Bulletin of the American Physical Society
62nd Annual Gaseous Electronics Conference
Volume 54, Number 12
Tuesday–Friday, October 20–23, 2009; Saratoga Springs, New York
Session JT1: Sheaths |
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Chair: Eric Joseph, IBM T.J. Watson Research Center Room: Saratoga Hilton Ballroom 1 |
Tuesday, October 20, 2009 4:00PM - 4:15PM |
JT1.00001: Different models of the plasma-sheath transition Karl-Ulrich Riemann The space charge formation in in the boundary layer of a quasi-neutral plasma is strongly influenced by the contribution of slow ions. As a consequence, the structure of the plasma-sheath transition depends in detail on the way how slow ion production is accounted for in the applied modeling approach. The ``intermediate scale'' connecting plasma sheath is therefore different (i) in fluid analysis, (ii) in kinetic analysis with cold ion source, and (iii) in kinetic analysis with hot ion source. We discuss the different models and present convenient analytical approximations for the cases (i) and (ii). The approximations supplement corresponding sheath approximations published previously [1]. The case (iii) is not solved until now. We derive the appropriate scaling and discuss the inherent difficulties. \\[4pt] [1] K.-U. Riemann, Plasma Sources Sci.~Technol.~{\bf 18}, 014007 (2009) [Preview Abstract] |
Tuesday, October 20, 2009 4:15PM - 4:30PM |
JT1.00002: Collisonal Friction Enhanced by Two-Stream Instabilities Determines the Bohm Criterion in Plasmas With Multiple Ion Species S.D. Baalrud, C.C. Hegna, J.D. Callen Ion-ion streaming instabilities are excited in the presheath region of plasmas with multiple ion species if the ions are much colder than the electrons. Streaming instabilities onset when the relative fluid flow between ion species exceeds a critical speed, $\Delta V_c$, of order the ion thermal speeds. Using a generalized Lenard-Balescu theory that accounts for instability-enhanced collective responses [1], one is able to show the instabilities rapidly (within a few Debye lengths) enhance the collisional friction between ion species far beyond the contribution from Coulomb collisions alone. This strong frictional force determines the relative fluid speed between species. When this condition is combined with the Bohm criterion generalized for multiple ion species, the fluid speed of each ion species is determined at the sheath edge. For each species, this speed differs from the common ``system'' sound speed by a factor that depends on the species concentration and $\Delta V_c$.\\[4pt] [1] S.D. Baalrud, J.D. Callen, and C.C. Hegna, Phys. Plasmas {\bf 15}, 092111 (2008). [Preview Abstract] |
Tuesday, October 20, 2009 4:30PM - 4:45PM |
JT1.00003: Experimental test of the Baalrud's model for ion loss from a two-species plasma Noah Hershkowitz, Chi-Shung Yip Recent experiments have shown that ions in plasmas containing two ion species reach a common velocity at the sheath-presheath boundary [1]. A new theory [2] suggest that collisional friction between the two ion species enhanced by two stream instability dominates the drift velocity of each ion species near the sheath edge but also suggest that there are differences in ion the velocity at the sheath-presheath boundary given by $\sqrt {\frac{1}{2\alpha }(v_{th1}^2 +\alpha v_{th2}^2 )} $. This suggests that significant differences in velocity will occur as the relative concentration varies. We report the first experimental test of this model. We measure ion velocity distribution functions (ivdfs) near sheath edge in an Argon/Xenon plasma as a function of the concentration ratios. The relative concentration of the two ion species is determined by the Ion Acoustic Wave phrase velocity measurements, the ivdfs are determined by Laser Induced Florescence, the electron temperature is measured by Langmuir probe and the plasma potential is measured by emissive probe. \\[4pt] [1] Lee, D; Hershkowitz, N; Severn, GD. Appl. Phys. Lett. \textbf{91,} 041505 (2007)\\[0pt] [2] S.D. Baalrud, J.D. Callen, and C.C. Hegna, GEC 2009 [Preview Abstract] |
Tuesday, October 20, 2009 4:45PM - 5:00PM |
JT1.00004: Field-enhanced Auger emission of electrons from metals B. Eismann, A.V. Phelps, L.C. Pitchford The electric field strength, E, at the cathode surface in microdischarges operating at high pressures is predicted to reach some 100 kV/cm. In this context, the objective of our work is to evaluate the influence of a high surface field on the ion-induced secondary electron emission coefficient from metal surfaces. Our starting point is the classical theory of Hagstrum (Phys. Rev., 96, 336, 1954), which we extended to include an electric field in the calculation of the probability for electron ejection following Auger neutralization of an incident ion. Among the various effects considered, the Schottky effect is by far the most important, and the secondary electron emission coefficient can be well approximated analytically as a constant (Hagstrum's original theory) plus a term depending on square root of E.~ The latter term is relatively more important for higher work function metals, and it is independent of the nature of the incoming ion. For argon ions incident on a tungsten surface, the calculated secondary electron emission coefficient is almost constant for E $<$ 100 kV/cm and thereafter increases from 0.05 to 0.08 for a factor of 50 increase in E. [Preview Abstract] |
Tuesday, October 20, 2009 5:00PM - 5:15PM |
JT1.00005: Ion Flux Formation in 2 MHz Capacitive Discharge at Different Gas Pressures Irina Schweigert We have studied the ion energy and angular distribution functions in a 2 MHz capacitive discharge in argon using 1D and 2D Combined Particle in Cell Monte-Carlo Collision simulations. We found that secondary electrons produced by ion bombardment from the electrodes make important contribution to the ionization at higher gas pressure. Calculations showed that the ion flux on the electrode is very sensitive to the plasma parameters. However, the ion angular distribution function weakly responds to a change of the gas pressure. For the given voltage amplitude the ions can have larger energy at higher gas pressure. The explanation of this phenomenon was found. For these conditions, variation of the sheath width can change the regime of ion motion. [Preview Abstract] |
Tuesday, October 20, 2009 5:15PM - 5:30PM |
JT1.00006: Charge separation in a magnetized plasma-sheath-lens Eugen Stamate Most of plasma processing technologies are based on radical-assisted ion-induced surface-modification where ions accumulate energy in the sheath, and then strike the surface modifying its properties in a desirable way. Plasma-sheath-lens is a three-dimensional potential distribution of customized shape, formed by the space charge surrounding a biased electrode-insulator interface. The discrete and modal focusing effects have been reveled for this type of electrostatic structures formed in plasma [1] and several applications including sheath thickness evaluation, negative ion detection and extraction of positive or negative ion beams have been developed. A non-magnetized plasma-sheath-lens act as a kinetic energy separator, but it is not mass sensitive. However, a magnetized plasma-sheath-lens exhibits mass separation, so that ions of different mass will impact the electrode at different locations on the biased electrode surface. The mass spectrum can be measured as the radial distribution of the ion current density over the plasma-sheath-lens's electrode. Relevant fluid and particles simulations of the magnetized plasma-sheath-lens structures and ion trajectories within them are presented for different plasma parameters and magnetic filed configurations. Practical aspects linked to the development of a new type of mass spectrometers are also investigated.\\[0pt] [1] E. Stamate and H. Sugai, Phys. Rev. Lett. (2005) 94, 125004 [Preview Abstract] |
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