Bulletin of the American Physical Society
66th Annual Gaseous Electronics Conference
Volume 58, Number 8
Monday–Friday, September 30–October 4 2013; Princeton, New Jersey
Session NR3: Plasma Boundaries: Sheaths, Boundary Layers, Others |
Hide Abstracts |
Chair: Greg Severn, University of San Diego Room: Nassau Room |
Thursday, October 3, 2013 10:00AM - 10:30AM |
NR3.00001: Experimental validation of sheath models at intermediate radio frequencies Invited Speaker: Mark Sobolewski Sheaths in radio-frequency (rf) discharges play a dominant role in determining important properties such as the efficiency of power delivery and utilization, plasma spatial uniformity, and ion energy distributions (IEDs). To obtain high quality predictions for these properties requires sheath models that have been rigorously tested and validated. We have performed such tests in capacitively coupled and rf-biased inductively coupled discharges, for inert as well as reactive gases, over two or more orders of magnitude in frequency, voltage, and plasma density. We measured a complete set of model input and output parameters including rf current and voltage waveforms, rf plasma potential measured by a capacitive probe, electron temperature and ion saturation current measured by Langmuir probe and other techniques, and IEDs measured by mass spectrometers and gridded energy analyzers. Experiments concentrated on the complicated, intermediate-frequency regime of ion dynamics, where the ion transit time is comparable to the rf period and the ion current oscillates strongly during the rf cycle. The first models tested used several simplifying assumptions including fluid treatment of ions, neglect of electron inertia, and the oscillating step approximation for the electron profile. These models were nevertheless able to yield rather accurate predictions for current waveforms, sheath impedance, and the peak energies in IEDs. More recently, the oscillating step has been replaced by an exact solution of Poisson's equation. This results in a modest improvement in the agreement with measured electrical characteristics and IED peak amplitudes. The new model also eliminates the need for arbitrary or nonphysical boundary conditions that arises in step models, replacing them with boundary conditions that can be obtained directly from measurements or theories of the presheath. [Preview Abstract] |
Thursday, October 3, 2013 10:30AM - 10:45AM |
NR3.00002: An algebraic RF sheath model for all excitation waveforms and amplitudes, and all levels of collisionality Ralf Peter Brinkmann, Abd Elfattah Elgendy, Homayoun Hatefinia, Mohammed Shihab, Torben Hemke, Alexander Wollby, Denis Eremin, Thomas Mussenbrock The boundary sheath of a low temperature plasma comprises typically only a small fraction of its volume but is responsible for many aspects of the macroscopic behavior. A thorough understanding of the sheath dynamics is therefore of theoretical and practical importance. This work focusses on the so-called ``algebraic'' approach which strives to describe the electrical behavior of RF modulated boundary sheaths in closed analytical form, i.e., without the need to solve differential equations. A mathematically simple, analytical expression for the charge-voltage relation of a sheath is presented which holds for all excitation wave forms and amplitudes and covers all regimes from the collision-less motion at low gas pressure to the collision dominated motion at gas high pressure. A comparison with the results of self-consistent particle-in-cell simulations is also presented. [Preview Abstract] |
Thursday, October 3, 2013 10:45AM - 11:00AM |
NR3.00003: Student Award Finalist - Instability and Inversion of the Sheath Potential Caused by Electron Emission Michael Campanell, Hongyue Wang, Alex Khrabrov, Igor Kaganovich Most theories of PSI with electron emission assume a temporally stable sheath exists and the plasma potential is positive relative to the wall [1]. Ions are assumed to be drawn to the wall via Bohm's criterion and the emission is treated as a fixed ``coefficient.'' We show if the emission is sufficiently strong, the presheath may disappear because there is no need for ions to reach the wall to maintain current balance or plasma shielding. In this ``inverse sheath'' regime, the wall charge is positive and the shielding charge is negative. The plasma potential is negative, ions are confined and plasma electrons are unconfined [2]. We also present simulations and theory on a class of sheath instabilities driven by secondary emission that can arise under general conditions [3,4]. Lastly, we analyze effects of emitted electrons transiting between surfaces in bounded plasmas; transit alters flux balance [5] compared to PSI models of one emitting wall [1].\\[4pt] [1] G.D. Hobbs and J.A. Wesson, Plas. Phys. 9, 85 (1967)\\[0pt] [2] M.D. Campanell, A.V. Khrabrov, and I.D. Kaganovich, PRL 108, 255001 (2012).\\[0pt] [3] M.D. Campanell,et al., PRL 108, 235001 (2012).\\[0pt] [4] M.D. Campanell, et al., POP 19, 123513 (2012).\\[0pt] [5] M.D. Campanell and H. Wang, submitted to APL (2013). [Preview Abstract] |
