Bulletin of the American Physical Society
50th Annual Meeting of the Division of Plasma Physics
Volume 53, Number 14
Monday–Friday, November 17–21, 2008; Dallas, Texas
Session UO5: Low Temperature Plasmas and Electric Propulsion |
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Chair: Philip Efthimion, Princeton Plasma Physics Laboratory Room: Reunion C |
Thursday, November 20, 2008 2:00PM - 2:12PM |
UO5.00001: Simulation of Sub-Scale Ion Optics on a Parallel Cluster Raed Kafafy, Joseph Wang Accurate prediction of the cross-over and perveance impingement limits of an ion optics system is very important to determine the feasible operational envelope of an ion thruster. Impingement of beamlet ions on the acceleration grid at either limit results in excessive sputter erosion of the acceleration grid, which is one of the major failure mechanisms. We developed the streamline hybrid-grid immersed-finite-element particle-in-cell (HG-IFE-PIC) model for three-dimensional simulations of plasma flow in ion optics. The model is designed to handle the boundary conditions at grid surfaces accurately while maintaining the computational speed of a standard PIC code. Direct three-dimensional simulations of quarter-subscale gridlets of seven apertures could be performed routinely even using powerful PCs. However, larger subscale gridlets having more apertures were out of reach of a single machine. In this paper, we parallelize the hybrid-grid immersed-finite-element (HG-IFE-IFE) using two-dimensional decomposition of the PIC and IFE physical domains to allow for the direct three-dimensional simulation of subscale gridlets including as many apertures as limited by the number of available computing nodes. Results will be compared against experimental measurements taken for a set of subscale gridlets with 7, 19, and 37 apertures. [Preview Abstract] |
Thursday, November 20, 2008 2:12PM - 2:24PM |
UO5.00002: Effects of sheath instability on properties of a Hall thruster Dmytro Sydorenko, Igor D. Kaganovich, Yevgeny Raitses, Andrei Smolyakov The sheath near the electron-emitting surface may become unstable if it is characterized by the negative volt-ampere characteristics, which occurs in presence of strong secondary electron emission. A 1d3v particle-in-cell code is applied to study the sheath instability effects on plasma-wall interaction in Hall thrusters. It is found that in stable stationary plasma state the final phase of cyclotron rotation of secondary electrons emitted from the thruster walls is not arbitrary but belongs to the discrete set of stability intervals [Sydorenko et al., Phys. Plasmas \textbf{15}, 053506 (2008); Kaganovich et al., Phys. Plasmas \textbf{14}, 057104 (2007)]. In the limit of high discharge voltages, a new regime with relaxation oscillations is identified. In this regime, the plasma constantly switches between a state with non-space charge limited emission and a state with a space charge limited emission. [Preview Abstract] |
Thursday, November 20, 2008 2:24PM - 2:36PM |
UO5.00003: Effect of nonlocal fast electrons on plasma properties in short discharges J. Blessington, S.F. Adams, V.I. Demidov, I. Kaganovich, M.E. Koepke, B.A. Tolson It is demonstrated experimentally that fast electrons, produced from the cold cathode in a short discharge without positive column, can charge walls to the negative potentials much higher than those corresponding to electron temperatures in the plasma. Those fast electrons are nonlocal and conserve their energies after many elastic collisions with atoms. This effect is similar to self-trapping of fast electrons in afterglow plasmas [1]. Application of additional potentials to the walls can change conditions of self-trapping of the fast electrons and modify structure of the discharge and plasma properties. The result can be used for controlling plasma properties, particular for changing intensities of spectral lines and lighting efficiency. Similarities and differences of the above effects for short discharges with cold and hot cathodes are discussed. Similar effects are important in cathode region (negative glow) of long discharges with positive column. This work was supported by the AFOSR. [1] V.I. Demidov, C.A. DeJoseph, Jr. and A.A. Kudryavstev, PRL 95, 215002 (2005). [Preview Abstract] |
