Bulletin of the American Physical Society
2006 59th Annual Gaseous Electronics Conference
Tuesday–Friday, October 10–13, 2006; Columbus, Ohio
Session FPT2: Poster IB |
Hide Abstracts |
Room: Holiday Inn Buckeye, 7:15pm - 9:15pm |
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FPT2.00001: PLASMA CHEMISTRY AND DIAGNOSTICS |
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FPT2.00002: Performance Model of the Hairpin Microwave Resonator and Experimental Results L.K. Warne, W.A. Johnson, R.S. Coats, R.E. Jorgenson, G.A. Hebner, A.M. Paterson, J.P. Holland Microwave hairpin resonator structures are finding increased applications in a range of plasma systems. This poster presents circuit models for a hairpin resonator probe used for measuring electron density. A transmission line model is used along the resonator length and lumped loads are used to capture the parasitic capacitance at the open end and the parasitic inductance at the drive or shorted end along with a lumped radiation resistance. The impact of a plasma sheath surrounding the resonator wires as well as the finite conductivity of the wires are addressed. Electromagnetic simulations using a frequency domain method of moments code are also included as comparisons to the circuit model results. Finally, the impact of the external cavity formed by the electrodes is ascertained. The model calculations will be compared with experimental measurements to demonstrate these effects. This work was supported by the Division of Material Sciences, BES, Office of Science, U. S. Department of Energy, Applied Materials, and Sandia National Laboratories, a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
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FPT2.00003: Sheath and collisional corrections in microwave hairpin resonators Nicholas Siefert, Brian Sands, Biswa Ganguly We report the effect of electron-neutral collisions on the quality factor $Q$ of hairpin resonators operating near 1 Torr. From the changes in $Q$, it is possible to determine change in the product of the electron-neutral collision frequency and the electron number density. The electron number density can be determined from the shift of the resonant frequency, so it may be possible to determine changes in the electron-neutral collision frequency. We also discuss effects of hairpin geometry and the sheath on electron number density measurements when considering a truncated transmission line model of the hairpin. Here, the location along the hairpin and the diameter of the wire are important for hairpin sensitivity and in determining the appropriate correction factor for the sheath. Additionally, we consider the collisionless sheath assumed by previous groups (e.g. Piejak et al. [\textit{J. Appl. Phys., }\textbf{95}, 3785 (2004)]) and its viability at higher pressures. We also report measurements of the steady state electron number density using four different hairpins, with resonant frequencies between 2 GHz and 4 GHz, in order to demonstrate the reproducibility of the density measurements and we compare these measurements with numerical calculations. [Preview Abstract] |
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FPT2.00004: Looking for information in the Q of the hairpin probe Nicholas Braithwaite, Eva Vasekova, Jafar Al-Kuzee A microwave resonator hairpin probe is used to measure the electron density of low pressure low temperature plasma. The probe acts as a quarter wave transmission line that is resonant at a frequency determined by the dielectric properties of the surrounding medium. The quality factor $Q$ of the hairpin resonance has been studied as a function of the pressure of the discharge gas and the power coupled to the discharge, for a number of gasses. The $Q$ values were inferred from the Gaussian fitting of the recorded resonant curves of the probe. The theoretical values of the $Q$ were calculated as a ratio of the energy stored in the cavity and the energy dissipated per one period. Since the dissipated energy depends on the collisions between electrons and the background gas, the quality factor tends to decrease with the gas number density, hence the pressure, as well as the electron number density. The measured values of $Q$ were compared with the theoretical values and found to be in good agreement. [Preview Abstract] |
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FPT2.00005: Charged Particle Dynamics in a Dual-Frequency Capacitively Coupled Fluorocarbon Plasma Dragana Mari\'c, Garrett Curley, Jean-Paul Booth, Pascal Chabert, Jean Guillon We are studying a customized 2 + 27.12~MHz industrial etch reactor, running in Ar/O$_2$ with c-C$_4$F$_8$ or CF$_4$ gas mixtures at pressures in the region of 50~mTorr. Independent control of ion flux and ion energy is an advantage of DFC plasmas, but little experimental data exists regarding the charged particle dynamics in complex industrial gas mixtures. Negative ions could play an important role in this type of plasma. The presence of negative ions will modify the positive ion flux arriving at a surface, and may even reach the surface and participate in etching. We measure the electron density using a microwave hairpin resonator and the positive ion flux with an ion flux probe: the ratio of these two quantities varies strongly with gas chemistry and gives evidence for the presence of negative ions. We have measured high electronegativity for high c-C$_4$F$_8$ flowrates. We have also examined the effect of varying the 2~MHz and 27.12~MHz powers on both the electron density and positive ion flux. This allows us to estimate the effect of varying power on the negative ion density. In addition CRDS was used to measure the F$^-$ density[1]. This optical measurement will be compared to the probe technique. \newline [1] Booth et al, Appl. Phys. Lett. 88 (2006) 151502 [Preview Abstract] |
