Bulletin of the American Physical Society
63rd Annual Gaseous Electronics Conference and 7th International Conference on Reactive Plasmas
Volume 55, Number 7
Monday–Friday, October 4–8, 2010; Paris, France
Session TF1: Basic Low-Temperature Plasma Physics |
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Chair: Leanne Pitchford, CNRS-Laplace Toulouse, France Room: 262 |
Friday, October 8, 2010 10:30AM - 10:45AM |
TF1.00001: Breakdown and operational regimes of low-pressure water vapour discharges Nikola Skoro, Dragana Maric, Gordana Malovic, Zoran Petrovic, William Graham We present our results on breakdown and different regimes of dc discharges in water vapour at low pressures. The discharge is established between parallel plate electrodes: copper cathode and transparent conductive anode, 5.4~cm in diameter with adjustable gap in-between. Discharge chamber allows side-on view so axial discharge profiles could be recorded. Water vapour is brought into the system from a test tube with water sample. Measurements of Paschen curves (breakdown voltage vs. pressure x electrode gap dependence) and Voltage-Current characteristics of the discharge were conducted for various \textit{pd} conditions and with water samples of different purity. Together with measurements of electrical properties axial discharge profiles were recorded. One of the most interesting features of the Paschen curve for water vapour is an inflection point around 2~Torr~cm which appears for smaller gaps (d $\le $ 1~cm) and higher pressures. The origin of the inflection point is not clear yet. Measurements with different water samples (bi-distilled and deionised water and tap water) produced the same values of breakdown voltages. Recorded profiles have shown that processes of excitation and ionization by heavy particles (probably by fast hydrogen atoms) dominate at \textit{pd} values lower than 0.5~Torr~cm. [Preview Abstract] |
Friday, October 8, 2010 10:45AM - 11:00AM |
TF1.00002: DC Breakdown of Low Pressure Gas in Long Tubes Valeriy Lisovskiy, Veronika Koval, Vladimir Yegorenkov The gas breakdown was experimentally investigated in DC electrical field in long discharge tubes. The measurements were performed in the tubes of radius R = 1.5 mm, 4 mm, 27.5 mm, whereas the inter-electrode gap values varied in the range L = 1 -- 250 mm. The conventional Paschen law was shown to hold in short discharge tubes for which L/R $\le $ 1. At L/R $>$ 1 the breakdown curves U(p) are shifted not only to lower pressure values but also to higher dc voltage values with the gap value increasing i.e., one must employ the modified law of gas breakdown U (pL, L/R). At L/R $>$ 20 increasing $L$ makes the dc breakdown curves to shift to higher U values, their minima being observed almost at the same gas pressure value. Our analytical model of gas breakdown in long tubes which takes into account ionization, transverse diffusion and electron drift as well as ion-induces secondary electron emission from the cathode surface predicts the experimental results quite well. [Preview Abstract] |
Friday, October 8, 2010 11:00AM - 11:15AM |
TF1.00003: RF breakdown in low pressure gases between small (millimetric) gap parallel plate electrodes with surface structures Boris Legradic, Alan Howling, Christoph Hollenstein We present an experimental investigation into RF breakdown for electrodes with holes or protrusions, approximating the situation in real reactors and providing a benchmark for fluid simulations. RF breakdown curves (voltage vs. pressure) generally show a steep left-hand branch at low pressures and a flatter right hand branch at higher pressures. Introducing protrusions or holes in parallel plate electrodes will lower the breakdown voltage in certain conditions. Yet experiments show that the breakdown curves are not perceptibly influenced by the increased electric field at sharp edges or ridges. Instead, both experiments and simulation show that breakdown at high pressure will occur at the protrusion providing the smallest gap, while breakdown at low pressure will occur in the aperture providing the largest gap. This holds true as long as the feature in question is wide enough: Features that are too narrow will lose too many electrons due to diffusion, either to the walls of the apertures or to the surroundings of the protrusion, which negates the effect on the breakdown voltage. The simulation we developed presents a tool to aid the design of complex RF parts for dark-space shielding. [Preview Abstract] |
Friday, October 8, 2010 11:15AM - 11:30AM |
TF1.00004: Depletion of High-Energy Electrons in Ar/Ne Inductively-Coupled Plasmas A.E. Wendt, R.O. Jung, John B. Boffard, Chun C. Lin Electrons in bounded, low-pressure plasmas are confined electrostatically by an electric potential difference between the plasma and vessel walls. Electrons with sufficiently large kinetic energy, however, can overcome this potential difference and escape to the walls. As a result, the electron energy distribution functions (EEDF) of inductively coupled plasmas often take the form of a Maxwell-Boltzmann distribution at low energies, but have a depleted number of high energy electrons. While the electrons in this high energy range ($>12$~eV) are often those most critical for driving plasma chemistry, this energy range is also difficult to measure with a Langmuir probe due to the low numbers of high energy electrons and non-negligible ion-current contribution. A simple analytic expression has been developed to account for electron losses in a system with an otherwise Maxwellian EEDF. After accounting for oscillations in the confining potential due to RF fluctuations in plasma potential, the modified Maxwellian agrees well with optical and Langmuir probe measurements of time-averaged EEDFs in Ar and Ar/Ne inductively coupled plasmas, and is well approximated by the two-parameter ($x,T_x$) form, $f_x(E)=c_1 T_x^{-3/2} E^{-1/2} \exp[-c_2(E/T_x)^x]$ with $x\approx 1.2$. [Preview Abstract] |
