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 CT1: Plasma Boundaries, Sheaths, and Basic Plasma Physics I |
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Chair: JP Sheehan, Plasmadynamics & Electric Propulsion Laboratory, University of Michigan Room: Ballroom EF |
Tuesday, November 4, 2014 8:00AM - 8:30AM |
CT1.00001: The Plasma-Sheath Boundary in Two-Ion-Species Plasmas Invited Speaker: Scott D. Baalrud The Bohm criterion is among the most important results in plasma physics because it provides the ion flow speed at the sheath edge under common plasma conditions. This is a useful boundary condition for modeling plasma-materials interactions, as well as for global plasma models. However, a difficulty arises when multiple ion species are present because the Bohm criterion provides only one constraint in as many unknowns as there are ion species. Conventional theory assumes that the ion species are decoupled, which leads to the prediction that each obtains its individual sound speed at the sheath edge: $V_i = \sqrt{T_e/m_i}$. However, experiments in Ar-Xe and He-Xe mixtures have revealed that the ion speeds can merge toward a common speed under typical low-temperature plasma conditions [1]. This merging of ion speeds suggests that ion-ion friction may be playing a role, but standard Coulomb collisions are far too weak to explain the measurements. In this work, we discuss how the experimental results can be understood by accounting for wave-particle collisions from ion-ion two-stream instabilities. These instabilities arise when the differential flow speed between the ion species exceeds a threshold value that depends on the ion species concentrations and the electron-ion temperature ratio. When this threshold is exceeded, wave-particle interactions rapidly increase the collision rate leading to an ion-ion friction force that effectively ``locks'' the differential flow speed to the instability threshold. This provides a second constraint that can be used to determine the speed of each ion species at the sheath edge. We present numerical calculations of the instability threshold, and new particle-in-cell simulations that show the presence of both the instabilites and enhanced friction force. Only by accounting for the instabilites can theory predict the simulated ion speeds at the sheath edge.\\[4pt] [1] Hershkowitz, Yip, Severn, Phys. Plasmas 18, 057102 (2011); and references therein. [Preview Abstract] |
Tuesday, November 4, 2014 8:30AM - 8:45AM |
CT1.00002: Ion Energy and Angular Distribution Functions at the Material Wall of a Magnetized Plasma Sheath Davide Curreli, Rinat Khaziev We present a calculation of the ion energy distribution and the ion angular distribution at the material wall of a magnetized plasma sheath. The calculation has been done using two different techniques: a Monte-Carlo method, propagating the trajectories of a Maxwellian population of ions across the ExB field of the magnetized sheath, and a Particle-in-Cell, giving a self-consistent treatment of the plasma behavior from the quasi-neutral region to the material boundary. Data are presented for magnetic fields inclined at angles from 0.0 to 88 degrees with respect to the normal to the surface, and field magnitudes up to 1.0 Tesla. The plasma sheath accelerates the ions up to energies scaled with the electron temperature. The ion angular distributions exhibit surprising non-linear trends, depending on both the plasma conditions and magnetic field. Ions can hit the wall at angles close to the surface normal with single-lobe IADF's, or at grazing angles with double-lobe IADF's. The energy-angle distributions strongly affect the material response, comprising electron secondary emission and material sputtering. [Preview Abstract] |
Tuesday, November 4, 2014 8:45AM - 9:00AM |
CT1.00003: Analytical model of plasma sheaths at intermediate radio frequencies Mark Sobolewski Analytical models of plasma sheaths provide physical insight and are useful in 2-d and 3-d plasma simulations, where numerical solution of the sheath equations at each boundary point is impractical. Analytical models have long been known for the high-frequency and low-frequency limits, where the ion transit time is either much greater than or much less than the rf period. At intermediate frequencies, however, sheath behavior is more complicated. In addition to the well-known narrowing of ion energy distributions (IEDs) there are other, lesser known effects, including changes in the ion current (which becomes strongly time-dependent within the sheath) and in IED peak intensities, average ion energy, sheath impedance, and sheath power. Here, we describe a new approach for modeling intermediate-frequency, collisionless sheaths. It captures the essential elements of ion dynamics yet still provides analytical expressions for most sheath properties. Predictions of the analytical model are compared to previous analytical models, numerical models, and, where possible, experimental data. The model yields new insights into ion dynamics and may serve to increase the accuracy of plasma simulations, particularly their predictions for average ion energy and power. [Preview Abstract] |
Tuesday, November 4, 2014 9:00AM - 9:15AM |
CT1.00004: Size dependent transitions induced by an electron collecting electrode near the plasma potential Edward Barnat, George Laity, Matt Hopkins, Scott Baalrud As the size of a positively biased electrode increases, the nature of the interface formed between the electrode and the host plasma undergoes a transition from an electron-rich structure (electron sheath) to an intermediate structure containing both ion and electron rich regions (double layer) and ultimately forms an electron-depleted structure (ion sheath). In this study, measurements are performed to further test how the key scaling relationship relating the area of the electrode to that of the area of the vessel containing the plasma discharge impacts this transition. This was accomplished using a segmented disk electrode in which individual segments were individually biased to change the effective surface area of the anode. Measurements on bulk plasma parameters such as the collected current density, plasma potential, electron density, electron temperature and optical emission are made as both the size and the bias placed on the electrode are varied. Size dependent transitions in the voltage dependence of the plasma parameters are identified in both argon and helium discharges and are compared to the interface transitions predicted by global current balance [1]. \\[4pt] [1] S. D. Baalrud, N. Hershkowitz, and B. Longmier, Phys. Plasmas 14, 042109 (2007). [Preview Abstract] |
Tuesday, November 4, 2014 9:15AM - 9:30AM |
CT1.00005: Sheath structure transition controlled by secondary electron emission at low gas pressure Irina Schweigert, Samuel J. Langendorf, Michael Keidar, Mitchell L.R. Walker Previously the experiments [1] demonstrated that the growth of the electron temperature with power in the Hall thruster is restricted by plasma-wall interaction if the wall has an enhanced secondary electron emission (SEE) yield. It is known that the plasma and wall is separated by the sheath potential drop to provide the condition of zero - current on the surface with floating potential. The rearrangement of the sheath structure near the plate with enhanced SEE is the subject of our experimental and theoretical study. The experiment was carried out in multidipole plasma device, where plasma is maintained by the negatively-biased emissive filament. The plate with sapphire surface is placed 50 cm apart from the filament. The plasma parameters were measured for different negative biases U$_{\mathrm{b}}$ and discharge currents J at P$=$10$^{-4}$ Torr. In our PIC simulations the plasma was calculated for the experimental conditions. We solved self-consistently the Boltzmann equations for the electron and ion distribution functions and Poisson equation for electrical field. Both in the experiment and simulation we found non-monotonic change in sheath structure near the plate depending on U$_{\mathrm{b}}$ and J. The kinetic simulations allowed us to describe the sheath rearrangement in terms of the electron energy distribution function.\\[4pt] [1] Raitses, Y., et al. \textit{Physics of Plasmas} 13 (2006): 014502. [Preview Abstract] |
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