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FPT2.00006: Characteristics of Spatial Electron Density Variation in a Pulsed-Rf and Bi-polar Pulsed-DC Magnetron Discharge S.K. Karkari, A.R. Ellingboe, C. Gaman, I. Swindells, J.W. Bradley Magnetron discharges are highly popular in reactive sputtering of metals and insulators for depositing value-added coatings onto a wide variety of substrates. A large problem associated with DC sputtering is due to deposition of insulating films on to the target itself; resulting in frequent arcing and de-stabilization of the plasma process. Arcing is elevated by modulating the magnetron target voltage at pulse frequencies between 20 kHz to 350 kHz in the range of 50{\%} to 90{\%} duty cycle. Time-modulation can be achieved either by Bi-polar Pulsed DC or by Pulsed rf voltages applied to the sputtering target. We apply a time-resolved floating hairpin resonance probe, to characterize the spatial electron density as the discharge progresses during the pulsed cycle. With the Bi-polar pulsed biasing, we observe an expanding pre-sheath during the rise of the on-phase (negative voltage) and anomalous growth in density during the reverse phase (positive voltage) at specific positions in the discharge. This is contrasting to the pulsed-rf case, where we observe the electron density in the off-phase decays uniformly at all positions, with faster decay rates closer to the target. The electron density perturbations in the case of Bi-polar pulsed DC are explained using the expanding sheath theory and the state of the plasma potential modulation in the discharge. [Preview Abstract] |
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FPT2.00007: Effect of microwave field on the Langmuir probe characteristics Akihiro Kono, Mitsutoshi Aramaki In measurements of electron energy distribution near the dielectric plate of surface-wave excited plasma, a discrepancy has been found between Thomson-scattering results and Langmuir-probe results. The reason might be due to possible distortion of the probe characteristics caused by the microwave field, but the phenomenon is not well understood yet. In this study, the effect of microwave field on the probe characteristics is investigated computationally using a fluid model. The effect of oscillating microwave filed was modeled as an oscillating probe bias potential $V_{0}+V_{1}$sin$\omega t$ against field-free plasma. Electron and ion continuity, electron and ion momentum balance, and Poisson's equations are solved to obtain the current to a cylindrical probe as a function of the DC probe bias potential $V_{0}$. The results indicate that the probe V-I characteristic is distorted and the second derivative $d^{2}I$/\textit{dV}$^{2}$ shows a hump as if the population of electrons having energies around the hump is increased. The obtained $d^{2}I$/\textit{dV}$^{2}$ curves resemble some of measured probe characteristics. [Preview Abstract] |
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FPT2.00008: Electrical and Optical Characterization of a pulsed plasma of N$_{2}$-H$_{2}$ Horacio Martinez, Farook Bashir Yousif This paper considers the electrical and optical characterization of glow discharge pulsed plasma in N$_{2}$/H$_{2}$ mixture at pressures of 0.5-4.0 Torr and discharge current between 0.2 and 0.6 A. The discharge current and the applied voltage are measured using conventional techniques. The emission from the pulsed plasma of a steady-state electric discharge in a N$_{2}$/H$_{2}$ mixture in the wavelength range 200-600 nm is investigated. It is shown that, at a range pressure of 0.5 to 4.0 Torr, the discharge mainly emits within the wavelength range 280-500 nm. The emission consists of N$_{2}$(C-X) 316 nm, 336 nm, 358 nm narrow peaks and a broad band with a maximum at $\lambda _{max}$= 427 nm. Also bands of N$_{2, }$N$_{2}^{+}$ and NH excited states were observed. All bands have their maximum intensity at the discharge current of 0.4 A. The intensities of the main bands are determined as functions of the pressure and discharge current. [Preview Abstract] |
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FPT2.00009: Measurement of ion density and electron temperature in Hanbit magnetic mirror device by using RF compensation triple probe Ikjin Choi, Kiho Hwang, Chinwook Chung, Sanggon Lee There are several operating diagnostic tools placed at Hanbit magnetic mirror plasma, such as Langmuir probe, magnetic probe, diamagnetic loop etc. Because of Langmuir probes are relatively easy to make and probe tips can be formed with many designs, it is used at Hanbit plasma device to measure electron temperature, density, and distribution of electron at Hanbit plasma in a ways of axiality and azimuth. The RF compensation triple probe was invented with an idea which is RF compensation technique used in single Langmuir probe. In case of DC, non-compensated triple probe is floated outside with high impedance, but in case of RF, it is connected with low impedance, non-floating. However, RF compensation triple probe is floated with high impedance which can measure plasma parameters with less distortion in case of RF. During checking plasma variables using RF compensation triple probe at Hanbit magnetic mirror plasma in axial way, many characteristics were observed which cannot observe with existing probes. [Preview Abstract] |
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FPT2.00010: Micro particles of different sizes as electrostatic probes in rf-plasma Ralf Basner, Gabriele Thieme, Florian Sigeneger, Holger Kersten, Gerald Schubert, Holger Fehske The plasma is bounded to the surrounding surfaces by the self-organizing structure of a sheath. In plasma diagnostics a relatively large uncertainty exists for the determination of the structure of these plasma sheaths. When a dust particle is immersed in plasma, it is charged by collecting electrons and ions. Negatively charged micro-particles can be confined in a horizontal plasma sheath. The equilibrium position is defined by the balance of the forces acting on the particles. Commonly, the electrostatic and the gravitational force are important in laboratory complex plasmas. Then the equilibrium position above a lower electrode depends mainly on the particle charge, the electric field, and the particle mass. The levitated particles react sensitively with their position to changes in the plasma sheath thus they may serve as electrostatic probes for the electric field. This approach has been successfully demonstrated in front of the powered electrode. In contrast, we present preliminary measurements on the behaviour of charged dust grains in front of the grounded or biased but not powered electrode. These situations are of interest in plasma technology for treatment of substrate surfaces. [Preview Abstract] |
