Bulletin of the American Physical Society
2006 59th Annual Gaseous Electronics Conference
Tuesday–Friday, October 10–13, 2006; Columbus, Ohio
Session MW2: Thermal Plasmas, Arcs, and Breakdown |
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Chair: Sergey O. Macheret, Lockheed Martin Room: Holiday Inn Salon B |
Wednesday, October 11, 2006 4:00PM - 4:15PM |
MW2.00001: Three -- dimensional mapping of electron temperature and electron density in an arc -- anode boundary layer Guang Yang, Joachim Heberlein A DC transferred argon arc system, which allows control of the anode boundary layer by introducing cold cross flow, has been setup in our lab to recreate the situation in the anode region of a typical plasma spray torch. In this study, a laser Thomson scattering system, which can probe a location as close as 50 micron from the anode surface, has been developed and has been used to obtain electron temperature and electron density maps at different planes in the anode boundary layer. The effects of different operating conditions with different cross flow gases (argon and nitrogen) are presented. Our results have shown that increasing argon cross flow rate changes the arc attachment from a diffuse mode to a transition mode with multiple unsteady attachment spots and then to a constricted mode. The electron temperature in the attachment increases from 9000K to 13000K and the electron density increases from 5*10$^{21}$m$^{-3}$ to 1*10$^{22}$m$^{-3}$ as the arc attachment changes from a diffuse mode to a constricted mode. In a constricted mode, an extended non-equilibrium region with low electron density ($<$10$^{21}$m$^{-3})$ and relatively high electron temperature ($\sim $5000K) is also formed. With nitrogen, the arc is constricted already at low cross flow rates. The three dimensional results allow the qualitative determination of the current and heat flux distribution in the anode boundary layer. [Preview Abstract] |
Wednesday, October 11, 2006 4:15PM - 4:30PM |
MW2.00002: 3D Finite Element Modeling of Arc and Jet Dynamics in a DC Plasma Torch Juan Trelles, Emil Pfender, Joachim Heberlein The unstable behavior of the plasma jet in direct current plasma torches has been well documented experimentally, but the nature of the instabilities is not well understood. This is in part due to our lack of understanding of the forcing effect on the plasma jet caused by the arc dynamics, mainly because the confinement of the arc inside the torch limits its direct observation. In this research, the dynamics of the electric arc and the plasma jet in a plasma torch are modeled using a 3D, time-dependent, LTE model. The fluid and electromagnetic equations are solved in a fully coupled manner by a variational multiscale finite element method, which implicitly accounts for the multiscale nature of the flow. Simulations of a commercial torch operating with Ar-He under typical operating conditions used for plasma spraying are presented. The simulation results reveal the highly unsteady and quasi-periodic behavior of the arc as well as the undulating nature of the plasma jet. Furthermore, our simulations indicate a clear correlation between the arc and jet dynamics, furthering our understanding of the interactions between thermal plasmas and cold gases, as typically found in plasma processing systems. [Preview Abstract] |
Wednesday, October 11, 2006 4:30PM - 4:45PM |
MW2.00003: Numerical investigation of stability of current transfer to thermionic cathodes Maria Jose Faria, Mikhail Benilov Considerable advances have been achieved in recent years in theory and modelling of current transfer from high-pressure arc plasmas to thermionic cathodes. Solutions describing the diffuse mode and different spot modes have been obtained and analyzed. However, this information is not yet sufficient for engineering practice: one needs to know also which of the modes are stable in some or other particular conditions. Unfortunately, a self-consistent stability theory is absent; hypotheses and speculations available in the literature are insufficient to explain experimental findings. In this work, stability of various modes of current transfer to thermionic cathodes of high-pressure arcs is studied numerically. The model of nonlinear surface heating is used. Particular attention is paid to the case where the arc is powered by a current source. It is found that the diffuse mode is stable at currents exceeding that corresponding to the first bifurcation point and is unstable at lower currents. The first spot mode is unstable between the bifurcation point and the turning point and is stable beyond the turning point. The second and subsequent spot modes are always unstable. [Preview Abstract] |
Wednesday, October 11, 2006 4:45PM - 5:00PM |
MW2.00004: Advanced Modeling of Thermal Plasmas for Industrial Applications Vittorio Colombo, Emanuele Ghedini Modeling results are presented for different industrial thermal plasma sources using a customized version of the commercial code FLUENT capable of 2D and 3D transient simulation with advanced CFD models that take into account turbulence effects using different approaches (Reynolds Stress Model and Large Eddy Simulation), transport of species and radiation (Discrete Ordinate Model with interaction between radiation and solid surfaces). Simulations results are presented in order to show the capabilities of this modeling tool, which is very useful for the design of a wide range of atmospheric pressure thermal plasmas devices and related assisted processes, such as: ICPTs with injection of powders for spheroidization, DC twin-torch transferred arc plasma systems for waste treatment, DC non-transferred arc torch for plasma spraying and DC transferred arc torch for high quality plasma cutting. [Preview Abstract] |
