a ICCD camera. A plastic polarizer plate was inserted in front of the entrance slit to selectively collect polarized light emission from helium atoms affected by the Stark effect. The Stark splitting and shifting of the He I line at 492.19 nm (2p 1P0 – 4d 1D) and of its forbidden counterpart (2p 1P0 – 4f 1F) is strongly dependent on the electric field strength. For the He I at 492.19 nm, the separation between allowed and forbidden lines or the intensity ratio of the allowed and forbidden component can be used to determine the electric field [1]. Time-resolved axial electric field measurements in a pin-pin discharge with a temporal resolution of 5 ns were performed for different applied voltage amplitudes from 3 to 6 kV when the first emission is observed at the breakdown event. The observed electric fields have values significantly larger than DC breakdown field of helium at atmospheric pressure confirming these discharges are generated with significant overvoltage. We observed a transition from diffuse filaments to filaments having a hollow structures at breakdown most likely due to step-wise excitation and ionization Figure 1 – π-polarizered emission spectra of the allowed processes. 1 1 1 1 (1s2p P1-1s4d D2) and forbidden (1s2p P1-1s4f F3) These measurements have been components of the He I 492 nm lines in the pin-pin extended to the measurement of the discharge. The applied voltage of the discharge is 3 and 5 kV electric field vector components of a He obtained at the breakdown event. plasma jet impinging on a dielectric substrate. We measured peak electric field value of ~15 kV/cm in the streamer head. As the jet operates in ambient air, the 492 nm emission line show strong interferences with N2 (C-B) (ν’=1 – ν”=7) emission at similar wavelength. In addition, we observed the presence of an unshifted field free component of the He I line in spite of 4-ns time-resolved measurements. *The authors gratefully acknowledge financial support from US Department of Energy (DE- SC0001939) and a grant through the general Plasma Science Program (AT4010100).

References [1] B M Obradović and M M Kuraica, Phys. Lett. A. 372, 137-140 (2008). 17

Analytical Formula for Cluster Diameter and Its Dispersion at the End of Nucleation Stage Mikael Tacu(a,b), Alex Khrabry(b,c), and Igor D. Kaganovich(b) (a) Ecole Normale Supérieure Paris-Saclay, 94230, Cachan, France (b) Princeton Plasma Physics Laboratory, Princeton, NJ, 08543 (c) Lawrence Livermore Laboratory, Livermore, CA, 94550

We analyze cluster size evolution when vapor cools down and start forming small clusters or droplets. Main focus of this study is nucleation stage when there is a barrier for nucleation of small clusters due to surface effects. In the nucleation stage, as gas cools down and the saturation pressure drops precipitously with the temperature faster than gas pressure, the gas becomes supersaturated and therefore out of equilibrium (the gas pressure is higher than saturated vapor pressure). The return to equilibrium occurs in the nucleation burst via Figure 1 – (a) N as a function of time for Aluminum the rapid formation of small clusters/droplets, 1 with ̇ = 1000K/s, T0= 1773K, γ=0.948N/m (b) Mean when the barrier to formation of small clusters cluster diameter simulated with two codes. due to surface tension can be overcome at sufficiently lower temperature when the supersaturation degree sufficiently increased. Figure 1 shows typical evolution of monomer density N1 and average cluster diameter as metal vapor cools down. We performed simulations with both the Friedlander’s momentum model and the NGDE solver for aluminum vapor cooling with cooling rate = 1000 K/s and initial temperature T0= 1773 K [1]. The time elapsed before the clusters are generated in the nucleation burst and Figure 2 -The first two moments of the particle size 5 corresponding value of the supersaturation distribution function for T0= 1773 K and = 10 K/s. degree are crucial parameters describing the process of cluster formation and growth. We have derived the relation between the time of nucleation burst, the corresponding value of the supersaturation degree, as well as the cluster size and its dispersion at the end of nucleation stage and rate of gas cooling [2]. We find that the cluster size and its dispersion are proportional to the gas pressure and inversely proportional to the cooling rate. The dispersion is much smaller than mean diameter as evident from Figure 2.

References [1] S.K. Friedlander, “Dynamics of Aerosol formation by chemical reaction”, Ann. N.Y. Acad. Sci. 354 (1983). [2] Mikael Tacu, Alex Khrabry, and Igor D. Kaganovich “Analytical Formula for Cluster Diameter and Its Dispersion at the End of Nucleation Stage” submitted to arXiv and Phys. Rev. E (2019).

Electrostatic Solitary Waves Generated in the Process of Ion Beam Charge Neutralization by Electron Emission from a Filament Chaohui Lan(a,b) and Igor D. Kaganovich(a) (a) Princeton Plasma Physics Laboratory, Princeton, NJ, 08543 ([email protected]) (b) Institute of Fluid Physics, China Academy of Engineering Physics, Mianyang, 621900, P. R. China ([email protected])

Ion beam neutralization has attracted much attention in the past few decades and finds applications in many fields involving astrophysics, accelerator applications, inertial fusion, in particular fast ignition and heavy ion fusion, as well as ion beam based surface engineering. In laboratory, these electrons can come from gas ionization caused by ion beam, electron emission of hot filament, secondary Figure 1 – Schematic of two-stream instability electron emission caused by ion bombardment generation due to potential well of ion beam pulse. on metal or pre-formed plasma in the channel of Dashed ellipse represents ion beam pulse. Electrons ion beam propagation. When electrons are are injected downstream. attracted by a positive ion beam pulse, electrons experience complex process in the potential well of ion beam pulse. Figure 1 shows how electrons interact with the potential well and are captured by the ion beam pulse. For the approaching space-charge potential well, downstream injected electrons are firstly accelerated and then Figure 2 – Dynamics of ESW: electron densities and reflected by the potential well. Some of these the potential profiles shown in the ion beam refernce reflected electrons then once again are reflected frame. Two figures correspond to the propagation of by the other side of the potential well. the solitary wave to the left (a) and to the right (b), Meanwhile, the potential of ion beam drops due respectively. to the filling of electrons into the potential well, leading to the escape of fast electrons and the capture of slow electrons. Thus, bouncing back and forth of trapped electrons naturally forms two streams in the potential well of the ion beam pulse, which will cause the occurrence of instability in phase space. We show that excitation of nonlinear electrostatic solitary waves (ESWs) during this process can essentially affect the degree of ion beam neutralization [1]. ESWs were originally discovered during simulations of the nonlinear stage of the two-stream instability, and have been observed in space and laboratory plasmas for many years. See Hutchinson’s paper for a recent review of ESWs [2]. But surprisingly, as far as we know they have never been reported in ion beam neutralization experiments, and even never been mentioned or investigated in related literatures. Figure 2 shows variations of electron density and the potential when the ESW moves in different directions, system parameters are given in Ref.1.

References [1] C. Lan and I. D. Kaganovich, “Electrostatic solitary waves in ion beam neutralization”, arXiv:1810.04655 and submitted to Phys. Plasmas (2019). [2] I. H. Hutchinson, Physics of Plasmas 24, 055601 (2017).

Electron Kinetics in Low Temperature Plasma Vladimir I Kolobov(a) and Valery Godyak(b) (a) University of Alabama in Huntsville ([email protected]) (b) University of Michigan ([email protected]) Low Temperature Plasma (LTP) is known for its highly nonequilibrium nature. The main causes of non-equilibrium for electrons are large spatial gradients, strong electric fields, fast temporal variations, and collisions. Another key feature of LTP is multiple time scales. The slowest time scale is the ion transport time, . The electron momentum and energy relaxation times, and , are much shorter than the ion transport time. Furthermore, both and depend strongly on electron energy. Thus, depending on the characteristic time scale and the corresponding spatial scale, different models for electron kinetics can be applied. For thermal electrons in noble gases, the momentum relaxation occurs faster than the energy relaxation ( ≪ ), and the electron mean free path is considerably smaller than the electron energy relaxation length . as shown schematically in Figure 1. When the device size L is larger than the phase velocity of electromagnetic field in plasma , full Maxwell solver should be used. At ≪ quasi-static (QS) models can be applied.

For spatial scales exceeding  , and the time scales exceeding , the electron transport can be described by fluid models with EDFs depending on local electric fields. For spatial scales exceeding m , and time scales exceeding Figure 1 – Selection of electron kinetic models depending , the EDF is close to isotropic in on characteristic temporal and spatial scales. velocity space, and can be described by a Fokker-Planck kinetic equation (FPE). For simulation of such plasma, it is convenient to use total electron energy as independent variable, where Vp is the plasma potential. At ≪ and for the time scales smaller than , full Boltzmann transport equation (BTE) must be used. For Coulomb interactions among electrons, m and  , are of the same order of magnitude and strongly decrease with decreasing electron energy. For molecular gases, m and  have large values at energies corresponding to excitation of rotational and vibrational states of molecules. Their values depend not only on electron energyes but also on specific populations of rotational and vibrational states. Thus, high-fidelity calculations of EDF in molecular gases requires self- consistent analysis of electron kinetics and vibrational kinetics of molecules. We will present an overview of recent advances in the theory and experimental studies of electron kinetics in LTP and provide our views on where the field is headed and on promising strategies for progress [1]. Using appropriate models for electrons in numerical plasma codes would be awarded with more realistic perception and understanding of LTP systems.

References [1] V. I. Kolobov and V. A. Godyak, Electron kinetics in low-temperature plasma, Invited Perspective Paper submitted to Phys Plasma (2019).

