Bulletin of the American Physical Society
56th Annual Meeting of the APS Division of Plasma Physics
Volume 59, Number 15
Monday–Friday, October 27–31, 2014; New Orleans, Louisiana
Session PO7: Low Temperature Plasmas: Diagnostics, Surfaces, Applications and Thrusters |
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Chair: John Foster, University of Michigan Room: Galerie 6 |
Wednesday, October 29, 2014 2:00PM - 2:12PM |
PO7.00001: ABSTRACT WITHDRAWN |
Wednesday, October 29, 2014 2:12PM - 2:24PM |
PO7.00002: The effect of ambipolar electric fields on the electron heating in capacitive RF plasmas Julian Schulze, Zoltan Donko, Aranka Derzsi, Ihor Korolov, Edmund Schuengel We investigate the electron heating dynamics in argon and helium capacitively coupled RF discharges driven at 13.56 MHz by Particle in Cell simulations and by an analytical model. Electrons are found to be heated by strong ambipolar electric fields outside the sheath during the phase of sheath expansion in addition to classical sheath expansion heating. Moreover, we find that electrons reflected multiple times from the expanding sheath edge within one RF period are the primary sources of ionization. In fact a synergistic combination of different heating events is required to sustain the plasma. The ambipolar electric field outside the sheath is found to be time modulated due to a time modulation of the electron mean energy caused by the presence of sheath expansion heating only during one half of the RF period at a given electrode. This time modulation results in more heating than cooling on time average. If an electric field reversal is present during sheath collapse, this time modulation will be enhanced. This ambipolar electron heating is found to represent an important heating mechanism, which should be included in models of capacitive RF plasmas. [Preview Abstract] |
Wednesday, October 29, 2014 2:24PM - 2:36PM |
PO7.00003: Investigation of self-excited plasma series resonance oscillations in multi-frequency capacitive discharges Edmund Sch\"ungel, Julian Schulze, Ihor Korolov, Aranka Derzsi, Zolt\'an Donk\'o The self-excitation of plasma series resonance (PSR) oscillations is a dominant feature in the current of asymmetric capacitively coupled radio-frequency discharges. The asymmetry can be caused by an asymmetry of the chamber geometry and/or that of the applied voltage waveform. We study the self-excitation of the PSR in a geometrically symmetric, electrically asymmetric capacitive argon discharge using PIC/MCC simulations, as well as an analytical model. The results show that increasing the number of subsequent harmonics in the driving voltage waveform enhances the asymmetry and, therefore, leads to a significant increase of the current amplitude of higher harmonics, which are generated due to the nonlinearities of the sheaths. These high-frequency resonance oscillations between the capacitive sheaths and the inductive plasma bulk can only be reproduced correctly by the analytical model, if the cubic sheaths charge - voltage relation and the temporal modulation of the bulk length and electron density within the RF period are taken into account. Furthermore, we demonstrate that the nonlinear electron resonance heating (NERH) associated with the presence of PSR oscillations significantly contributes to the total electron heating and causes a spatial asymmetry of the ionization. [Preview Abstract] |
Wednesday, October 29, 2014 2:36PM - 2:48PM |
PO7.00004: Formation and dynamics of striations in an annular inductive plasma Nicolas Plihon, Victor Desangles, Pascal Chabert We present an experimental characterization of the dynamics of a low pressure, radio-frequency inductively coupled plasma with an internal coil (resulting in an annular geometry) as described in [1]. At low pressure, the resulting plasma equilibrium is axisymmetric. We show that the cylindrical symmetry of the system is broken at sufficiently high pressure (above 20 mTorr) and low coupled power. In these non-axisymmetric configurations, striations occur along the azimuthal direction. The number of plasma lobes (or striations) increases as pressure increases (from 2 to 7 lobes as pressure increases from 50 to 2500 mTorr). Both stationary and rotating lobes have been observed. The transition between the axisymmetric configuration and non-axisymmetric configurations is shown to be subcritical, resulting in bistability. The transitions between non-axisymmetric configurations with various numbers of lobes are supercritical. High-speed imaging of the emitted light and time-resolved Langmuir probe measurements allow to precisely characterize the dynamics of the lobes, as well as the transitions between configurations. \\[4pt] [1] J. Arancibia Monreal, P. Chabert and V. Godyak, \textit{Phys. Plasmas}, \textbf{20}, 103504 (2013) [Preview Abstract] |
