Bulletin of the American Physical Society
65th Annual Gaseous Electronics Conference
Volume 57, Number 8
Monday–Friday, October 22–26, 2012; Austin, Texas
Session LW2: Basic Plasma Physics Phenomena in Low-Temperature Plasmas |
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Chair: Yevgeny Raitses, Princeton Plasma Physics Laboratory Room: Classroom 203 |
Wednesday, October 24, 2012 1:30PM - 1:45PM |
LW2.00001: STUDENT AWARD FINALIST: Control of Electron Energy Distributions Through Interaction of Electron Beams and the Bulk in Capacitively Coupled Plasmas Sang-Heon Song, Mark J. Kushner The control of electron energy distributions, $f(\varepsilon)$, in capacitively coupled plasmas is necessary to optimize the fluxes of reactive species to the substrate. Beams of electrons $>$100s eV are produced by secondary electron emission and acceleration through the sheaths. These beams occur from the rf biasing or can be augmented with an additional dc bias. Although the beam electrons mostly collide with the gas, collisions with bulk electrons also occur. Previous work investigated beam-Langmuir wave interactions that may transfer energy to the bulk electrons [1]. In this paper, we report on a computational investigation of the purely kinetic interaction between the beam and bulk through electron-electron (e-e) collisions, and the consequences on $f(\varepsilon)$. A CCP with and without dc augmentation was investigated with a 2-d plasma hydrodynamics model including an electron Monte Carlo simulation with e-e collisions. Secondary electrons are produced by ion and electron impact on surfaces. The beam electrons collide with low energy bulk electrons, delivering energy to the bulk and depleting the beam, and can shape the $f(\varepsilon)$ in ways not otherwise attainable in self-sustained rf equilibrium plasmas. We will discuss shaping of $f(\varepsilon)$ and changes in plasma properties through the beam-bulk interactions, and use of a dc bias to control $f(\varepsilon)$. \\[4pt] [1] L. Xu et al., Appl. Phys. Lett. \textbf{93}, 261052 (2008). [Preview Abstract] |
Wednesday, October 24, 2012 1:45PM - 2:00PM |
LW2.00002: The Maxwell Demon and its instabilities Chi-Shung Yip, J.P. Sheehan, Umair Suddiqui, Noah Hershkowitz, Greg Severn Previous experiments have shown that in a low pressure, low temperature plasma, positively biasing an array of thin wires can increase electron temperature. This works, it is thought, by creating an angular momentum trap to absorb cold electrons. In this experiment, such a Maxwell demon device was reproduced by welding 0.025mm tungsten wires onto stainless steel shafts, which were coated with a ceramic coating. However, we found that the effect of such device is identical to a plate with the same total surface area. These devices was used to more than double the plasma electron temperatures in a multi-dipole chamber operating in the mTorr regime. Moreover, the demon is observed to reduce the cold electron population in a plasma with a bi-Maxwellian electron distribution, leaving a single Maxwellian electron distribution. In addition, at high positive voltage, relaxation instabilities in the kHz range occured, as MacKenzie et al. had observed. The instability was determined to be pulsing anode spots, and the measurement of time-resolved plasma parameters in this instability was achieved by using a slow sweeping Langmuir probe. Relaxation time of the instability was modeled by a production-lost balanced method. [Preview Abstract] |
Wednesday, October 24, 2012 2:00PM - 2:15PM |
LW2.00003: 2D fluid simulations of acoustic waves in pulsed ICP discharges: Comparison with experiments Emilie Despiau-Pujo, Gilles Cunge, Nader Sadeghi, N. St. J. Braithwaite Neutral depletion, which is mostly caused by gas heating under typical material processing conditions, is an important phenomenon in high-density plasmas. In low pressure pulsed discharges, experiments show that additional depletion due to electron pressure (Pe) may have a non-negligible influence on radical transport [1]. To evaluate this effect, comparisons between 2D fluid simulations and measurements of gas convection in Ar/Cl2 pulsed ICP plasmas are reported. In the afterglow, Pe drops rapidly by electron cooling which generates a neutral pressure gradient between the plasma bulk and the reactor walls. This in turn forces the cold surrounding gas to move rapidly towards the center, thus launching an acoustic wave in the reactor. Time-resolved measurements of atoms drift velocity and gas temperature by LIF and LAS in the early afterglow are consistent with gas drifting at acoustic wave velocity followed by rapid gas cooling. Similar results are predicted by the model. The ion flux at the reactor walls is also shown to oscillate in phase with the acoustic wave due to ion-neutral friction forces. Finally, during plasma ignition, experiments show opposite phenomena when Pe rises.\\[4pt] [1] Cunge et al, APL 96, 131501 (2010) [Preview Abstract] |
