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1. Helium metastable species generation in atmospheric pressure RF plasma jets driven by tailored voltage waveforms in mixtures of He and N 2

   https://doi.org/10.1088/1361-6463/ab6d97  

   Korolov, I. ; Leimkühler, M. ; Böke, M. ; Donkó, Z. ; Schulz-von der Gathen, V. ; Bischoff, L. ; Hübner, G. ; Hartmann, P. ; Gans, T. ; Liu, Y. ; et al ( February 2020 , Journal of Physics D: Applied Physics)

   Spatially resolved tunable diode-laser absorption measurements of the absolute densities of He-I (2³S₁) metastables in a micro atmospheric pressure plasma jet operated in He/N₂and driven by ‘peaks’- and ‘valleys’-type tailored voltage waveforms are presented. The measurements are performed at different nitrogen admixture concentrations and peak-to-peak voltages with waveforms that consist of up to four consecutive harmonics of the fundamental frequency of 13.56 MHz. Comparisons of the measured metastable densities with those obtained from particle-in-cell/Monte Carlo collision simulations show a good quantitative agreement. The density of helium metastables is found to be significantly enhanced by increasing the number of consecutive driving harmonics. Their generation can be further optimized by tuning the peak-to-peak voltage amplitude and the concentration of the reactive gas admixture. These findings are understood based on detailed fundamental insights into the spatio-temporal electron dynamics gained from the simulations, which show that voltage waveform tailoring allows to control the electron energy distribution function to optimize the metastable generation. A high degree of correlation between the metastable creation rate and the electron impact excitation rate from the helium ground state into the He-I ((3s)³S₁) level is observed for some conditions which may facilitate an estimation of the metastable densities based on phase resolved optical emission spectroscopy measurements of the 706.5 nm He-I line originating from the above level and metastable density values at proper reference conditions.

    

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2. A surface mechanism for O 3 production with N 2 addition in dielectric barrier discharges

   https://doi.org/10.1088/1361-6595/ace95d  

   Meyer, Mackenzie ; Foster, John ; Kushner, Mark J. ( August 2023 , Plasma Sources Science and Technology)

   Ozone, O₃, is a strong oxidizing agent often used for water purification. O₃is typically produced in dielectric barrier discharges (DBDs) by electron-impact dissociation of O₂, followed by three-body association reactions between O and O₂. Previous studies on O₃formation in low-temperature plasma DBDs have shown that O₃concentrations can drop to nearly zero after continued operation, termed the ozone-zero phenomenon (OZP). Including small (<4%) admixtures of N₂can suppress this phenomenon and increase the O₃production relative to using pure O₂in spite of power deposition being diverted from O₂to N₂and the production of nitrogen oxides, N_xO_y. The OZP is hypothesized to occur because O₃is destroyed on the surfaces in contact with the plasma. Including N₂in the gas mixture enables N atoms to occupy surface sites that would otherwise participate in O₃destruction. The effect of N₂in ozone-producing DBDs was computationally investigated using a global plasma chemistry model. A general surface reaction mechanism is proposed to explain the increase in O₃production with N₂admixtures. The mechanism includes O₃formation and destruction on the surfaces, adsorption and recombination of O and N, desorption of O₂and N₂, and NO_xreactions. Without these reactions on the surface, the density of O₃monotonically decreases with increasing N₂admixture due to power absorption by N₂leading to the formation of nitrogen oxides. With N-based surface chemistry, the concentrations of O₃are maximum with a few tenths of percent of N₂depending on the O₃destruction probability on the surface. The consequences of the surface chemistry on ozone production are less than the effect of gas temperature without surface processes. An increase in the O₃density with N-based surface chemistry occurs when the surface destruction probability of O₃or the surface roughness was decreased.

    

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3. O 2 based resonantly ionized photoemission thermometry analysis of supersonic flows

   https://doi.org/10.1364/OE.471021  

   McCord, Walker ; Gragston, Mark ; Plemmons, David ; Zhang, Zhili ( October 2022 , Optics Express)

   Characterization of the thermal gradients within supersonic and hypersonic flows is essential for understanding transition, turbulence, and aerodynamic heating. Developments in novel, impactful non-intrusive techniques are key for enabling flow characterizations of sufficient detail that provide experimental validation datasets for computational simulations. In this work, Resonantly Ionized Photoemission Thermometry (RIPT) signals are directly imaged using an ICCD camera to realize the techniques 1D measurement capability for the first time. The direct imaging scheme presented for oxygen-based RIPT (O₂RIPT) uses the previously established calibration data to direct excite various resonant rotational peaks within the S-branch of theC³Π, (v = 2) ← X³Σ(v^′ = 0) absorption band of O₂. The efficient ionization of O₂liberates electrons that induce electron avalanche ionization of local N₂molecules generating N₂⁺, which primarily deexcites via photoemissions of the first negative band of$N_2^{+}(B^2{\mathrm{\Sigma <\#comment/>}}_u^{+}-<\#comment/>X^2{\mathrm{\Sigma <\#comment/>}}_g^{+})$. When sufficient lasing energy is used, the ionization region and subsequent photoemission signal is achieved along a 1D line thus, if directly imaged can allow for gas temperature assignments along said line; demonstrated here of up to five centimeters in length. The temperature gradients present within the ensuing shock train of a supersonic under expanded free jet serves as a basis of characterization for this new RIPT imaging scheme. The O₂RIPT results are extensively compared and validated against well-known and established techniques (i.e., CARS and CFD). The direct imaging capability fully realizes the technique’s fundamental potential and is expected to be the standard of implementation going forward. The direct imaging capability can play instrumental roles in future scientific studies that rely upon acute characterization of thermal gradients within a medium that cannot be easily resolved by a point. Furthermore, the removal of the spectrometer greatly reduces the cost, complexity, and optical alignment associated with prior RIPT measurements.