Wednesday, October 29, 2014 2:48PM - 3:00PM |
PO7.00005: Ultraviolet Rayleigh scatter imaging in atmospheric microdischarges for spatial temperature profiles James Caplinger, Steven Adams, Amber Hensley, Boyd Tolson Spatially resolved temperature measurements within a microdischarge in atmospheric pressure air have been conducted using Rayleigh scattering of a pulsed ultraviolet laser. The scatter image intensity along the laser beam axis is proportional to the background gas target density and thus, according to the ideal gas law, is inversely proportional to gas translational temperature. By measuring the scatter image with and without a discharge, the temperature was determined in 1-dimension along the laser beam passing radially through the discharge. The 1- dimensional scattering intensity profiles were then used to generate 2- dimensional cross-sectional slices of temperature by transitioning the height of the laser beam. The cross-sectional temperature profiles exhibited a high degree of cylindrical symmetry with the radial width of the high temperature region expanding with increasing discharge current. Peak temperatures determined by Rayleigh scattering for each current were compared to temperatures derived from standard optical emission spectral analyses of N$_{\mathrm{2}}$(C-B) bands, where the calculated rotational temperatures from emission were in reasonable agreement with the Rayleigh translational temperature profiles. [Preview Abstract] |
Wednesday, October 29, 2014 3:00PM - 3:12PM |
PO7.00006: Investigation of possible sheath disappearance near a electrode biased at the plasma potential Chi-Shung Yip, Noah Hershkowitz, Greg Severn It is well established that when an electrode is biased negative with respect to the plasma potential, an ion sheath forms and when it is biased positive with respect to the plasma potential, an electron sheath forms provided that the electrode is small (Aplate/Achamber \textless (me/Mi)1/2) [1]. However, when a small electrode is biased at the plasma potential, it is unknown whether an ion sheath, an electron sheath or no sheath will form. Movable small (3-5cm diameter) plates biased at the plasma potential are immersed in a filament discharge in a multi-dipole chamber. Plasma potential and IVDFs near the plate are measured to determine whether an ion sheath, an electron sheath or no sheath formed. Ion velocities are determined by Laser-Induced Florescence, the electron temperature and electron density are measured by a planar Langmuir probe and the plasma potential is measured by an emissive probe. \\[4pt] [1] S. D. Baalrud, N. Hershkowitz and B. Longmier, Phys. Plasmas 14, 042109 (2007) [Preview Abstract] |
Wednesday, October 29, 2014 3:12PM - 3:24PM |
PO7.00007: Direct measurements of ion dynamics in magnetic presheaths M. Umair Siddiqui, Cory Jackson, Justin Kim, Noah Hershkowitz Ion velocities and temperatures are measured in the presheath of a grounded plate downstream from a helicon plasma source using laser-induced fluorescence. The plate is held 16 to 60 degrees relative to the 1 kG background magnetic field. Velocity profiles are compared to a 1D ion fluid model and are shown to agree well. Implications for ion flow to tokamak and Hall thruster walls are discussed. [Preview Abstract] |
Wednesday, October 29, 2014 3:24PM - 3:36PM |
PO7.00008: Ion and Electron Energy-Angle Distribution Functions at the Material Wall in Magnetized Plasmas Davide Curreli, Rinat Khaziev The supersonic acceleration occurring at the plasma-material interface in presence of an oblique magnetic field is analyzed performing kinetic-kinetic particle-in-cell simulations (kinetic ions, kinetic electrons), comprising the effect of collisions. The energy-angle distribution functions of both ions and electrons are obtained at several locations from the bulk quasi-neutral plasma to the wall, in order to show how the plasma kinetics changes during the acceleration across the presheath up to the material wall. We highlight how collisional processes affect the structure of the sheath and presheath and modify the energy-angle distributions at the material wall. We present the scaling factor of the average ion and electron energy and peak pitch angle at the wall as a function of the bulk plasma conditions, for use in the correlation of fluid plasma models to material models. [Preview Abstract] |
