Bulletin of the American Physical Society
68th Annual Gaseous Electronics Conference/9th International Conference on Reactive Plasmas/33rd Symposium on Plasma Processing
Volume 60, Number 9
Monday–Friday, October 12–16, 2015; Honolulu, Hawaii
Session QR4: Modeling and Simulation III |
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Chair: Igor Kaganovich, Princeton Plasma Physics Laboratory Room: 303 AB |
Thursday, October 15, 2015 3:30PM - 4:00PM |
QR4.00001: Multidimensional, fully implicit, exactly conserving electromagnetic particle-in-cell simulations Invited Speaker: Luis Chacon We discuss a new, conservative, fully implicit 2D-3V particle-in-cell algorithm for non-radiative, electromagnetic kinetic plasma simulations, based on the Vlasov-Darwin model.\footnote{Nielson and Lewis, \textit{Methods Comput. Phys.} \textbf{16} p.367 (1976)} Unlike earlier linearly implicit PIC schemes and standard explicit PIC schemes, fully implicit PIC algorithms are unconditionally stable and allow exact discrete energy and charge conservation. This has been demonstrated in 1D electrostatic\footnote{Chen, Chac\'on, and Barnes, \textit{J. Comput. Phys.} \textbf{230} p.7018 (2011)} and electromagnetic\footnote{Chen and Chac\'on, \textit{Comput. Phys. Commun.} \textbf{185} p.2391 (2014)} contexts. In this study, we build on these recent algorithms to develop an implicit, orbit-averaged, time-space-centered finite difference scheme for the Darwin field and particle orbit equations for multiple species in multiple dimensions.\footnote{Chen and Chacon, \textit{Comput. Phys. Commun.}, submitted} The Vlasov-Darwin model is very attractive for PIC simulations because it avoids radiative noise issues in non-radiative electromagnetic regimes. The algorithm conserves global energy, local charge, and particle canonical-momentum exactly, even with grid packing. The nonlinear iteration is effectively accelerated with a fluid preconditioner, which allows efficient use of large timesteps, $O(\sqrt{\frac{m_i}{m_e}}\frac{c}{v_{eT}})$ larger than the explicit CFL. In this presentation, we will introduce the main algorithmic components of the approach, and demonstrate the accuracy and efficiency properties of the algorithm with various numerical experiments in 1D and 2D. [Preview Abstract] |
Thursday, October 15, 2015 4:00PM - 4:15PM |
QR4.00002: Modeling and control of ion energy distribution functions at the electrodes of multi-frequency capacitively coupled plasmas Edmund Sch\"ungel, Zolt\'an Donk\'o, Ihor Korolov, Aranka Derzsi, Julian Schulze The energy of ions flowing onto boundary surfaces in technological plasmas is of crucial importance for applications. In particular, the shape of the ion energy distribution function (IEDF) determines the surface processes. This is why capacitive radio-frequency (RF) plasmas are widely used. It has been found that the mean energy and flux of ions can be controlled separately in dual-frequency discharges. However, advanced methods should allow for a control of not only the mean ion energy, but also of the shape of the IEDF. Here, we present such an approach based on voltage waveform tailoring. A RF voltage consisting of up to 5 harmonics is applied to one electrode. The outcome of PIC/MCC simulations is compared to an analytical model, which tracks the motion of ions in the electric field of the RF sheath and takes charge exchange collisions into account. The IEDF width, i.e. the maximum and mean ion energy, is controlled by tuning the applied harmonics' phases according to the Electrical Asymmetry Effect. Based on a fundamental understanding of the ion dynamics, the IEDF can be customized and specific features of the distribution -- such as peaks at intermediate energies -- can be generated and shifted along the energy axis by adjusting the shape of the driving voltage waveform. [Preview Abstract] |
Thursday, October 15, 2015 4:15PM - 4:30PM |
QR4.00003: Two- and three-dimensional particle-in-cell simulations of ExB discharges Johan Carlsson, Igor Kaganovich, Alexander Khrabrov, Yevgeny Raitses, Andrei Smolyakov The Large-Scale Plasma (LSP) Particle-In-Cell with Monte-Carlo Collisions (PIC-MCC) code has been used to simulate several crossed-field (ExB) discharges in two and three dimensions. Two-dimensional (2D) simulations of a cold-cathode electric discharge with power-electronics applications and a Penning discharge will be presented. Three-dimensional (3D) simulation results of a cylindrical Hall thruster with scaled plasma parameters will also be shown and compared to experiment~[Ellison2012]. To enable the 2D and 3D ExB discharge simulations, several improvements to the LSP code were made, including implementation of a new electrostatic field solver, external-circuit model and models for particle injection and secondary-electron emission. To ensure the correctness of the collision models used (and particularly important for the cold-cathode-discharge simulations), validation and code benchmarking was done with the LSP and EDIPIC codes in 1D for a glow discharge. Results and conclusions will be presented.\\[4pt] C. L. Ellison, Y. Raitses and N. J. Fisch, ``Cross-field electron transport induced by a rotating spoke in a cylindrical Hall thruster,'' Physics of Plasmas \textbf{19}, 013503 (2012). [Preview Abstract] |
Thursday, October 15, 2015 4:30PM - 4:45PM |