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FPT2.00011: Real-time, Noninvasive Monitoring of Ion Energy at Insulating Electrodes Mark Sobolewski The dc self bias voltage is often monitored during plasma processing to provide a rough estimate of ion bombardment energies. However, many plasma reactors use electrostatic chucks, which have a large dc impedance that makes dc bias measurements impossible. A chuck may also have a large rf impedance that produces a significant rf voltage drop across the chuck. In this study chuck impedance effects were investigated in an inductively coupled plasma reactor by incorporating insulating structures into the rf-biased lower electrode. Measurements were made to characterize the capacitive impedance of the insulating electrode itself and the combined impedance of the electrode plus the wafer. This impedance was included in a numerical model of the plasma and its sheaths and the combined model was used to analyze measured rf bias current and voltage waveforms. This approach allows a real-time, noninvasive monitoring technique developed for bare metallic electrodes to be extended to insulating electrodes, including electrostatic chucks. The technique not only determines the dc self bias voltage but also the total ion current and ion energy distributions at the wafer or chuck surface. [Preview Abstract] |
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FPT2.00012: Real time multi-dimensional control in a two-frequency, confined plasma etcher Vladimir Milosavljevic, Andrey M. Islyaikin, Cezar Gaman, Albert R. Ellingboe Multi-dimensional control of plasma processes is of interest for reducing excursions in semiconductor manufacturing. Key parameters for plasma etching of dielectric films include ion-flux and gas-density of oxygen containing species in the plasma. The ion-flux is measured by with an isolated collection area built into the electrode surface, biased to -18Volts. Density of chemical species are measured using Mass-spectrometry or OES/Actinometry. The response-surface of the sensors in the process-space was collected over the process space. Sensor signals are not orthogonal, and do not directly map onto the input control parameters. The sensor data is compared to external setpoints for ion flux and chemical density. The functional dependencies of the response-surface, in conjunction with simple physical models, are used to deconvolute the sensor signals onto the control parameters. In this paper we demonstrate real-time control of ion-electrode-flux independent of plasma chemistry (O, O$_{2}$, CO, or CO$_{2})$ in a modified Exelan chamber (Lam Research). The control is stable to external perturbations to the operating point of the chamber. [Preview Abstract] |
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FPT2.00013: O atoms loss coefficient on porous SiO2 and TiO2 measured by plasma induced fluorescence Katia Allegraud, Lina Gatilova, Olivier Guaitella, Jean Guillon, Antoine Rousseau The time evolution of O atoms density in the gas phase during the post-discharge of a pulsed plasma is studied using a plasma induced fluorescence technique (PIF): a main long pulse creates the plasma and a shorter one re-excites atoms in the time post-discharge was used. The gas pressure is 133 Pa in N2/O2 mixture and the plasma is a pulsed DC discharge. The surface loss coefficient of O atoms on pyrex, porous silica, porous TiO2 is measured, this latter being a photocatalytic material. It is shown that the presence of porous silica or TiO2 leads to a stong increase of the O atom surface loss coefficient. When nano cluster of TiO2 are deposited on porous silica, the loss coefficient is first high and comparable to the case of the porous silica, but decreases after few milliseconds. Such a decrease of the surface loss coefficient has recently been reported in a pulsed microwave discharge [1]. The effect of the pre-irradiation of the porous materials by external ultraviolet is also studied. \newline \newline [1] G. Cartry, X. Duten and A. Rousseau Plasma Sources Sci. Technol. 15 (2006) 479--488 [Preview Abstract] |
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FPT2.00014: Production of carbonaceous particulates by interaction between a graphite plate and high-density hydrogen plasmas K. Sasaki, T. Maeda, N. Takada, M. Aramaki, K. Shibagaki This work is a simulation experiment for the divertor region of a nuclear fusion reactor. A graphite plate was irradiated by high-density hydrogen plasmas produced by helicon-wave discharges. The helicon source was a linear machine with uniform magnetic field, by which the high-density plasma column was confined radially. It was observed by the naked eye that the surface of the graphite plate was changed by the irradiation of the plasma column. The close observation using a secondary electron microscope indicated the formation of many particulates with diameters less than 10 $\mu$m. The diameter and the density of particulates were dependent on the radial position. The sources for the formation of particulates may be hydrocarbon and carbonic radicals produced by the interaction between the high- density H$_2$ plasma and the graphite plate. By considering the setting of the helicon source, it is speculated that the growth of particulates occurs on the surface of the graphite plate. We adopted laser-desorption time-of-flight mass spectrometry for analyzing carbonaceous particulates. As a result, we found that H atoms were included in carbonaceous particulates, which is a critical problem from the viewpoint of safety hazards such as tritium inventory in D-T nuclear fusion reactors. [Preview Abstract] |
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FPT2.00015: Effect of electron detachment on the wall potential in the afterglow of an oxygen discharge E.A. Bogdanov, A.A. Kudryavtsev, V.I. Demidov, C.A. DeJoseph, Jr. Using a detailed, self-consistent model of a low pressure O$_{2}$ discharge, it is shown that detachment of electrons in the afterglow can lead to a significant increase in the negative wall potential with respect to the plasma potential. This effect can be used to modify the near-wall sheath electric field and thickness, which may be important for some plasma processing applications. In addition, the effect can lead to an increase in electron density with time, and a reduction in diffusion cooling of electrons and can thus be used to modify the electron temperature. The simulation was performed on a 100{\%} modulated rf discharge operating at 13.56 MHz, using a commercial software package (www.cfdrc.com) with modifications. The density and mean energy of the electron component is obtained by solving either the fluid balance equations or the kinetic equations for the EEDF. The self-consistent electric field is found from Poisson's equation. Heavy particles are described using the fluid model and includes 160 plasmachemical reactions. [Preview Abstract] |