Thursday, November 20, 2008 2:36PM - 2:48PM |
UO5.00004: Ambipolar and nonambipolar flow across magnetic field Amnon Fruchtman, Gennady Makrinich The two-dimensional steady-state of plasma confined radially by an axial magnetic field is studied. An analytical solution is found by variable separation for the case that the transport coefficients are constant, without an assumption of ambipolar flow. In two such analytical descriptions, of ambipolar and nonambipolar diffusion, are compared. By ambipolar flow we mean equal ion and electron flow locally. Since the sources of ions and electrons are equal the total ion and electron flows through the boundaries are equal even when the flow is not ambipolar. We then formulate the nonlinear diffusion problem where the nonlinearity results from the dependence of the transport coefficients on the plasma and neutral densities. We solve the nonlinear 1D cross-field transport for the two different cases of ambipolar and nonambipolar diffusion. The nonambipolar diffusion corresponds to electrons crossing field lines at the end walls, in what is called the short-circuit effect. Neutral depletion, recently studied for the unmagnetized case,\footnote{A. Fruchtman, G. Makrinich, J.-L. Raimbault, L. Liard, J.-M. Rax, and P. Chabert, Phys. Plasmas \textbf{15}, 057102 (2008).} is included here for the magnetized plasma. [Preview Abstract] |
Thursday, November 20, 2008 2:48PM - 3:00PM |
UO5.00005: Magnetic Field Effects on Simulating the Plume of an Anode Layer Hall Thruster Yongjun Choi, Michael Keidar, Iain Boyd Two dimensional axi-symmetric simulations of xenon plasma plume flow fields from a D55 anode layer Hall thruster are performed with a hybrid particle-fluid method. The magnetic field surrounding the Hall thruster exit is included in the calculation. In this simulations, the Boltzmann model and a detailed fluid model are used to compute the electron properties, the direct simulation Monte Carlo method models the collisions of heavy particles, and the Particle-In-Cell method models the transport of ions in an electric field. The plasma properties obtained from the hydrodynamic model are used as boundary conditions for the simulations. The accuracy of the simulation is assessed through comparison with various measured data. It is found that a magnetic field significantly affects the profile of the plasma in the Detailed model. For instance, in the case of zero magnetic field, the plasma has a potential about 80 V at 10 mm from the thruster exit, while in the case of a magnetic field included, the plasma potential is about 60 V. Results predicted by the Detailed model with the magnetic field are found to be in better agreement with experimental data. [Preview Abstract] |
Thursday, November 20, 2008 3:00PM - 3:12PM |
UO5.00006: Numerical Investigation of Plasma Detachment in Magnetic Nozzle Experiments Kamesh Sankaran, Kurt Polzin There exists a need for a comprehensive model that describes plasma detachment from an externally-imposed magnetic nozzle. To make progress towards that end, plasma flow in a magnetic nozzle experiment was simulated using a multidimensional code that includes theoretical models of various dispersive and dissipative processes. The aim is to compare the computational results with various proposed detachment theories to estimate the importance of possible mechanisms. An applied magnetic field topology is created using a magnetostatic solver, and is then superimposed on the time-dependent magnetic field induced in the plasma to provide a self-consistent field description. The applied field model matches those found in actual experiments, thus making way for validating the computational results. In addition, comparison of the simulation results with the experimentally obtained plasma parameters will provide insight into mechanisms that lead to plasma detachment, revealing how they scale with different input parameters. [Preview Abstract] |
Thursday, November 20, 2008 3:12PM - 3:24PM |