Striations in Atmospheric Pressure He/2%H2O Plasma Discharges M.A. Lieberman, E. Kawamura and A.J. Lichtenberg University of California, Berkeley ([email protected])

Narrow gap atmospheric pressure plasma (APP) discharges have wide ranging applications, especially in the biomedical field. Discharge control for applications can be disrupted by instabilities. 1D particle-in-cell (PIC) simulations of an rf current-driven 1 mm gap He/2%H2O APP showed standing striations (spatial oscillations) in the bulk [1]. We developed a linear striation theory, which found that these striations are due to non-local electron kinetics that cause the ionization rate coefficient Kiz to decrease with increasing rms electric field E. Discharges with lower ion mobility and larger bulk recombination rates tended to be more unstable. We determined a critical wavelength such that shorter wavelengths are suppressed by diffusion while longer wavelengths may be restricted by the finite gap width for narrower gaps and a transition to local electron kinetics for wider gaps. We extended the gap size of the He/2%H2O APP's in the PIC simulations to 2 and 4 mm driven by either dc or rf current sources [2]. We found that wider gap discharges tend to be more unstable, as they can accommodate a wider range of wavelengths. The mixture of the various excited modes in the wider gaps can lead to distinctly non-sinusoidal spatial oscillations. We also conducted 1D PIC simulations of a 1 mm gap He/2%H2O APP, driven at low frequencies well below the ionization frequency, which showed reasonable agreement with a time-varying global model [3]. For wider gaps, we assumed that a transition to locality was the stabilizing factor. We have recently conducted 1D PIC simulations of a 4 mm gap rf current-driven He/2%H2O APP with multi- mode striations, in order to study the effect of striation wavelength on the discharge instability and directly observe the transition to locality. In the striation theory, we assume that Kiz(x) is related to q E(x) by Kiz/Kiz0 = (E/E0) , where Kiz0 and E0 are the equilibrium values of Kiz and E in the bulk. The excited modes are cut off at shorter wavelengths by diffusion and at larger wavelengths by a transition to locality. In the window of instability, non-local electron kinetics cause the exponent factor q < 0. At longer wavelengths q becomes less negative, indicating a transition to locality, in which local calculations give q. All our previous models assumed a simplified chemistry with no negative ions. Figure 1 shows the PIC results for densities of a full chemistry He/2%H2O APP driven at 0.23A/cm2@27MHz. The ions have two peaks for every electron peak. A non-linear striation model including negative ions was developed which showed reasonable agreement with the PIC results, but also some differences, since the model assumption of quasi- neutrality in the bulk was not satisfied for these discharges.

Figure 1 – PIC results for densities of a He/2%H2O APP driven at 0.23A/cm2@27MHz.

References [1] E. Kawamura, M.A. Lieberman and A.J. Lichtenberg, Plasma Sources Sci. Technol. 25, 054009 (2016). [2] E. Kawamura, M.A. Lieberman and A.J. Lichtenberg, J. Phys. D: Appl. Phys. 50, 145204 (2017). [3] E. Kawamura, M.A. Lieberman and A.J. Lichtenberg, Phys. Plasmas 25, 013535 (2018).

Characterization of Non-Equilibrium Flow in Inductively Coupled Plasma Torches Savio J. Poovathingal(a), Juliusz Kruszelnicki(b), Iain D. Boyd(a) and Mark J. Kushner(c) University of Michigan, Ann Arbor, MI, USA 48109 (a) Department of Aerospace Engineering ([email protected], [email protected]) (b) Department of Nuclear Engineering and Radiological Sciences ([email protected]) (c) Department of Electrical Engineering and Computer Science ([email protected]) Experimental facilities using inductively coupled plasmas (ICPs) are being used to evaluate materials for high-temperature applications in the aerospace industry. In these facilities, gas is heated to temperatures of 5,000-10,000 K by the plasma produced by an electromagnetic field formed inside a plasma torch consisting of a cylindrical quartz tube with solenoidal coils. The coils are usually near the gas inlet (see Fig. 1) resulting in a high degree of non-equilibrium in the excitation reaction with quenching and recombination processes dominating as the plasma flows downstream. The material test conditions can be extrapolated to aerospace flight conditions only if the gas exiting the plasma torch is in local thermodynamic equilibrium. A recent study has shown that the final thermodynamic state is a strong function of the non-equilibrium state of the plasma near the coils of the torch [1]. ICP facilities are also being used to validate new gas- material chemical kinetic models for high- temperature applications. Characterizing the state of the plasma inside the torch and determining the thermodynamic conditions will provide key inputs for test conditions and enable a framework to optimize the operating conditions of the plasma torch. To meet these objectives, simulations were performed using nonPDPSIM, a 2-dimensional, non-equilibrium plasma hydrodynamics model [2] for an ICP sustained in Ar [3]. The nonPDPSIM modules employed were the plasma transport module (charged particle continuity equations and Poisson's equation), the electromagnetic module (2D frequency domain solution for coil generated Figure 1 – a) Schematic of the plasma torch with electric fields), and the fluid module (Navier Stokes the number density of electrons. b) Density of equations). The modules were combined in a time- electrons and excited states at r = 1.75 cm. splicing manner until the steady state is reached. The plasma torch at the University of Vermont was simulated [4], where spectroscopic data is being collected for validation purposes. A schematic of the torch and the electron density are shown in Fig. 1a, which demonstrates the formation of a localized plasma near the coils. Axial profiles of the density of electrons and excited states at r = 1.75 cm are shown in Fig. 1b. A region of strong non-equilibrium occurs near the coils, where the electron and excited state densities are significantly higher than nominal equilibrium values. However, the densities decrease inside the tube as the plasma flow downstream while retaining a significant gradient in the radial direction for all quantities. The influence of thermodynamic non- equilibrium on the exit conditions in the torch will be discussed. References [1] W. Zhang, A. Lani, and M. Panesi, Phys. Plasmas, 23, 073512 (2016). [2] S. A. Norberg, E. Johnsen and M. J. Kushner, Plasma Sources Sci. Technol. 24, 035026 (2015). [3] P. Tian and M.J. Kushner, Plasma Sources Sci. Technol., 24, 034017 (2015). [4] W. Owens, J. Uhl, M. Dougherty, A. Lutz, J. Meyers, and D.G. Fletcher, AIAA, 4322 (2010).

Generation of Non-thermal Plasmas at Moderate and Atmospheric Pressures Yevgeny Raitses, Shurik Yatom and Sophia Gershman Princeton Plasma Physics Laboratory ([email protected], [email protected], [email protected])

In this talk, we review results of recent studies of microchannel plasma sources developed at PPPL. One of the sources is based on a cold cathode glow discharge operating in a dc steady state mode in a moderate pressure range of 2–10 torr. The source consists of a flat cathode and a hollowl anode. Ion-induced secondary electron emission (SEE) from the flat cathode is the source of electrons accelerated to high energies in the cathode sheath potential. The source geometry and the applied DC voltage are keys to the availability and the extraction of the nonthermal portion of the electron population [1]. In experiments, the source was operated in the discharge voltage range of 400- a) 700 V. Langmuir probe measurements revealed the presence of electrons with mean energies of ~15 eV outside of the microplasma source. Independently, two-dimensional Particle-in-Cell simulations confirmed that this plasma source can indeed generate high energy electrons b) (energy > 100 eV), but only when the applied 311.67 nm 250000 voltage exceeds 500 V [2]. At lower voltages, C3 -B3 (3-2) N 2+nd system u g energetic electrons dissipate almost all their 2 energy in the plasma column. The developed 200000 plasma source can be useful for applications 150000 310.4 nm 3 3 which require selective processing. C u-B g (3-2) 100000 We have also studied a flexible plasma (arb.u.) Intensity OH (A-X) (0-0) source based on a dielectric barrier discharge (DBD) operating at atmospheric pressure. The 50000 flexibility is achieved due to the use a printed 0 circuit board (each opening is less than 0.1 cm in 3060 3080 3100 3120 diameter). The source is well-suited for plasma Wavelength (A) treatment of complex shape and large area Figure 1- a) PPPL flexible DBD discharge and b) Emission of hydroxyl radical surfaces with potential applications for human healthcare [3]. In recent experiments, electrical characteristics and optical emission spectra of the plasma generated by the flex DBD-based source were characterized. The focus of this parametric study was on scaling of the power with the source area and geometry. For the operation in air, the emission spectrum is dominated by emission of molecular N2. In addition, the emission of OH (A- X)(0-0) band is also detected. This serves as an evidence for dissociation of water molecules present in the ambient atmosphere. The N2 bands were used to calculate the rotational and vibrational temperatures. Additionally, the ratio of the intensities of the bandheads of N2 second positive system + (2-5) and the N2 first negative system (0-0) were utilized for evaluation of the reduced electric field [4], for a range of operating powers. Results of these studies will be discussed in this talk.

