Bulletin of the American Physical Society
70th Annual Gaseous Electronics Conference
Volume 62, Number 10
Monday–Friday, November 6–10, 2017; Pittsburgh, Pennsylvania
Session FT2: Plasma Boundaries and Sheaths |
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Chair: Scott Baalrud, University of Iowa Room: Duquesne |
Tuesday, November 7, 2017 1:30PM - 1:45PM |
FT2.00001: Modeling wall ion fluxes in an RF discharge: insights from 2D PIC simulation Romain Lucken, Trevor Lafleur, Vivien Croes, Antoine Tavant, Anne Bourdon, Pascal Chabert Global models of plasma discharges have been widely used to simulate plasma reactors in the fields of plasma processing and space propulsion [Chabert et al 2012, Grondein et al. 2016]. These models rely on accurate description of the ion current leaving the plasma: after undergoing a pre-sheath drop, the ions enter the sheath at Bohm velocity. The pre-sheath drop is characterized by an edge-to-center plasma density ratio $h_L$ and heuristic models were formerly derived to understand how this parameter varies with plasma temperatures and ion mean free path, based on one-dimensional (1D) transport theory [Chabert et al. 2011], and validated by 1D simulation [Lafleur et al. 2015]. A model of inductively coupled plasma (ICP) discharges was implemented into a 2D benchmarked particle-in-cell (PIC) code [Turner et al. 2012], running with various gases (Ar, He, Xe). These simulations show that the ion flux has a strong spatial dependency -- in agreement with former results [Lafleur et al. 2012] -- and that it is affected by the aspect ratio of the discharge reactor. The influence of dielectric walls is also investigated. [Preview Abstract] |
Tuesday, November 7, 2017 1:45PM - 2:00PM |
FT2.00002: Student Excellence Award Finalist: 3D ion and neutral distribution measurements and simulations of the boundary region of a magnetized plasma Derek S. Thompson, Shane Keniley, Rinat Khaziev, Davide Curreli, M. Umair Siddiqui, Miguel F. Henriquez, David D. Caron, Andrew J. Jemiolo, Jacob W. McLaughlin, MIkal T. Dufor, Luke A. Neal, Earl E. Scime We present the first paired 3D laser induced fluorescence measurements of ion and neutral velocity distribution functions (I/NVDFs) in a plasma boundary. These measurements are performed in the presheath region of an absorbing boundary immersed in a background magnetic field that is obliquely incident to the boundary surface ($\psi = 74^{\circ}$). Parallel and perpendicular flow measurements demonstrate that cross-field flows occur and that ions within several gyro-radii of the surface are accelerated in the $\vec{E} \times \vec{B}$ direction. We present electrostatic probe measurements of electron temperature, plasma density, and electric potential in the same region. Ion, neutral and electron measurements are compared to particle-in-cell and Boltzmann simulations, allowing direct comparison between measured and theoretical distribution functions for all three species. [Preview Abstract] |
Tuesday, November 7, 2017 2:00PM - 2:30PM |
FT2.00003: Hot Cathode Current Mode Transitions Invited Speaker: Michael Campanell Hot cathodes are a key component of many plasma physics experiments and applications. Examples include thermionic converters, thermionic tethers, emissive probes, neutralizers (e.g. in thrusters), and the Large Plasma Device. In the literature, it is often assumed that when the thermionic current is limited, the sheath is "space-charge limited" (SCL) and ions accelerate into the cathode [1]. In recent studies we showed that SCL sheaths cannot exist at floating surfaces because charge-exchange (CX) ion trapping in the virtual cathode forces a transition to a state with an inverse sheath [2]. In this talk, we show on theoretical grounds, and with continuum kinetic simulation videos, that stable SCL sheaths cannot exist at biased hot cathodes either. Whenever a virtual cathode first forms, CX ions will start collecting at the potential minimum until the ion density reaches the electron density at a point. Further ion collection makes the new neutral region grow from the cathode sheath towards the anode, leading to the creation of another plasma where ions are confined and both electrodes have inverse sheaths. The transitions from temperature-limited mode to anode glow mode seen in previous experiments [3,4] and PIC simulations [4] of thermionic discharges are consistent with this explanation. We conclude that the existence of operating modes with inverse cathode sheaths needs to be considered for other plasma applications that rely on hot cathodes. [1] S. Takamura et al., Contrib. Plasma Phys. \textbf{44}, 126 (2004). [2] M.D. Campanell and M.V. Umansky, Phys. Rev. Lett. 116, 085003 (2016) and Phys. Plasmas 24, 057101 (2017). [3] L. Malter, E. O. Johnson and W. M. Webster, RCA Review \textbf{12}, 415 (1951). [4] F. Greiner et al., Phys. Rev. Lett. \textbf{70}, 3071 (1993). [Preview Abstract] |
Tuesday, November 7, 2017 2:30PM - 2:45PM |
