Bulletin of the American Physical Society
65th Annual Gaseous Electronics Conference
Volume 57, Number 8
Monday–Friday, October 22–26, 2012; Austin, Texas
Session DT1: Diagnostics I |
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Chair: Keiichiro Urabe, University of Tokyo Room: Amphitheatre 204 |
Tuesday, October 23, 2012 10:00AM - 10:15AM |
DT1.00001: Overview of the electric propulsion plasma diagnostics suite for the VASIMR VX-200 testbed Christopher Olsen, Benjamin Longmier, Maxwell Ballenger, Jared Squire, Tim Glover, Mark Carter, Edgar Bering, Matthew Giambusso Descriptions of the various plasma diagnostics and data analysis methods are given for instruments used in high power ($>$ 100 kW) electric propulsion testing. These include planar Langmuir probes, an articulating retarding potential analyzer, a double Langmuir probe, a multi-axis magnetometer, a high frequency electric field probe, microwave interferometer, and momentum flux targets. These diagnostics have been used to measure the efficiencies of the thruster, plasma source, ion cyclotron resonance booster, and magnetic nozzle as well as used to explore physical phenomena in the plume such as ion/electron detachment, plasma turbulence, and magnetic field line stretching. Typical plume parameters range up to 10$^{13}$ cm$^{-3}$ electron density, 1 kG applied magnetic fields, ion energies in excess of 150 eV, and cold electrons (2 -- 5 eV) with a spatial measurement range over 2 m. [Preview Abstract] |
Tuesday, October 23, 2012 10:15AM - 10:30AM |
DT1.00002: Elucidating turbulence dynamics with advanced flow and electric field diagnostics in the large-scale, low temperature, EC heated Texas Helimak plasma W.L. Rowan, K.W. Gentle, Alvaro Garcia de Gorordo, G.A. Hallock Understanding flow shear is essential to most applications of fluid dynamics in that it generally leads to instability and turbulence in three-dimensional flows. However, in a magnetized plasma -- where the equations can often be reduced to two dimensions, shear in the plasma flow velocity transverse to the magnetic field is a very general mechanism for stabilizing turbulence. This theoretical concept is widely invoked, but demonstrations of correlation between flow shear and turbulence suppression in controlled experiments are few. This presentation describes the new research opportunities offered in this area by the The Texas Helimak experiment and its diagnostics. Sharply focused experiments on this novel device require advanced diagnostics. Plasma flow is measured with an imaging spectrometer. More than a hundred Langmuir probes measure turbulence, plasma temperature, plasma density, and electric field. A heavy neutral beam was constructed and is capable of high resolution plasma potential measurements in the plasma. Additional spectroscopic diagnostics are used for supplementary measurement of temperature and density. The data is acquired, stored, and made available within a framework that can be maintained and upgraded by scientists and students. [Preview Abstract] |
Tuesday, October 23, 2012 10:30AM - 10:45AM |
DT1.00003: The Multiple Resonance Probe: A Novel Device for Industry Compatible Plasma Diagnostics Ralf Peter Brinkmann, Robert Storch, Martin Lapke, Jens Oberrath, Christian Schulz, Tim Styrnoll, Christian Zietz, Peter Awakowicz, Thomas Musch, Thomas Mussenbrock, Ilona Rolfes To be useful for the supervision or control of technical plasmas, a diagnostic method must be i) robust and stable, ii) insensitive to perturbation by the process, iii) itself not perturbing the process, iv) clearly and easily interpretable without the need for calibration, v) compliant with the requirements of process integration, and, last but not least, vi) economical in terms of investment, footprint, and maintenance. Plasma resonance spectroscopy, exploiting the natural ability of plasmas to resonate on or near the electron plasma frequency, provides a good basis for such an ``industry compatible'' plasma diagnostics. The contribution will describe the general idea of active plasma resonance spectroscopy and introduce a mathematical formalism for its analysis. It will then focus on the novel multipole resonance probe (MRP), where the excited resonances can be classified explicitly and the connection between the probe response and the desired electron density can be cast as a simple formula. The current state of the MRP project will be described, including the experimental characterization of a prototype in comparison with Langmuir probes, and the development of a specialized measurement circuit. [Preview Abstract] |