References [1] S. Gershman and Y. Raitses, J. Phys. D: Appl. Phys. 51, 235202 (2018) [2] D. Levko, Phys. Plasmas 26, 013517 (2019) [3] B. Boekema et al., J. Phys. D: Appl. Phys. 49, 044001 (2015) [4] I. N. Kosarev et. al., Plasma Sources Sci. Technol. 21, 045012 (2012)

Self-Organization in 1 atm DC Glows: Current Understanding and Potential Applications John E. Foster(a), Yao Kovach(a), Janis Lai(a) and Maria C. Garcia(b) (a) University of Michigan ([email protected]) (b) Universidad de Cordoba, Spain ([email protected]) Self-organization refers to the spontaneous generation of spatially or temporally organized patterns in an otherwise disordered system. Self-organization is ubiquitous in plasma physics particularly in the low pressure regime such as those observed in astrophysical jets or plasma loaded flux loops that form on the surface of the sun.[1] In recent times, the observation of self-organization in atmospheric pressure plasmas has captured the attention of researchers. The mechanism of formation is still not well understood. Here we survey the broad high pressure occurrence of self- organized plasmas and review the most recent theories on their formation. We also describe experiments aimed at understanding their formation as well as suggest applications. While self- organization in 1 atm plasmas includes a range of different discharge types, this survey will focus in particular on self-organization in 1 atm DC glows with a liquid anode.[1] While these systems produce intricate self-organization patterns, they also the basis for a number of technological applications. An unusual pattern observed in a DC glow experiment with liquid water anode is shown in Fig. 1, representing a precise spatial distribution of luminous plasma on the liquid surface. What physical mechanisms at the surface support this plasma production? What role does fluid effects such as circulation and localized evaporation play in the maintenance of these steady state patterns? What role does ion flow play at the interface in regard to chemistry induced and charge transfer? Correlations between patterns with solution properties such as pH and conductivity have been studied. Studies also suggest the importance of negative ions in the formation of patterns.[3] Results will be discussed from DC glow experiments aimed at understanding self- organization. Here the dependence of pattern formation on electrolyte type has been explored. The interplay between localized evaporation and plasma column formation has been investigated spectroscopically, illustrating the surprising role of electrolytes in the gas phase. Recently observed particle emission is also discussed in the presence of self-organization. We also discuss a recently observed new form of self-organization associated with coupling between fluid and plasma dynamics. These observations have been made in a 2-D bubble apparatus where streamers couple with elastic fluid oscillations to give rise to self-organized patterns. The use of plasmas with liquids for technologically applications require an understanding of plasma liquid interaction and transport for scale up. Self-organization may play a key role in scale- up. We speculate on the impact of self-organization on the optimization of reactors for technology applications which can give rise to mixing and plasma contact area modification which changes depending on the nature of the self-organization pattern shape.

References [1] astro: arXiv:1708.03394v2 [2] Trellis, J.P., Journal of Applied Physics D. 49, 393002 (2016) [3] Shirai, N., Uchida, S., and Tochikubo, F., Plasma Source Sci. Technol. 23, 54010 (2014).

Figure 1 – Self-organization pattern observed in DC glow with liquid anode. 24

Studies of Plasma Surface Interactions of Plasma Catalysis (a) (a) (a) (a) (b) (a) S. Zhang , Y. Li , A. Knoll , P. Luan , P. J. Bruggeman and G. S. Oehrlein (a) University of Maryland, College Park ([email protected]) (b) University of Minnesota, Twin Cities ([email protected]) Low temperature plasma has shown potential to be capable of enhancing catalytic reactions. The understanding of plasma catalyst surface interactions is required to reveal the origin of plasma catalyst synergy and optimize plasma-catalysis performance and product selectivity. Using a similar approach to prior collaborative work on polymer etching [1,2], a well-characterized atmospheric pressure plasma jet (APPJ) was employed to study plasma-surface interactions of plasma catalysis at the University of Maryland (UMD) whereas gas phase processes are being investigated at the University of Minnesota. At UMD we employed Fourier transform infrared spectroscopy (FTIR)-based techniques for understanding the plasma catalysis of partial methane oxidation by the Ar/O2 APPJ-Ni catalyst system. The composition of gaseous product from plasma catalysis was studied downstream by FTIR absorption spectroscopy [3], whereas diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used for in-situ (a) characterization of plasma catalyst surface 0.04 Ar/O switched on interactions. Figure 1(a) shows a schematic of the 2 Plasma power: 2 W. Ar/O : 200/1 sccm 2 plasma assisted catalysis reactor that integrates a Ar/CH :400/2 sccm 0.03 4 DRIFTS set up. Surface-bound CO can be 3 mm 25 oC detected on the supported Ni catalyst during 0.02 plasma treatment by DRIFTS along with other signals. The absorbance of surface bonded CO C-O bond (A.U.) bond C-O 0.01 8 mm 500 oC depends strongly on the distance between plasma- o 25 C o source and catalyst surface, catalyst temperature, 500o C 0.00 500 C, no plasma gas composition, plasma power, and exposure -5 0 5 10 15 20 25 30 35 time (see Fig. 1 (b)). DRIFTS enables us to study Time (min) (b) CO2, CH4, CHn (n=1, 2, 3) and H2O , and the dependence of the observed surface moieties Figure 1 – (a) Schematic of plasma assisted changes with the variables mentioned above. catalysis reactor using a supported Ni powder catalyst that integrates a DRIFTS set up for real- Ultimately, the observed changes on surface time plasma-surface interaction studies; (b) chemistry will be correlated to the incident DRIFTS C-O signal versus time for different plasma species, which will provide more detailed APPJ-catalyst distances and catalyst insights on plasma-induced processes at catalyst temperatures for an Ar/O2 discharge. surfaces. * The authors gratefully acknowledge financial support from US Department of Energy (DE-SC0001939) and NSF (CBET-1703211).

References [1] P. Luan, A. J. Knoll, P. J. Bruggeman, and G. S. Oehrlein, J. Vac. Sci. Technol. A 35, 05C315 (2017). [2] P. Luan, A.J. Knoll, H. Wang, V.S.S.K. Kondeti, P. J. Bruggeman, and G. S. Oehrlein, J. Phys. D: Applied Physics, 50, 03LT02 (2017) [3] A. J. Knoll, S. Zhang, M. Lai, P. Luan, and G. S. Oehrlein, J. Phys. D: Applied Physics, 52, 225201 (2019).

Controlling Micro-gap Breakdown with Engineered Surface Morphology Yangyang Fu, Janez Krek and John P. Verboncoeur Michigan State University (fuyangya, krek, [email protected])

Gas breakdown in microscale gaps has become an active area of investigation with growing attention on micro-discharges and their wide applications. Even though gas breakdown has been extensively investigated in gaps between finished planar electrodes, electrode surface defects cannot be completely avoided, and the discharge properties can be greatly affected by the electrode surface status [1]. As the gap distance shrinks to microscales, the electrode surface morphology, such as surface roughness and protrusions, becomes more pronounced with respect to the breakdown processes. In this work, we study the effects of single and multiple protrusions on the cathode electrode, with mutual interactions (e.g., electric shielding effect) on micro-gap breakdown processes, seeking engineered surface control of breakdown voltage. The breakdown voltages are quantified in the Townsend discharge regime based on the voltage-current characteristics. With a single surface protrusion, the breakdown (b) path can automatically shift from the top of the protrusion to its side surface, and then to the wider non-protrusion electrode gap, with dependence on the gas pressure. This phenomenon results in a combined Paschen’s curve, which transits from the long-gap behavior at low pressure to the short-gap behavior at high pressure, with Figure 1 – Effect of the multiple cathode protrusions on relatively low breakdown voltage in a wide the normalized electrical potential distriubtion (a) and range of gas pressure [2]. Multiple distributed current density on the protrusion tips (b). protrusions on the electrode surface (Fig. 1) smooth the perturbation of the electric field in a short distance due to the electrical shielding effect, with the current density mainly distributed on the protrusion tips due the field enhancement [3]. The breakdown can occur along the longest discharge path at low pressures when the protrusion spacing is larger and the shielding effect is not significant or along the shortest path at high pressures when the electric field is enhanced on the protrusion tips. This work elucidates micro-gap gas breakdown characteristics with various surface morphologies, enabling design of devices with predictable breakdown characteristics across a wide range of electrode surface conditions, pressures, and gap spacing.

This work was supported by the U.S. Department of Energy (Grant No. DESC0001939) and the U.S. Air Force Office of Scientific Research (Grant No. FA9550-18-1-0061, FA9550-18-1-0062). References [1] A. Venkattraman, J. Phys. D: Appl. Phys. 47, 425205 (2014). [2] Y. Fu, P. Zhang, and J. P. Verboncoeur, Appl. Phys. Lett. 113, 054102 (2018). [3] Y. Fu, Janez Krek, P. Zhang, J. P. Verboncoeur, Appl. Phys. Lett. 114, 014102 (2019).

What Have We Learned About Controlling Atmospheric Pressure Plasmas? Natalia Babaeva, Juliusz Kruszelnicki, Kenneth Engeling, John E. Foster, Eric Johnsen, Amanda Lietz, Soheila Mohades, Seth Norberg, Guy Parsey, Wei Tian, Jun-Chieh Wang, Zhongmin Xiong and Mark J. Kushner

University of Michigan, Ann Arbor, MI, 48109-2122 USA ([email protected]) During the course of the Plasma Science Center, atmospheric pressure plasmas have come to the forefront of the field of low-temperature plasmas as sources of reactive species for materials processing and biological applications. Fundamental plasma processes occurring in atmospheric pressure plasma jets (APPJs) and dielectric barrier discharges (DBDs) are being investigated with the goal of controlling reactive fluxes delivered to hard, soft and liquid surfaces. The field has come to a consensus that controlling APPJs and DBDs is more difficult than low pressure plasmas due to the shorter equilibration timescales and smaller collisional scale lengths inherent to these systems, particularly when treating surfaces. When treating soft, complex and liquid surfaces, those shorter spatial and time scales couple the plasma more closely to the surface, thereby making control even more difficult. Computational investigations have been performed for a large variety of APPJs and DBDs with the goal of understanding the fundamental plasma processes that will enable development of real-time-control strategies. Results from those investigations will be discussed. To control the species produced in plasma activated liquid first requires control of the plasma produced reactants, though the mapping is not one-to-one due to the need to transport that reactivity across the gas- liquid boundary. Examples will be discussed for investigations of He APPJs into ambient air addressing operational and environment variability. Operational variability includes, for example, repetition rate, flow rate, impurities, electrode design and flat liquid surfaces vs liquid droplets. Environmental variability includes, for example, location of electrical ground planes, ambient gas flow and angle of APPJs with respect to the surface. The coupling of plasma kinetics and chemistry with gas dynamics is an underutilized control strategy. This is particularly true for the reactivity produced with air containing plasmas where reactive oxygen species (ROS) typically require fewer reactions to produce compared to RNS (reactive nitrogen species). Here gas residence time, the repetitive production of primary species amongst reaction products from previous pulses, penetration of ionization waves (IWs) into the ambient and gas temperature can be leveraged to control the ratio of ROS to RNS. The coupling of plasma kinetics with surfaces is highly dependent on the complexity of that surface, and generalizations are difficult to make. For example, porosity not only increases surface area but also changes the local value of pd (pressure  transport distance), which impacts excitation rates as a function of E/N (electric field/gas number density). The character of surface ionization waves (SIWs) depends on the polarization of microstructures of complex dielectric surfaces, onset of secondary processes (e.g., electric field emission) and surface charging. The solvation of charged and neutrals species into liquids depends on the past history of treatment. Due to species dependent rates of transport of plasma produced species into droplets immersed in an APPJ or DBD, RNS-to-ROS ratios in and pH of the droplet are sensitive functions of droplet diameter.

