Bulletin of the American Physical Society
70th Annual Gaseous Electronics Conference
Volume 62, Number 10
Monday–Friday, November 6–10, 2017; Pittsburgh, Pennsylvania
Session JW2: Diagnostics II |
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Chair: Terry Sheridan, Ohio Northern University Room: Duquesne |
Wednesday, November 8, 2017 8:00AM - 8:15AM |
JW2.00001: An Electron Density Diagnostic Based on Doppler-free Measurement of Stark Broadening Abdullah Zafar, Elijah Martin, Steve Shannon Passive spectroscopic measurements of Stark broadening have been reliably used to determine electron density for decades. However, a low-density limit (\textasciitilde 1013 cm-3) exists due to Doppler and instrument broadening of the spectral line profile. A synthetic diagnostic for measuring electron density capable of high temporal (ms) and spatial (mm) resolution is currently under development at Oak Ridge National Laboratory. The diagnostic is based on measuring the Stark broadened, Doppler-free, spectral line profile of a Balmar series transition using a laser-based technique. The diagnostic approach outlined here greatly reduces line broadening using Doppler-free saturation spectroscopy (DFSS), allowing access to Stark broadening regimes at lower densities than previously realized. This technique has been successfully employed to measure spectral data in an electron cyclotron resonance (ECR) source for an electron density range of 1011-1012 cm-3. Theoretical modeling continue to improve as diagnostic artifacts, such as crossover peaks, are better understood and captured in the simulations. Details of diagnostic implementation and agreement between experimental data and theoretical results is discussed. [Preview Abstract] |
Wednesday, November 8, 2017 8:15AM - 8:30AM |
JW2.00002: Optical Emission Diagnostics of a Non-equilibrium Helium Plasma Jet at 1 atm in Ambient Air Vincent M Donnelly, Tam Nguyen, Demetre J Economou We studied a He 200 kHz rf plasma jet emerging into open air from a quartz tube wrapped by a grounded and an rf-powered electrode. The jet impinged on a dielectric substrate (MgF$_{\mathrm{2}}$ or fused silica). VUV to near IR emission spectra were recorded through the substrate either along the discharge axis, or at a steep angle to isolate emission close to the surface. Time-resolved emission was observed close to the surface only during a brief period near to just past the peak in the positive applied rf voltage. No emission was observed during the negative voltage with the exception of a weak emission from N$_{\mathrm{2}}$(C$^{\mathrm{3}}\Pi _{\mathrm{u}}\to $B$^{\mathrm{3}}\Pi_{\mathrm{g}})$ just prior to peak negative voltage. With the exception of N$_{\mathrm{2}}$(C$^{\mathrm{3}}\Pi_{\mathrm{u}})$, emissions along the discharge axis from impurities mixing into the He flow just outside the nozzle were dominated by dissociative excitation via He metastables (He*). Axial emission from N$_{\mathrm{2}}^{\mathrm{+}}$ was also produced by collisions with He* (i.e. Penning ionization of N$_{\mathrm{2}})$. These emissions were only modulated to a small degree during the rf period, and were shifted in phase with respect to the peak positive and negative voltages, reflecting the lifetime of He*. Detailed analysis of the emission temporal dependences revealed details of discharge kinetics. [Preview Abstract] |
Wednesday, November 8, 2017 8:30AM - 8:45AM |
JW2.00003: State-by-state spectra fitting tool for highly-non-equilibrium plasmas: discharge in contact with water Tomas Hoder, Jan Vorac, Petr Synek Recently, the interest in discharges in contact with water increased enormously. Often, the discharges are ignited in a noble gas and the atoms and water fragments are the only available spectral signature. In such cases, the spectrum of hydroxyl radical may seem attractive for neutral gas thermometry. This contribution brings an extensive analysis of OH(A-X) spectrum obtained on special case of kHz driven surface DBD in contact with water. We have observed a spectrum that can be interpreted as a superposition of emission from several groups of OH. We have distinguished three groups - \textit{cold group}, best observable for low $N$' quantum numbers, \textit{hot group}, best observable for higher $N$' \quad quantum numbers and the third group influenced by iso-energetic vibrational energetic transfer OH(A,v'$=$1 -\textgreater v'$=$0), best observable for 8 \textless $N$'\textless 14. The data was processed by the novel method of state-by-state fitting. This approach combines spectral simulation and traditional Boltzmann plot construction procedure. A synthetic spectrum is simulated for each rovibronic upper state, including the instrumental broadening and matched with the measurement. This functionality was incorporated to the \underline {massiveOES} software. [Preview Abstract] |
Wednesday, November 8, 2017 8:45AM - 9:00AM |
JW2.00004: IEDF distortion in high voltage retarding field energy analyzers Steven Shannon, Matthew Talley, John Verboncoeur, Lee Chen There is a need for IEDF measurement at increasingly high voltages in the kV range, particularly (but not limited to) the characterization of industrial systems that rely on high energy ions to achieve desired process conditions for material removal, deposition, or modification. Field variation, surface charge accumulation, and space charge formation within an RFEA diagnostic can limit the energy resolution of the sensor and in some cases provide distorted IEDF's when typical first derivative analysis of the sensor's VI characteristic is employed. Design considerations for an RFEA capable of operation in a capacitive RF system with sheath voltages in the kV range are presented, and show how IEDF energy resolution is impacted through design considerations such as grid geometry, spacing, and between-grid channel design. Space charge induced IEDF distortion when employing VI differentiation techniques will be presented. Specifically, distortion of the low energy portion of an IEDF due to space charge effects as well as pathways for resolving this distortion phenomenon through data analysis will be presented. [Preview Abstract] |
Wednesday, November 8, 2017 9:00AM - 9:30AM |
JW2.00005: Single Emission-Line-Ratio Techniques for Correlating Reduced Electric Field, Electron Energy Distribution, and Metastable-Atom Density in a Pulsed Argon Discharge Invited Speaker: Jim Franek Argon emission lines, particularly those in the near-infrared region (700-900nm), are used to determine plasma properties in low-temperature, partially ionized plasmas to determine effective electron temperature and argon excited state density using appropriately assumed electron energy distributions. While the effect of radiation trapping is included in the interpretation of plasma properties from emission-line ratio analysis, eliminating the need to account for these effects by directly observing the 3px-to-1sy transitions is preferable in most cases as this simplifies the analysis. The extended coronal model is used to acquire an expression for 420.1-419.8nm emission-line ratio, which is sensitive to direct electron-impact excitation of argon excited states as well as stepwise electron-impact excitation of argon excited states for the purpose of inferring plasma quantities from experimental measurements. Initial inspection of the 420.1-419.8nm emission-line ratio suggests the pulse may be empirically divided into three distinct stages. Using equilibrium electron energy distributions from simulation to deduce excitation rates in the extended coronal model affords agreement between predicted and observed metastable density. Applying this diagnostic technique to lower-resolution spectroscopic systems is not straightforward, however, as the 419.8nm and 420.1nm emission-line profiles are convolved and become insufficiently resolved for treating the convolution as two separate emission-lines. To remedy this, the argon 425.9nm emission-line is evaluated as a proxy for the 419.8nm emission-line as they are both attributed to direct excitation from the argon ground state. The intensity of the 425.9nm emission-line is compared to the intensity of the 419.8nm emission-line over a range of plasma conditions to infer the same plasma quantities from similar experimental measurements. Discrepancies between the observed intensities of the emission-lines are explained by electron-impact cross-sections of their parent states and the electron energy distribution. [Preview Abstract] |
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