Friday, October 8, 2010 11:30AM - 11:45AM |
TF1.00005: Plasma and neutral-gas flow from a radial plasma source Gennady Makrinich, David Zoler, Amnon Fruchtman The mixed flow of collisional plasma and of a neutral-gas out of a Radial Plasma Source (RPS) [1] is studied. In the RPS, an argon gas is ionized and accelerated radially outward by the electric field applied across an axial magnetic field between an inner anode and a cathode neutralizer located outside the source. The impulse delivered to the ion flow by the applied electric field was measured and found to be larger than the maximal impulse that can be delivered if the ions are collisionless. We show that this impulse enhancement could result from the electric force being felt by ions for a longer time; their residence time in the acceleration region is increased due to their slowing-down collisions with neutrals. In addition, the plasma potential at different distances from the source axis was deduced from the measured potential of a cylindrical emissive probe at saturation. When the RPS is not magnetized, a plasma ball is produced near the anode. The plasma flux near the plasma ball and the amplitude of forced oscillations of a pendulum induced by the flow in the vicinity of the plasma ball, both seem to be much larger than expected by the Langmuir relation in double layers.\\[4pt] [1] G. Makrinich and A. Fruchtman, Phys. Plasmas \textbf{16}, 043507, 2009; Appl. Phys. Lett. \textbf{95}, 181504 (2009). [Preview Abstract] |
Friday, October 8, 2010 11:45AM - 12:00PM |
TF1.00006: Gas convection caused by electron pressure drop in the afterglow of a pulsed ICP discharge Gilles Cunge, David Vempaire, Nader Sadeghi Neutral depletion is an important phenomenon in a CW high-density plasmas. Under typical conditions used for material processing (medium density), it is mostly caused by the gas heating. However, we show that in \textit{pulsed} discharges, the neutral depletion caused by the electron pressure P$_{e}$ plays an important role on radical transport. In the afterglow, P$_{e}$ drops in 10 $\mu $s due to the electron cooling. The conservation of the total (neutral plus electron) pressure through the reactor volume, imposes thus a neutral pressure gradient $\Delta $P$_{N}$ between the plasma bulk and the reactor walls. In turn, this forces the cold surrounding gas to move rapidly towards the reactor center. Measured drift velocity of Al atoms in the early afterglow of Cl$_{2}$/Ar discharge by time-resolved LIF is as high as 250 m.s$^{-1}$. This is accompanied by a rapid gas cooling. The opposite phenomenon is expected to take place during the plasma ignition when the electron pressure rises. The transfer of pressure between electrons and neutrals is expected to take place through ambipolar diffusion: ions, accelerated by the ambipolar field exerts a friction force on the neutrals. [Preview Abstract] |
Friday, October 8, 2010 12:00PM - 12:15PM |
TF1.00007: Gas temperature and N2(A) density measurements in the afterglow of pulsed nanosecond discharge Eugene Mintoussov, Deanna Lacoste, Scott James Pendleton, Gabi Stancu, Christophe Laux, Nikolay Popov, Svetlana Starikovskaia In the present work the experiments combining (i) electrical measurements of current, voltage, reduced electric field and electron density in a nanosecond time scale, (ii) gas temperature, densities of N2(A) and O-atoms in a microsecond time scale have been suggested. Temperature increase was measured by emission spectroscopy technique in the afterglow of pulsed nanosecond discharge in air and nitrogen for 3-9 Torr, with delays 1.5 and 2.5 microseconds relative to the discharge. The typical values of heating for this timescale are 60-80 K. The preliminary calculations are in reasonable agreement with experiments. Cavity ring-down spectroscopy (CRDS) technique has been used to measure N2(A) temporal behavior for different (0,1, and 2) vibrational levels. The experiments demonstrate that N2(A) density decay in nitrogen occurs significantly slower than in air. [Preview Abstract] |
Friday, October 8, 2010 12:15PM - 12:30PM |
TF1.00008: Pure Rotational CARS Studies of Thermal Energy Release and Ignition in Nanosecond Repetitively Pulsed Hydrogen-Air Plasmas Yvette Zuzeek, Igor Adamovich, Walter Lempert Pure rotational CARS thermometry is used to study kinetics of low-temperature plasma assisted fuel oxidation and ignition in a repetitive nanosecond pulse discharge in hydrogen-air mixtures, with number of pulses in a 40 kHz burst varying from a few to a few hundred. Results are shown to agree well with predictions of a new hydrogen-air plasma chemistry model, which incorporates non-equilibrium plasma processes, H2 -- air chemistry, non-empirical scaling of nanosecond pulse energy coupled to the plasma, and quasi-one-dimensional conduction heat transfer. In particular, the results demonstrate that the heating rate in low temperature hydrogen-air plasmas is much faster than in air plasmas, primarily due to energy release from exothermic reactions of fuel with O and H atoms generated in the plasma. At intermediate temperatures, 500 -- 600 K, OH formation from chain branching processes increases, with rapid concurrent increase in heat release, leading to rapid temperature rise and, in some cases, volumetric ignition. Kinetic sensitivity analysis is used to identify dominant plasma and chemical processes and demonstrates that removal of the radical generation processes by the nanosecond pulsed plasma from the model completely blocks subsequent exothermic chemical reactions, thus making ignition impossible. [Preview Abstract] |
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