Bulletin of the American Physical Society
74th Annual Gaseous Electronics Conference
Volume 66, Number 7
Monday–Friday, October 4–8, 2021;
Virtual: GEC Platform
Time Zone: Central Daylight Time, USA
Session GT51: Plasma Diagnostics: Laser Diagnostics I |
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Chair: Sergey Leonov, University of Notre Dame Room: Virtual GEC platform |
Tuesday, October 5, 2021 3:45PM - 4:00PM |
GT51.00001: Imaging of Methyl Radical in a Plasma Jet by Photofragmentation Laser-Induced Fluorescence Dirk van den Bekerom, Erxiong Huang, Jonathan H Frank The methyl radical (CH3) is a key intermediate species in plasma assisted processes such as catalysis and combustion. For example, in oxidative coupling of methane, the initial extraction of a hydrogen atom from methane results in the formation of methyl, which can subsequently react to form desired products such as C2 species. However, methyl cannot be detected directly by laser-induced fluorescence. We present a technique for imaging the two-dimensional distribution of methyl near catalytic surfaces, using photofragmentation laser-induced fluorescence (PF-LIF). In this technique, methyl is photodissociated by a 213 nm laser beam to produce CH fragments. These photofragments are then detected with LIF imaging by exciting a transition in the B-X band of CH with a second laser at 390 nm. Collisional B-A electronic energy transfer then populates the A-state of CH, after which fluorescence is detected in the A-X band at 430 nm with a gated ICCD camera and suitable band pass filter. This non-resonant detection scheme enables interrogation near surfaces. The PF-LIF diagnostic is calibrated by producing a known amount of methyl through photodissociation of acetone. We demonstrate the PF-LIF approach by imaging the methyl distribution in a nanosecond pulsed helium plasma jet with a coflow containing hydrocarbon fuels. |
Tuesday, October 5, 2021 4:00PM - 4:15PM |
GT51.00002: Temperature and Density Profile of Xe I and Xe II Inside the Discharge Channel of a Hall Effect Thruster Mustafizur Rahman, Richard Branam Plasma species transport inside the discharge channel, specifically in the ionization and acceleration zones of a Hall Effect Thruster is the key factor that involves with thruster’s efficiency and lifetime. Spatially resolving the ionization and acceleration zones requires temperature and density profiles of neutrals and ions. A Laser Induced Fluorescence (LIF) technique is employed to determine the excited state temperature and relative density of Xe I, Xe II in different discharge conditions. Spatially distributed plasma potential maps are estimated from the top of the anode to the plume of the thruster. |
Tuesday, October 5, 2021 4:15PM - 4:30PM |
GT51.00003: Atomic oxygen densities in pulsed inductively-coupled plasmas: Laser spectroscopy & energy resolved actinometry Michael Mo, Kari Niemi, Andrew R Gibson, Christopher Bowman, Deborah O'Connell, Timo Gans Oxygen-containing pulsed Inductively Coupled Plasma (ICP) enable high-precision semiconductor etching with nanoscale features and low substrate damage. Atomic oxygen plays a key role in the plasma-material interactions, and reliable absolute density measurements can improve plasma processing precision. Two-photon Absorption Laser Induced Fluorescence (TALIF) spectroscopy can accurately measure these with high spatial and temporal resolution. However, implementing TALIF as a routine industrial method is challenging. Alternative diagnostics that balance ease of use and accuracy are desired. Energy Resolved Actinometry (ERA) has previously shown to provide reliable information in capacitively-coupled plasmas. In this contribution, the ERA concept has been adapted to different operation regimes of an ICP and also extended to implement Bayesian analysis using different optical emission lines for improved reliability. It is compared with direct TALIF measurements taken in the early afterglow. Generally, good agreement has been found between the two techniques, promising the potential application of ERA in future real-time process control. |
Tuesday, October 5, 2021 4:30PM - 4:45PM |
GT51.00004: Absolute N atom density measurements in an Ar/N2 Micro-Hollow Cathode Discharge by means of nanosecond Two-Photon Laser Induced Fluorescence Alice Remigy, Guillaume Lombardi, Xavier Aubert, Swaminathan Prasanna, Nelson De Oliveira, Claudia Lazzaroni Hexagonal boron nitride (h-BN) has attracted lots of interest because of its high band gap and compatibility with graphene. To provide the N atoms necessary for the deposition of h-BN, a Micro Hollow Cathode Discharge (MHCD) ignited in an Ar/N2 gas mixture is used to dissociate the strongly bonded N2 molecules. The MHCD is placed in a reactor with two chambers communicating only through the hole of the MHCD. A pressure differential between the two chambers, with tens of millibars in the high pressure chamber and 1 mbar in the low pressure chamber, creates a plasma jet towards the low pressure side. The absolute density of N atoms is measured by means of nanosecond Two-Photon Laser Induced Fluorescence (TALIF), scanning along the plasma jet axis over 4 cm from the hole. A parametric study is performed, varying the pressure in the high pressure chamber and the percentage of N2 in the Ar/N2 gas mixture. These measurements are compared to those previously obtained by the Vacuum Ultra Violet high resolution Fourier Transform spectrometry set-up of the DESIRS beamline at the SOLEIL synchrotron. These measurements are necessary to better understand and optimize the production of N atoms in the MHCD source and their propagation towards the substrate for h-BN deposition. |
Tuesday, October 5, 2021 4:45PM - 5:00PM |
GT51.00005: SNR comparison experiments of DC heated LaB6 rod and tungsten filament cathodes diagnosed by laser-induced fluorescence Di Jiang, Chi-Shung Yip, Wei Zhang, Chenyao Jin, Guosheng Xu, Liang Wang Multi-dipole confined hot cathode discharges are extensively used in low temperature plasma studies to investigate sheath-presheath formation, develop plasma diagnostics and study plasma-wave interactions, etc. Typically, high-energy primary electrons in multi-dipole devices are generated by direct hot cathode discharge from very thin (d< 0.3 mm) tungsten filaments, which are readily heated via a DC current to the temperature emitted by electrons to produce plasma, but is very luminous such that the filaments themselves may become a sizable source of noise in laser induced fluorescence (LIF) measurements [1]. Recently, tungsten filaments are increasingly being replaced by less luminous alternatives, such as barium oxide and lanthanum hexaboride (LaB6) cathodes. These materials can emit electrons at temperatures close to 1000 K lower than the tungsten material, so the background light emitted will be greatly reduced, which involves an important improvement in active spectral diagnostic studies such as laser-induced fluorescence, i.e. the decrease in background black body radiation under the same discharge parameters will be conducive to the improvement of the signal-to-noise ratio of effective fluorescent signals. However, it is more difficult to heat these cathodes via a DC current, complicating their associated cathode assembly designs. In this presentation, we present a simple design to directly heat a LaB6 cathode manufactured at suitable dimensions, and performed comparison of LIF signal-to-noise ratio of this LaB6 hot cathode discharge with that of a typical tungsten filament discharge. |
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