Wednesday, October 11, 2006 5:00PM - 5:15PM |
MW2.00005: Electric field measurements in moving ionization fronts during plasma breakdown Erik Wagenaars, Gerrit Kroesen, Mark Bowden We have performed time-resolved, direct measurements of electric field strengths in moving ionization fronts during the breakdown phase of a pulsed plasma. Plasma breakdown, or plasma ignition, is a highly transient process marking the transition from a gas to a plasma. Some aspects of plasma breakdown are reasonably well understood, but many details remain unknown, mainly because of a lack of direct measurements of plasma properties. Most of the important processes in breakdown, such as electron multiplication in avalanches and propagation of ionization fronts, are controlled by the electric field distribution in the discharge region. We have developed an experimental laser technique capable of measuring spatially and temporally resolved electric field distributions in both plasma and neutral gas. The technique is based on detecting the Stark shift and mixing of high-lying Rydberg levels of xenon atoms, using a 2+1 photon excitation scheme with fluorescence-dip detection. With this experimental arrangement, we measured absolute, time-resolved electric field strengths during the breakdown phase of a low-pressure plasma between parabolic electrodes. Characteristic features of breakdown, such as a moving ionization front with electric field enhancement and the formation of a plasma sheath, were observed. [Preview Abstract] |
Wednesday, October 11, 2006 5:15PM - 5:30PM |
MW2.00006: Steady-State Model for Laser-Guided Lightning-Like Discharges M. Lampe, R. Fernsler, S. Slinker, D. Gordon, P. Sprangle We have developed a reduced model for laser-guided discharges, which can be solved efficiently over the complete length and duration of the discharge, and also provides analytic insights. We assume the laser pre-pulse designates a long thin channel with given low conductivity, and that current flows entirely in this channel. Maxwell's equations can be reduced to a diffusion equation for E$_{z}$(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. By specifying that the discharge propagates at a constant speed u, the diffusion equation is further reduced to a first-order O.D.E. in $\tau \equiv $t--z/u, which must be solved self-consistently with rate equations that determine D($\tau )$. The required driving voltage pulse V($\tau )$ is an output of the calculation. Even in absence of deionization processes, we show the discharge propagates only if T$_{e}$ is large enough throughout the channel to drive continuing ionization. If the channel is narrow enough, this can occur at sustainable levels of E$_{z}$ due to saturation of the N$_{2}$ vibrational energy sink. [Preview Abstract] |
Wednesday, October 11, 2006 5:30PM - 5:45PM |
MW2.00007: Simulation of the Plasma Dynamics at the Ionization Front of High Pressure Discharges Using Monte Carlo Methods on an Adaptive Mesh Ananth N. Bhoj, Mark J. Kushner During breakdown of high pressure discharges, such as coronas, a thin ionization front propagates across the gap. The ionization front has steep spatial gradients in the electric field and electron temperature which produce commensurate gradients in excitation rates. In spite of operating at high pressures where the electrons are highly collisional, these gradients may be severe enough that electron transport is non-local. To address these conditions, a new computational technique was developed that captures the non-local nature of the electron energy distribution (EED) at the ionization front. The basic computational platform is a 2-dimensional plasma hydrodynamics model based on unstructured meshes that addresses electrostatics and multi-fluid charged particle transport. The EED at the ionization front is captured using an electron Monte Carlo Simulation executed on an adaptive, rectilinear mesh. The location of the adaptive mesh is determined by sensors that select regions where non-local transport might occur. From the EEDs computed in the non-local regions, electron transport coefficients and sources are obtained and transferred to the fluid modules. Results will be discussed for positive and negative corona discharges in air and the non-local character of the ionization front will be described. [Preview Abstract] |
Wednesday, October 11, 2006 5:45PM - 6:00PM |
MW2.00008: Measuring the electrical breakdown of air for very small electrode separations Emmanouel Hourdakis, Garnett W. Bryant, Neil M. Zimmerman Understanding the basic principles of electrical breakdown in air for small electrode separations is becoming very important in the design and operation of microscale devices such as MEMS sensors and actuators. This work presents a new method [1] for measuring the value of breakdown voltage in air for electrode separations from 400 nm to 45 $\mu $m. The method consists of bringing 2 evaporated Au electrodes on sapphire together in a parallel plate geometry. Amongst the improvements of our method are the measurement of plate separation and the very small surface roughness ( average of 6 nm ). We demonstrate the ability to deduce the value of the separation of the plates by the value of the capacitance. We analyze the data for small separations, using the theory of standard field emission and field amplification on the surface of a conductor. We come to a prediction about the geometry and size of the electrode surface protrusions that would produce the observed emission. For the first time, we look for these predicted protrusions using an AFM. We find several reasons why the standard theory does not appear to explain our data. \newline \newline [1] Rev. Sci. Instrum. \textbf{77}, 034702 (2006) [Preview Abstract] |
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