Tuesday, October 23, 2012 10:45AM - 11:00AM |
DT1.00004: Novel diagnostic tool, curling probe, for monitoring electron density during plasma processing Anil Pandey, Kimitaka Kato, Shunjiro Ikezawa, Keiji Nakamura, Hideo Sugai A new type of microwave resonator probe, \textit{curling probe}, has recently been proposed [1] which enables direct measurement of electron density even in plasma deposition process. The FDTD simulation of 10-mm-diam curling probe shows a sharp resonance at the frequency from 1 to 6 GHz uniquely determined by the electron density. The resonance frequencies measured by the curling probe in ICP and microwave plasma were explained well by the FDTD simulation result as well as the analytical formula. When a dielectric layer is deposited on the probe surface, the resonance frequency decreases with the increasing layer thickness. Using this probe characteristic, one can \textit{in situ} monitor the thickness of dielectric layer deposited onto a wall of plasma vessel, where the curling probe is positioned to just the same surface as the inner wall. The FDTD simulation shows $\sim $2 MHz shift in the resonance frequency for deposition of 3-$\mu $m-thick dielectric layer, in good agreement with the experimental observation. Thus, the wall deposition layer during chamber cleaning or CVD process can be \textit{in situ} monitored by the curling probe.\\[4pt] [1] I. Liang, K. Nakamura, and H. Sugai, Appl. Phys. Express 4, 066101 (2011). [Preview Abstract] |
Tuesday, October 23, 2012 11:00AM - 11:15AM |
DT1.00005: Electrical and optical diagnostics of CO2 microwave plasmas produced by a radial-line slot antenna Se Youn Moon, Demetre J. Economou, Vincent M. Donnelly, Jianping Zhao, Lee Chen, Radha Sundararajan, Yoshio Susa, Toshihisa Nozawa CO2 plasmas generated by microwave power supplied to a radial-line slot antenna were studied by Langmuir probe and optical spectroscopic methods. At a distance of 190 mm from the slot antenna, the electron temperature and electron density were 2.4 eV and 8.5E10 cm$^{-3}$, respectively, at 10 mTorr with 3 kW microwave power. At a pressure of 150 mTorr, the electron temperature decreased to below 1 eV while the electron density dropped to 8.6E9 cm$^{-3}$ because the plasma was then localized near the antenna. Low electron temperature is advantageous for reducing some forms of device damage. In addition, the gas temperature, obtained from the rovibrational spectra of added N2 gas, increased from 650 K at 10 mTorr to 1160 K at 50 mTorr. Further increase in gas pressure up to 150 mTorr resulted in a slight decrease of the gas temperature, again due to plasma localization near the antenna. The atomic oxygen density, derived using actinometry, was 5.7E1012 cm$^{-3}$ at 10mTorr and 5.6E13 cm$^{-3}$ at 150 mTorr with 3kW. Results using trace rare gas optical emission spectroscopy (TRG-OES) and vacuum ultra-violet absorption spectroscopy will also be presented and compared with Langmuir probe measurements and actinometry. [Preview Abstract] |
Tuesday, October 23, 2012 11:15AM - 11:30AM |
DT1.00006: Langmuir probes in the intense rf environment inside a helicon discharge Francis F. Chen High-density helicon discharges are usually studied downstream from the antenna region. Langmuir probes are hard to use there because of the intense rf environment and heavy bombardment by ions and electrons. The plasma studied here is a small helicon discharge with permanent magnets to provide the dc B-field. A very thin probe was designed for the small discharge (5 cm diam $\times $ 5 cm long) without disturbing it. A floating rf-compensation electrode (CE) supplemented the self-resonant chokes to filter out the rf pickup of 30-50V peak-to-peak. The effect of CE size is shown. Saturation ion current varies with the square root of probe potential $V_{p}$ well beyond to expected validity range of the Langmuir Orbital Motion Limited formula. Fitting the $I^{2} - V$ curve with a straight line, we subtract the fitted ion current from total current to obtain the ln($I_{e})-V_{p}$ curve. This is almost always straight, indicating Maxwellian electrons. With high B-fields and collisions, the transition region of the $I - V$ characteristic is very short, and sometimes absent. However, $T_{e}$ can be obtained correctly in the ion-subtraction region. This is fortunate, since drawing saturation electron current can drive the probe to emission. Interesting effects of electron emission will be shown. [Preview Abstract] |
