Bulletin of the American Physical Society
60th Gaseous Electronics Conference
Volume 52, Number 9
Tuesday–Friday, October 2–5, 2007; Arlington, Virginia
Session VF2: Plasma Diagnostics I |
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Chair: Alex Paterson, Applied Materials Room: Doubletree Crystal City Crystal Ballroom B |
Friday, October 5, 2007 8:00AM - 8:30AM |
VF2.00001: Reflections on Electric Probes Invited Speaker: One of the more immediate temptations for an experimental plasma physicist is to insert some kind of refractory, conducting material into a plasma, as a simple means of probing its charge composition. Irvine Langmuir tried it in the 1920s and was one of the first to develop an electrical probe method in his early work on electrical discharge plasmas. There are now numerous variations on the theme including planar, cylindrical and spherical geometry with single, double and triple probes. There are also probes that resonate, propagate and reciprocate. Some probes are electrostatic and others are electromagnetic; some are effectively wireless; most absorb but some emit. All types can be used in steady and transient plasmas, while special schemes have been devised for RF plasmas, using passive and active compensation. Magnetised plasmas pose further challenges. Each configuration is accompanied by assumptions that constrain both their applicability and the analytical methods that translate the measured currents and voltages variously into charge densities, space potentials, particle fluxes, energy distributions and measures of collisionality. This talk will take a broad look at the options and opportunities for electric probes, principally in the environment of non-equilibrium plasma. [Preview Abstract] |
Friday, October 5, 2007 8:30AM - 8:45AM |
VF2.00002: Xenon operation of the Non-ambipolar Electron Source Noah Hershkowitz, Ben Longmier The Non-ambipolar Electron Source (NES) is an RF plasma-based electron source that does not rely on electron emission at a cathode. All of the random electron flux in NES is extracted through an electron sheath resulting in total non-ambipolar flow when the ratio of the ion loss area to the electron loss area is approximately equal to the square root of the ratio of the ion mass to the electron mass, and the ion sheath potential drop at the chamber walls is much larger than Te/e. Operation with Xe has increased the output current (from previous results with Ar at 15 A) to 30 A when using 2.2 sccm Xe, and 1.2 kW RF power at 13.56 MHz and increased the gas utilization, the ratio of extracted electron current to neutral supply current, to 180. Operation with a graphite ion collector/Faraday shield has significantly reduced sputtering. NES could replace hollow cathode electron sources in a wide variety of applications. The physics behind the improved Xe operation compared to Ar is described. [Preview Abstract] |
Friday, October 5, 2007 8:45AM - 9:00AM |
VF2.00003: Nanoparticle charging in weakly ionized gases Marco Francesco Gatti, Uwe Kortshagen Charging dominates the behavior of nanoparticles immersed in weakly ionized gases. The nanoparticle charge is virtually always modeled through the Orbital Motion Limited (OML) theory, even though serious doubts exist about its validity. The approach adopted in this study allows one to overcome most of the simplifying assumptions of the OML theory, providing insight into the behavior of nanoparticles in real laboratory plasmas. The method adopted is a self-consistent molecular dynamic-Monte Carlo simulation in which the ion motion is tracked, and the collisions between ions and background gas are treated statistically, while electrons are modeled through an analytical expression. This method allows for the investigation of the effects of ion-neutral collisions (elastic scattering and charge exchange) and of high particle density. Simulations are performed over a wide range of particle concentration, gas pressure, particle size, and electron temperature. The results show a strong dependence of the nanoparticle charge on the particle density, and a non monotonic dependence on gas pressure. Furthermore, the role of particle size and electron to ion temperature ratio is highlighted. An analytical model capable of predicting the dependence of the nanoparticle charge upon all the aforementioned parameters is derived. [Preview Abstract] |
Friday, October 5, 2007 9:00AM - 9:15AM |