Abstracts - Poster Presentations

Low Temperature Plasma Similarities from Low to High Ionization Regimes Yangyang Fu, Janez Krek, and John P. Verboncoeur Michigan State University ({fuyangya, krek, johnv}@egr.msu.edu)

Similarity laws of gas discharge physics have been historically derived and continuously developed to correlate fundamental characteristics among two or more compared plasma systems [1, 2]. Based on the similarity law theory, under certain conditions physical parameters in geometrically similar plasma vessels of different dimensions can be linearly scaled with dimensional factors [3]. For weakly ionized low temperature plasma discharges, the similarity laws were previously found to be valid for geometrically similar gaps, which could be used to extrapolate the discharge characteristics. In this study, transition characteristics of low-temperature plasma similarity laws are evaluated from low to 100 n /n high ionization degree regimes, using a e1 e10 kinetic global model with a coupled n /n e1 e3 Boltzmann solver in argon. The similarity n /n e1 e2 relation of plasma density to ionization 10 degree in geometrically similar gaps is presented. The validity of the classical density ratio similarity laws holds at higher ionization degree for low pressure than for higher 1 pressure. For high pressure cases, the -10 -8 -6 -4 -2 10 10 10 10 10 deviations of the similarity laws are X observed with different scaling factors, as 0 shown in Figure 1. With the validity of Figure 1 – The plasma density ratio versus ionization degree X in geometrically similar gaps with scaling factor the similarity laws in the low ionization 0 k. ne1 is the unscaled case (p = 760 Torr, d = 0.1 cm, r = regime, the time-dependent scaling of the 0.1 cm), and the corresponding scaling factors are k = 2 for individual species density as well as the ne1/ne2, k = 3 for ne1/ne3, and k = 10 for ne1/ne10. electron energy distributions are also validated. The transition characteristics are affected by the significance of nonlinear reaction processes. The motivation for this study is to seek a method to estimate bulk plasma characteristics based on geometric similarity of discharges at different scales. The results are beneficial for understanding and utilizing the similarity laws in a wide range of plasma ionization degree regimes, and allow predictable scaling of plasma devices within regimes of validity.

This work was supported by the U.S. Department of Energy (Grant No. DESC0001939).

References [1] M. U. Lee, J. Lee, J. K. Lee, and G. S. Yun, Plasma Sources Sci. Technol. 26, 034003 (2017). [2] Y. Fu, G. M. Parsey, J. P. Verboncoeur, and A. J. Christlieb, Phys. Plasmas 24, 113518 (2017). [3] X. Tan and D. B. Go, J. Appl. Phys. 123, 063303 (2018).

Electric Field Distribution in an Atmospheric Pressure, Ns Pulse, Helium Plasma Jet Measured by Ps Second Harmonic Generation Keegan Orr(a), Yong Tang(b), Marien Simeni Simeni(c), Igor V. Adamovich(d), Dirk Van Den Bekerom(d) (a) Ohio State University ([email protected]) (b) Tsinghua University, Beijing ([email protected]) (c) University of Minnesota ([email protected]) (d) Ohio State University ([email protected]) (e) Ohio State University ([email protected]) Temporal and spatial distributions of electric field in an atmospheric pressure, ns pulse discharge He plasma jet are measured by picosecond Electric Field Induced Second Harmonic (E- FISH) laser diagnostics [1,2]. This experimental technique is species-independent, such that it can be used for measurements of electric field distributions in plasmas sustained essentially in any high- pressure gas mixture, with high (sub-ns) temporal resolution. The measurements have been done in a two-dimensional He plasma jet exhausting from a 44 mm x 4.5 mm rectangular cross section channel and impinging on a liquid water surface. Electric field distributions are measured using a ~10 mm wide laser sheet, at different heights above the water surface, up to 2.0 mm. The discharge is sustained by positive and negative polarity, repetitive ns pulse voltage waveforms. Absolute calibration of the electric field is provided by measuring a known electrostatic electric field in the same geometry and at the same flow conditions. The vertical component of the electric field vector is determined by measuring the second harmonic signal with vertical polarization. The electric field is averaged over the span of the two-dimensional plasma jet (in the direction of the laser sheet), with the spatial resolution across the laser beam of approximately 20 µm and temporal resolution of 5 ns. The line-of-sight averaging is justified by the plasma emission images, Figure 1 – Vertical eletric field distributions in a which illustrate that the plasma remains positive polarity ns pulse discharge plasma jet taken diffuse an apparently uniform across the span with a laser sheet at different heights above the water of the jet. The spatial resolution of the surface, 45 ns after the voltage rise. present measurements is limited by the size of the E-FISH signal image on the ICCD camera used as a detector. Fig. 1 shows the distributions of the vertical component of the electric field in the He plasma jet. The two well-pronounced peaks in the signal are caused by the plasma layers near the walls of the channel where the plasma jet is formed. The electric field data will be accompanied by schlieren imaging of the plasma jet, illustrating the effect of the plasma on the jet flow. The new experimental results provide essential new insight into kinetics of plasma-liquid interaction, for biomedical applications, and produce extensive sets of data for detailed validation of high-fidelity kinetic models of these plasmas.

References [1] B.M. Goldberg, T.L. Chng, A. Dogariu, and R.B. Miles, Appl. Phys. Lett. 112 (2018) 064102. [2] 2. M. Simeni Simeni, Y. Tang, K. Frederickson, and I.V. Adamovich, Plasma Sources Sci. Technol. 27 (2018) 104001

Electrical Discharge in Gas Bubbles in Gel S. Gershman and Y. Raitses Princeton Plasma Physics Laboratory ([email protected])

This work presents electrical and spectral characteristics of an electrical discharge in a hydrogel. In parallel with fast development of the biological applications of plasmas, the interest in hydrogels has been growing due to their significance in biological applications from drug delivery to wound healing and as tissue models. Hydrogels may be used in conjunction with plasma treatment in plasma medicine and for the study of plasma interaction with biological systems. [1] The polar nature of water molecules makes hydrogels significantly different from the well-studied silicone gels used as insulation. Hence, partial electrical discharges in hydrogels are interesting for their biological applications as well as for advancing our understanding of the interactions of plasmas with water based soft materials. [2] The hydrogel in this study contained 9% of dissolved hydrogen peroxide by weight and <1 mm size gas bubbles composed of air/H2O2. The gel was placed in quartz cuvette between needle and plane electrodes. 1 s rectangular 5 – 10 kV pulses were applied to the electrodes. [3] In addition to the micro- Figure 1 – Spectra of the 306.4 nm OH band emission from Ar bubble at 7 kV bubbles, an Ar bubble in negative and positive (needle) polarity. was introduced around the needle electrode. The voltage pulses applied at ~10 Hz resulted in a consistent partial discharge in the bubble, repeatable over hundreds of shots with ~10 A current pulses. The optical emission spectroscopy showed the presence of active species such as OH radicals. The spectra showed significant difference in the OH 306.4 nm system for the positive and negative (needle) polarity discharges at the same applied potential difference between the electrodes (Fig. 1). The discharge in positive polarity is initiated at a lower voltage for the same size bubble and the spectrum at the same voltage, shows a much more intense OH emission. In the future, gas and electronic temperatures will be determined from the OH and the N2 band systems and Ar of H lines. In addition, imaging will be used to study the propagation of the discharge and its mechanical effects on the gel. It is expected that the viscoelasticity of the gel will result in fast dampening of the bubble oscillations and motion. [4] Experiments show that an inert gas, such as Ar can be injected to produce stable, reproducible partial discharge that generates active species by interacting with the surface of the gel.

References [1] L. H. Yahia, et al., J Biomedical Sci. 4 100013 (2015) [2] J. Ino, et al. Biomatter., 3 25414 (2013) [3] S. Gershman and A. Belkind, Eur. Phys. J. D: Appl. Phys. 60, 661 (2010) [4] M. Sato et al., J. Phys. D: Appl. Phys. 47 155201 (2014)

Electron Densities and Temperatures Measurements in Atmospheric Pressure Nanosecond Pulse Helium Discharges

V.S. Santosh K. Kondeti(a), Marien Simeni Simeni(a), Nader Sadeghi(b), Peter J. Bruggeman(a) (a) University of Minnesota ([email protected]) (b) Université Joseph Fourier Grenoble, France ([email protected])

In non-equilibrium plasmas, electrons are responsible or directly enable all major processes occurring in the plasma, this includes ionization, the generation of reactive species and photons. As a consequence, the electron density (ne) and temperature (Te) are one of the main fundamental plasma parameters and an understanding of plasma processes requires the knowledge of ne and Te. Different techniques have been used to measure ne including Stark broadening, collision laser induced fluorescence, MW interferometry and Thomson scattering [1]. Thomson scattering has the major advantage that it can be used to simultaneously measure ne and Te and in principle with very high spatial and temporal resolution only limited by the laser beam profile and pulse duration. Thomson scattering intensities in non-equilibrium low temperature plasmas are typically much smaller than Rayleigh scattering and hence the Rayleigh signal needs to be removed. Traditionally a triple grating spectrometer has been used but recently volume Bragg gratings have been produced with sufficient narrow bandwidth and sufficient reduction in Rayleigh scattering intensity Figure 1 – Room air pure rotational Raman spectrum together with the (~ 10-4) to enable Thomson best synthetic fit. scattering measurements with a regular monochromator [2]. We implemented such a Bragg filter and Figure 1 shows the capability of the system to reduce the Rayleigh scattering in air by more than 5 × 10-3 enabling the measurement of rotational Raman scattering signal in air. We also obtained preliminary Thomson scattering results for an atmospheric pressure plasma jet. For incoherent Thomson scattering the scattering is a Doppler profile. The width relates to Te and the area relates to the ne. The density calibration is performed recording room air pure rotational Raman spectrum. While current measurements are point measurements, the advantages of this new approach include a higher efficiency (more signal) thanks to less optical components in the optical path and a simpler and more compact apparatus system (single stage VS triple stage monochromators).

