Bulletin of the American Physical Society
67th Annual Gaseous Electronics Conference
Volume 59, Number 16
Sunday–Friday, November 2–7, 2014; Raleigh, North Carolina
Session NR1: Plasma Boundaries, Sheaths, and Basic Plasma Physics II |
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Chair: Scott Baalrud, Department of Physics, University of Iowa Room: State EF |
Thursday, November 6, 2014 10:00AM - 10:30AM |
NR1.00001: Modeling Sheaths in DC Discharges Invited Speaker: Scott Robertson Textbook presentations on sheaths are often limited to a discussion of Bohm's criterion because more detailed analysis results in equations that can be solved only by numerical methods. There are both fluid and kinetic models for sheaths that can be solved by packaged numerical integration routines in a mathematical spreadsheet such as Mathematica, Matlab, or Mathcad. The potential profiles and the currents for sheaths at boundaries usually have monotonic profiles that are easily modeled using a Boltzmann distribution for electrons and for ions using the fluid momentum equation and the continuity equation with a source term describing plasma production. Additional ion species and bi-Maxwellian electron distributions are easily included. Virtual cathodes may form above emissive surfaces which divide the distribution function of emitted electrons into a passing population and a reflected population that can be modeled only by a kinetic approach. For sheaths at inserted objects such as probes and dust particles, it is customary to prescribe the plasma characteristics at infinity, to ignore creation of new plasma by ionization, and to solve for the radial variation of the density near the object and for the current collected by the object. A kinetic model is required for sheaths at inserted objects because the distribution function must be divided into passing particles and collected particles. [Preview Abstract] |
Thursday, November 6, 2014 10:30AM - 10:45AM |
NR1.00002: Electric field profiles in obstructed helium discharge Peter Fendel, Biswa Ganguly, Peter Bletzinger Axial and radial variations of electric field have been measured in dielectric shielded 25 mm diameter parallel plate electrode for 2 mA, 2250 V helium dc discharge at 1.75 Torr with 6.5 mm gap. The axial and radial electric field profiles have been measured from the polarization dependent Stark splitting of 2$^{1}$S $\to$ 11 $^{1}$P transition through collision induced fluorescence from 4$^{3}$D $\to$ 2$^{3}$P. The electric field values showed a strong radial variation peaking up to 5 kV/cm near the cathode radial boundary, and decreasing to about 1 kV/cm near the anode, suggesting the formation of an obstructed discharge for this low Pd condition. Also, the on-axis electric field was nearly constant across the gap indicating a radially non-uniform current density. In order to obtain information about the space charge distribution in this obstructed discharge, it was modeled using the 2-d axisymmetric Poisson solver with COMSOL finite element modeling program. The model discharge dimensions were selected to match the experimental dimensions. The best fit to the measured electric field distribution was obtained with a space charge variation of $\rho $(r)$ = \rho _{0}$(r/r$_{0})^{3}$, where $\rho $(r) is the local space charge density, $\rho_{0}$ is the maximum space-charge density, r the local radial value and r$_{0}$ the radius of the electrode. [Preview Abstract] |
Thursday, November 6, 2014 10:45AM - 11:00AM |
NR1.00003: Electric field measurements in a nanosecond pulse discharge by picosecond CARS / 4-wave mixing Ben Goldberg, Ivan Shkurenkov, Igor Adamovich, Walter Lempert Time-resolved electric field measurements in hydrogen by picosecond CARS / 4-wave mixing are presented. Measurements are carried out in a high voltage nanosecond pulse discharge in hydrogen in plane-to-plane geometry, at pressures of up to several hundred Torr, and with a time resolution of 0.2 ns. Absolute calibration of the diagnostics is done using a sub-breakdown high voltage pulse of 12kV/cm. A diffuse discharge is obtained by applying a peak high voltage pulse of 40 kV/cm between the electrodes. It is found that breakdown occurs at a lower field, 15-20 kV/cm, after which the field in the plasma is reduced rapidly due to plasma self shielding The experimental results are compared with kinetic modeling calculations, showing good agreement between the measured and the predicted electric field. [Preview Abstract] |
Thursday, November 6, 2014 11:00AM - 11:15AM |