Wednesday, October 24, 2012 2:15PM - 2:30PM |
LW2.00004: Head-on Collision of Two Solitary Waves in a Plasma Ravinder Kumar, Ajay K. Singh, Omveer Singh, Hitendra K. Malik, Raj P. Dahiya Solitary waves have been very fascinating due to their applications in various fields of science and engineering. A solitary wave when retains its shaper after having collision with another solitary wave is called a soliton. The soliton structure can trap particles and convect them over large distances in the laboratory, astrophysical and space related plasmas. Therefore, they can contribute to the transportation of anomalous particles and the energy from one region to another. The aim of the present work is to investigate the head-on collision of two solitary waves in a multi-component plasma having ions, two types of electrons and dust grains under the effect of magnetic field and charge fluctuation of the dust grains. Using extended Poincare-Lighthill-Kuo (PLK) method, we derive a coupled equation that carries the contribution of oppositely propagating solitary waves and their phase relationship. By solving this coupled equation, we obtain the trajectory of both the solitary waves and finally calculate the phase shift taken place during their head-on collision. [Preview Abstract] |
Wednesday, October 24, 2012 2:30PM - 2:45PM |
LW2.00005: Oscillations of current in dc discharge induced by an auxiliary electrode Irina Schweigert, Vladimir Demidov, Igor Kaganovich The plasma parameters of dc discharge with thermionic emission cathode can be actively controlled by addition of the second anode made as a diaphragm with a hole in the center (see for example, [1]). This control is based on fundamental property of nonlocal electron kinetics and can be beneficial for variety of applications. We simulated parameters of this dc discharge plasma making use of two-dimensional PIC MCC code. The geometry of device and plasma parameters are taken from Ref. [1]. The dc discharge operates in helium at gas pressure 0.5 - 4 Torr and is supported by the thermionic emission of electrons from cathode. The inter-electrode distance is 1.1 cm and the diaphragm is placed at distance 0.1 - 0.15 cm from anode. The discharge current ranges from 0.05A to 0.3A. The plasma characteristics, potential, electron and ion density profiles and EEDF are studied for different current densities, gas pressures and radius of diaphragm hole. The presence of the diaphragm leads to increased ionization only for a certain range of gas pressures and the radii of diaphragm hole. An increase of a size of diaphragm hole leads to development of current oscillations. \\[4pt] [1] V. I. Demidov, C. A. DeJoseph, Jr., V. Ya. Simonov, Appl. Phys. Lett. 91 (2007) 201503. [Preview Abstract] |
Wednesday, October 24, 2012 2:45PM - 3:00PM |
LW2.00006: Excitation of Ion Acoustic Waves by Electron Beams I.D. Kaganovich, D. Sydorenko, E. Tokluoglu, E.A. Startsev, A.V. Khrabrov, L. Chen, P. Ventzek, R. Sundararajan, A. Ranjan, K. Kumar The interaction of an electron beam with plasma is of particular importance for hybrid DC/RF coupled plasma sources used in plasma processing. A high frequency (HF) electron plasma wave resonant with the high-energy beam may decay into another HF wave and an ion acoustic wave. The new HF wave may have lower phase speed than the original HF wave. Electron acceleration by the slower HF wave may explain the low-energy peak in the electron energy distribution function measured in plasma processing devices [1]. In the present paper, the collisionless electron heating in a hybrid RF-DC plasma source is studied using the particle-in-cell code EDIPIC [2,3]. In simulation, electrons emitted from the cathode surface are accelerated through a dc bias electric field and form an 800 eV electron beam entering the bulk plasma. The beam excites electron plasma waves through the two-stream instability. High localized plasmon pressure creates ion acoustic waves in the process similar to the modulation instability. Eventually, coupling between electron plasma waves and ion acoustic waves deteriorates HF oscillations, which leads to bursting behavior.\\[4pt] [1] L. Chen, Proceedings of ICRP 2010\\[0pt] [2] D. Sydorenko, Phys. Plasmas, 14, 013508 (2007)\\[0pt] [3] D. Sydorenko, Phys Rev Lett., 103, 145004 (2009) [Preview Abstract] |
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