FT2.00004: Charge-exchange ions in a weakly collisional sheath Uwe Czarnetzki, Tsanko Tsankov The interaction of plasma ions with surfaces is determined by their velocity distribution function (IVDF). The IVDF is formed primarily in the sheath region in front of the surface. Here, an approximate approach is presented for a weakly collisional sheath that relies on an expansion of the ion velocity distribution function. The expansion is based on the smallness parameter sheath width to ion mean free path. With this approach the distribution function of the ions colliding in the sheath is estimated. This allows accurate prediction of various ion characteristics in the sheath (mean ion energy, effective ion temperature, ion mean velocity). Results for a floating sheath and a high-voltage sheath are presented. [Preview Abstract] |
Tuesday, November 7, 2017 2:45PM - 3:00PM |
FT2.00005: Transition in sheath structure near emissive grooved surface in discharge plasma controlled by electron beam. Irina Schweigert, T. S. Burton, G. B. Thompson, S. Langendorf, M. L. R. Walker, M. Keidar The plasma sheath characteristics, particularly the transition between different types of sheath near the grooved plate made from hBN was studied in the experiment and kinetic simulation. The discharge plasma is sustained by an electron beam from the emissive heated cathode. These beam electrons provide the secondary electrons emission from the grooved plate placed with some distance from the cathode. In the experiment, the critical voltage for `collapse of sheath' was measured for a planar surface and grooved surfaces for a 1 mm and 5 mm grooved depth. The 2d3V PIC MCC simulations were performed for the experimental conditions. The measured response of sheath structure near the grooved emissive surface on the change of discharge voltage shows an increase the critical voltage with grooved depth. In the 2d3V PIC MCC simulations, the collapse voltage was obtained with an increase of groove depth which is in good agreement with experimental data. It was shown in simulations that the increase of critical voltage for grooved surface is attributed to a variation of the potential distribution near the grooved surface and the redistribution of electron flux from the plasma to the plate. [Preview Abstract] |
Tuesday, November 7, 2017 3:00PM - 3:15PM |
FT2.00006: Stability of the plasma sheath in the presence of secondary electrons emission Roberto Martorelli, Vivien Croes, Romain Lucken, Antoine Tavant, Trevor Lafleur, Anne Bourdon, Pascal Chabert We propose a systematic analysis of the structural stability of the sheath in the presence of secondary electron emission induced by energetic plasma electron impacting the wall. The analysis is performed using the Sagdeev potential, in a similar fashion as in the study of nonlinear ion-acoustic waves. In this context, the study of the Poisson's equation for the sheath potential is reduced to an initial value problem. Setting the proper initial conditions, corresponding to the electric field and the electrostatic potential at the wall, different possible solutions for the sheath potential can be obtained, from Bohm-like to oscillatory ones. The main characteristics of the classical monotonic sheath can be reproduced as well through this approach. The inclusion of the emitted electrons provides additional degrees of freedom to the system, specifically the density and the energy of the emitted electrons. We show that a transition between different types of solutions might be induced by changing the value of the parameters of the system. In particular we are able to reproduce a transition to a non-monotonic sheath for critical values of the parameters, in a similar fashion as in the space charge limited regime. [Preview Abstract] |
Tuesday, November 7, 2017 3:15PM - 3:30PM |
FT2.00007: Spatiotemporal analysis of the electric field reversals in capacitively coupled SiH$_{\mathrm{4}}$/Ar RF discharge Wang Xi-Feng, Jia Wen-Zhu, Song Yuan-Hong, Dai Zhong-ling, Wang You-Nian A capacitively coupled RF SiH$_{\mathrm{4}}$/Ar discharge is investigated by a fluid/MC hybrid model, in which we focus our main attention on the influences of gas ratio, pressure and voltage amplitude on the electric field reversals. It is found that as a small proportion of SiH$_{\mathrm{4}}$ is added in Ar discharge, a weak reverse electric field is obtained near the collapse sheath edge, mainly due to the accumulation of electrons on account of drift and ambipolar diffusion. Results show that electrons might be heated by the reverse fields during the sheath contraction. However, these heated electrons are not sufficient for background gas ionization. As the SiH$_{\mathrm{4}}$ ratio increases, the electron field reversal is enhanced apparently and becomes the dominate electron heating method, contributing to high energy tails of electron energy probability functions (EEPFs) which would be responsible for significant ionization at sheath collapse phase. Further, the electric field reversal could be enhanced by increasing the pressure and voltage, leading to an enhanced heating compared with that in a pure Ar discharge. [Preview Abstract] |
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