References [1] P. J. Bruggeman, and R. Brandenburg, J. Phys. D. 46, 464001 (2013). [2] B. Vincent, S. Tsikata, S. Mazouffre, T. Minea, and J. Fils, PSST. 27, 055002 (2018).

Infrared Studies of Catalyst Surface During Plasma-catalysis: CHn Groups (a) (a) (b) (b) (a) Y. Li , S. Zhang , J. Jiang , P. J. Bruggeman and G. S. Oehrlein (a) University of Maryland, College Park ([email protected]) (b) University of Minnesota, Twin Cities ([email protected]) Cold atmospheric plasma (CAP) sources can produce different types of reactive chemical species which are useful in many applications such as waste destruction, materials processing, biomedical engineering, and others. The enhancement of thermal catalysis by cold atmospheric plasma has also attracted a great deal of attention. In this study, we examined the behavior of CHn (n=1,2,3) species on the catalyst surface during plasma-assisted catalysis. Using a 0.02 well-characterized atmospheric pressure 3 mm Plasma off plasma jet (APPJ) and a commercial Plasma on nickel catalyst supported on Al2O3 and SiO2 particles, plasma catalysis using 0.01 Ar/O2 interacting with Ar/CH4 was studied for partial oxidation of methane. 200 sccm Ar and 1 sccm O2 was supplied to the APPJ, whereas 2 sccm 0.00 CH CH CH CH methane was supplied in 400 sccm Ar 8 mm 3 2 2 CH downstream near the catalyst surface. IR absorbance 3 Plasma power was 2 W [1]. Real-time catalyst surface characterization was 0.01

performed by diffuse reflectance infrared Fourier spectroscopy (DRIFTS). We observed formation of CHn groups on the catalyst surface before the plasma 0.00 3000 2950 2900 2850 2800 treatment. The infrared modes of the -1 Wavenumber(cm ) CHn layer are located in the wavenumber -1 -1 range 2800 cm to 3000 cm . When the Figure 1 - Measured CHn spectral regions prior and after APPJ was switched on, the surface CHn 2 min of Ar/O2 plasma exposure at 3 mm and 8 mm intensities changed rapidly and APPJ nozzle-to-catalyst surface distance for a catalyst o systematically for certain APPJ temperature of 500 C. operating conditions. Representative data for two conditions are shown in figure 1 and show intensity changes of the CHn DRIFTS data that dependent strongly on APPJ nozzle-to-catalyst surface distance. Data for other conditions and detailed analysis will be presented. We will examine to what extent the observed modifications of CHn on the catalyst surface can be explained by the action of plasma-produced reactive species like atomic O, ozone, and others [2]. For comparison we will report the behavior of the sputter- deposited Ni films on SiO2/Si for similar treatment conditions. * The authors gratefully acknowledge financial support by US Department of Energy (DE- SC0001939) and NSF (CBET-1703211).

References [1] P. Luan, A. J. Knoll, P. J. Bruggeman, and G. S. Oehrlein, J. Vac. Sci. Technol. A 35, 05C315 (2017). [2] A. J. Knoll, S. Zhang, M. Lai, P. Luan and G. S. Oehrlein, J. Phys D Appl Phys. ( in press, 2019)

Modeling Interactions Between Artificial Bone Scaffolding and Atmospheric Pressure Plasmas* Juliusz Kruszelnicki, Runchu Ma and Mark J. Kushner

University of Michigan, Ann Arbor, MI, 48109-21122 USA ([email protected]) Porous ceramic materials are used as tissue scaffolding for bone regeneration. Recently, plasma treatment of porous calcium hydroxyapatite, a bone scaffolding material, has been shown to increase cell attachment and adhesion, therefore enhancing its performance as a bone substitute.[1] The precise mechanisms which produce this improvement are not well known, but surface coverages of adsorbed O, OH, OH-, and N have been linked with increased wettability of the scaffolds, possibly leading to increased cell proliferation.[2] Performing the plasma functionalization at atmospheric pressure would improve the economics of the process. In this investigation, plasma discharges propagating into the tens-of-micrometer-scale pores in ceramic media were investigated using the 2-d plasma modeling platform, nonPDPSIM.[3] Plasma was sustained in a co-planar dielectric barrier reactor (DBD) by negative nanosecond voltage pulses. The bottom dielectric was modeled as a ceramic with four pores and 100% interconnectivity. Discharges in Air and He/O 2 Figure 1 – Electron density in the gas gap gas mixtures were simulated, and fluxes of plasma- and bone scaffold at t = 9 ns. produced species to the surfaces of the pores were calculated. The width and angle of the inter-pore channel then were then varied to determine the impact of topology on the distribution of surface-adsorbed species. We found that a microdischarge first forms in the DBD gas gap, followed by plasma propagation into the pores, as is shown in Fig. 1 for a discharge in air. The ceramic has dielectric constant of 61. Electrons are seeded in the opening to the pores primarily by drift transport and photoionization. Electric field enhancement at the curved and pointed surfaces and surface charging then leads to additional breakdown through the pore chain. The alignment of the pores with respect to the applied electric field has a significant impact on plasma properties. For example, surface ionization waves (SIWs) inside the pore-chain most readily formed at angles far from vertical. High-energy electrons in the ionization front of the SIWs lead to higher rates of production and deposition of reactive species onto the surfaces when the pores are angled. However, the asymmetry in topology also led to decreased uniformity in distribution of those species. Similarly, narrower openings between pores resulted in non-uniform propagation of plasma, leading to higher rates of local surface charging, and less uniform plasma densities throughout the openings.

References [1] Y. Moriguchi et al., PLoS ONE 13, 3 (2018). [2] F. Tan et al., Acta Biomaterialia 8, 1627 (2012). [3] S. A. Norberg, E. Johnsen, M. J. Kushner, Plasma Sources Sci. Technol. 24, 035002 (2015). * Work supported by the DOE Office of Fusion Energy Science and National Science Foundation.

Optimizing Power Delivery using Impedance Matching Networks with Set-Point and Frequency Tuning for Pulsed Inductively Coupled Plasmas* Chenhui Qu(a), Joel Brandon(b) Carl Smith(b), David Coumou(c), Steven C. Shannon(b) and Mark J. Kushner(a) (a) University of Michigan, Ann Arbor, MI 48109-2122 USA ([email protected], [email protected]) (b) North Carolina State University, Raleigh, NC 27695-7909 ([email protected]) (c) MKS Instruments, ENI Power Division, Rochester NY ([email protected]) The delivery of the radio frequency (RF) power to low pressure plasma processing reactors for microelectronics fabrication is maximized by utilizing impedance matching networks. In the scenario where pulsed power is used, the impedance of the plasma can significantly change through one period, thereby requiring a real-time-adjustment of the matching network components to maintain an efficient power transfer. However, typically the duration of the pulse is too short for any adjustment on the electrical component during the pulse, whereas other strategies are needed to perform the impedance matching. One strategy is to select the value of network components (typically capacitors) to match the power pulse at a specific instant to optimize power delivery either in the early or late pulse. This has been shown to provide the tailoring of the power on a transient in the pulsed systems. Matching pulsed inductively coupled plasmas (ICPs) by this method is challenged by the capacitive-to-inductive (E-H) transition at the beginning of the pulse. Another strategy for impedance matching is varying the RF frequency, which can be done rapidly but requires a feed-forward frequency trajectory instead of the traditional reflected power driven Figure 1 – Power reflection coefficient with and without feedback loops employed in frequency frequency tuning. Frequency varies in the range of 8-12 tuning generators. Because both the MHz. matching network components and plasma impedance are frequency dependent, adjusting RF frequency enables a real-time matching. Impedance matching of pulsed ICPs was computationally investigated using the Hybrid Plasma Equipment Model (HPEM) and the results are compared to the experiment carried out on the ICAROS ICP reactor at NCSU. Pulsed ICPs were operated in 20 mTorr with an Ar/Cl2 gas mixture. Set-point matching at different instants can suppress or enhance the capacitive portion of the power, thereby controls the oscillation in plasma potential. Impedance matching by frequency tuning over a few MHz for a base frequency of 10 MHz was performed over a limited range of change in plasma conditions. (See Fig. 1.) The sensitivity of frequency tuning on the matching network components and termination capacitance of the antenna was also studied.

* Work supported by DOE Office of Fusion Energy Sciences, National Science Foundation, and Samsung Electronics Co. Ltd.

