Bulletin of the American Physical Society
60th Gaseous Electronics Conference
Volume 52, Number 9
Tuesday–Friday, October 2–5, 2007; Arlington, Virginia
Session PR1: Laser and Air Plasmas |
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Chair: Ed Barnat, Sandia National Laboratories Room: Doubletree Crystal City Crystal Ballroom A |
Thursday, October 4, 2007 8:00AM - 8:30AM |
PR1.00001: Transmission line analysis of laser-guided streamers and leaders Invited Speaker: Plasma Physics Division, NRL. We have developed a 1-D transmission-line model for laser-guided discharges, which can be used to analyze both streamers and leaders over the complete length and duration of the discharge, and which facilitates analytic insight as well as providing a simplified, quickly solvable semi-quantitative simulation capability. It is assumed that the laser designates a specified seed electron density within a long thin channel, which is connected directly to a high-voltage source. In this way, the physical situation differs somewhat from natural lightning, which is driven by a uniform electric field, rather than via connection to a voltage source. The mathematics reduces to a 1-D diffusion equation for the electric field E(z,t), with a diffusion coefficient D(z,t) proportional to the channel conductance, very small ahead of the discharge and rapidly increasing at the discharge head. This equation can be solved directly, or the model can be further reduced by requiring that the discharge propagates in a self-similar fashion at a constant propagation speed u; the diffusion equation then reduces to a first-order O.D.E. in t$'$ = t - z/u, which must be solved self-consistently with rate equations that determine D(t$'$). In analyzing streamers (in cold air), we represent the rates as functions of E/n; this simple model yields immediate insights. In analyzing leaders, where the air is heated and excited, we use a complete air chemistry model. The model provides estimates for the minimum propagation speed of negative waves, the minimum level of pre-ionization required for propagation of positive waves, the electric field in the discharge head and body, and the radius and range of leaders, and is especially useful for understanding the streamer-to-leader transition. Work done in collaboration with R. F. Fernsler, S. P. Slinker, D. F. Gordon, and P. Sprangle. [Preview Abstract] |
Thursday, October 4, 2007 8:30AM - 8:45AM |
PR1.00002: Plasma Excited Chemical-Oxygen-Iodine Lasers: Optimizing Injection and Mixing for Positive Gain Natalia Y. Babaeva, Luis A. Garcia, Ramesh A. Arakoni, Mark J. Kushner Chemical oxygen-iodine lasers achieve oscillation on the $^{2}$P$_{1/2}\to ^{2}$P$_{3/2}$ transition of atomic iodine at 1.315 $\mu $m by a series of excitation transfers from O$_{2}(^{1}\Delta )$. In electrically plasma excited devices (eCOILs), O$_{2}(^{1}\Delta )$ is produced in a flowing plasma, typically He/O$_{2}$, at a few to tens of Torr. The iodine is injected into the flow as a He/I$_{2}$ mixture immediately upstream (or in) a supersonic nozzle. A small positive gain with I* limited to a narrow boundary layer near the wall indicates slow mixing when the I$_{2}$ is injected from the wall. This results in low utilization of O$_{2}(^{1}\Delta )$. In this paper we discuss results from 1- and 2-dimensional computational investigations of means to optimize gain in eCOILs by using different I$_{2}$ injection strategies. It was found that due to the plasma generated distribution O$_{2}(^{1}\Delta )$, placement of injectors closer to the axis significantly increased gain by facilitating complete O$_{2}(^{1}\Delta )$/I$_{2}$ mixing. This is partly a function of the inlet flow of NO through the discharge which regulates the density of O atoms produced by electron impact dissociation of O$_{2}$. By optimizing the nozzle dimensions, their location, and I$_{2}$ and NO flow rates, the yield of O$_{2}(^{1}\Delta )$ required to achieve positive gain can be minimized. [Preview Abstract] |
Thursday, October 4, 2007 8:45AM - 9:00AM |
PR1.00003: Kinetics of the Electron Beam Driven Ar-Xe Laser on NRL's Electra Generator J.P. Apruzese, J.L. Giuliani, M.F. Wolford, A. Dasgupta, G.M. Petrov, J.D. Sethian, D.D. Hinshelwood, M.C. Myers, F. Hegeler, Ts. Petrova Due to its efficiency and potentially high power, the Ar-Xe IR laser (1.733 microns) has been the subject of investigation by several groups around the world since the 1980's. Nonetheless, there is still no clear resolution of some of the key physics and kinetics issues that affect its properties. We are addressing these issues in a coordinated program of experiments and modeling at the Naval Research Laboratory. For our experiments we employ NRL's Electra facility, with its extensive suite of diagnostics, developed as a KrF UV laser for the Department of Energy's High Average Laser Power program. We present results showing that Xe$_{2}^{+}$ as well as ArXe$^{+}$ significantly contributes to the pumping of the laser, and that dissociation of ArXe$^{+}$ accounts for most of the laser's temperature sensitivity. We have also found that for an amplifier with dimensions 30 x 30 x 100 cm, the optimum e-beam power deposition density is 50-100 kW/cc for the 140 nanosecond Electra diode pulse. [Preview Abstract] |
Thursday, October 4, 2007 9:00AM - 9:15AM |
PR1.00004: Inferring argon plasma properties from optical emission: the role of metastable atoms R.O. Jung, John B. Boffard, Chun C. Lin, R. Ding, Y.-H. Ting, Y. Yang, A.E. Wendt Collisions between electrons and Ar atoms are primarily responsible for the characteristic plasma glow of Ar discharges. The intensity of a given emission line depends upon both the electron energy distribution (EED) and the excitation cross sections for populating the excited levels. Since the EED also drives the plasma chemistry, there is a need for non-invasive diagnostics of the EED in plasmas for industrial processing. One obstacle in using optical emission spectroscopy (OES) of the plasma glow as a diagnostic is that only electrons in the highest energy range of the EED have enough energy to excite atoms directly from the Ar $3p^6$ ground state. Due to the much lower energy threshold, and much larger cross sections, excitation from atoms in $3p^44s$ metastable levels can contribute substantially to plasma emissions. Recent measurements of excitation cross sections into $3p^55p$ levels ($\lambda$: 395-470~nm) from the Ar metastable levels [1] allow us to exploit the role of metastable atoms to probe the low energy range of the EED. Verification of this OES technique with simultaneous Langmuir probe (for the EED) and optical absorption (for the metastable density) measurements is underway. \\ {[1] R. O. Jung, et al, Phys. Rev. A {\bf 75}, 052707 (2007). \vspace*{-1ex}} [Preview Abstract] |
Thursday, October 4, 2007 9:15AM - 9:30AM |
PR1.00005: Electrical and Optical Diagnostics of an Electron-Beam Generated Air Plasma Robert Vidmar, Kenneth Stalder A pulsed 1-ms 100-keV 20-mA electron beam injected through a transmission window produces air plasma in a 400-liter test cell filled with laboratory air. The beam current is monitored by a current sensor up-stream to a transmission window and supported against air pressures in the test cell from 1 mTorr to 640 Torr. RF amplitude and phase measurements at 10 GHz quantify electron density. Optical emissions from the plasma are monitored by a diode array spectrometer and quantify nitrogen emissions. Ozone concentration is monitored with a UV absorption system. Concentrations of other species are monitored by tunable diode laser absorption spectroscopy. Representative single-shot data from these diagnostic systems will be discussed. This work is supported by the Air Force Research Laboratory under grant numbers FA9550-04-1-0015 and FA9550-04-1-0444; and State of Nevada matching funds. [Preview Abstract] |
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