Bulletin of the American Physical Society
65th Annual Gaseous Electronics Conference
Volume 57, Number 8
Monday–Friday, October 22–26, 2012; Austin, Texas
Session MW2: Negative Ion and Dust Particle Containing Plasmas |
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Chair: Ane Aanesland, LPP, Ecole Polytechnique/CNRS Room: Classroom 203 |
Wednesday, October 24, 2012 3:30PM - 4:00PM |
MW2.00001: Large and powerful rf-driven hydrogen plasmas: negative ions for the heating systems of ITER Invited Speaker: Ursel Fantz Negative ion sources are an excellent example for the manifold of applications of low temperature plasmas which ranges from basic research to industrial applications. One of the outstanding application areas is in fusion, where a large and powerful negative hydrogen ion source is as a central component of the neutral beam injection systems for heating and current drive of the international fusion project ITER. The challenge to extract an ion current of 57 A (D) and 69 A (H) from a low temperature hydrogen plasma at low pressure (0.3 Pa) is accompanied by the challenge to accelerate the beam to 1 MeV. Large RF sources with the size of a door operating at a power of up to 800 kW must deliver a uniform and stable negative hydrogen ion current density higher than 200 A/m$^2$ over the total area for one hour. Simultaneously, the amount of co-extracted electrons should be kept below one in order to avoid severe damages of the extraction system. These requirements can be met only by combining the disciplines of low temperature plasma physics, plasma surface interaction, ion beam optics, beam physics, and mechanical and electrical engineering. The state of the art and prospects of the negative hydrogen ion source development will be discussed with emphasis on the physical aspects. [Preview Abstract] |
Wednesday, October 24, 2012 4:00PM - 4:15PM |
MW2.00002: Nanoparticle-Plasma Interactions in Dusty Argon-Hydrogen Plasmas Uwe Kortshagen, Meenakshi Mamunuru We studied the role of hydrogen in altering the plasma-nanoparticle interactions in low pressure dusty Ar-H$_{2}$ plasma. Most dusty plasmas in which particles form through chemical nucleation, are multi-component plasmas containing hydrogen. As hydrogen's ionization potential is close to that of argon, both gases may be ionized. The presence of the light mass hydrogen ions has the potential to modify the plasma and plasma-nanoparticle interactions. We developed a global model for dusty argon-hydrogen plasma. For given absorbed power, nanoparticle density, pressure, and chamber size, we solved the power balance, plasma species balance, and particle current balance equations. We included a system of rate equations for important argon-hydrogen plasma chemical reactions and obtained electron energy distribution function (EEDF) using ZDPlasKin. A trace amount of H$_{2}$ gas in Ar discharge causes Ar$^{+}$, ArH$^{+}$, and H$_{3}^{+}$ to be the dominant ions. Their relative densities are dependent on chamber pressure, gas composition, and the nanoparticle density. Increase in H$_{2}$ gas fraction reduces the plasma density. The presence of light ions reduces the average particle charge. Electron collisions with hydrogen and with the nanoparticles affect the EEDF shape. Overall, we find that the presence of H$_{2}$ in the discharge significantly alters the plasma properties and the fundamental plasma-nanoparticle interactions. This work was supported by the US Dept. of Energy Plasma Science Center and~DOE grant DE/SC-0002391. [Preview Abstract] |
Wednesday, October 24, 2012 4:15PM - 4:30PM |
MW2.00003: Plasma Crystallization of Silicon Nanoparticles Rebecca Anthony, Nicolaas Kramer, Eray Aydil, Uwe Kortshagen Using nonthermal plasmas for synthesis of silicon nanocrystals is well-established. However, nanoparticle heating in the plasma, which leads to particle crystallinity, is poorly understood. The mechanism behind heating of these particles has only been studied through modeling. In-situ measurement of particle temperature during plasma processes is difficult, but particles themselves can serve as thermometers, as their crystallinity will change depending on heating in the plasma. Here we investigate the heating and crystallization of nanoparticles using a double-plasma configuration, examining both the particles and the plasma. Amorphous silicon nanoparticles are formed in a low-power plasma, then injected into a separate plasma which is operated with variable power. Nanoparticle characterization confirms that crystallization of the particles occurs at a threshold power to the secondary plasma, around 30W (nominal) for 5nm particles. Optical emission spectroscopy on the plasma provides estimates of the electron temperature during nanoparticle crystallization, and capacitive probe measurements reveal ion densities at varying plasma powers. We will compare our outcomes to previous modeling results to build a complete picture of nanoparticle heating in plasmas. [Preview Abstract] |
Wednesday, October 24, 2012 4:30PM - 4:45PM |