Particle Decharging and Agglomeration in Pulsed, Particle Dense Dusty RF Plasmas Toshisato Ono, Christopher J. Hogan and Uwe R. Kortshagen Dept. of Mechanical Engineering, University of Minnesota ([email protected], [email protected] and [email protected])

The spatiotemporal evolution of dust particles in transient plasmas is of interest for applications in the particle synthesis in plasmas. The purpose of this research is to carry out fundamental investigations of the decharging and agglomeration of particles in transiently pulsed plasmas. In general, negatively charged particles repel one another and do not coagulate in steady-state plasmas [1]. Less understood is particle behavior, when the steady-state plasma is disturbed, i.e. when it is pulsed or turned off. The charging and decharging of particles are the most important reactions governing the motion of particles in transient plasmas (pulsed plasmas or afterglows). If particles decharge quickly in the afterglow relative to the time scale of particle motion, the space-charge effect on their motion will be minimal, and neutral drag, gravitational forces, diffusion, and possibly thermophoretic forces, will control particle motion [2, 3]. However, if decharging is relatively slow compared to timescale of particle motion, because of residual fields in a transient plasma or afterglow, and because of space charge effects, particles may display unique behavior, and furthermore, their behavior may be highly size and chemical composition (material) dependent. In this research, laser light scattering (LLS) measurements have been conducted to monitor particle migration and trapping. Rectangular wave modulation has been added to RF discharge power, which enables us to conduct repeatable afterglow experiments. RF pulses hence induce particles to have differential velocities in both speed and direction. Upon reigniting the plasma, such motion is reversed, and all particles return to the trapping location. Differential motion is known to be an efficient method of agglomeration, and we find that when decharging occurs, particle agglomeration occurs rapidly in a pulsed plasma system, with larger agglomerates evident simply from laser light scattering images. We provide evidence that the mechanism of decharging is not simply determined by ion-particle collision, but may be driven by electron emission, which is material (dielectric constant) dependent. Figure 1 shows a photograph comparison of particle clouds of different materials exposed to identical pulsed plasmas with initial similar submicrometer range particle size distributions and the same loaded particle mass. High dielectric constant particles appear to have agglomerated significantly less, Figure 1 – LLS images of particle clouds after 14 minutes of RF suggesting they decharge more pulsing with variable frequency. (a) Barium titanate particles (b) slowly, consistent with electron Silicon dioxide particles. emission as the main mode of decharging.

References [1] M. Gatti and U. Kortshagen, Phys. Rev. 78, 4, 1–6 (2008). [2] H. Setyawan, M. Shimada, K. Ohtsuka, and K. Okuyama, Chem. Eng. Sci., 57, 3, 497–506 (2002). [3] H. H. Hwang and M. J. Kushner, J. Appl. Phys., 82, 5, 2106–2114 (1997). 35

The Etching Probability of Polystyrene by H, OH and O Radicals in an RF Driven Atmospheric Pressure Plasma Jet Y. Zheng(a), V.S.S.K. Kondeti(a), P. Luan(b), G.S. Oehrlein(b) and Peter J. Bruggeman(a) (a) Department of Mechanical Engineering, University of Minnesota, 111 Church Street SE, Minneapolis, MN 55455 ([email protected]) (b) Department of Materials Science and Engineering, University of Maryland, Energy Research Facility (Bldg. #223), 8279 Paint Branch Drive, College Park, MD 20742 ([email protected])

Atmospheric pressure plasmas have a wide range of applications ranging from surface modification to bacterial inactivation [1]. They are widely employed for the etching of polymers [2,3]. However, the reactive species from the plasma responsible for the etching of polymers is often not well understood. Previous work has been associated with understanding the role of VUV photons and O radicals in the etching of polystyrene [2,3]. In this work, the etching probability of polystyrene (PS) by O, H and OH radicals were estimated by correlating the experimentally measured etching depth with the simulated densities of the radical species at the substrate surface. The H and OH densities obtained by the 2-dimensional COMSOL simulation model were compared with laser induced fluorescence measurements. The etching effect was studied for the plasma jet operating in Ar with O2, H2, or H2O in a N2 atmosphere to enable us to assess the role of atomic O, H and OH radicals in the etching of PS films. For Ar/O2 plasma, a linear relationship was found between the amount of etched C atoms from PS and the O flux for different treatment distances, suggesting that the etching reaction probability of atomic O is in the Figure 1 – Comparison of measured and simulated results of -6 -5 the axial H and OH densities of the Ar/H2O plasma jet. range of 6.4×10 - 1.2×10 . For Ar/H2 we found that the etching depth of PS by Ar/H2 plasma was independent of the substrate distance while the H density decays exponentially, suggesting H does not play an important role in etching. The deduced maximum value for the etching reaction probability for H is 8×10-6. Figure 1 shows the comparison between the experimental and simulated H and OH densities for Ar/H2O plasma. The corresponding etching reaction probabilities of -3 PS by OH is in the range of 1.3-48×10 . While the OH density generated by Ar/H2O plasma is two orders of magnitude smaller than the H and O densities, OH radicals still dominate the etching of PS. Similarly, PS etching by Ar/H2 plasma at large distances could be due to the generation of OH radicals caused by O2 impurity in the feed gas.

References [1] I Adamovich et al., J. Phys. D: Appl. Phys. 50, 323001 (2017). [2] P. Luan, A. J. Knoll, P. J Bruggeman & G. S. Oehrlein, J. Vac. Sci. & Tech. A, 35(5), 05C315 (2017). [3] P. Luan et al., J. Phys. D: Appl. Phys., 50(3), 03LT02 (2016).

Benchmark of EEDF Evaluations in Global Modeling of Low Temperature Plasmas Janez Krek, Yangyang Fu and John P. Verboncoeur Michigan State University ({krek, fuyangya, johnv}@egr.msu.edu)

Global (volume-averaged) models present valuable tools in predicting macroscopic plasma behavior, including evaluating the importance of individual reactions and species in plasmas, helping identify the key reactions and chains for spatial-dependent simulations [1, 2]. The Kinetic Global Model framework (KGMf) was further extended, using either BOLOS [3] (two-term spherical approximation) or MultiBolt [4] (multi-term spherical approximation), to self-consistently incorporate temporal evolution of electron energy distribution function (EEDF). Temporal evaluation of EEDF, while enabling improved fidelity of results, comes at a cost of higher computational complexity. Reducing the frequency of EEDF evaluation is imperative to preserve the advantage of the global model while maintaining the accuracy of the solutions. In this work, we first confirm the correctness of the results by running the KGMf with EEDF evaluated each time step and compare the results to ZDPlasKin (ZDPK) with BOLSIG+ [5]. For atmospheric discharge with enforced quasi-neutrality, Maxwellian EEDF, and constant absorbed power density 3 Pabs = 50 W/cm , we compared temporal evolution of species densities (electrons, atomic ions, and molecular ions), with good agreement - Figure 1 – Comparison of electron density from the (see Fig. 1); electron density difference at t = 10 KGMf and ZDPK for high pressure (p = 760 Torr) 4 s is 10%. Reaction set included ionization, argon plasma, Maxwellian EEDF with common excitation, step-wise ionization, and three-body dynamic effective temperature via KGMf. collisions. For lower pressure and power (p = 1 3 Torr, Pabs = 1 W/cm ) for the same reaction set, results from both codes agree well, resulting in a maximum difference of electron density of 4% at t = 0.1 s. The KGMf extended with an intermittent Boltzmann solver to update the EEDF enables rapid study of plasma systems with large numbers of species and reactions, to quantify the most important reactions based on specific goals. For example, in order to optimize UV at a particular wavelength, one must consider multiple reaction chains and species which may play a complicated and interconnected role in production of that particular wavelength, and not just the final excited states and its immediate transition.

This work was supported by the U.S. Department of Energy (Grant No. DESC0001939).

References [1] G. M. Parsey, Ph.D. thesis, Michigan State University (2017). [2] S. K. Nam and J. P. Verboncoeur, Appl. Phys. Lett. 92, 231502 (2008). [3] A. Luque, https://pypi.python.org/pypi/bolos (2004). [4] J. Stephens, J. Phys. D: Appl. Phys. 51, 125203 (2018). [5] G. J. M. Hagelaar and L. C. Pitchford, Plasma Sources Sci. Technol. 14, 722 (2005).

Two Dimensional Simulations of Carbon Arc Jian Chen(a,b), Alex Khrabry(a,c), Andrei Khodak(a) and Igor D. Kaganovich(a) (a) Princeton Plasma Physics Laboratory, Princeton, NJ, 08543 (b) Department of Engineering Physics, Tsinghua University, Beijing, China, 100084 (c) Lawrence Livermore Laboratory, Livermore, CA, 94550

Short atmospheric-pressure arcs with ablating carbon anode are used for production of carbon nanoparticles. Many arc parameters, such as the arc current and inter-electrode gap width, can significantly affect the carbon ablation rate and, consequently, the production of nanoparticles. In this work, we employed a previously developed self-consistent 3D model [1] to study plasma properties of the short carbon arc in helium atmosphere. For initial study, 2D axisymmetric set-up was used. Fluid model for the gas and the plasma is coupled to models of heat transfer and current flow in the electrodes [1]. Temperature profiles were determined from the heat balance equations which accounts for radiation, electron emission, recombination of ions, space-charge sheaths and joule heating. A sheath model was used to determine the sheath voltage drop and particle fluxes on the electrodes. We do not resolve sheath in the simulations and rather account for narrow sheath regions by using effective boundary conditions for the particle and energy transport equations. This model was implemented into ANSYS CFX code and detailed description can be found in Ref. [1]. Using this code, we performed parametric study of arc properties as function of the arc current and inter-electrode gap. The current density streamlines and the radial current density profiles for cross sections of the arc taken at different y positions (anode, y=0, cathode, y=4mm) are shown in figure 1 for the arc current 50 A and inter-electrode gap 4 mm. Arc parameters correspond to experimental set up used in Ref. [2]. As evident from Fig.1, current streamlines start uniformly at the anode surface and converge to the cathode surface. Current density gradually increases along the streamlines while the current channel becomes narrower near the cathode manifesting arc current constriction near the cathode.

