Bulletin of the American Physical Society
72nd Annual Gaseous Electronics Conference
Volume 64, Number 10
Monday–Friday, October 28–November 1 2019; College Station, Texas
Session PR3: Plasma Diagnostics II |
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Chair: Christopher Limbach, Texas A&M University Room: Century III |
Thursday, October 31, 2019 10:00AM - 10:15AM |
PR3.00001: Selective Measurement of Rayleigh-Brillouin Scattering in Weakly Ionized Plasmas Using Dispersion from a Mercury Vapor Prism Anuj Rekhy, Mikhail Shneider, Richard Miles We present here an approach to the instantaneous measurement of the local temperature and density in a weakly ionized plasma through spectral dispersion of the Rayleigh-Brillouin Scattering (RBS) by mercury vapor. A mercury atomic vapor cell in the form of a single or multiple prisms causes light near resonances to be strongly refracted, so the RBS spectrum is dispersed and can be imaged onto a camera or detector array. Mercury has seven stable isotopes and, due to hyperfine splitting, there are 10 distinct absorption lines in the vicinity of 253.7 nm. High dispersion can be achieved by increasing the vapor pressure in the cell containing mercury, and, at relatively high vapor pressure, those lines merge into a single absorption feature of 30 GHz. By selecting a narrow linewidth laser with the wavelength tuned just within the edge of the mercury vapor absorption feature, direct scattering from particles, windows and walls is blocked. However, the wing of the RBS spectrum extends into the transparent region and is refracted by the mercury vapor, and provides a spatial image of the RBS spectrum on a detector placed at a distance from the prism. The linewidth of the RBS yields the kinetic temperature of the weakly ionized gas and the integrated line strength yields the density. [Preview Abstract] |
Thursday, October 31, 2019 10:15AM - 10:30AM |
PR3.00002: Velocity distribution function measurement of heavy species in weakly ionized plasma flows via coherent Rayleigh-Brillouin scattering Alexandros Gerakis, Mikhail Shneider, Kentaro Hara We suggest the applicability of single shot coherent Rayleigh-Brillouin scattering (CRBS) for the determination of the velocity distribution function (VDF) of heavy species in a weakly ionized plasma, from which macroscopic quantities can be extracted. CRBS, a four-wave mixing technique, relies on the interaction between a high energy optical lattice of precisely tailored chirped frequency and the VDF of the medium to which it is applied. The variation of the CRBS signal intensity at different optical lattice phase velocities enables the restoration of the VDF, directly mapping it in the CRBS lineshape. CRBS has already been demonstrated to be the coherent analogue of spontaneous Rayleigh-Brillouin scattering in measuring the temperature, pressure, bulk and shear viscosity, speed of sound and polarizability of a gas or gas mixture, in a single laser shot [1]. Nanoparticles produced in an arc discharge have also been measured \textit{in situ} using CRBS [2]. We will discuss the recent progress of single shot CRBS as a gas flow measurement technique and its use in a weakly ionized plasma flow. 1. A Gerakis, MN Shneider and PF Barker 2013 Opt. Lett. 38(21), pp.4449-4452. 2. A Gerakis, YW Yeh, MN Shneider, JM Mitrani, BC Stratton and Y Raitses 2018 Phys. Rev. Appl. 9(1), p.014031. [Preview Abstract] |
Thursday, October 31, 2019 10:30AM - 10:45AM |
PR3.00003: Student Excellence Award Finalist: Laser-induced fluorescence measurement of enhanced ion-acoustic fluctuations in an electron sheath Ryan Hood, Scott Baalrud, Lucas Beving, Robert Merlino, Fred Skiff We present ion fluctuation spectra, resolved spatially through the presheath region of a positively and negatively biased electrode using laser-induced fluorescence (LIF) [1]. Ion-acoustic fluctuations are observed near 500 kHz, about half the ion plasma frequency, throughout the presheath and for positively and negatively biased electrodes. The fluctuation power is observed to increase significantly when the electrode is biased above the plasma potential. However, the fluctuation power does not vary greatly with distance from the electrode. These observations are consistent with a recent theory that predicts the presence of a long-range electron presheath, in which the fast electron flow drives ion-acoustic fluctuations [2]. [1] S. W. Mattingly and F. Skiff, Rev. Sci. Instrum. 89, 043508 (2018). [2] B. Scheiner, S. D. Baalrud, B. T. Yee, M. M. Hopkins, and E. V. Barnat, Phys. Plasmas 22, 123520 (2015). [Preview Abstract] |
Thursday, October 31, 2019 10:45AM - 11:00AM |
PR3.00004: Towards Remote Magnetic Field Measurements via Microwave Scattering by a Laser-Generated Plasma Christopher Galea, Mikhail Shneider, Arthur Dogariu, Richard Miles The presence of an external magnetic field has been shown to depolarize the microwave scattering from a small laser-generated plasma. This effect has been demonstrated experimentally with a femtosecond Ti:Sapphire laser focused into a quartz cell containing 7 milliTorr xenon with an external magnetic field ranging from 0.02 -- 0.08 T. This magnetically induced depolarization depends on both the magnitude and direction of the magnetic field and occurs locally at the laser-generated plasma, suggesting the possibility of a remote vector magnetic field diagnostic in gases and weakly ionized plasmas. In this talk, we further investigate the effect both theoretically and experimentally: validating our model for a larger variety of conditions and assessing the diagnostic capability of this effect. Recent experiments have shown the mitigation of the magnetically induced depolarization at higher microwave frequencies (i.e., frequencies much greater than the electron cyclotron frequency) as well as the presence of background depolarization due to scattering by the quartz cell. Potential limitations of the proposed diagnostic technique will also be discussed. [Preview Abstract] |