Thursday, October 3, 2013 11:00AM - 11:15AM |
NR3.00004: The Langmuir's Paradox: Can the Ion Acoustic Instability at the Sheath Edge Thermalize the Ions Too? Chi-Shung Yip, Noah Hershkowitz, Greg Severn Recently a theoretical prediction was that in single-species plasmas, ion-ion collisional friction is enhanced by the ion acoustic instability [1]. The theory predicted that the instability will not only enhance the thermalization of the electrons, but will also, near the sheath-edge, thermalize the non-Maxwellian tail of the ion velocity distribution function (IVDF), caused by charge exchange in the presheath. The theory also predicted that this instability disappears through collisional damping as neutral pressure of the plasma increases. This experiment aims to verify this theory by measuring the IVDFs near the sheath edge in a multi-dipole chamber discharge in Argon and Xenon gas for a variety of neutral pressures and electron temperatures. The threshold parameters of the phenomenon are explored. The IVDFs are determined by Laser-Induced Florescence, the electron temperature is measured by a Langmuir probe and the plasma potential towards the boundary is measured by an emissive probe.\\[4pt] [1] S.D. Baalrud, et al. ``Instability-enhanced collisional effects and Langmuir's paradox,'' Phys. Rev. Lett, {\bf 102}, 245005 (2009) [Preview Abstract] |
Thursday, October 3, 2013 11:15AM - 11:30AM |
NR3.00005: End-boundary sheath potential, Langmuir waves, electron and ion energy distribution in the low pressure DC powered Non-ambipolar Electron Plasma Lee Chen, Zhiying Chen, Merritt Funk The non-ambipolar electron plasma (NEP) is heated by electron beam extracted from the electron-source Ar plasma through a dielectric injector by an accelerator located inside NEP. NEP pressure is in the 1-3mTorr range of N$_{2}$ and its accelerator voltage varied from $V_{A}=+$80 to $V_{A}=+$600V. The non-ambipolar beam-current injected into NEP is in the range of 10s Acm$^{-2}$ and it heats NEP through beam-plasma instabilities. Its EED$f$ has a Maxwellian bulk followed by a broad energy-continuum connecting to the most energetic group with energies above the beam-energy. The remnant of the injected electron-beam power terminates at the NEP end-boundary floating-surface setting up sheath potentials from $V_{S}=$80 to $V_{S}=$580V in response to the applied values of $V_{A}$. The floating-surface is bombarded by a space-charge neutral plasma-beam whose IED$f$ is near mono-energetic. When the injected electron-beam power is adequately damped by NEP, its end-boundary floating-surface $V_{S}$ can be linearly controlled at almost 1:1 ratio by $V_{A}$. NEP does not have an electron-free sheath; its ``sheath'' is a widen presheath that consists of a thermal presheath followed by an ``anisotropic'' presheath, leading up to the end-boundary floating-surface. Its ion-current of the plasma-beam is much higher than what a conventional thermal presheath can supply. If the NEP parameters cannot damp the electron beam power sufficiently, $V_{S}$ will collapse and becomes irresponsive to $V_{A}$. [Preview Abstract] |
Thursday, October 3, 2013 11:30AM - 12:00PM |
NR3.00006: Particle-in-cell simulations of discharges with intense electron emission Invited Speaker: Dmytro Sydorenko In many plasma devices, the plasma is bounded by walls which emit electrons due to secondary electron emission or thermionic emission. At low pressures, the electron mean free path exceeds the plasma dimensions, and the emitted electrons accelerated by the intense electric field of the near-wall sheath propagate through the plasma as an electron beam. The beam dynamics in a finite length system is different from theoretical predictions for infinite or periodic plasmas. This presentation gives a summary of numerical studies of beam-plasma interaction in Hall thrusters and dc discharges carried out with a particle-in-cell code [1]. The code resolves one spatial coordinate and three velocity components, it is based on the direct implicit algorithm [2], the electron-to-ion mass ratio is realistic, numerous collisions between electrons and neutrals and the Coulomb collisions are included, code performance is enhanced with the help of MPI parallelization. The following effects are discussed: vanishing of the two-stream instability due to modification of the bulk electron velocity distribution [3], sheath instability in Hall thrusters [4], intermittency and multiple regimes of the two-stream instability in dc discharges. \\[4pt] [1] D. Sydorenko et al., Phys. Plasmas 13, 014501 (2006).\\[0pt] [2] M. R. Gibbons and D. W. Hewett, J. Comput. Phys. 120, 231 (1995).\\[0pt] [3] D. Sydorenko et al., Phys. Plasmas 14, 013508 (2007).\\[0pt] [4] D. Sydorenko et al., Phys. Plasmas 15, 053506 (2008). [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700