Bulletin of the American Physical Society
63rd Annual Gaseous Electronics Conference and 7th International Conference on Reactive Plasmas
Volume 55, Number 7
Monday–Friday, October 4–8, 2010; Paris, France
Session UF3: Plasma Boundaries and Sheaths |
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Chair: Julian Schulze, Ruhr-University of Bochum Room: 162 |
Friday, October 8, 2010 2:00PM - 2:15PM |
UF3.00001: Temporally-Resolved Measurements of the Ion Distribution Function in a Radio Frequency Sheath Walter Gekelman, Brett Jacobs, Patrick Pribyl, Mike Barnes The time-dependent argon ion velocity distribution function above and within the plasma sheath of a radio frequency (rf) biased substrate has been measured using laser induced fluorescence (LIF) in a commercial plasma processing tool. The measurements were acquired at eight different phases of the 2.2 MHz rf waveform and show the ion dynamics to vary dramatically throughout a cycle. Discharge parameters were such that the rf bias period was on the order of the ion transit time through the sheath ($\tau_{ion}$/$\tau_{rf}$ = 0.435). The heat flux and plasma flow is derived from the ion distribution function. This work embodies the first time resolved measurement of ion velocity distribution functions (IVDFs) within an rf biased sheath over a large area (30 cm diameter) substrate. [Preview Abstract] |
Friday, October 8, 2010 2:15PM - 2:30PM |
UF3.00002: Plasma-Sheath Transition: Electron Rich Sheath Klaus Wiesemann, Philipp Mertmann In a collisonfree sheath charged particle trajectories can be
described as those of single particles falling freely under the
influence of an external electric potential. In plasma field-
driven motion is drift -- as long as friction by electron-ion
collisions balances the field force $e$\underline {\textit{E}}.
It depends only on local quantities. The transition from field
driven electronic drift to free fall resembles run-away and we
treat it analogously by a fluid model for a stationary
inhomogeneous plasma region in front of a (plane) electrode at
positive potential. Ions are described by Boltzmann equilibrium,
electrons as drifting Maxwellian (similarly to H. Dreicer, Phys.
Rev. \textbf{115} (1959) 238). We present numerical solutions
of the fluid equation together with the resp. Poisson equation
for initial plasma densities of 10$^{10}$ and 10$^{11}$cm$^{-3}$
at \textit{kT}$_{e}$=1eV and 10eV. Initial condition is set by an
initial electron drift $v_{D}$. Solutions exhibit quasineutral
plasma followed by a sheath -- the extension of plasma
depending strongly on the initial condition. For $v_{D}$=
0.5*10$^{-3}$*$ |
Friday, October 8, 2010 2:30PM - 2:45PM |
UF3.00003: Experimental test of instability enhanced collisional friction for determining ion loss in two ion species plasmas Greg Severn, Chi-Shung Yip, Noah Hershkowitz Recent experiments have shown that ions in weakly collisional plasmas containing two ion species of comparable densities nearly reach a common velocity at the sheath edge. A new kinetic theory of Baalrud et al. suggests that collisional friction between the two ion species enhanced by a two stream instability reduces the drift velocity of each ion species relative to each other near the sheath edge, and finds that the difference ($\Delta V = V_2-V_1$) in velocities at the sheath edge depends on the relative concentrations of the species, and on the ion temperatures. The difference is small when the concentrations are comparable and is large, with each species reaching its own Bohm velocity, when the relative concentration differences are large. To test these findings, ion drift velocities were measured near the near sheath edge in Argon-Xenon plasmas as a function of the concentration ratio using the laser-induced fluorescence technique. We showed that the predictions are in excellent agreement with our measurements. These are the first experimental tests of the new model. [Preview Abstract] |
Friday, October 8, 2010 2:45PM - 3:00PM |
UF3.00004: Double sheaths on cathodes emitting neutral atoms Mikhail Benilov, Larissa Benilova The model of a collisionless near-cathode space-charge sheath with ionization of atoms emitted by the cathode surface is considered. Apart from being of interest due to its potential application to vacuum arcs, the model is of interest due to its relation to the classic Tonks-Langmuir model of a plane glow discharge. Numerical calculations showed that the mathematical problem is solvable and its solution is unique. In the framework of this model, the sheath represents a double layer with a potential maximum, with the ions which are produced before the maximum returning to the cathode surface and those produced after the maximum escaping into the plasma. Numerical results are given in a form to be readily applicable in analysis of discharges burning in cathode vapor. It is found that the height of the potential hump is within the range (0.85{\ldots}1.26)kT$_{e}$/e. This value is insufficient to explain the observed velocities of ions in cathode jets of vacuum arcs, in agreement with the belief of many researchers that the main contribution to acceleration of ions is given by the plasma pressure gradient. The ion backflow coefficient is at least 53{\%}, in agreement with the experiment. [Preview Abstract] |
Friday, October 8, 2010 3:00PM - 3:15PM |
UF3.00005: Kinetic Theory of the Presheath and Bohm Criterion S.D. Baalrud, C.C. Hegna A kinetic theory of the Bohm criterion is developed that is based on positive-exponent velocity moments of the plasma kinetic equation. Our result is contrasted with the conventional kinetic Bohm criterion, which is based on a $v^{-1}$ moment of the Vlasov equation. The salient difference between the two results is that low velocity particles dominate in the conventional theory, but are essentially unimportant in our model. The presence of low-velocity particles can cause unphysical divergences in the conventional model, which are not present in the new theory. A kinetic equation that accounts for wave-particle scattering by convective instabilities is also used to show that ion-acoustic instabilities in the presheath of low-temperature plasmas (where $T_e \gg T_i$) can cause both ions and electrons to obtain Maxwellian distribution functions in the presheath of a single-ion-species plasma. The same theory is also used to show how a strong collisional friction force can arise between ion species when ion-ion streaming instabilities arise in the presheath of a multiple-ion-species plasma. We show how this effect can determine the flow speed of each ion species at the sheath edge. [Preview Abstract] |
Friday, October 8, 2010 3:15PM - 3:30PM |
UF3.00006: Theoretical and experimental study of ion flux formation in an asymmetric high-frequency capacitive discharge I. Schweigert, D. Ariskin, T. Chernoizyumskaya, A. Smirnov Parameters of a high-frequency capacitive discharge in argon in axially symmetric chambers of different geometries are studied in experiments and by means of two-dimensional kinetic modeling by the Particle-in-Cell Monte Carlo collision method. It is demonstrated that a change in the ratio of the areas of the driven and grounded electrodes can substantially increase the ion energy on the electrode practically without disturbing the plasma parameters. Particular attention is paid to studying the self-bias voltage and the ion distribution function on the electrode for gas pressures ranging from 15 to 70 mtorr. In the experiment, the ion flux was studied by an energy analyzer placed behind the grounded electrode with an orifice in the middle. Several reactors with different ratios of the areas of the driven and grounded electrodes were considered to study the effect of the chamber geometry on the ion flux. The plasma potential relative to the grounded electrode and, therefore, the maximum energy of the ions are demonstrated to increase with increasing area ratio. The measured and calculated parameters of the plasma, such as the electron concentration and temperature, and also the ion energy distribution functions are in good agreement. [Preview Abstract] |
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