Bulletin of the American Physical Society
73rd Annual Gaseous Electronics Virtual Conference
Volume 65, Number 10
Monday–Friday, October 5–9, 2020; Time Zone: Central Daylight Time, USA.
Session XF2: Diagnostics III: Optical DiagnosticsLive
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Chair: Matthew Goeckner, University of Texas, Dallas |
Friday, October 9, 2020 9:45AM - 10:00AM Live |
XF2.00001: Towards Single Shot Plasma Diagnostics Stephan Reuter, Benjamin M. Goldberg, Luka Hansen, Arthur Dogariu, Richard B. Miles Single shot measurements accurately represent real time events. Highly sensitive measurements, however, typically require averaging procedures to sufficiently increase signal to noise ratio. For random events in space or time, averaging can easily lead to unwanted smearing and distortion of results. Post processing of data presents the possibility to combine the accuracy of single shot data recording with the benefits of averaging procedures. We discuss three approaches in which we collect and post-process single shot data of stochastic events such as the electric field and the flow field of an atmospheric pressure plasma jet. Firstly, time binning sorts electric field measurements in a post-processing routine according to their random delay with respect to the applied voltage in order to increase the time resolution. Secondly, advanced averaging yields an increased signal to noise ratio from post-processing that averages electric field measurements sorted by an intensity threshold. Finally, space slicing allows ``discretized 3D-measurements'', spreading multiple laser sheets by a rotating mirror. We demonstrate the space slicing approach by Rayleigh scattering across a 3D-volume to capture the plasma jet's flow field. [Preview Abstract] |
Friday, October 9, 2020 10:00AM - 10:15AM Live |
XF2.00002: In -situ Raman thermometry as a tool to study Low temperature plasma -- material surface interactions Carla Berrospe Rodriguez, Joseph Schwan, Lorenzo Mangolini Low temperature plasmas are commonly used to grow and process high melting point materials. The synthesis of these offers advantages over other production methods, which are used for energy storage, highly sensitive detection, disease diagnosis and treatment, among others. The heating mechanisms occurring at the interface between these plasmas and materials are not fully understand and comprehending this, is crucial to further improve plasma-driven processes. We present for the first time an insight of non-thermal plasma surface interaction with nanoparticles by means of in-situ Raman spectroscopy. We tested this method on chemical vapor deposition carbon thin films, carbon nanoparticles and multilayered graphene, where the temperature of the particles immersed in the plasma was obtained by Raman thermometry. The temperature of the materials showed a significant dependence on the plasma gas composition, where higher values were observed in pure argon compared to hydrogen diluted argon. Carbon nanoparticles presented higher temperature compared to graphene and carbon films, due to is lower thermal conductivity. A temperature up to 570 K for a 20 W coupled power was reached, this means 6 times the initial temperature (320 K, induced by the laser). Finally, heat transfer modeling was carried out to discard temperature increment due to gas heating. This non-invasive laser-based method opens the opportunity for a synthesis monitoring tool in real time. [Preview Abstract] |
Friday, October 9, 2020 10:15AM - 10:30AM Live |
XF2.00003: Machine learning and optical emission spectroscopy Tahereh Mansouri, Paul Maguire Applying machine learning to plasma spectra has potential to expand access to plasma conditions as well as applications in e.g. gas sensors or clinical breath analysis. We have recently demonstrated CH$_{\mathrm{4}}$ detection at 1~ppm using a He low-temperature RF plasma jet and low-cost spectrometer. This identification occurs without dependence on carbon-based spectral lines. However, the high number of data variables (wavelengths) poses a severe challenge for machine learning and to progress to fully realistic gas environments requires greater understanding of the relationship between plasma, spectral features and model development. To this end we investigate the PLS-DA algorithm family which can provide feedback as to the significance of particular variables to a model's success or failure. Here we report models capable of differentiating different molecular species (CH$_{\mathrm{4}}$, H$_{\mathrm{2}}$ and C$_{\mathrm{2}}$H$_{\mathrm{2}})$ down to 1ppm. We use the Variable Importance on Projection (VIP) scores as a primary feature reduction technique to select the most influential wavelengths, which include mainly the major lines (He, H$_{\mathrm{x}})$ as well as impurity lines with C$_{\mathrm{x}}$H$_{\mathrm{y}}$ species having limited effect. [Preview Abstract] |
