Bulletin of the American Physical Society
71st Annual Gaseous Electronics Conference
Volume 63, Number 10
Monday–Friday, November 5–9, 2018; Portland, Oregon
Session FT4: Plasma Boundaries: Sheaths, Boundary Layers, Others |
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Chair: Sebastian Wilczek, Ruhr-University Bochum Room: Oregon Convention Center A107-A109 |
Tuesday, November 6, 2018 2:00PM - 2:30PM |
FT4.00001: Electron Sheaths and Fireballs Invited Speaker: Scott Baalrud Sheaths form to balance electron and ion losses from plasmas. Because electrons are much more mobile than ions, most sheaths are ion sheaths. These act to reflect electrons back into the plasma and accelerate ions toward the boundary. Although less common, a number of novel sheath structures including electron sheaths, double sheaths and anodic double layers (aka fireballs) can form near electrodes biased positive with respect to the plasma. These structures exhibit interesting properties, such as self-organization, wave excitation and global influence on bulk plasma properties. They can also be utilized for applications such as flow control, electron sources, control of plasma electron energy distributions, and plasma surface modification. This presentation will review a number of recent advances in our understanding of sheath structures near biased electrodes, including the existence of an electron presheath [1] and associated electron Bohm criterion [2], as well as the excitation of instabilities at both the ion and electron plasma frequencies in the electron presheath. It will also review recent progress in understanding fireball formation including a new model for the onset [3], as well as its validation from two-dimensional particle-in-cell simulations and a novel non-invasive laser collision induced fluorescence diagnostic [4]. [1] Yee, Scheiner, Baalrud, Barnat and Hopkins, PSST 26, 025009 (2017) [2] Scheiner, Baalrud, Yee, Hopkins and Barnat, Phys. Plasmas 22, 123520 (2015) [3] Scheiner, Barnat, Baalrud, Hopkins, Yee, Phys. Plasmas 24, 113520 (2017) [4] Scheiner, Barnat, Baalrud, Hopkins, and Yee, Phys. Plasmas 25, 043513 (2018) [Preview Abstract] |
Tuesday, November 6, 2018 2:30PM - 2:45PM |
FT4.00002: Influence of Neutral Pressure on Instability Enhanced Friction and Ion Velocities at the Sheath Edge of Two-Ion-Species Plasmas Patrick Adrian, Scott Baalrud, Trevor Lafleur Ions are well-known to enter the sheath at the Bohm velocity in single-ion plasmas. However, for two ion species plasmas, the ions' sheath edge speed has been observed to deviate from the individual Bohm velocity [1]. The Instability-Enhanced Friction (IEF) theory [2] accurately predicts this deviation by accounting for an enhanced friction between the ions that merges their velocities. The enhanced friction is caused by ion-ion two-stream instabilities in the presheath. Here we report an advancement of the IEF theory which includes the effect of neutral pressure when predicting sheath edge ion speed in a two ion species plasma [3]. The predictions for the ions' sheath edge flow were tested against Particle-in-Cell Monte-Carlo Collision (PIC-MCC) simulations for a range of neutral pressures and were shown to be accurate. The theory and simulations indicate that the two-stream instability can persist up to 10's of mTorr. This result implies ion-ion streaming instabilities can affect the ions' sheath edge flow speed in plasma based manufacturing devices, which can operate in the 10's of mTorr of pressure. [1] G. D. Severn, \textit{et. al.}, PRL.~90, 145001 (2003) [2] S. D. Baalrud, \textit{et. al.} PRL 103, 205002 (2009) [3] P. J. Adrian, \textit{et. al.}, POP 24, 123505 (2017) [Preview Abstract] |
Tuesday, November 6, 2018 2:45PM - 3:00PM |
FT4.00003: Ion flux on emissive surface with debye-scale erosion trenches Irina Schweigert, Michael Keidar The surface of walls confining the low temperature plasma can erode with time due to plasma-wall interaction that leads to variations of electron and ion fluxes over the surface. In this work, the sheath structure modification near the emissive surface with a Debye length scale erosion trenches is analyzed with increasing the beam electron energy. In 2D3V Particle-in-cell Monte Carlo collision simulations, we study the effect of the secondary electron emission from the floating grooved plate on the plasma sheath at low gas pressure and compare our results with the known experimental data. The discharge operation and secondary electron emission from the hBN plate are controlled by an electron beam from heated cathode. The Boltzmann equations are solved to find the distribution functions for electrons and ions together with the Poisson equation for the electrical potential. Since the BN-plate is under the floating potential the total current on it equal zero. In simulations, the wall material sample has four identical trenches. It is shown that the potential distribution acts as a focusing lens on the ion current and on low energy electron current and redirect the ion current inside to the trenches and the electron current to the front surface. The ion flux to the front surface and bottom of grooves is analyzed in the terms of the ion energy distribution function. We consider also a segmented plane surface with the different secondary electron coefficients. It is shown that an non-uniformity of emissive surface properties affects the ion flux distribution, and consequently increases locally etching rate. [Preview Abstract] |
Tuesday, November 6, 2018 3:00PM - 3:15PM |
FT4.00004: Magnetic field induced anode sheath transition in modified hollow cathode discharge Ramkrishna Rane, Kushagra Nigam, P Bharathi, Alphonsa Joseph, Subrato Mukherjee The present work reports on the study of the anode sheath behaviour in the magnetically enhanced hollow cathode plasma. A uniform magnetic field in the range of 0 to 50 Gauss induces a change in the potential which results in the transition from ion to electron sheath. The global plasma response to this transition is investigated using Langmuir probe and Optical Emission Spectroscopy. A systematic study showed that during the transition, the electron temperature increases and plasma density decreases in the bulk plasma. The emission spectra of the plasma showed the presence of strong atomic and ionic lines of Argon. The intensity of these spectral lines showed a dip during the transition between two sheaths. The discharge showed an onset of anode spot or fireball at critical magnetic field. The plasma potential locks on to the ionization potential of argon gas when anode spot is completely formed. Further, oscillations of the order of 5-20 KHz frequency are observed in the floating potential due to the extra ionization and excitations in the electron sheath. The reason of the electron sheath formation at particular magnetic field is attributed to the reduction of the electron flux reaching to the anode in the direction perpendicular to the magnetic field [Preview Abstract] |
Tuesday, November 6, 2018 3:15PM - 3:30PM |
FT4.00005: The design of uniform Poynting Vector RF antenna for the surface wave plasma. Hema Swaroop Mopidevi, Thomas Anderson The power-coupling intermediary of inductive coupled plasma (ICP) is the coil's B-field whereas for RF surface wave plasma (SWP; e.g., 27Mhz) it is the near-field Poynting vector of radiation. Thus, the antenna need not be placed close to vacuum window alleviating capacitive coupling from the high voltage tips. To accommodate the large RF wavelength inside a small space, the antennas are often coiled or made into spiral shapes. This paper reports the calculations of near-field Poynting vector spatial uniformity between 2 types of antenna. First, a spiral dipole antenna is shown to have strong near-field in the high-current ring-region of the spiral; the surface wave could then cause plasma non-uniformity in the ring-region. Then, a quadrupole antenna is shown to have overcome the dipole's issue through radiations from both high- and low-current portions of the antenna (the high voltage tips): the antenna's geometry takes advantage of the tips' high voltage turning it into strong E-fields that will radiate by the displacement current term of the Maxwell-Ampere Equation. In other words, this antenna combines real- and displacement-current radiations to result in a spatially uniform magnitude of Poynting vector. [Preview Abstract] |
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