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FPT2.00016: Plasma-photocatalysis combination for air pollutant removal: identification of the synergy mechanisms O. Guaitella, F. Thevenet, A. Rousseau, C. Guillard, G. Stancu, J. Roepcke The coupling of a photocatalyst with a non thermal plasma (DBD) is studied; based on experimental results we discuss separately the contributions of (i) the chemistry involved as a function of the porosity of the material, and (ii) the influence of the photocatalytic activity on the chemistry of C2H2 oxidation. C2H2 removal is strongly increased by the presence of a porous material (SiO2 or TiO2): the destruction of C2H2 is driven by species created by the plasma and concentrated by a porous [1]. Our experiments confirm that C2H2 removal rate increases with the porosity of the material, whereas the selectivity also depends on the chemical composition of the surface. In parallel, the temporal evolution of C2H2 concentration was measured by Tuneable Diode Laser Absorption Spectroscopy (TDLAS) in the mid infrared region in a low pressure discharge during a single plasma pulse (one shot). The contribution of external ultraviolet radiation and plasma exposure were quantified, both with and without photocatalyst. The synergetic effect was clearly demonstrated [2]. \newline [1] U. Roland, F. Holzer, F.-D. Kopinke 2002 Catalysis Today 73 315--323 [2] A. Rousseau, O. Guaitella, L.V. Gatilova, F. Thevenet, C. Guillard, J. Roepcke, G. D. Stancu , Appl. Phys. Let. 87, 221501 (2005). [Preview Abstract] |
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FPT2.00017: Kinetic Modeling of Electron Loss Kinetics in high pressure O$_{2}$/N$_{2}$ Electron Beam Sustained Plasmas Kraig Frederickson, Walter Lempert Previous work has demonstrated that the effective rate of electron loss by three-body attachment in O$_{2}$ containing plasmas is significantly decreased in vibrationally excited nonequilibrium environments, an effect which has been attributed to an increased rate for the electron detachment pathway. Specifically it is postulated that the rate of detachment is greatly enhanced by the presence of vibrationally excited ``rapid detacher'' species, since in a collision between O$_{2}^{-}$ and a molecule possessing two or more quanta of vibrational energy the low electron affinity of O$_{2}$ ($\sim $0.43eV) is exceeded. Assuming that the detachment cross section for such collisions is approximately gas kinetic, modeling indicates that the calculated effective electron detachment rate is nearly equal to or in excess of the experimentally determined net electron attachment rate. Analysis of the vibrational level populations indicates that in the nonequilibrium environment created in this study there is sufficient population in the vibrational levels responsible for detachment to account for the observed 5-6 order of magnitude increase in the effective detachment rate. [Preview Abstract] |
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FPT2.00018: \textit{in-situ} FTIR characterization of the plasma chemistry as functions of the plasma duty cycle and peak power in a 1,3-butadiene discharge in an inductively coupled Gaseous Electronics Conference (GEC) Cell. Matthew Goeckner, Ashish Jindal, Lawrence Overzet Time averaged \textit{in-situ} Fourier Transform Infrared Spectroscopy is used to characterize the plasma chemistry of pulsed 1,3 Butadiene (H$_{2}$C=CHCH=CH$_{2})$ discharges as functions of both the plasma duty cycle and on phase power in a GEC Cell. Various ratios of plasma on to off times for equivalent duty cycles are investigated at peak powers of 40, 50, 60, and 70 W. Variations in densities associated with the major observed spectral bands are examined and the possible dissociation mechanisms responsible for all observed vibrations are investigated. For example, the data shows that free CH$_{2}$ stretching vibrations increase in a sub-linear fashion with increasing duty cycle. Approximately 44{\%} of the CH$_{2}$ density is due to free daughter species at the largest (90{\%}) duty cycle. This indicates that reaction kinetics are changing from cleavage of primarily the $\pi $ bond of the C=C bond at lower duty cycles to cleavage of both $\pi $ and $\sigma $ (complete dissociation) at duty cycles approaching continuous wave biasing. This data will be tied to film growth. [Preview Abstract] |
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FPT2.00019: Asymptotic Expressions for the Parameters Describing Low Pressure Electronegative Plasmas Raoul Franklin Experimentalists working in plasma processing need relatively simple expressions characterizing their plasmas. To that end we have revisited the Tables given by Franklin and Snell (1992) to obtain asymptotic i.e. algebraic expressions for the values of potential, ion speed and densities at the `plasma edge' and the eigenvalue in terms of the parameters $\varepsilon $ = Te/Tn and $\alpha $ = nn0/ne0, for the case where a single negative ion species is dominant. Given the geometry of most plasma processing systems, there is more interest in plane geometry, and so we have concentrated on that, but we have found analogous expressions for cylindrical geometry. Over almost all of the parameter space, we have obtained values that approximate well to the accurate computed solutions. Franklin R. N. and Snell J. (1992)\textit{ J Phys D : Appl Phys }\textbf{25} 435-7 [Preview Abstract] |
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FPT2.00020: Ion-ion Plasmas production and investigation Gary Leray, Pascal Chabert, Nicolas Plihon, Jean-Luc Raimbault A magnetized low-pressure electronegative discharge was used to produce ion-ion plasmas. We used both $Ar$/$O_2$ and $Ar$/$SF_6$ gas mixtures. The cylindrical plasma core is produced by an helicon wave, generated by a Boswell-type antenna excited at 13.56 MHz. The magnetic field strength is such that electrons are magnetized while ions are not, resulting in an electron-free plasma at the edge of the cylinder. The negative ion fraction was measured as a function of the radius by electrostatic probes and laser-induced fluorescence. The electrons represent less than one percent of the negative charge at the edge which allowed measuring the ion energy distribution function for both positive and negative ions. Finally, the criterion for sheath formation in electron-free plasmas was investigated. [Preview Abstract] |