Friday, October 9, 2020 10:30AM - 10:45AM Live |
XF2.00004: Development of low-pressure oxygen discharge emission model for electron temperature determination Jessica Pachicano, John B. Boffard, Chun C. Lin, Amy Wendt The development of an oxygen emission model is motivated by interest in non-invasive diagnostics based on optical emission spectroscopy (OES) to determine plasma properties, including electron temperature, $T_e$. Model development for O and O$_2^+$ emission intensities includes experimental (OES, multipole resonance probe, Langmuir probe) determination of $T_e$-dependent rate constants for collisional electronic excitation of O$_2^+$ from the ion ground state as well as parameters to quantify electron quenching of emitting states. Excitation and quenching parameters for the O$_2^+$ first (FNS) and second (SNS) negative system bands have been determined by fitting emission intensities as a function of electron density using inductively coupled plasma (ICP) measurements recorded for a range of pressures (2.5-30 mTorr) and RF powers (100-2000 W). A comparison of fit results for a FNS band based on Maxwellian and ``depleted tail'' electron energy distribution functions (EEDF) will be presented. Including the weaker O$_2^+$ SNS emissions will enhance the ability of the completed model to resolve the EEDF using OES alone since the functional dependences of their intensities on plasma conditions exhibit different trends compared to those of the stronger FNS bands. [Preview Abstract] |
Friday, October 9, 2020 10:45AM - 11:00AM Live |
XF2.00005: Mass spectrometric study of O, O$_{\mathrm{2}}(a^{\mathrm{1}}\Delta _{\mathrm{g}})$ and O$_{\mathrm{3}}$ in time-modulated RF driven atmospheric pressure plasma jets Jingkai Jiang, Yolanda Aranda Gonzalvo, Peter Bruggeman A molecular beam mass spectrometer (MBMS) was established to measure fluxes of neutral and ionic species from atmospheric pressure plasma at a substrate. In this work, we report the first measurements of absolute densities of O$_{\mathrm{2}}(a^{\mathrm{1}}\Delta_{\mathrm{g}})$ in an atmospheric pressure plasma jet (APPJ) by MBMS. The ability to measure spatial profiles of O$_{\mathrm{2}}(a^{\mathrm{1}}\Delta _{\mathrm{g}})$ impinging on a substrate in the effluent of the APPJ is a key advantage of the MBMS over previously reported optical methods. The measured large O$_{\mathrm{2}}(a^{\mathrm{1}}\Delta_{\mathrm{g}})$ densities in the APPJ effluent, up to one order of magnitude higher than the O density, underline the potential importance of O$_{\mathrm{2}}(a^{\mathrm{1}}\Delta_{\mathrm{g}})$ in many applications. In addition, we also investigate the change of species fluxes impinging on a dielectric substrate for touching and non-touching conditions by equipping the MBMS with a dielectric sampling plate. Spatially resolved measurements of neutral and ionic species in an He$+$1{\%} O$_{\mathrm{2}}$ RF-driven APPJ are reported. The spatially resolved distribution of O, O$_{\mathrm{2}}(a^{\mathrm{1}}\Delta_{\mathrm{g}})$ and O$_{\mathrm{3\thinspace }}$is dominated by convection and, remarkably shows minimum differences between touching and non-touching conditions. [Preview Abstract] |
Friday, October 9, 2020 11:00AM - 11:15AM Live |
XF2.00006: SPRIGHT -- Small Plasma Reflection Interrogation by GHz Transmitter Andrei Khomenko, Sergey Macheret A new diagnostic method for small-size and microplasmas is proposed. Microplasma diagnostics is quite difficult: microwave interferometry does not work due to the small size and the surrounding walls, probes perturb the small plasma, and optical diagnostics is challenging when the walls are not transparent. The new method is dubbed SPRIGHT (Small Plasma Reflection Interrogation with GigaHertz Transmitter) and is based on applying a weak RF probing signal to the plasma and measuring the reflected signal's amplitude and phase over a wide frequency range. Thus, both real and imaginary parts of the impedance are found over the wide frequency range. If several criteria are satisfied, a simple lumped-parameter equivalent circuit can be used to infer the key plasma parameters, such as the sheath thickness, the electron density, and the momentum transfer collision frequency (and the electron temperature). An experimental setup was developed and implemented to enable application of the weak probing signal to the same electrodes that are used for RF plasma excitation while isolating the two dissimilar-frequency circuits from each other. The method was applied to different plasma cells and was experimentally validated for a small RF plasma at a gas pressure of 1-5 Torr. [Preview Abstract] |