Wednesday, October 29, 2014 3:36PM - 3:48PM |
PO7.00009: Erosion due to ion sputtering in absence of Debye Sheath at Divertor plates: PIC simulation K.S. Goswami, S. Adhikari A 2D-3V Particle-in-Cell code with Monte Carlo Collision and a Plasma Surface Interaction Code written in Matlab is used to study the effect of grazing angle ($\alpha$) on solid surface (divertor) erosion due to ion sputtering in magnetic fusion devices, where $\alpha$ is the angle between the magnetic field and the surface tangent. The ion distribution in front of an absorbing wall is computed using a kinetic model. Important factors like ion energy and impact angle for wall erosion and sputtering are highlighted. The dependence of these two parameters on grazing angle is investigated in detail. Physical Sputtering for ion bombardment is strongly dependent on incident ion energy and this energy is mainly gained by the ions when they travel through the potential drop across the combined Chodura Sheath and Debye Sheath. The present work contains the study of two scenario. In the first one we have studied the usual case to compare our result to the other similar work i.e. in presence of both Chodura Sheath and Debye Sheath. In the second one with the idea of previous work [1] we have created the scenario where Debye Sheath cease to appear. The second scenario provides us the result that was never expected that the incident energy profile got reversed. The study is focused on the effect of grazing angle and its relation with the material erosion. Our study covers different materials (e.g. Be, Fe, W etc.) which are used as plasma facing components.\\[4pt] [1] P.C. Stangeby\textit{ Nucl. Fusion} \textbf{52} (2012) 083012 [Preview Abstract] |
Wednesday, October 29, 2014 3:48PM - 4:00PM |
PO7.00010: Adapting Particle-In-Cell simulations to the study of short pulse laser damage Robert Mitchell, Douglass Schumacher, Enam Chowdhury We present novel Particle-In-Cell (PIC) simulations of the full femtosecond-pulse laser damage process and the resulting damage spot morphology. At the heart of these simulations is the implementation, for the first time, of a Lennard-Jones pair potential model (LJPPM) for PIC codes. The use of PIC facilitates the first ab-initio treatment of realistic target sizes, retaining the strengths of PIC including self-consistent treatment of the laser-particle interaction and subsequent generation of plasma waves and electron heating, while the LJPPM allows a PIC code to treat a system of particles as a medium which can ablate, melt, and resolidify. Combining these two approaches, we model the effect of a femtosecond-pulse laser on metal targets near and above the damage threshold and compare to recent experimental results. In particular, we present the first simulations of the emergence of Laser-Induced Periodic Surface Structure (LIPSS) upon femtosecond-pulse laser irradiation. [Preview Abstract] |
Wednesday, October 29, 2014 4:00PM - 4:12PM |
PO7.00011: Simulation of Nanofilm Formation in Low-Temperature Plasmas Jan Willem Abraham, Michael Bonitz Metal-polymer nanocomposites are of growing interest in many fields because the diverse physical features of their constituents allow for the production of materials with interesting novel properties. Recent experiments [1] and simulations [2,3,4] have shown that co-evaporation of the metallic and organic components in a simple single-step process can give rise to the formation of ultrahigh-density Fe-Ni-Co nanocolumnar structures embedded in a fluoropolymer matrix. We show new results from kinetic Monte Carlo simulations that are expected to answer the question whether similar structures can also be produced in a plasma environment with an enhanced influence of surface defects.\\[4pt] [1] Greve et al., Appl. Phys. Lett. 88, 123103 (2006)\newline [2] L. Rosenthal et al., J. Appl. Phys. 144, 044305 (2013)\newline [3] L. Rosenthal et al., Contrib. Plasma Phys. 51, 971 (2011)\newline [4] Chapter in ``Complex Plasmas: Scientific Challenges and Technological Opportunities,'' Michael Bonitz, Jose Lopez, Kurt Becker and Hauke Thomsen (Eds.), Springer Series on Atomic, Optical, and Plasma Physics, Volume 82 2014 [Preview Abstract] |