    

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4. Vertical Variations in Thermospheric O/N 2 and the Relationship Between O and N 2 Perturbations During a Geomagnetic Storm

   https://doi.org/10.1029/2023EA002988  

   Yu, Tingting ; Wang, Wenbin ; Ren, Zhipeng ; Cai, Xuguang ; He, Maosheng ( September 2023 , Earth and Space Science)

   The ratio of O to N₂number densities (O/N₂) at different altitudes is an important parameter in describing thermospheric neutral composition changes and their effects on the ionosphere during geomagnetic storms. However, storm‐induced vertical variations in O/N₂and its dependence on the O and N₂perturbations are still not fully understood. Here, the Thermosphere/Ionosphere Electrodynamics General Circulation Model simulations were used to investigate the responses of thermospheric composition at different pressure levels to the super geomagnetic storm occurred on November 20 and 21 in 2003. Our analysis shows that the behaviors of O/N₂perturbations on different pressure levels are similar above ∼180 km altitude. In the middle and low thermosphere of below ∼300 km, the storm‐time O/N₂decrease is mainly caused by a large reduction of O number density. However, N₂enhancement plays a vital role in O/N₂decreases in the upper thermosphere. The O/N₂enhancement is mainly attributed to the N₂decreases at all pressure levels. The changes of O and N₂number densities at a constant pressure level can be explained by the perturbations of their mass mixing ratio (mmr) and total mass density (ρ). The regions of the O/N₂decrease are characterized by the O mmr decrease and N₂mmr enhancement, whereas the regions of the O/N₂increase are characterized by the O mmr increase and N₂mmr decrease. Theρvalue that shows the decrease globally at most pressure levels during the storm either enhance or reduce the O and N₂perturbations.

    

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5. Plasma-droplet interaction study to assess transport limitations and the role of ⋅ OH, O ⋅ ,H ⋅ ,O 2 (a 1 Δ g ),O 3 , He(2 3 S) and Ar(1s 5 ) in formate decomposition

   https://doi.org/10.1088/1361-6595/ac2676  

   Nayak, Gaurav ; Oinuma, Gaku ; Yue, Yuanfu ; Santos Sousa, João ; Bruggeman, Peter J ( November 2021 , Plasma Sources Science and Technology)

   Abstract Plasmas interacting with liquid microdroplets are gaining momentum due to their ability to significantly enhance the reactivity transfer from the gas phase plasma to the liquid. This is, for example, critically important for efficiently decomposing organic pollutants in water. In this contribution, the role of ⋅ OH as well as non- ⋅ OH-driven chemistry initiated by the activation of small water microdroplets in a controlled environment by diffuse RF glow discharge in He with different gas admixtures (Ar, O 2 and humidified He) at atmospheric pressure is quantified. The effect of short-lived radicals such as O ⋅ and H ⋅ atoms, singlet delta oxygen (O 2 ( a 1 Δ g )), O 3 and metastable atoms of He and Ar, besides ⋅ OH radicals, on the decomposition of formate dissolved in droplets was analyzed using detailed plasma diagnostics, droplet characterization and ex situ chemical analysis of the treated droplets. The formate decomposition increased with increasing droplet residence time in the plasma, with ∼70% decomposition occurring within ∼15 ms of the plasma treatment time. The formate oxidation in the droplets is shown to be limited by the gas phase ⋅ OH flux at lower H 2 O concentrations with a significant enhancement in the formate decomposition at the lowest water concentration, attributed to e − /ion-induced reactions. However, the oxidation is diffusion limited in the liquid phase at higher gaseous ⋅ OH concentrations. The formate decomposition in He/O 2 plasma was similar, although with an order of magnitude higher O ⋅ radical density than the ⋅ OH density in the corresponding He/H 2 O plasma. Using a one-dimensional reaction–diffusion model, we showed that O 2 ( a 1 Δ g ) and O 3 did not play a significant role and the decomposition was due to O ⋅ , and possibly ⋅ OH generated in the vapor containing droplet-plasma boundary layer. 

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