Figure 1 – Current density streamlines in the arc and the radial current density profiles for different y positions from anode placed at y=0, cathode - at y=4 mm; the arc current is 50 A; gap is 4.0 mm, anode radius is 3.2 mm, cathode radius 4.8 mm.

References [1] A. Khrabry, I. D. Kaganovich, A. Khodak, V. Vekselman and Y. Raitses, “Validated Modeling of Atmospheric-Pressure Anodic Arc”, arXiv (2019) https://arxiv.org/abs/1902.09991. [2] V. Vekselman, M. Feurer, T. Huang, B. Stratton, Y. Raitses, “Complex structure of the carbon arc discharge for synthesis of nanotubes”, Plasma Sources Sci. Technol. 26, 065019 (2017).

Convenient Analytical Solution for Vibrational Spectrum of H2 Alexander V. Khrabrov(a), Wei Yang(a,b) and Igor D. Kaganovich(a) (a) Princeton Plasma Physics Laboratory, Princeton NJ 08543 ([email protected]) (b) School of Physics, Dalian University of Technology, Dalian 116024, China ([email protected])

We investigate formation of the Vibrational Distribution Function (VDF) : of molecules in a gas at low pressure from experiment: when vibrational levels are predominantly excited by electron impact from the ground state and deactivated in collisions with chamber walls. As an example, we study the vibrational kinetics of hydrogen for conditions of a negative-ion source. The wall collisions are described by a repopulation probability ′, for a vibrational state ′ to relax into a lower state , with 0 . The ground- state density is assumed constant. For Figure 1 – Vibrational spectra of H2 in plasma hydrogen, experimental values reported reactor, calculated using measured wall-collision by Stutzin et al. are widely used in repopulation probabilities (solid lines) and from analytical discharge models.[1,2] We find that the solution Eq. (1) (dashed lines). calculated VDF is not sensitive to the functional form of the repopulation probability , when experimental values are replaced with a uniform distribution among lower levels, i.e., , 1/, . Here, is the initial vibrational state and is the final state after a wall collision. This simplification allows to solve the resulting linear system for VDF (i.e. to invert the corresponding triangular matrix) analytically. Given the source of excitation , the solution is conveniently expressed as

∑ . (1)

Here, is the loss rate of excited molecules to the wall. We call this approach Reduced Linear Model (RLM) for VDF. We validate the RLM by using numerical results from our recently published global model of negative-ion source.[3] As seen in Fig. (1), the RLM result is quite close to the full numerical solution. Thus RLM can be utilized to obtain the VDF in global models (with no real loss of accuracy, given the underlying approximations and uncertainties in reaction rates).

References [1] G. C. Stutzin, A. T. Young, A. S. Schlachter, and W. B. Kunkel, Bull. Am. Phys. Soc. II 33, 2091 (1988). [2] J. R. Hiskes and A. M. Karo, Appl. Phys. Lett. 54, 508 (1989). [3] W. Yang, S. N. Averkin, A. V. Khrabrov, I. D. Kaganovich, Y.-N. Wang, S. Aleiferis, and P. Svarnas, Phys. Plasmas 25, 113509 (2018).

In-Plasma Photo-Assisted Etching of Silicon Demetre J. Economou and Vincent M. Donnelly University of Houston ([email protected]) Si etching in halogen-containing plasmas (diluted with Ar), was investigated using a nearly monoenergetic ion flux, extracted from the afterglow of a pulsed plasma, sustained in a Faraday- shielded ICP reactor. A threshold energy of Eth=16 eV was measured for ion-assisted etching (IAE) of Si with chlorine [1], in agreement with published beam studies. Above threshold, the etching rate scaled with ion energy, E, as (E½ - Eth½), also in agreement with published works. Surprisingly, considerable etching of p-type silicon was observed (Fig. 1), independent of ion energy, even for ions with energies below the 16 eV threshold. Such “sub-threshold etching” of p-type Si in a plasma had not been reported previously. Carefully controlled experiments showed conclusively that the sub-threshold etching was due to photo-assisted etching (PAE) of Si by chlorine. In particular, it was found that PAE was most effective for <170 nm. PAE was also observed with Br2, HBr, Br2/Cl2 and HBr/Cl2 feed gases, diluted with Ar. It was found that sub- threshold etching rates scaled with the product of surface halogen coverage (measured by XPS) and the 750.4 nm Ar- line intensity measured by optical emission spectroscopy.[2] OES was also used to monitor the etching rate of Si in Ar/Cl2 Figure 1 – Relative etching rates of Si at different plasmas, under PAE conditions, by pressures (left axis), and absolute etching rate at 50 recording the time evolution of the ratio of mTorr (hollow triangles, right axis), as a function of E1/2 Si to Ar emission intensities, ISi/IAr. The (E=ion energy) in an Ar/Cl2 plasma pulsed at 10kHz observation of substantial modulation of with a duty cycle of 20%. ISi/IA in 1 kHz pulsed-power plasmas (for power levels between 300 and 500 W) ruled out a lattice damage-induced mechanism of in-plasma PAE since, if this mechanism were dominant, ISi/IAr would not be modulated. Further data analysis suggested that the etching rate was proportional to the instantaneous VUV ﬂux.[3] Recent high resolution XPS experiments revealed that, under PAE conditions, SiCl was the only SiClx adsorbate, while under IAE conditions SiCl2 and SiCl3 were also found on the Si surface.[4] Further experiments indicated no synergy between PAE and IAE. For p-type, and presumably un-doped or lightly doped n-type Si, the PAE rate is signiﬁcant, compared to that of ion-assisted etching, for processes that require low ion energies (10s of eV) to achieve high selectivity and low damage, such as atomic layer etching. Under commonly employed conditions, PAE likely plays an important role in the evolution of artefacts in sub-micron features etched in Si using halogen-containing plasmas.

References [1] S. H. Shin et al., J. Vac. Sci. Technol. A, 30, 021306 (2012). [2] W. Zhu et al., J. Appl. Phys. 115, 203303 (2014) [3] S. Sridhar et al., J. Vac. Sci. & Technol. A 34, 161303 (2016). [4] E. Hirsch et al., submitted for publication.

DRIFTS Study of the Enhancement of Catalytic Partial Oxidation of Methane by Cold Atmospheric Plasma (a) (a) (a) (b) (b) (a) S. Zhang , Y. Li , A. Knoll , J. Jiang , P. J. Bruggeman and G.S. Oehrlein (a) University of Maryland, College Park, MD ([email protected]) (b) University of Minnesota, Twin Cities, MN ([email protected]) Recently cold atmospheric pressure plasma (CAP) assisted catalysis has attracted a great deal of attention because of synergistic enhancements of thermal catalysis by CAP-related plasma catalyst surface interaction [1]. We investigated CAP assisted catalytic partial 0.03 Ar plasma Ar/O plasma surface bond C-O oxidation of methane to syngas using a 2 Ar plasma well-characterized atmospheric pressure plasma jet (APPJ). The catalyst studied is nickel supported by silica and alumina. The 0.02 APPJ is RF excited Ar plasma jet with or without 1% O2. CH4/Ar is fed downstream o from the APPJ close to the temperature- 0.01 Catalyst T: 500 C Plasma power: 2 W. controlled catalyst. As reported in our Ar/O : 200/1 sccm 2 Ar/CH : 400/2 sccm previous work [2], a plasma power C-O bond(A.U.) byDRIFTS 4 dependent synergistic effect of CO 0.00 8 mm distance production is seen for this system for gas 0 255075100 Time (min) phase CO probed downstream by FTIR (a) absorption spectroscopy [2]. The magnitude  o of the plasma-catalyst synergistic (PCS) Ar plasma Ar/O plasma Ar plasma Catalyst T:500 C 2 Plasma power:2 W. ) effect is reduced when increasing catalyst -3 Ar/O :200/1 sccm 3 2 Ar/CH :400/2 sccm temperature. 4 8mm distance Here we use diffuse reflectance infrared Fourier transform spectroscopy 2 CO (DRIFTS) to study plasma surface interactions of the CO PCS effect. By 1 changing the amount of oxygen added to the phase density (cm Gas CO 2 APPJ, the oxidation of surface-bound CO 0 can be followed using DRIFTS (see figure 1 0 10203040 Time (min) (a), for a plasma source-catalyst separation (b) of 8 mm). At elevated catalyst temperature Figure 1 - (a) Surface bonded C-O behaviour and (b) gas (500 °C) the abundance of surface-bound phase CO and CO2 production under same conditions CO is reduced compared to room temperature when 1% O2 is introduced into the APPJ. The CO results are mirrored in the behavior of CO2 in the gas phase. The gas phase results obtained for CO and CO2 are shown in figure 1 (b). The gas phase CO production correlates with the behavior of surface-bonded CO. Furthermore, a decrease in CO correlates with an increase in CO2 and vice versa. This correlation between surface CO and gas phase CO was also seen for a plasma source – catalyst separation distance of 3 mm. A possible mechanism consistent with these results is CO(s) + O(g)  CO2,(g). Surface bonded C-O is oxidized by atomic oxygen (or ozone) and desorbs as CO2. The authors gratefully acknowledge financial support by US Department of Energy (DE- SC0001939) and NSF (CBET-1703211). References [1] J. C. Whitehead, J. Phys. D: Appl. Phys., 49, 243001 (2016). [2] A. J. Knoll, S. Zhang, M. Lai, P. Luan, and G. S. Oehrlein, J. Phys. D: Applied Physics, 52, 225201 (2019). 41