MW2.00004: Numerical Modeling of an RF Argon-Silane Plasma with Dust Particle Nucleation and Growth Steven Girshick, Pulkit Agarwal We have developed a 1-D numerical model of an RF argon-silane plasma in which dust particles nucleate and grow. This model self-consistently couples a plasma module, a chemistry module and an aerosol module. The plasma module solves population balance equations for electrons and ions, the electron energy equation under the assumption of a Maxwellian velocity distribution, and Poisson's equation for the electric field. The chemistry module treats silane dissociation and reactions of silicon hydrides containing up to two silicon atoms. The aerosol module uses a sectional method to model particle size and charge distributions. The nucleation rate is equated to the rates of formation of anions containing two Si atoms, and a heterogeneous reaction model is used to model particle surface growth. Aerosol effects considered include particle charging, coagulation, and particle transport by neutral drag, ion drag, electric force, gravity and Brownian diffusion. Simulation results are shown for the case of a 13.56 MHz plasma at a pressure of 13 Pa and applied RF voltage of 100 V (amplitude), with flow through a showerhead electrode. These results show the strong coupling between the plasma and the spatiotemporal evolution of the nanoparticle cloud. [Preview Abstract] |
Wednesday, October 24, 2012 4:45PM - 5:00PM |
MW2.00005: Soliton Reflection in a Magnetized Cold Plasma having Dust Grains and Trapped Electrons Hitendra K. Malik, Omveer Singh, Raj P. Dahiya A solitary wave is said to be a soliton if it retains its shape after collision with another solitary wave. The solitons get reflected from a boundary or the density gradient present in the plasma. In the present work, the reflection of a soliton is studied in a magnetized cold plasma having dust grains and trapped electrons. Considering the density inhomogeneity in the plasma, we derive relevant modified Korteweg-deVries (mKdV) equations for the right and left going solitary waves and then after coupling these equations at the point of reflection we solve the coupled equation for obtaining the expression for the reflection coefficient based on which the soliton reflection is examined under the effect of magnetic field, dust grain density, and the temperature of trapped electrons. Specifically the role of trapped electrons and dust grains is uncovered for the excitation of solitary waves and their reflection. [Preview Abstract] |
Wednesday, October 24, 2012 5:00PM - 5:15PM |
MW2.00006: Effects of impurities on negative ion mobility in O$_{2}$ Yui Okuyama, Susumu Suzuki, Haruo Itoh We have investigated the effects of impurities on the negative ion mobility in O$_{2}$ at atmospheric pressure using a high-pressure ion drift tube with a positive point plate gap that acts as a negative ion detector. We reported a reduced mobility, in particular ``zero field mobility'' of 2.31 cm$^{2}$/V$\cdot$s for O$_{2}^{-}$ in O$_{2}$.\footnote{Y. Okuyama et al.: J. Phys. D: Appl. Phys., 45, 195202 (2012).} This value is in good agreement with values in other reports of 1.95 to 2.42 cm$^{2}$/V$\cdot$s. Although many studies have been carried on the measurement of negative ion mobility over the last 50 years, discrepancies between the values obtained remain and the origin of the discrepancies has not been clarified until now. We found that one of the reasons for the discrepancies originates from impurities in O$_{2}$ that are considered to be released from the surface of the chamber as an absorbed gas (N$_{2}$ or CO$_{2})$ or to already exist in the O$_{2}$. These impurities form negative ions such as CO$_{3}^{-}$, CO$_{4}^{-}$, NO$_{3}^{-}$ and N$_{2}$O$_{2}^{-}$ with the O$_{2}$ in the chamber. The mobilities of these ions are slightly larger than that of O$_{2}^{-}$. Therefore, if small amounts of impurities such as N$_{2}$ and CO$_{2}$ exist in O$_{2}$, an increased negative ion mobility is observed at $E$/$N >$ 2.54$\times $10$^{-1}$ Td. Moreover, the negative ion mobility of O$_{2}^{-}$ was also measured in high-purity O$_{2 }$(99.9999{\%}) and ultrahigh-purity O$_{2}$(99.99995{\%}) while employing a gas filter that can reduce the water content to less than 100 ppt. As a result, the mobility of O$_{2}^{-}$ was increased to 2.39 cm$^{2}$/V$\cdot$s. This value is close to the values reported by Dutton and Howells (J. Phys. B, 1, 1160, '68), Rees (Aust. J. Phys., 18, 41, '65) and Voshall et al. (J. Chem. Phys., 43, 1190, '65). [Preview Abstract] |
Wednesday, October 24, 2012 5:15PM - 5:30PM |
MW2.00007: STUDENT AWARD FINALIST: Measurements of Positive and Negative Energy Distribution Function obtained from a Langmuir probe in an ion-ion plasma Jerome Bredin, Pascal Chabert, Ane Aanesland An ion-ion plasma, created downstream of a magnetic barrier, has been studied with a Langmuir probe. In classical electron-ion plasmas the second derivative of the I-V characteristic below the plasma potential can be used to deduce the electron energy distribution function (EEDF). In nearly electron-free ion-ion plasmas, we propose to use the second derivative to deduce both the electrons and the ion (positive and negative) EDFs. Below the plasma potential, the second derivative involves two distributions; the negative ions at low energy and the electrons, which are in very small quantity, at high energy. Above the plasma potential, the second derivative analysis leads to the positive ion distribution. The exact procedure to analyze the data will be detailed during the presentation. We found that downstream of the magnetic barrier, where the ion-ion plasma forms, ion temperatures are fairly high, from 0.5~eV to 0.1~eV. The temperature of the positive ions is slightly higher than that of the negative ions. The ion densities can also be deduced from the I-V characteristic and they are in the order of $10^{17}$~m$^{-3}$, that is almost three order of magnitude higher than the electron density in this region of the plasma. [Preview Abstract] |
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