Friday, October 9, 2020 11:15AM - 11:30AM Live |
XF2.00007: Velocity Map Imaging for Electron Energy Distribution Measurements in REMPI-initiated Plasma Jonathan Frank, David Chandler, Martin Fournier, Eric Smoll Improved understanding of the dynamics of pulsed plasmas is needed for accurate modeling of plasma physics and plasma-enhanced chemistry. We developed a technique for studying dynamics of transient low-temperature plasmas in a unimolecular beam velocity map imaging experiment. This approach enables measurements of the temporal evolution of the electron energy distribution function (EEDF) with ns resolution and facilitates studies of transient plasmas without complications from surface chemistry and electrical properties of walls. Low-energy electrons (3.3eV) are generated in a narrow energy distribution (\textless 1meV) by photoionizing a jet-cooled beam of krypton with a pulsed UV laser (214.7nm) in a 2$+$1 REMPI (resonance-enhanced multiphoton ionization) scheme. Electron trajectories are confined by coulombic attraction of Kr$+$ cations, resulting in a transient plasma that persists for microseconds in the field-free region of a pulsed ion/electron imaging apparatus. Electrons are rapidly extracted and accelerated into an electrostatic lens. The resulting velocity map image provides the EEDF at the instant that the repeller plate is pulsed. The EEDF temporal evolution is measured by varying the time between the REMPI laser pulse and the high voltage pulse. [Preview Abstract] |
Friday, October 9, 2020 11:30AM - 11:45AM Live |
XF2.00008: The Effect of Decoherence on Microwave Scattering from a Small Expanding Laser-Generated Plasma Christopher Galea, Mikhail Shneider, Arthur Dogariu, Richard Miles The scattering of microwaves with wavelength $\lambda$ by a plasma of size $L_p$ that is small compared to the wavelength ($L_p \ll \lambda$, so that the scattering is considered coherent) has been studied extensively in the literature and has resulted in many useful diagnostics, including Radar Resonance-Enhanced Multiphoton Ionization (Radar REMPI) and Rayleigh Microwave Scattering (RMS). Recent Radar REMPI experiments conducted at pressures below 1 Torr show that changing from 12 GHz to 94 GHz microwaves results in a faster decay rate of the scattering signal. Assuming the microwave scattering is coherent, we would expect little change between the two normalized frequency cases. However, if we consider the expansion of the plasma (e.g., due to ambipolar diffusion), at some point the spatial phases will differ enough to destructively interfere and our measured signal will drop drastically -- we will call this decoherence. We expect stronger decoherence for the 94 GHz case because the plasma size will become comparable to the shorter wavelength (94 GHz) first, which explains why there is faster decay in the 94 GHz signal. In this talk, we present a model of the decoherence effect and show how it explains this measurement discrepancy for a xenon-helium cell at 152 mTorr. [Preview Abstract] |
Friday, October 9, 2020 11:45AM - 12:00PM On Demand |
XF2.00009: Characteristics of an energy selective mass spectrometer with a Bessel-Box type energy filter Christian Schulze, Zoltan Donko, Jan Benedikt Ions are responsible for the majority of plasma surface interactions. Due to its high scientific interest and widespread use in commercial applications, the precise measurement of ion energy distributions (IED) for specific ion species is essential for their understanding and control. In contrast to other ion diagnostics, energy selective mass spectrometry (ESMS) allows energy and mass selectivity under the drawback of suffering from distortions like chromatic aberration and energy dependent acceptance angles. Therefore, ESMS is typically used for qualitative measurements only. Here, results of ion trajectory simulations are presented that show the focusing behavior of the ion lens system and transmission characteristics of the Bessel-Box energy filter. A guideline to set ion lens voltages in order to minimize distortions is provided. Additionally, measured IEDs are compared with the angular and energy resolved ion flux provided by 1D-PIC simulations. [Preview Abstract] |
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