Wednesday, October 29, 2014 4:12PM - 4:24PM |
PO7.00012: Toxicity study of water transferred graphene-based nanostructures for cell culture substrate Fabricio Borghi, Tim Van der Laan, Musarat Ishaq, Shailesh Kumar, Kostya Ostrikov Graphene has attracted enormous attention due to its unique physical and chemical properties. Early researches had focused on it electrical properties for device applications. Nowadays graphene has attracted increased interest in bio-medical applications, such as cell culture substrates. Substrates are critical for: investigating early stage development of cells, new drugs tests and tissue engineering. Benefits of graphene for this application are: it can be produced with desired structural morphology, its surface properties can be modified via plasma or chemical treatment (decorated with specific functional groups), and it can be transferred to a plethora of substrates (high influence of cells fate). Successful applications of graphene-based materials for bio-med applications are predominantly produced via chemical methods. When produced via Thermal CVD, the transfer to the desired substrate involves chemical treatment, potentially contaminating the graphene. In this work, we use a unique plasma produced graphene, transferred to glass via a chemical-free process, as cell culture substrates. This work aims graphene's bio-toxicity. Our results show that our material is non toxic, and cells morphology and proliferation indicates similar growth among all samples and the control. [Preview Abstract] |
Wednesday, October 29, 2014 4:24PM - 4:36PM |
PO7.00013: A permanent-magnet helicon thruster Francis F. Chen Gridded ion thrusters are the classical method for propelling spacecraft to their designed orbital velocities. These thrusters generate electrons with a thermionic cathode and accelerate them with positive grids, creating a plasma. Ions are extracted from the plasma and accelerated with another grid and ejected from the spacecraft to propel it. An external electron source is used to neutralize the ion beam, preventing the spacecraft from charging up negatively. Hall thrusters also accelerate ions electrostatically, but the electrons are held back not by grids but by a magnetic field. A cool electron source is needed here also. Helicon thrusters eject neutral plasma, and the ions are given a kick in an external ``double layer,'' which forms as a sheath in free space. We have miniaturized a helicon thruster by using a permanent magnet over a small discharge tube. The ejected plasma is measured with a retarding-field ion analyzer. At low pressures, the RFID peaks around 27eV and can be increased by biasing the top plate, thus achieving a reasonable specific impulse. [Preview Abstract] |
Wednesday, October 29, 2014 4:36PM - 4:48PM |
PO7.00014: A Simple Model of Cross-Field Diffusion in Hall Thrusters based on Turbulence Energy Cascade Mark A. Cappelli, Eunsun Cha, Eduardo Fernandez We present a Hall plasma thruster model based on turbulence energy cascade to smaller scales characterized by the electrons gyro-radius. We employ scaling arguments originally developed for viscous energy dissipation in turbulent fluid mechanics with the assumption that the electron scattering rate is expected to scale as the strain-rate in the electron fluid, and that the size of the largest turbulent eddies scale as the electron gyro-radius and local drift velocity. Using this framework, expressions are derived for the entropy production rate which can be used in an independent entropy transport equation from which the transport coefficient can be derived. Alternatively, if one assumes that the main source of electron energy dissipation is turbulent energy cascade, then the energy dissipation replaces the Ohmic heating term in the electron energy equation. We will present the general results of this analysis, as well as initial results obtained from 2-D hybrid simulations that incorporate this model and its variants. [Preview Abstract] |
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