Mapping of 2-D Plasma-induced Fluid Flow Using Particle Image Velocimetry Janis Lai and John E. Foster University of Michigan ([email protected]) Plasmas coming into contact with liquid water is the basis of a range of technological applications for environmental remediation and medicine. These technologies are enabled by complex plasma-driven chemical and physical processes at the plasma-liquid interface. The production of plasma-induced reactive species accounts for plasma’s potency for inducing cell proliferation in wound healing, and against harmful contaminants in water for decontamination. At the interface, however, plasmas can also drive Marangoni flow in the bulk liquid. This effect can thus circulate reactive species in the bulk liquid, to regions far away from the plasma-liquid interface. Mapping this plasma-drive fluid flow gives insight into how plasmas locally alters the contact area chemically and physically. Accounting for plasma-driven flow can ultimately impact the design and application of technologies using plasmas. A 2-D plasma-in-liquid apparatus was used to study the plasma-liquid interfacial region. The observed plasma-driven flow was mapped using particle image velocimetry (PIV). Previous PIV results [1] showed that sharp velocity shear was Figure 1 – Velocity field of plasma-induced bulk fluid present near the interface region, which led flow. to the presence of stable Kelvin-Helmholtz perturbation. Additionally, sharp sheer created vortices in the bulk liquid, resulting in large-scale circulation that transported plasma-derived reactive species far away from the interface. This observed circulation was consistent with Marangoni flow, which was driven by gradients of surface tension in the liquid. The gradient of surface tension can be induced via localized changes in temperature and concentration of ions. Joule heating by the streamers can significantly heat up the liquids locally which lead to temperature gradients in the bulk liquid; while diffusion and in-situ production of reactive species can cause concentration gradient to arise. In this work, we present results of mapped plasma-induced fluid flow in the presence of varying liquid conditions. Conductivity and surface tension impact how streamers propagate along the interface, and as a result alter the induced chemistry and fluid flow. Furthering the understanding on transport processes at the interface thus becomes relevant for plasma applications seeking to process any liquid or liquid containing media.

References [1] J. Lai, V. Petrov and J.E Foster, IEEE Trans. Plasma Sci. 46, 875-881 (2018).

Hybrid Method of Moments to Predict Nanoparticle Nucleation, Growth and Charging in Dusty Plasmas Sai R. Narayanan, Suo Yang and Steven L. Girshick Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN ([email protected]) A number of previous studies of dusty plasmas undergoing particle nucleation and growth have utilized sectional methods to represent the particle size distribution, self-consistently coupled to plasma models. Several of these studies modeled the spatiotemporal evolution of a plasma in a 1-D parallel-plate configuration [1]. The particle size distribution in this approach is discretized into sections, and population balance equations are solved for the number density of particles in each section. Sectional methods are able to solve for particle size distributions in cases where significant polydispersity exists, as well as for particle charge distributions that arise due to stochastic charging of particles of given size. However, when such models are self-consistently coupled to plasma models in a 2-D system, as in [2], the computational cost can become quite high, as a large number of sections must be considered to minimize errors due to numerical diffusion. The method of moments (MoM) is a promising alternative approach, which reduces the number of equations to be solved, and thereby the computational cost. Evolution equations are solved only for a small set of statistical moments of the size distribution. While MoM approaches have been widely used to model neutral aerosols, no such model currently exists for dusty plasmas. Aerosols undergoing simultaneous nucleation and particle growth commonly develop bimodal particle size distributions. The small-size mode develops through fresh nucleation, while, in plasmas, the large-size mode develops through the growth of negatively-charged nanoparticles that are trapped in the bulk plasma by the electric field. As seen in Fig. 1, to capture the bimodal size distribution in the hybrid method of moments, a delta function is used to represent the small-size mode, and an interpolation of a lognormal distribution is used to represent the large- size mode. Such a technique has been utilized in the field of combustion soot aerosols [3,4]. To model particle charge in MoM, an analytical expression can be used to obtain the average particle charge for a given particle size in the large-size mode [5], while the fraction of particles in the Figure 1 – Comparison of particle size distribution obtained using a sectional method with small-particle mode that have -1, 0 or +1 reconstruction using moments of the distribution. charge can be solved as in [6]. References [1] P. Agarwal and S.L. Girshick, Plasma Source. Sci. Technol. 21, 055023 (2012). [2] R. Le Picard, A.H. Markosyan, D.H. Porter, S.L. Girshick and M.J. Kushner, Plasma Chem. Plasma Process. 36, 941 (2016). [3] M.E. Mueller, G. Blanquart and H. Pitsch, Combust. Flame 156, 1143 (2009). [4] S. Yang and M.E. Mueller, Proc. Combust. Inst. 37, 1141 (2019). [5] R. Le Picard and S.L. Girshick, J. Phys. D 49, 095201 (2016). [6] M. Mamunuru, R. Le Picard, Y. Sakiyama and S.L. Girshick, Plasma Chem. Plasma Process. 37, 701 (2017). 43

Atomic Hydrogen Generation in the Ionizing Plasma Region and Effluent of a Helium-Water Atmospheric Pressure Plasma Jet by Two-Photon Absorption Laser Induced Fluorescence (TALIF) Yuanfu Yue, V. Santosh S. K. Kondeti and Peter J. Bruggeman Department of Mechanical Engineering, University of Minnesota, 111 Church Street SE Minneapolis, MN 55455 USA ([email protected], [email protected])

Non-equilibrium atmospheric pressure plasma jets received a lot of attention in the last decade due to promising applications in the biomedical field and for material processing. [1] Reactive species generated by plasmas, such as hydroxyl are considered to play essential roles for these applications. Atomic hydrogen, another dominant reactive species, shows great potentials for ion reduction in solution enabling nanoparticle synthesis. [2] In this study, we investigate the H production in a cold atmospheric -15 -5 0 Unit: mm pressure plasma jet. The plasma jet is Plasma emission profile driven by a high voltage pulser while flowing helium with 0.1% H2O through the jet. The one dimensional two-photon absorption laser induced fluorescence signal of atomic hydrogen (H-TALIF) is recorded along the axis of symmetry of a plasma jet by aligning the laser beam into the quartz capillary tube of the jet. The plasma is ignited and propagates along the capillary tube towards the jet Flow direction effluent so that the H- TALIF can be performed both inside the tube and in Figure 1 - Time resolved H-TALIF density distribution the plasma jet effluent outside the in axis from inside to outside of the tube. The plasma is tube. This allows us to uniquely study driven by 1.5 μs voltage pulse at a pulse repetition rate the generation of atomic H in a plasma of 2 kHz. The position with positive numbers denotes effluent direction. jet. The absolute density calibration is achieved by performing Kr-TALIF in situ. In this report, the generation, transport and recombination of H atoms are investigated with particular emphasis on the impact of plasma generation and pulse repetition rates. Figure 1 shows the transportation of H radicals in axial direction. The H atoms are mainly generated between the two ring electrodes by the first voltage pulse and are subsequently transported by the gas flow to the jet effluent outside the tub while recombining. The spatial generation of H at the rising and falling edge of H.V. pulse is also clearly illustrated by the first two (red and green) H density profiles in Figure 1. Subsequent plasma pulses lead to an increase of the H density, depending on pulse repetition rate. Additional results also show memory effects for repetition frequencies in excess of 2 kHz. Combined with information of the plasma-dissipated energy and emission intensity, we will use the spatial and temporal variation of the atomic H density to discuss its main production and destruction.

References [1] X. Lu. G.V. Naidis, M. Laroussi, S. Reuter, D. Graves and K. Ostrikov, Phys. Rep. 630 pp1-84 (2016). [2] S. Kondeti, U. Gangal, S. Yatom and P. Bruggeman, J. Vac. Sci. Technol. A 35 061302 (2017).

List of Participants

Last Name First Name Institution Email Adamovich Igor Ohio State University [email protected] Barnat Ed Sandia National Laboratories [email protected] Bruggeman Peter University of Minnesota [email protected] Chen Jian PPPL Donnelly Vince University of Houston [email protected] Economou Demetre University of Houston [email protected] Engelmann Sebastian IBM [email protected] Foster John University of Michigan [email protected] Fu Yangyang Michigan State University [email protected] Gershman Sophia PPPL [email protected]; [email protected] Girshick Steven University of Minnesota [email protected] Godyak Valery University of Michigan [email protected] Ioannis Kassaskeris University of Maryland Kaganovich Igor PPPL [email protected] Khrabrov Alexander PPPL [email protected] Kolobov Vladimir CFDRC/University of Alabama at [email protected]; Huntsville [email protected] Kortshagen Uwe University of Minnesota [email protected] Krek Janez Michigan State University [email protected] Kruszelnicki Juliusz University of Michigan [email protected] Kushner Mark University of Michigan [email protected] Lai Janis University of Michigan [email protected] Li Chen University of Maryland [email protected] Li Yudong University of Maryland Lieberman Mike UC-Berkeley [email protected] Lin Kang-Yi University of Maryland [email protected] Lin Peng University of Houston Narayanan Sai Ranjeet University of Minnesota [email protected]

Last Name First Name Institution Email Oehrlein Gottlieb University of Maryland [email protected] Ono Toshisato University of Minnesota [email protected] Orr Keegan Ohio State University [email protected] Podder Nirmol DOE [email protected] Poovathingal Savio University of Michigan [email protected] Pranda Adam University of Maryland [email protected] Qu Chenhui University of Michigan [email protected] Raitses Yevgeny PPPL [email protected] Seo YoungSik Samsung/University of Maryland [email protected] Simeni Simeni Marien University of Minnesota [email protected] Thomas Edward Auburn University [email protected] Verboncoeur John Michigan State University [email protected] Yue Yuanfu University of Minnesota Zhang Shiqiang University of Maryland [email protected] Zheng Yashuang University of Minnesota