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FPT2.00021: How many particles must a two-dimensional dusty plasma have to appear infinite? T.E. Sheridan A complex (dusty) plasma disk (CPD) is a two-dimensional system of $n$ particles interacting through a shielded Coulomb potential with Debye length $\lambda$ and confined in an isotropic parabolic well. The emergence of macroscopic behavior in a CPD is studied by considering the dependence of the breathing frequency $\omega_{{\rm br}}$ on $n$, $\lambda$, the disk radius $R_{0}$, and the nearest neighbor distance $a$. An approximate analytical expression for $\omega_{{\rm br}}$ is derived for $R_{0}\gg\lambda$ with $a/\lambda$ finite. In the plasma regime $a < \lambda$, so that each particle interacts with many other particles, $\omega_{{\rm br}}^{2}\approx4$ independent of $n$. In the ``condensed-matter'' regime $a > \lambda$, nearest-neighbor interactions dominate and $\omega_{{\rm br}}^{2}\sim a/\lambda$. Exact solutions for $n=100$ to 3200 particles approach the unbounded-plasma limit as $n$ increases. Solutions with $n=3200$ and $a/\lambda$ between 0.25 and 0.5 are found to provide the best approximation to an infinite plasma. [Preview Abstract] |
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FPT2.00022: Breathing oscillation of elliptical Debye cluster K.D. Wells, M.J. Garee, A.C. Herrick, T.E. Sheridan An elliptical Debye cluster is a system of $n$ identical particles confined in a two-dimensional anisotropic potential well and interacting through a shielded Coulomb potential. A model for this system has been developed which has three parameters: the number of particles, the well ellipticity, and the Debye shielding parameter. From numerical solutions of the model, we find as the ellipticity increases from circular to highly elliptical that the breathing mode persists and that the breathing frequency increases. For highly elliptical clusters, the breathing mode consists of particle motions predominantly in the direction of the minor axis of the ellipse. An elliptical cluster with $n = 49$ monodisperse microspheres was studied in the Dusty O.N.U. experimenT (D.ONU.T). The anisotropic parabolic well was created using a rectangular aperture (17.5~mm $\times$ 30.2~mm) placed on the lower electrode of an rf discharge. Plasma was created using $\approx 11$~W of rf power for 18~mtorr of argon. The well ellipticity was determined by measuring the center-of-mass frequencies along the major and minor axes of the resulting elliptical cluster using both driven and thermally-excited oscillations. These two methods give results that are in good agreement. Cluster parameters were determined by measuring the frequency of the breathing mode and comparing with numerical eigenmode solutions of the model. The measured Debye shielding parameter was $\approx 2$. The measured cluster temperature was 400~K. [Preview Abstract] |
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FPT2.00023: HIGH PRESSURE PLASMAS AND LIGHTING |
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FPT2.00024: Local field approximation and runaway electron generation in streamer tip conditions W.J.M. Brok, Chao Li, J.J.A.M. van der Mullen, U. Ebert Recent advances in lightning and streamer physics indicate that the kinetic behaviour of particles plays a role: the detection of x-rays emanating from lightning events indicates that processes occur in which individual particles have higher energies than can be accounted for in the models that have been used in the past. In order to investigate the influence of microscopic processes such as individual electron avalanches in front of a streamer tip, we are in the process of developing a hybrid fluid -- particle model. As a step towards this goal, to study the manner in which the coupling between a fluid and Monte Carlo model can be established, we developed a Particle In Cell Monte Carlo simulation of a planar front. By means of this model the electrons kinetics in the tip of the front are studied and common fluid models assumptions such as the local field approximation are re-evaluated. Enhancement of the electric field, due to space charges in the front, is shown to augment the probability of runaway electron generation in the conditions typical for negative streamers. [Preview Abstract] |
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FPT2.00025: Density and lifetime evaluation of weakly ionized plasma for laser-triggered lightning by means of laser absorption Michiteru Yamaura The potential ability of lasers to control lightning can be improved by using a train of pulses with sub-millisecond separations [1-2]. Laser-triggered experiments in a small-scale (10 mm gap) atmospheric discharge facility show that the triggering is dramatically enhanced when a five-pulse train of sub-Joule energy is used instead of a single pulse. This effect increases rapidly as the pulse interval is reduced. In order to evaluate the trigger effect quantitatively, the plasma density produced by a pulsed KrF excimer laser with high repetition rate of kHz order was measured by means of laser absorption [3-4]. It appears that at a sub-millisecond pulse interval, sufficient positive and negative ions survive in subsequent pulses, thus enabling easy deionization. Hence, significant plasma build-up occurs from one pulse to the next. However, this persistence of ions would appear to imply that the rate of recombination (effectively a charge transfer between ions) is considerably lower than previously believed. \newline References \newline [1] M.Yamaura, et al: J.Appl.Phys.\textbf{ 95}, 6007 (2004). \newline [2] M.Yamaura,et al : Appl.Phys Lett. \textbf{86} 131502 (2005). \newline [3] M.Yamaura: J.Appl.Phys.\textbf{98} 043101 (2005) \newline [4] M.Yamaura,et al : Appl.Phys Lett. \textbf{88 }to be appeared in June (2006) [Preview Abstract] |