Thursday, October 31, 2019 11:00AM - 11:30AM |
PR3.00005: Electric Field Measurements in Nanosecond Pulsed Discharges Invited Speaker: Marien Simeni Simeni Nanosecond pulsed electrical gas discharges are generated by applying a voltage in excess of the breakdown threshold across two electrodes. The applied voltage creates an electric field responsible for accelerating the first free electrons present to higher energies, transferring electrical energy to electrons. This results in electron avalanches and in streamer formation, parts of the breakdown process. The electric field hence controls input energy partition in the plasma (vibrational and electronic excitation, dissociation, ionization) and also the rate of gas temperature increase. In return, the electric field in the plasma is controlled by the ionization, electron and ion transport, electron emission from electrodes and surface charge accumulation on dielectrics. Therefore high-resolution spatio-temporal electric field measurements are of great interest for insights into kinetics of ionization, charge transport and also for validation of kinetic models. In this work we use Stark splitting polarization spectroscopy and electric field-induced second harmonic (E-FISH) to perform spatio-temporal electric field measurements. The former technique is used for discharges in helium, investigating shifting of the He I line at 492.2 nm and of its forbidden counterpart in presence of an electric field. E-FISH is employed in a variety of discharges (surface DBD, quasi-2D DBD and pin-to-pin) with an absolute calibration provided by measurements of a known Laplacian field. E-FISH measurements were performed in air, argon and krypton plasmas and in plasmas-enhanced flames. Both picosecond and femtosecond laser pulses were used. [Preview Abstract] |
Thursday, October 31, 2019 11:30AM - 11:45AM |
PR3.00006: In Situ Measurement of Two Dimensional, Two Component Electric Field Dynamics of a ns-SDBD Plasma with Sub-Nanosecond Resolution by Femtosecond EFISH Kristofer Meehan, Andrey Starikovskiy, Arthur Dogariu, Richard B. Miles Nanosecond pulsed Dielectric Barrier Discharge (DBD) plasmas have gained popularity as an efficient method of producing quasi-uniform plasmas at atmospheric pressure, with uses in fields such as from combustion, aerodynamic flow control, and plasma medicine. In situ measurements of Surface DBD's (SDBD) have traditionally been difficult to achieve due to surface effects, fast timescales, and the sub-millimeter plasma thickness. The recent development of the nonlinear laser diagnostic Electric Field Induced Second Harmonic (EFISH) allows for electric field measurement within the plasma volume that are directionally sensitive and temporally resolved. In this study, we have scanned a cylindrically focused, 200 fs FWHM Ti:Sapphire laser beam to produce two-dimensional maps of sub-nanosecond resolved, two component electric field dynamics in a ns-SDBD plasma. Discharges in Argon and Air are studied at low pressure ($<$200 Torr). Reduced electric fields of 1000 Td in the ionization wave are suggested by the measurement, which are supported by previous spectroscopic results. Both the incident and reverse ionization wave are apparent, demonstrating fs-EFISH to be a useful tool for studying and developing ns-SDBD actuators. [Preview Abstract] |
Thursday, October 31, 2019 11:45AM - 12:00PM |
PR3.00007: Computational Imaging of Electron Densities and Temperatures of a Cathodic Arc using Laser Excited Emission Data Brian Z. Bentz, Edward V. Barnat This communication reports computational methods to extract spatially resolved images of electron densities and temperatures from laser-collision induced fluorescence (LCIF) data in low-pressure helium environments. The primary advantage of the approach is the capability to determine electron densities up to 10$^{\mathrm{16}}$ cm$^{\mathrm{-3}}$, whereas previous LCIF diagnostics were limited to electron densities up to about 10$^{\mathrm{12}}$ cm$^{\mathrm{-3}}$. This was accomplished using the maximum likelihood estimator (MLE) to estimate the electron density and temperature within the plasma by minimizing the difference between LCIF measurements and collisional radiative model predications. The method is demonstrated with a cathodic arc expanding in 65 mTorr of helium. Unexpectedly, a discontinuity is found about 4 mm above the arc where the electron temperature increases from 0.5 eV to 5 eV and the electron density drops from about 10$^{\mathrm{14}}$ cm$^{\mathrm{-3}}$ to about 10$^{\mathrm{12}}$ cm$^{\mathrm{-3}}$. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy National Nuclear Security Administration under contract DE-NA0003525. [Preview Abstract] |
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