NR1.00004: EEDF and Plasma Parameters of an Argon Positive Column Valery Godyak, Bengamin Alexandrovich, George Petrov The existing experimental data base on plasma properties of the positive column in noble gases was obtained during the past century with optical spectroscopy and Langmuir probe technique. The latter is based on the assumption of a Maxwellian electron energy distribution function (EEDF). However, numerous calculations for EEDFs and experiments in Ramsauer-type gases, such as Ne, Ar, Kr and Xe, have shown Druyvesteyn-like distributions in the elastic energy range, unless strong e-e collisions at large plasma density were able to Maxwellize the EEDF. Another source of error in Langmuir probe diagnostics in Ramsauer gases is a large uncertainty in determining the plasma potential that may result in significant error in estimation of the plasma density. It has been shown [1] that the only reliable way to find basic plasma parameters in such plasmas is the EEDF measurement with plasma parameters determined as appropriate integrals of the measured EEDF. In the present work, we carried out EEDF measurements in Ar and found plasma parameters as EEDF integrals in wide ranges of pressure (1 mTorr -- 1 Torr) and discharge current (3mA -3A) in a positive column of DC discharge. The experimental results were compared with simulations based on solution of the one-dimensional electron Boltzmann equation [2] coupled with a set of equations for the plasma density and plasma potential [3]. The problems associated with EEDF measurements in DC plasmas prone to different kind of instabilities, as well as the area of the model applicability are discussed in this presentation. [1] V. Godyak, et al, J. Appl. Phys. \textbf{73}, 3657 (1993). [2] D. Uhrlandt and R. Winkler, J. Phys. D \textbf{29}, 115 (1996). [3] U. Kortshagen et al, Plasma Sources Sci. Tech. \textbf{5}, 1 (1996). [Preview Abstract] |
Thursday, November 6, 2014 11:15AM - 11:30AM |
NR1.00005: A self-consistent view on plasma-neutral interaction near a wall: plasma acceleration by momentum removal and heating by cold walls Gerard van Rooij, Niek den Harder, Teofil Minea, Amy Shumack, H. de Blank In plasma physics, material walls are generally regarded as perfect sinks for charged particles and their energy. A special case arises when the wall efficiently reflects the neutralized plasma particles (with a significant portion of their kinetic energy) and at the same time the upstream plasma is of sufficiently high density to yield strong neutral-ion coupling (i.e. reflected energy and momentum will not escape from the plasma). Under these conditions, plasma-surface interaction will feedback to the upstream plasma and a self-consistent view on the coupling between plasma and neutrals is required for correct prediction of plasma conditions and plasma-surface interaction. Here, an analytical and numerical study of the fluid equations is combined with experiments (in hydrogen and argon) to construct such a self-consistent view. It shows how plasma momentum removal builds up upstream pressure and causes plasma acceleration towards the wall. It also shows how energy reflection causes plasma heating, which recycles part of the reflected power to the wall and induces additional flow acceleration due to local sound speed increase. The findings are relevant as generic textbook example and are at play in the boundary plasma of fusion devices. [Preview Abstract] |
Thursday, November 6, 2014 11:30AM - 12:00PM |
NR1.00006: Effects of Emitted Electron Temperature on the Sheath Invited Speaker: J.P. Sheehan It has long been known that electron emission from a surface significantly affects the sheath surrounding that surface, reducing the sheath potential and electric field. Typical fluid theory of a planar sheath with emitted electrons assumes that the plasma electrons follow the Boltzmann relation and the emitted electrons are emitted with zero energy, predicting a potential drop of Te across the sheath when the surface is allowed to float. A one-dimensional kinetic theory of sheaths surrounding planar, electron-emitting surfaces is presented which accounts for plasma electrons lost to the surface and the temperature of the emitted electrons. It is shown that ratio of plasma electron temperature to emitted electron temperature significantly affects the sheath potential when the plasma electron temperature is within an order of magnitude of the emitted electron temperature. The sheath potential goes to zero as the plasma electron temperature equals the emitted electron temperature, which can occur in the afterglow of an rf plasma and some low-temperature plasma sources. These results were validated by particle-in-cell simulations. The theory was tested by making measurements of the sheath surrounding a thermionically emitting cathode in the afterglow of an rf plasma. The measured sheath potential shrunk to zero as the plasma electron temperature cooled to the emitted electron temperature, as predicted by the theory. [Preview Abstract] |
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