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FPT2.00026: Dependence of the Current on the Hollow Cathode Dimensions and Seed Electron Properties in a Pseudospark Discharge Initiation Selma Cetiner, Peter Stoltz, Peter Messmer Hollow cathode discharges can be triggered by different mechanisms, including laser irradiation on the front or back wall of the cathode or beam triggering from another hollow cathode source, resulting in different developments of the discharge. The two-dimensional kinetic plasma simulation code OOPIC Pro and the two and three-dimensional code VORPAL are used to study the properties of both standard and compact hollow cathode devices. This investigation studies variations in the peak initiation current with the hollow cathode dimensions and the location and energy of the seed electrons. The relative importance of secondary electrons due to both ion and electron impact is also studied. It is demonstrated that that the magnitude of the peak current has a dependency on all these factors combined and trends cannot be established by varying a parameter in isolation. [Preview Abstract] |
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FPT2.00027: Apoptosis in vascular cells induced by cold atmospheric plasma treatment Raymond Sladek, Eva Stoffels Apoptosis is a natural mechanism of cellular self-destruction. It can be triggered by moderate, yet irreversible damage. Apoptosis plays a major role in tissue renewal. Artificial apoptosis induction will become a novel therapy that meets all requirements for tissue-saving surgery. Diseased tissues can disappear without inflammation and scarring. This is particularly important in treatment of blockages in body tracts (e.g. cardiovascular diseases). Artificial induction of apoptosis can be achieved by means of cold plasma treatment. In this work an atmospheric micro-plasma operated in helium/air has been used to induce apoptosis in vascular cells. Parametric studies of apoptosis induction have been conducted; the efficiency is almost 100{\%}. The apoptotic factors are ROS/RNS (reactive oxygen and nitrogen species). Their densities in the plasma have been measured by mass spectrometry. For apoptosis induction, RNS seem to be more important than ROS, because of their relative abundance. Moreover, addition of a ROS scavenger (ascorbic acid) to the cell culture medium does not reduce the occurrence of apoptosis. Cold plasma is a very efficient tool for fundamental studies of apoptosis, and later, for controlled tissue removal in vivo. [Preview Abstract] |
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FPT2.00028: The Plasma Pencil: A Novel Pulsed Plasma Source* Tamer Akan, Xinpei Lu, Mounir Laroussi One of the attractive characteristics of nonequilibrium plasmas is their enhanced plasma chemistry without the need for elevated gas temperatures. Using nanoseconds high voltage pulses the electron energy distribution function can be controlled in a way to shift it towards the high-energy tail. Higher electrons energies lead to enhanced gas phase chemistry. This is advantageous in material processing applications where advanced chemical processes under low temperature conditions are desirable. In this paper, we report on a novel pulsed cold plasma source, the ``plasma pencil'' [1], and its unique characteristics. The plasma pencil is capable of generating a cold plasma plume several centimeters in length. It exhibits low power requirements and by using helium as a carrier gas the gas temperature remains low for extended periods of operation. The plasma plume emitted by the plasma pencil can be applied to bare skin without causing any heating or painful sensation. In this paper, the characteristics of the plasma pencil will be discussed. These include the current-voltage characteristics, plume temperature, emission spectra, and the time and spatial evolution of the plasma plume. *Work supported by the Air Force Office of Scientific Research (AFOSR). \newline [1] M. Laroussi and X. Lu, ``Room Temperature Atmospheric Pressure Plasma Plume for Biomedical Applications'', \textit{Applied Physics Letters}, Vol. 87, 2005. [Preview Abstract] |
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FPT2.00029: Model calculations of O2(1D) production in microcathode sustained discharges in argon/oxygen mixtures E. Munoz-Serrano, G. Hagelaar, J.P. Boeuf, L.C. Pitchford It is now well established that non-thermal, high-pressure plasmas can be initiated and sustained between a microhollow cathode discharge (MHCD) acting as a plasma cathode and a third electrode placed some distance away. To investigate the properties of the plasma created in such a microcathode sustaind (MCS) discharge configuration, we have developed a 2D quasi-neutral model of a radially expanding ``positive-column'' in which the current crossing the exit plane of the MHCD is input as a boundary condition. We are particularly interested in determining operating conditions leading to high yields of singlet delta (metastable) oxygen molecules O2(1D), and thus the model includes a kinetic scheme to describe the plasma chemistry in pure O2 and in Ar/O2 mixtures. For 10{\%} O2 in a 50 torr Ar/O2 mixture, a discharge current of 1 mA, a 200 micron MHCD hole diameter and 0.6 cm gap spacing, we find that the reduced electric field, E/N, on-axis at the mid-plane is about 15 Td. The calculated O2(1D) yield on-axis near the exit of the MHCD is 10{\%}. For higher O2 partial pressures, quenching of O2(1D) in 3-body collisions with O2 and O atoms leads to a decrease in the predicted yield, but the optimum pressure depends on the assumed values for the 3-body quenching rates. Details of the model and results of species density profiles for a range of conditions will be presented. [Preview Abstract] |
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FPT2.00030: On the Influence of the Dielectric Barriers on the Atmospheric Pressure Glows in Helium Peng Zhang, Ning Zhou, Uwe Kortshagen Based on a two-dimensional fluid model with a local field approximation, the atmospheric pressure glow discharges (APGs) with two dielectric barriers in helium with nitrogen impurities are studied. The model self-consistently solves the Poisson equation for the electric field and the continuity equations for the densities of all species. The momentum equations are simplified by the drift-diffusion flux. The electrons, helium atomic and molecular ions, helium metastables, and nitrogen molecular ions are included in the simulation. The model successfully predicts the formation of self-organized filaments in the discharge gap. The results showed that smaller number of filaments forms for dielectric material with lower permittivity. And a uniform Townsend-like discharge can be obtained by using of a material with lower permittivity. Based on the simulation model, the APG initiation process in the reactor with a single dielectric barrier is studied. And the influence of the thickness of the dielectric is also investigated. [Preview Abstract] |