UO5.00007: Exhaust Plume Measurements of the VASIMR VX-200 Benjamin Longmier, Edgar Bering III, Jared Squire, Tim Glover, Franklin Chang-Diaz, Michael Brukardt Recent progress is discussed in the development of an advanced RF electric propulsion concept: the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) VX-200 engine, a 200 kW flight-technology prototype. Results from high power Helicon only and Helicon with ICRH experiments are performed on the VX-200 using argon plasma. Recent measurements of axial plasma density and potential profiles, magnetic field-line shaping, charge exchange, and force measurements taken in the plume of the VX-200 exhaust are made within a new 125 cubic meter cryo-pumped vacuum chamber and are presented in the context of RF plasma thruster physics. [Preview Abstract] |
Thursday, November 20, 2008 3:24PM - 3:36PM |
UO5.00008: High Power Electric Propulsion Using The VASIMR{\texttrademark} VX-200: A Flight Technology Prototype Edgar Bering III, Benjamin Longmier, Tim Glover, Franklin Chang-Diaz, Jared Squire, Michael Brukardt The Variable Specific Impulse Magnetoplasma Rocket (VASIMR{\texttrademark}) is a high power magnetoplasma rocket, capable of Isp/thrust modulation at constant power. The plasma is produced by a helicon discharge. The bulk of the energy is added by ion cyclotron resonance heating (ICRH.) Axial momentum is obtained by adiabatic expansion of the plasma in a magnetic nozzle. Thrust/specific impulse ratio control in the VASIMR{\texttrademark} is primarily achieved by the partitioning of the RF power to the helicon and ICRH systems, with the proper adjustment of the propellant flow. Ion dynamics in the exhaust were studied using probes, gridded energy analyzers (RPA's), microwave interferometry and optical techniques. Results are summarize from high power ICRH experiments performed on the VX-100 using argon plasma during 2007, and on the VX-200 using argon plasma during 2008. The VX-100 has demonstrated ICRH antenna efficiency $>$90{\%} and a total coupling efficiency of $\sim $75{\%}. The rocket performance parameters inferred by integrating the moments of the ion energy distribution corresponds to a thrust of 2 N at an exhaust velocity of 20 km/s with the VX-100 device. The new VX-200 machine is described. [Preview Abstract] |
Thursday, November 20, 2008 3:36PM - 3:48PM |
UO5.00009: ABSTRACT WITHDRAWN |
Thursday, November 20, 2008 3:48PM - 4:00PM |
UO5.00010: Wire Ablation Plasma Source Development at Sandia National Laboratories M.D. Johnston, K. Hahn, B.V. Oliver, T.A. Mehlhorn, D.W. Droemer, M.D. Crain, Y. Maron, V. Bernshtam, E. Stambulchik Experiments are being conducted to investigate fine wires as potential plasma sources for applications such as intense electron beam transport and propagation. For these studies, a microsecond long, low inductance, capacitive discharge (40kA, 50kV) is driven through a wire(s) to generate a plasma. The plasma expansion is determined by JxB forces which are controlled via changes in geometry and current. High resolution visible spectroscopy is used to spatially and temporally measure plasma parameters throughout the pulse. Lineshapes, ratios, and intensities are compared with time-dependent CR calculations to obtain plasma densities and temperatures. Results are compared with MHD calculations and scaling laws for mass ablation rates from wires. [Preview Abstract] |
Thursday, November 20, 2008 4:00PM - 4:12PM |
UO5.00011: A time-resolution study of converging point of atmospheric microwave plasma Chuji Wang, Ping-Rey Jang, Nimisha Srivastava, Susan Scherrer, Theodore S. Dibble, Yixiang Duan Extensive studies on atmospheric microwave induced plasma or microwave plasma torch have been reported in literature. One of the well-known phenomena created in such a plasma is the existence of a converging point in the plasma plume. It is widely published in literature that this converging point divides a plasma plume into two distinctive regions for plasma diagnostics and applications and that the location of the converging point on the axis of the plume depends on gas flow rates at a given plasma power. We investigated the plasma generation and the plasma plume dynamics using a time-resolution imaging. It was found for the first time that the converging point is actually a time-averaged visual effect and does not exist at all when the plasma plume is examined under high time-resolution, e.g., $<$1 ms. Images of the plasma, which operated at 120 W with a central and supporting argon gas flow rates of 1.0 and 0.5 lpm, respectively, were digitally captured at different time-resolutions, ranging from 1 to 600 $\mu $s. A tentatively gas glow dynamic model was proposed to interpret the phenomenon. [Preview Abstract] |
