Browsing by Author "Schulze, Julian"

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    2D spatially resolved O atom density profiles in an atmospheric pressure plasma jet: From the active plasma volume to the effluent
    (Bristol : IOP Publ., 2021) Steuer, David; Korolov, Ihor; Chur, Sascha; Schulze, Julian; Schulz-von der Gathen, Volker; Golda, Judith; Böke, Marc
    Two-dimensional spatially resolved absolute atomic oxygen densities are measured within an atmospheric pressure micro plasma jet and in its effluent. The plasma is operated in helium with an admixture of 0.5% of oxygen at 13.56 MHz and with a power of 1 W. Absolute atomic oxygen densities are obtained using two photon absorption laser induced fluorescence spectroscopy. The results are interpreted based on measurements of the electron dynamics by phase resolved optical emission spectroscopy in combination with a simple model that balances the production of atomic oxygen with its losses due to chemical reactions and diffusion. Within the discharge, the atomic oxygen density builds up with a rise time of 600 µs along the gas flow and reaches a plateau of 8 1015 cm-3. In the effluent, the density decays exponentially with a decay time of 180 µs (corresponding to a decay length of 3 mm at a gas flow of 1.0 slm). It is found that both, the species formation behavior and the maximum distance between the jet nozzle and substrates for possible oxygen treatments of surfaces can be controlled by adjusting the gas flow.
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    Multi-diagnostic experimental validation of 1d3v PIC/MCC simulations of low pressure capacitive RF plasmas operated in argon
    (Bristol : IOP Publ., 2021) Schulenberg, David A.; Korolov, Ihor; Donkó, Zoltán; Derzsi, Aranka; Schulze, Julian
    The particle-in-cell/Monte Carlo collisions (PIC/MCC) simulation approach has become a standard and well-established tool in studies of capacitively coupled radio frequency (RF) plasmas. While code-to-code benchmarks have been performed in some cases, systematic experimental validations of such simulations are rare. In this work, a multi-diagnostic experimental validation of 1d3v electrostatic PIC/MCC simulation results is performed in argon gas at pressures ranging from 1 Pa to 100 Pa and at RF (13.56 MHz) voltage amplitudes between 150 V and 350 V using a custom built geometrically symmetric reference reactor. The gas temperature, the electron density, the spatio-temporal electron impact excitation dynamics, and the ion flux-energy distribution at the grounded electrode are measured. In the simulations, the gas temperature and the electrode surface coefficients for secondary electron emission and electron reflection are input parameters. Experimentally, the gas temperature is found to increase significantly beyond room temperature as a function of pressure, whereas constant values for the gas temperature are typically assumed in simulations. The computational results are found to be sensitive to the gas temperature and to the choice of surface coefficients, especially at low pressures, at which non-local kinetic effects are prominent. By adjusting these input parameters to specific values, a good quantitative agreement between all measured and computationally obtained plasma parameters is achieved. If the gas temperature is known, surface coefficients for different electrode materials can be determined in this way by computationally assisted diagnostics. The results show, that PIC/MCC simulations can describe experiments correctly, if appropriate values for the gas temperature and surface coefficients are used. Otherwise significant deviations can occur.
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    QDB: A new database of plasma chemistries and reactions
    (Bristol : IOP Publ., 2017) Tennyson, Jonathan; Rahimi, Sara; Hill, Christian; Tse, Lisa; Vibhakar, Anuradha; Akello-Egwel, Dolica; Brown, Daniel B.; Dzarasova, Anna; Hamilton, James R.; Jaksch, Dagmar; Mohr, Sebastian; Wren-Little, Keir; Bruckmeier, Johannes; Agarwal, Ankur; Bartschat, Klaus; Bogaerts, Annemie; Booth, Jean-Paul; Goeckner, Matthew J.; Hassouni, Khaled; Itikawa, Yukikazu; Braams, Bastiaan J; Krishnakumar, E.; Laricchiuta, Annarita; Mason, Nigel J.; Pandey, Sumeet; Petrovic, Zoran Lj.; Pu, Yi-Kang; Ranjan, Alok; Rauf, Shahid; Schulze, Julian; Turner, Miles M.; Ventzek, Peter; Whitehead, J. Christopher; Yoon, Jung-Sik
    One of the most challenging and recurring problems when modeling plasmas is the lack of data on the key atomic and molecular reactions that drive plasma processes. Even when there are data for some reactions, complete and validated datasets of chemistries are rarely available. This hinders research on plasma processes and curbs development of industrial applications. The QDB project aims to address this problem by providing a platform for provision, exchange, and validation of chemistry datasets. A new data model developed for QDB is presented. QDB collates published data on both electron scattering and heavy-particle reactions. These data are formed into reaction sets, which are then validated against experimental data where possible. This process produces both complete chemistry sets and identifies key reactions that are currently unreported in the literature. Gaps in the datasets can be filled using established theoretical methods. Initial validated chemistry sets for SF6/CF4/O2 and SF6/CF4/N2/H2 are presented as examples.