QR4.00004: Numerical parameter constraints for accurate PIC-DSMC simulation of breakdown from arc initiation to stable arcs Christopher Moore, Matthew Hopkins, Stan Moore, Jeremiah Boerner, Keith Cartwright Simulation of breakdown is important for understanding and designing a variety of applications such as mitigating undesirable discharge events. Such simulations need to be accurate through early time arc initiation to late time stable arc behavior. Here we examine constraints on the timestep and mesh size required for arc simulations using the particle-in-cell (PIC) method with direct simulation Monte Carlo (DMSC) collisions. Accurate simulation of electron avalanche across a fixed voltage drop and constant neutral density (reduced field of 1000 Td) was found to require a timestep $\sim$ 1/100 of the mean time between collisions and a mesh size $\sim$ 1/25 the mean free path. These constraints are much smaller than the typical PIC-DSMC requirements for timestep and mesh size. Both constraints are related to the fact that charged particles are accelerated by the external field. Thus gradients in the electron energy distribution function can exist at scales smaller than the mean free path and these must be resolved by the mesh size for accurate collision rates. Additionally, the timestep must be small enough that the particle energy change due to the fields be small in order to capture gradients in the cross sections versus energy. [Preview Abstract] |
Thursday, October 15, 2015 4:45PM - 5:00PM |
QR4.00005: A tunable microplasma gradient-index lens for millimeter waves Ayyaswamy Venkattraman Field-induced electron emission from the cathode and its interaction with microdischarges has gained significant attention in the last few years particularly in the context of microscale gas breakdown. Recent advances in nanofabrication have led to the development of novel cathodes that demonstrate impressive field emission properties with turn-on fields as low as 1 V/$\mu $m and field enhancement factors as high as 1000 implying that field emission could play an important role in microplasmas as large as 500 $\mu $m. This work presents proof of concept of a novel application of field emission assisted (FEA) microplasmas that exploits the relatively high plasma number densities encountered in these devices. We hypothesize that the number density gradients and the resulting gradient in the microplasma relative permittivity/refractive index can be utilized as a tunable diverging lens with on/off ability to defocus waves in the Terahertz regime. Electron number density profiles obtained from one-dimensional particle-in-cell with Monte Carlo collisions (PIC-MCC) simulations for a typical FEA microplasma are used to determine the relative permittivity and conductivity profiles. Frequency domain wave propagation simulations using these profiles show that sub-mm waves can be controlled using the microplasma lens with the degree of defocusing depending on the wavelength. In spite of the non-zero conductivity, it is shown that the medium is not significantly lossy at the frequencies considered. [Preview Abstract] |
Thursday, October 15, 2015 5:00PM - 5:15PM |
QR4.00006: 2D Axisymmetric vs 1D: A PIC/DSMC Model of Breakdown in Triggered Vacuum Spark Gaps Stan Moore, Chris Moore, Jeremiah Boerner Last year at GEC14, we presented results of one-dimensional PIC/DSMC [1-2] simulations of breakdown in triggered vacuum spark gaps. In this talk, we extend the model to two-dimensional axisymmetric and compare the results to the previous 1D case. Specially, we vary the fraction of the cathode that emits electrons and neutrals (holding the total injection rates over the cathode surface constant) and show the effects of the higher dimensionality on the time to breakdown.\\[4pt] Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.\\[4pt] [1] C. K. Birdsall and A. B. Langdon, \textit{Plasma Physics via Computer Simulation}, McGraw-Hill, New York (2005).\\[0pt] [2] G. A. Bird, \textit{Molecular Gas Dynamics and the Direct Simulation of Gas Flows}, Oxford University Press, Oxford, UK (1994). [Preview Abstract] |
Thursday, October 15, 2015 5:15PM - 5:30PM |
QR4.00007: Breakdown in Atmospheric Pressure Plasma Jets: Nearby Grounds and Voltage Rise Time Amanda Lietz, Mark J. Kushner Atmospheric pressure plasma jets (APPJs) are being investigated to stimulate therapeutic responses in biological systems. These responses are not always consistent. One source of variability may be the design of the APPJs -- the number and placement of electrodes, pulse power format -- which affects the production of reactive species. In this study, the consequences of design parameters of an APPJ were computationally investigated using \textit{nonPDPSIM}, a 2 d model. The configuration is a cylindrical tube with one or two ring exterior electrodes, with or without a center pin electrode. The APPJ operates in He/O$_{\mathrm{2}}$ flowing into humid air. We found that the placement of the electrical ground on and around the system is important to the breakdown characteristics of the APPJ, and the electron density and temperature of the resulting plasma. With a single powered ring electrode, the placement of the nearest ground may vary depending on the setup, and this significantly affects the discharge. With two-ring electrodes, the nearest ground plane is well defined, however more distant ground planes can also influence the discharge. With an ionization wave (IW) that propagates out of the tube and into the plume in tens of ns, the rise time of the voltage waveform can be on the same timescale, and so variations in the voltage rise time could produce different IW properties. The effect of ground placement and voltage waveform on IW formation (ns timescales) and production of reactive neutrals (ms timescales) will be discussed. [Preview Abstract] |
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