Tuesday, October 23, 2012 11:30AM - 11:45AM |
DT1.00007: Electron Energy Distribution Function Measurements in a Negative Hydrogen Ion Source John Blandino, Zachary Taillefer, Lynn Olson A High Pressure Discharge Negative Ion Source (HPDNIS) operating on hydrogen has been built and tested. The HPDNIS uses an RF discharge operating in a pressure range of 10s to 100s of Torr. Gas from this discharge flows through an orifice into a lower pressure (10s of mTorr), negative ion production region designed to maintain low electron temperature and enhance negative ion formation through dissociative attachment. Plasma exiting the negative ion production region is extracted through a biased grid set with separation of negative ions and electrons achieved via an applied magnetic field. This presentation will describe measurements of the electron energy distribution function (EEDF), in the negative ion production region, made using a single Langmuir probe based on the Druyvesteyn method. The presentation will discuss challenges to using this method as a result of complications arising from the presence of negative ion species, collisionality, and RF induced distortion of the EEDF. The presentation will also summarize estimates of the rate constant for collisional processes such as dissociative attachment, made using the measured EEDFs in the negative ion production region of the HPDNIS. [Preview Abstract] |
Tuesday, October 23, 2012 11:45AM - 12:00PM |
DT1.00008: Langmuir Probe RF Plasma Compensation using Simulation Method Aasim Yousif Azooz The problem Langmuir probe data deformation due to RF pickup by the probe is treated through computer simulation method. It is pointed out that proper RF effects compensations can be obtained by proper software treatment of the RF contaminated data. It is demonstrated that correct RF unaffected probe I-V characteristics can be accurately reproduced from the RF contaminated data. This eliminates the need for the use of any filters or other hardware procedures. User friendly matlab based software is presented. The software automatically retrieves the correct RF unaffected I-V characteristics for single and double Langmuir probe data which consequently allows for proper evaluation of plasma parameters such as the plasma electron temperature, electron number density and the electron energy distribution function (EEDF). [Preview Abstract] |
Tuesday, October 23, 2012 12:00PM - 12:15PM |
DT1.00009: On multiple component detection in molecular plasmas using cw external-cavity quantum cascade infrared lasers Dmitry Lopatik, Norbert Lang, Uwe Macherius, Henrik Zimmermann, Juergen Roepcke Several cw external cavity quantum cascade lasers (EC-QCLs) have been tested as radiation sources for an absorption spectrometer focused on the analysis of molecular plasmas. Based on the wide spectral tunability of EC-QCLs multiple species detection is demonstrated in low pressure Ar/N$_{2}$ MW plasmas containing CH$_{4}$ as hydrocarbon precursor. Using the direct absorption technique the evolution of the concentrations of CH$_{4}$, C$_{2}$H$_{2}$, HCN and H$_{2}$O has been monitored depending on the discharge conditions (p= 0.5 mbar, f= 2.45 GHz) in a planar MW plasma reactor. The concentrations were found to be in the range of 10 $^{11}$ -- 10 $^{14}$ molecules cm$^{-3}$. Based on the profiles of absorption lines the gas temperature T$_{g}$ has been calculated in dependence on the discharge power. Changing the discharge power from 0.2 kW to 1 kW leads to an increase of T$_{g}$ from 400 to 700 K. The typical spectral line width of the EC-QCLs under the study was about 30 MHz. Varying the power values of an EC-QCL for direct absorption measurements at low pressure conditions no saturation effects in determining the concentrations of CH$_{4}$ and C$_{2}$H$_{2}$ could be found under the used conditions. [Preview Abstract] |
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