VF2.00004: Electric probes for characterization of microwave produced plasma. Vipin K. Yadav Electric probes namely Langmuir and capacitive probes are designed to characterize microwave produced plasma. In microwave or electron cyclotron resonance (ECR) produced plasmas, the simple Langmuir probe gives error in the measurement of plasma parameters due to the presence of a steady magnetic field. To eliminate these errors along with the already existing sheath effects, some modifications are mandatory to be incorporated in the design of a Langmuir probe. Two such probes, a Langmuir probe to measure plasma parameters and a capacitive probe to detect plasma oscillations are designed. The plasma parameters measured by the designed probe matches well with the theoretically estimated values. [Preview Abstract] |
Friday, October 5, 2007 9:15AM - 9:30AM |
VF2.00005: The Bohm Criterion Revisited Natalia Sternberg, Valery Godyak The plasma-sheath model and the Bohm criterion introduced by Bohm is one of the earliest attempts to model the plasma and the sheath separately, and to find a way to join the plasma and the sheath solutions. Although there is hardly a paper on plasma-sheath modelling that does not quote the Bohm criterion, Bohm's paper and his results are widely misunderstood. The reason for this is that in his paper, Bohm himself misinterpreted his result by concluding that the sheath edge coincides with the reference point of his plasma-sheath model. As a result, the criterion for the reference point obtained by Bohm to ensure monotonicity of his sheath solution (i.e., the Bohm criterion) was erroneously applied to the sheath edge and used in the literature as a criterion for sheath formation. We show that the Bohm criterion when applied to the sheath edge contradicts Bohm's own definition of the sheath, and cannot be obtained from Bohm's plasma-sheath model. [Preview Abstract] |
Friday, October 5, 2007 9:30AM - 9:45AM |
VF2.00006: First experimental test of the generalized Bohm criterion using Ar$^{+}$ and Xe$^{+}$ LIF in Ar-Xe plasmas Dongsoo Lee, Noah Hershkowitz, Greg Severn The Bohm sheath criterion in single- and two-ion species plasmas is studied with laser-induced fluorescence (LIF) using two diode lasers in Xe and Ar--Xe plasmas. The plasmas are generated in a weakly-collisional ($<$ 1 mTorr) unmagnetized dc hot filament discharge confined by multidipole magnetic fields. Two LIF schemes are employed to measure the argon and xenon ion velocity distribution functions (ivdfs) near a negatively biased boundary plate. The Ar II transition sequence at 668.614 nm and Xe II at 680.574 nm are adopted to detect each ion's fluorescence. The results show that the argon and xenon ion velocities appear to approach the ion sound speed of the system near the sheath-presheath boundary, which excludes the possibility that both the ions will have their own Bohm velocities. In addition, the generalized Bohm criterion is also satisfied with the measured data. This is the first experimental test of the generalized Bohm criterion. [Preview Abstract] |
Friday, October 5, 2007 9:45AM - 10:00AM |
VF2.00007: Numerical Solutions for Langmuir Probes Richard Fernsler A simple numerical technique is presented for computing the electrostatic potential around spherical and cylindrical Langmuir probes residing in plasmas consisting of hot electrons and cold, collisionless, positive ions. Rather than solving Poisson's equation directly, its derivative is solved instead using a ``shooting'' method. The new equation is one order higher, but it is linear in the field and can thus be solved implicitly. The scheme is therefore stable numerically, but it requires initial values for the potential, electric field, and derivative of the field at some starting location. Fortunately, these parameters are all easy to obtain in the bulk plasma. The numerical results show that the sheath around the probe can be surprisingly wide, and as a result the ion saturation current has a more complicated dependence on probe voltage than is commonly assumed. (That dependence is related to but separate from the variation seen with probes smaller than a Debye length.) The numerical technique presented can in principle be used with any nonlinear, ordinary differential equation (including with multipoint boundary conditions or unknown eigenvalues), provided a suitable set of initial values can be obtained. [Preview Abstract] |
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