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FPT2.00031: Diagnostics of an rf-excited micro atmospheric pressure plasma jet S. Reuter, K. Niemi, V. Schulz-von der Gathen, T. Mussenbrock, T. Gans The ``standard'' 13.56\,MHz rf-excited plasma jet operates at ambient conditions. It generates a homogeneous plasma in helium or argon with small admixtures (about 1 vol.-\%) of oxygen$^1$. Absolute concentrations of atomic oxygen have been measured in the effluent of the plasma jet by two-photon laser-induced fluorescence (TALIF). Even at several centimeter distance from the nozzle still there is 1\% of the initial atomic oxygen density of $10^{16}$\,cm$^{-3}$ present$^2$. Here we present a modified $\mu-$APPJ version particularly designed for investigation of the discharge interior. First emission spectroscopic investigations and tests of applicability are presented. The wettability of polymer Petri dishes could be adjusted in a wide range (wetting angle from 60$^\circ$ to below 10$^\circ$).\par \noindent$^1$ S. Wang, V. Schulz-von der Gathen, and H.F. Doebele, Appl. Phys. Lett. {\bf 83}, 3272 (2003)\\ $^2$ K. Niemi, S. Reuter, V. Schulz-von der Gathen, and H.F. Doebele, Plasma Sources Sci. Technol. {\bf 14}, 375 (2005) [Preview Abstract] |
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FPT2.00032: Nonlinear lumped circuit modeling of an atmospheric pressure rf discharge M. Lapke, D. Ziegler, T. Mussenbrock, T. Gans, V. Schulz-von der Gathen The subject of our modeling approach is a specifically modified version of the atmospheric pressure plasma jet (APPJ, originally proposed by Selwyn and coworkers$^1$) with reduced discharge volume, the \emph{micro} atmospheric pressure plasma jet ($\mu$-APPJ). The $\mu$-APPJ is a homogeneous nonequilibrium discharge operated with Argon or Helium as the feedstock gas and a percentage volume admixture of a molecular gas (O$_2$, H$_2$, N$_2$). The efficiency of the discharge is mainly due to the dissociated and activated molecules in the effluent that can be selected depending on the application. A variety of applications in surface treatment have already been demonstrated, e.g., in semiconductor technology, restoration and bio-medicine. In this contribution we present and analyze a nonlinear lumped circuit model of the $\mu$-APPJ. We apply a two-scale formalism. The bulk is modeled by a generalized Ohm's law, whereas the sheath is described on a considerably higher level of mathematical sophistication. The main focus lies on the spectrum of the discharge current in order to support the characterization of the discharge via model-based diagnostics, i.e., the estimation of the spatially averaged electron density from the frequency of certain self-excitated collective resonance modes.\par \noindent J. Park et al., Appl. Phy. Lett. {\bf 76}, 288 (2000) [Preview Abstract] |
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FPT2.00033: Measurement of metastable Ar atom density in atmospheric-pressure microgap discharge using laser absorption spectroscopy Akihiro Kono, Tomoyuki Shibata, Mitsutoshi Aramaki Atmospheric-pressure Ar glow discharge in a microgap between two knife-edge electrodes (10-mm length, 100-$\mu $m gap separation) driven by 2.45-GHz microwave is being studied aiming at an application to VUV excimer light source. One of the knife-edge electrodes has a gas sink at its ridge, enabling introducing gas flow through the discharge plasma. The density of metastable Ar atoms, which are precursors of excimer molecules, is studied using laser absorption spectroscopy. The beam of a tunable diode laser at wavelengths around 696.5 nm is arranged to pass through the microgap obliquely to have an absorption path length of $\sim $1 mm. At a microwave power of 10 W, the observed absorption at the line center was $\sim $10{\%} with a pressure broadened line width of $\sim $13 GHz, giving metastable Ar atom density of 3$\times $10$^{13}$ cm$^{-3}$. In a similar condition, the electron density measured using a laser Thomson scattering technique was 3$\times $10$^{14}$ cm$^{-3}$. The behavior of metastable atom density for varying discharge conditions is under investigation. (Work supported by Grant-in-aid 15075205 from MEXT Japan.) [Preview Abstract] |
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FPT2.00034: Atmospheric Pressure non-thermal plasmas for surface treatment of polymer films Hsiao-Feng Huang, Chun-Hsiang Wen, Hsiao-Kuan Wei, Chwung-Shan Kou Interest has grown over the past few years in applying atmospheric pressure non-thermal plasmas to surface treatment. In this work, we used an asymmetric glow dielectric-barrier discharge (GDBD), at atmospheric pressure in nitrogen, to improve the surface hydrophilicity of three kinds of polymer films, biaxially oriented polypropylene (BOPP), polyimide (PI), and triacetyl cellulose (TAC). This set-up consists of two asymmetric electrodes covered by dielectrics. And to prevent the filamentary discharge occur, the frequency, gas flow rate and uniformity of gas flow distribution should be carefully controlled. The discharge performance is monitored through an oscilloscope, which is connected to a high voltage probe and a current monitor. The physical and chemical properties of polymer surfaces before and after GDBD treatment were analyzed via water contact angle (CA) measurements, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) techniques. [Preview Abstract] |
