Bulletin of the American Physical Society
2006 59th Annual Gaseous Electronics Conference
Tuesday–Friday, October 10–13, 2006; Columbus, Ohio
Session VF2: Transport Theory and Electron Distribution Functions |
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Chair: Rainer Johnson, University of Pittsburgh Room: Holiday Inn Salon B |
Friday, October 13, 2006 8:00AM - 8:30AM |
VF2.00001: Moment Theory of Ion Motion in Devices with Fields that Depend Upon Time and Space Invited Speaker: Recent work has extended the momentum-transfer theory of drift tubes to ion traps and similar devices where the external fields vary with both position and time. Four such extentions will be discussed: two-temperature (2T) and multi-temperature (MT) theories for atomic ion-atom systems, and spherical basis and Cartesian basis theories for molecular systems. In first approximation, the various theories give sets of differential equations with collision frequencies that vary with the effective temperature(s) characterizing the relative kinetic energy of the ion-neutral collisions. Solutions of the sets of equations provide the ion number density, average velocities, average energies and average temperatures as functions of time and of position in the apparatus. Such solutions will be discussed for the Maxwell model, for rigid spheres, and for general ion-neutral interactions. Emphasis will be placed on two new predictions obtained by using the 2T and MT theories to consider non-ideal quadrupole ion traps. [Preview Abstract] |
Friday, October 13, 2006 8:30AM - 8:45AM |
VF2.00002: Non-isotropic non-Maxwellian Electron Velocity Distribution Functions in Low-pressure Plasmas Igor D. Kaganovich, Dmytro Sydorenko, Yevgeny Raitses, Andrei Smolyakov We show that at very low pressures, the Electron Velocity Distribution Functions (EVDF) can become non-isotropic and non-Maxwellian. Specifically, plasmas in Hall thrusters are studied. Such plasmas are sustained at low neutral gas pressure, where the electron mean free path is large compared with thruster dimensions and the electron motion is almost collisionless. Particle-in-cell simulations show that electrons tend to stratify into different groups depending on their origin and confinement condition (i.e. whether they are trapped or not by the plasma potential). These different electron groups have to be treated separately, as they have completely different properties and cannot be lumped together into one Maxwellian EVDF, which is implicitly assumed by the fluid approach. Moreover, the EVDF is found to be strongly non-isotropic due to the large electric field directed parallel to the walls and high plasma losses to the wall, especially in presence of strong secondary electron emission pertaining to Hall thrusters. Typically, the temperature in the direction of the electric field is a factor of two larger than that in the direction towards the walls. [Preview Abstract] |
Friday, October 13, 2006 8:45AM - 9:00AM |
VF2.00003: Townsend Discharge in Methane at Very High E/N Zeljka Nikitovic, Aleksandra Strinic, Vladimir Stojanovic, Olivera Sasic, Gordana Malovic, Zoran Petrovic We show preliminary comparisons between experimental data and Monte Carlo simulations for the spatial profiles of excitation coefficients for the molecular band CH $(A2-X2)$ produced in dissociative excitation by electron swarms and fast neutrals in methane. Measurements were made in parallel plate drift tube for E/N values between 500 Td and 11000 Td (E- electric field, N- gas density, 1 Td = 10-21 Vm2). The spatial profiles of emission reveal significant heavy particle excitation even at moderately high E/N. Calculated absolute profiles are in excellent agreement with the experimental results. The calculations were based on the heavy particle cross sections of Petrovi\'c and Phelps [1]. \newline [1] Z.Lj.Petrovi\'c A.V.Phelps (1992) unpublished. [Preview Abstract] |
Friday, October 13, 2006 9:00AM - 9:15AM |
VF2.00004: Probe diagnostics in low pressure dc discharge. Does the Langmuir Paradox exist? Valery Godyak, Ben Alexandrovich, Abdur Rahman Maxwellian electron energy distributions in a highly non-equilibrium plasma of low pressure dc discharges is one the oldest and fascinating mysteries of gas discharge physics. There is extensive literature and many hypotheses attempting to explain this paradox, but the problem still remains unsolved. In this report we present results on the EEDF measurement in the positive column of a dc discharge in mercury vapor with differently oriented probes placed along the positive column over a wide range of discharge current showed that: a) - the EEDF is not Maxwellian, b) - is essentially anisotropic, c) - is not in equilibrium with discharge current (i.e. EEDF changes along the positive column), d) - the electron temperature inferred from the measured EEDF and that determined by the slope of the probe characteristic in semi-log scale are essentially different, e) - the linearity of the probe characteristic in semi-log scale (the sign of a Maxwellian EEDF) may occurs at essentially nonlinear dependence of the second derivative of the probe characteristic on the probe voltage in semi-log scale. The main conclusions of this study are: a) - the absence of Maxwellian EEDF in the low pressure dc discharge and b) - the Druyvesteyn method is not applicable for measurement of highly anisotropic EEDF typical for the Langmuir Paradox condition. [Preview Abstract] |
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