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    Three-dimensional density distributions of NO in the effluent of the COST reference microplasma jet operated in He/N2/O2
    (Bristol : IOP Publ., 2020) Preissing, Patrick; Korolov, Ihor; Schulze, Julian; Schulz-von der Gathen, Volker; Böke, Marc
    Laser induced fluorescence spectroscopy (LIF) is used to measure absolute ground state densities of nitric oxide (NO) in the effluent of the COST reference microplasma jet (COST-jet) with three-dimensional spatial resolution. The jet is operated in helium with a nitrogen/oxygen admixture. The experiments are performed with the jet expanding into open air and into a controlled He/synthetic air atmosphere. The most efficient production of NO is found at a 0.5% admixture of N2/O2 at a ratio of 4/1, that is considered to be synthetic air. Maximum NO densities of 3.25 × 1014 cm-3 and 4.5 × 1014 cm-3 are measured in the air and He/synthetic air atmosphere, respectively, at an axial distance of 2 mm from the nozzle. The distribution patterns are found to transit into a turbulent regime for air atmosphere at greater axial distances, while in the He/synthetic air atmosphere this effect is not observed. It is found that the expansion of the region of high NO density in the effluent is strongly coupled to the helium flow. Furthermore, the NO density is found to depend on the absolute feed gas flow, i.e. its maximum decreases as a function of the gas flow. This is a result of the longer residence time of the gas in the active plasma volume at lower gas flows and higher energy densities. For very high values of the applied radio frequency power the NO density is saturated. From time resolved measurements of the LIF signals the quenching coefficient for the NO(A 2Σ+) state by air is found to be k u,air = 4.2(±0.5) × 10-11 cm3 s-1, while quenching by He is negligible, k u,He 1 × 10-14 cm3 s-1. The amount of ambient air intruding the helium effluent is determined as well.
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    Zero-dimensional and pseudo-one-dimensional models of atmospheric-pressure plasma jets in binary and ternary mixtures of oxygen and nitrogen with helium background
    (Bristol : IOP Publ., 2021) He, Youfan; Preissing, Patrick; Steuer, David; Klich, Maximilian; Schulz-von der Gathen, Volker; Böke, Marc; Korolov, Ihor; Schulze, Julian; Guerra, Vasco; Brinkmann, Ralf Peter; Kemaneci, Efe
    A zero-dimensional (volume-averaged) and a pseudo-one-dimensional plug-flow (spatially resolved) model are developed to investigate atmospheric-pressure plasma jets operated with He, He/O2, He/N2 and He/N2/O2 mixtures. The models are coupled with the Boltzmann equation under the two-term approximation to self-consistently calculate the electron energy distribution function. An agreement is obtained between the zero-dimensional model calculations and the spatially averaged values of the plug-flow simulation results. The zero-dimensional model calculations are verified against spatially resolved simulation results and validated against a wide variety of measurement data from the literature. The nitric oxide (NO) concentration is thoroughly characterized for a variation of the gas mixture ratio, helium flow rate and absorbed power. An 'effective' and a hypothetical larger rate coefficient value for the reactive quenching N2(A3Σ, B3Π) + O(3P) → NO + N(2D) are used to estimate the role of the species N2(A3Σ, B3Π; v > 0) and multiple higher N2 electronically excited states instead of only N2(A3Σ, B3Π; v = 0) in this quenching. The NO concentration measurements at low power are better and almost identically captured by the simulations using the 'effective' and hypothetical values, respectively. Furthermore, the O(3P) density measurements under the same operation conditions are also better predicted by the simulations adopting these values. It is found that the contribution of the vibrationally excited nitrogen molecules N2(v ≥ 13) to the net NO formation rate gains more significance at higher power. The vibrational distribution functions (VDFs) of molecular oxygen O2(v < 41) and nitrogen N2(v < 58) are investigated regarding their formation mechanisms and their responses to the variation of operation parameters. It is observed that the N2 VDF shows a stronger response than the O2 VDF. The sensitivity of the simulation results with respect to a variation of the VDF resolutions, wall reaction probabilities and synthetic air impurity levels is presented. The simulated plasma properties are sensitive to the variation, especially for a feed gas mixture containing nitrogen. The plug-flow model is validated against one-dimensional experimental data in the gas flow direction, and it is only used in case an analysis of the spatially resolved plasma properties inside the jet chamber is of interest. The increasing NO spatial concentration in the gas flow direction is saturated at a relatively high power. A stationary O2 VDF is obtained along the direction of the mass flow, while a continuously growing N2 VDF is observed until the jet nozzle.