Thursday, November 20, 2008 4:12PM - 4:24PM |
UO5.00012: Effect of surface conditions affecting voltage breakdowns Randolph Flauta, Maro Aghazarian, John Caughman, David Ruzic The maximum power transferred by ion cyclotron range of frequency (ICRF) antennas is dependent on the breakdown threshold when operated at high voltages. The voltage that these antennas can withstand is lowered and hence breakdowns occur due to many factors. The Surface Plasma Arcs by Radiofrequency - Control Study or SPARCS facility has a 0-15kV DC power supply to deliver power to flat cathode surface and semi-spherical anode made of Cu and Al under 10$^{-8}$-10$^{-6}$ torr vacuum conditions. The effects of different surface conditions on the breakdown threshold were then investigated. Also, as the ICRF antennas used for heating plasmas may come into contact with contaminants from the plasma, Li was also deposited on the cathode surface through in-situ evaporation coating and its effect on the breakdown threshold was investigated. Results on surface roughness showed no significant dependence of the breakdown threshold on macroscopic surface roughness in the cathode arithmetic roughness range of $\sim $77-1139nm. Microscopic surface features such as grain boundaries, impurities and imperfections may play a more visible role in affecting the vacuum breakdown. [Preview Abstract] |
Thursday, November 20, 2008 4:24PM - 4:36PM |
UO5.00013: Atmospheric microwave plasma system using hybrid resonant structures Hoyoung Song, Jung Mi Hong, Keun Ho Lee, Jin Joo Choi Cold atmospheric pressure plasmas have the potential to replace many traditional vacuum-based plasma processing practices as well as opening up entirely new plasma applications. An atmospheric microwave plasma system with power up to 6 kW and frequency of 2.45 GHz has been designed and constructed using 3D FEM code. Mode analysis, resonant condition, and dimension of the cavity have been carefully designed and investigated to optimize the resonant structure. The system consists of a hybrid coaxial cavity structure which couples magnetically into the central cavity. The microwave plasma was reliably sustained up to atmospheric pressure with good uniformity. The density of radicals increased with increasing pressure. The atmospheric microwave plasma presented in this paper can be applied to various applications which require high rate and large area material processing. It provides more effective and competitive process compared to the conventional low pressure plasma systems. [Preview Abstract] |
Thursday, November 20, 2008 4:36PM - 4:48PM |
UO5.00014: Study of atmospheric air AC glow discharge using optical emission spectroscopy and near infrared diode laser cavity ringdown spectroscopy Nimisha Srivastava, Chuji Wang, Theodore S. Dibble AC glow discharges were generated in atmospheric pressure by applying high voltage AC in the range of 3500-15000 V to a pair of stainless steel electrodes separated by an air gap. The discharges were characterized by optical emission spectroscopy (OES) and continuous wave cavity ringdown spectroscopy (cw-CRDS). The electronic (T$_{ex})$, vibrational (T$_{v})$, and rotational (T$_{r})$ temperatures were measured. Spectral stimulations of the emission spectra of several vibronic bands of the 2$^{nd}$ positive system of N$_{2}$, the 1$^{st}$ negative system of N$_{2}^{+}$, the (0,1,2,3-0) bands of NO (A-X), and the (0-0) band of OH (A-X), which were obtained under various plasma operating conditions, show that T$_{r}$, T$_{v}$, and T$_{ex}$ are in the ranges of 2000 - 3800, 3500 - 5000, and 6000 - 10500$^{ }$K, respectively. Emission spectra show that OH concentration increases while NO concentration decreases with an increase of electrode spacing. The absorption spectra of H$_{2}$O and OH overtone in the near infrared (NIR) were measured by the cw-CRDS with a telecommunications diode laser at wavelength near 1515 nm. [Preview Abstract] |
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