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FPT2.00035: Spatially Resolved Gas Temperature Measurements in an Atmospheric Pressure DC Glow Microdischarge with Raman Scattering S. Belostotskiy, Q. Wang, V. Donnelly, D. Economou, N. Sadeghi Spatially resolved rotational Raman spectroscopy of ground state nitrogen N$_{2}$(X$^{1}\Sigma _{g}^{+})$ was used to measure the gas temperature (T$_{g})$ in a nitrogen dc glow microdischarge (gap between electrodes d$\sim $500 $\mu $m). An original backscattering, confocal optical system was developed for collecting Raman spectra. Stray laser light and Raleigh scattering were blocked by using a triple grating monochromator and spatial filters, designed specifically for these experiments. The optical system provided a spatial resolution of $<$100 $\mu $m. Gas temperatures were determined by matching experimental spectra to model spectra obtained by convolution of theoretical line intensities with the apparatus spectral resolution, with Tg as the adjustable parameter. T$_{g}$ was determined as a function of pressure and discharge current density (P = 400-760 Torr, j$_{d}$ = 200-1000 mA/cm$^{2})$. Midway between the electrodes, T$_{g}$ increased linearly with j$_{d}$, reaching 500 K at 1000 mA/cm$^{2}$ j$_{d}$ for a pressure of 720 Torr. Spatially resolved gas temperature measurements will also be presented and discussed in combination with a mathematical model for gas heating in the microplasma. This work is supported by DoE/NSF. [Preview Abstract] |
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FPT2.00036: Experimental Study on Atmospheric Pressure RF Capacitive He/CF$_4$/O$_2$ Discharges Takashi Kimura, Takamasa Hanai Electrical and optical measurements of atmospheric pressure capacitive radio frequency (13.56 MHz) He/CF$_4$/O$_2$ discharges were carried out by changing the mixture compositionsof CF$_4$ and oxygen at the fixed He content of 99$\%$. Those discharges were produced between two planar electrodes of 40mm-$\phi$ at the gap length of 1.0 mm. The impedance of the discharge gradually decreased as oxygen was mixed to He/CF$_4$ discharges, and then reached the minimum around the oxygen content of 0.1$\%$, beyond which it increased with increasing oxygen content. Optical emission spectroscopy has been used in order to observe the excited species generated in the capacitive He/CF$_4$/O$_2$ discharges. Optical emission spectra used in this study exhibit emission lines of excited Ar, O and F atoms. The intensity at 704 nm emitted from the excited atomic fluorine increased markedly as oxygen was mixed to CF$_4$ discharges, and then reached the maximum around the oxygen content of 0.2 - 0.3$\%$, beyond which it decreased with increasing oxygen content. The intensity at 845 nm emitted from the excited atomic oxygen was also investigated in order to grasp the dependence of the atomic oxygen density on the oxygen content. The intensity at 845 nm was approximately proportional to O$_2$ content, resulting in the linear relationship between the atomic oxygen density and the O$_2$ content. [Preview Abstract] |
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FPT2.00037: The Influence of Polymer Films on an APGD in Helium Damian Della Croce, Gagik Nersisyan, William Graham Electrical and optical diagnostic techniques have been used to study the influence of various polymers in the gap of a Helium APGD. A gated ICCD was used to record short exposure time images (2$\mu $s) through the development of the discharge current pulse. The APGD was generated between two parallel, glass 4mm thick) plates which cover copper mesh electrodes. The gap was 5mm. Typically a 4.4kV (peak to peak) sinusoidal voltage was applied to the powered electrode with a frequency of 30kHz. The other electrode was grounded. The system was housed in an evacuated chamber, previously evacuated to a base pressure of 10$^{-4}$ Pa, before Helium was introduced to static atmospheric pressure. A spectrometer was used to record the emission spectra from the discharge. To date studies on polypropylene (PP) and polyester (PET) have been conducted and polyamide will follow. Interesting trends are evident when they are compared to those for the He APGD with no polymer present. Electrically the traces for PET are dramatically different to those for PP and no polymer, which are comparable. Imaging shows that PP yields a filamentary discharge. PET on the other hand produces a glow --type discharge. We are currently studying if the different results are intrinsic to the polymer or the anti-cling surface treatments that the polymer suppliers may be applying. DD is supported by EPSRC and Dow Corning Plasma Solutions. [Preview Abstract] |
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FPT2.00038: Diagnostics of an Electron Beam Integrated Thruster Tsitsi Madziwa-Nussinov, Max Light, Pat Colestock, Ron Kashuba, Rick Faehl Since the 1970s, Russian scientists have been utilizing Plasma cathode electron (PCE) sources for production of electron beams [1], [2]. We have utilized a PCE source in our Electron Beam Integrated Thruster (EBIT) experiment. Using an ECR source at 2.45GHz, we made our PCE source (described in detail in another presentation at this conference) by biasing a conducting plasma chamber to negative voltages up to -140V. We left a small aperture of 2cm in diameter through which an electron beam is extracted into a downstream Pyrex glass chamber with magnetic coils for plasma confinement. The plasma-electron beam system was diagnosed using three methods: a Langmuir probe (for electron temperatures, space potentials and electron densities), spectroscopy (for electron temperatures) and a retarded electron potential energy analyzer (for electron energies and space potentials). In this paper we will briefly describe our experiment, the PCE source, and give details of the diagnostics and our initial results on EBIT. \newline \newline [1] Yu. E. Kreindel, \textit{Plasma Cathode Electron Sources }$\sim$Atomizdat, Moscow, 1977, p. 144. \newline [2] E. M. Oks, Plasma Sources Sci. Technol. \textbf{1}, 249 $\sim $1992. [Preview Abstract] |
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FPT2.00039: Use of a Plasma Cathode Electron (PCE) source in an Electron Beam Integrated Thruster (EBIT) Max Light, Tsitsi Madziwa-Nussinov, Pat Colestock, Ron Kashuba, Rick Faehl The electron Beam Integrated Thruster (EBIT) plasma propulsion concept centers around the use of an electron beam to ionize a propellant; a more efficient ionization mechanism than conventional electric propulsion concepts. In this paper we outline the EBIT concept, in particular, the generation of the electron beam in a Plasma Cathode Electron (PCE) source. The PCE beam source utilizes a plasma as an electron beam cathode, eliminating lifetime and heating issues associated with material cathodes. Our PCE source was created using 1.5kW of microwave power at 2.45GHz delivered in a static magnetic field of 875Gauss. We were able to drive electron beams of greater than 100A in our source with very high beam efficiencies by biasing the ECR source chamber to -120V. [Preview Abstract] |
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