Bulletin of the American Physical Society
62nd Annual Gaseous Electronics Conference
Volume 54, Number 12
Tuesday–Friday, October 20–23, 2009; Saratoga Springs, New York
Session DM: Kinetics Workshop: Electron Kinetics |
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Chair: Laxminarajan Raja, University of Texas at Austin Room: Saratoga Hilton Ballroom 1 |
Monday, October 19, 2009 4:00PM - 4:30PM |
DM.00001: Accurate reconstruction of non-Maxwellian electron energy distribution functions Invited Speaker: Low temperature plasmas, particularly those used for materials processing, are likely to have electron energy distribution functions (EEDF's) that are non-Maxwellian. Analysis of Langmuir probe voltage-current (VI) characteristics using the Druyvesteyn relationship has provided researchers with a means of obtaining these distributions to study phenomena in areas ranging from plasma chemistry to RF heating. Two aspects of obtaining non-Maxwellian EEDF's with Langmuir probes are reviewed. The first is how to address the ill-posed nature of the integral Druyvesteyn problem to obtain an accurate representation of the actual EEDF. The second is how to incorporate non-Maxwellian distributions into more advanced probe models, specifically models that account for conditions where the bias sheath on a Langmuir probe approaches the physical extent of the probe itself, commonly referred to as the thick sheath approximation. In collaboration with Ahmed Elsaghir, North Carolina State University. [Preview Abstract] |
Monday, October 19, 2009 4:30PM - 5:00PM |
DM.00002: 3D hybrid simulations for run-away electrons from streamers Invited Speaker: X-ray and gamma ray bursts with quantum energies from hundreds of eV to tens of MeV have been observed in lightning and in long sparks in laboratory. In the lab, hard X-rays are observed during the streamer-leader phase. Therefore the generation of very energetic run-away electrons in streamers has to be investigated, and the consecutive generation of energetic photons through Bremsstrahlung. Following the precise electron energy distribution in the high field region of the streamer head requires a Monte Carlo approach. MC is also suited to study streamer branching triggered by particle fluctuations or streamer inception from few electrons. But a long streamer demands enormous computational power and storage while so-called super-particle methods create numerical artifacts. Fluid approximations, on the other hand, are computationally efficient in regions with large particle densities like the interior of a streamer finger. Therefore a hybrid model that couples fluid and particle model in suitable regions has been developed. The coupling needs a consistent description of the electron dynamics in particle and fluid model, especially in their buffer region. The consistency of the transport coefficients is studied both for swarms and for planar fronts. The fluid model has to be extended to include the non-local effects at the streamer ionization front. The 3D hybrid model can simulate long streamers while following the electron motion at the most active region. The correct prediction of run-away electrons requires reliable cross sections and differential cross sections, which is lacking in the energy range from tens of keV to MeV. We investigate the generation of run-away electrons and present how keV electrons are produced in a growing streamer. The work was performed together with U. Ebert, W. Hundsdorfer, W. Brok and J.J.A.M. van der Mullen. [Preview Abstract] |
Monday, October 19, 2009 5:00PM - 5:30PM |
DM.00003: Kinetic Theory of Instability-Enhanced Collisional Effects Invited Speaker: A generalization of the Lenard-Balescu collision operator is derived which accounts for the scattering of particles by instability amplified fluctuations that originate from the thermal motion of discrete particles (in contrast to evoking a fluctuation level externally, as is done in quasilinear kinetic theory) [1]. Emphasis is placed on plasmas with convective instabilities. It is shown that an instability-enhanced collective response results which can be the primary mechanism for scattering particles, being orders of magnitude more frequent than conventional Coulomb collisions, even though the fluctuations are in a linear growth phase. The resulting collision operator is shown to obey conservation laws (energy, momentum, and density), Galilean invariance, and the Boltzmann ${\mathcal{H}}$-theorem. It has the property that Maxwellian is the unique equilibrium distribution function; again in contrast to weak turbulence or quasilinear theories. Instability-enhanced collisional effects can dominate the physics of low-temperature plasmas. For example, this theory has been applied to two outstanding problems: Langmuir's paradox [2] and determining Bohm's criterion for plasmas with multiple ion species. Langmuir's paradox is a measurement of anomalous electron scattering rapidly establishing a Maxwellian distribution in gas discharges with low temperature and pressure. This may be explained by instability-enhanced scattering in the plasma-boundary transition region (presheath) where convective ion-acoustic instabilities are excited. Bohm's criterion for multiple ion species is a single condition that the ion fluid speeds must obey at the sheath edge; but it is insufficient to determine the speed of individual species. It is shown that an instability-enhanced collisional friction, due to streaming instabilities in the presheath, determines this criterion.\\[4pt] [1] S.D. Baalrud, J.D. Callen, and C.C. Hegna, Phys. Plasmas {\bf 15}, 092111 (2008).\\[0pt] [2] S.D. Baalrud, J.D. Callen, and C.C. Hegna, Phys. Rev. Lett. (to appear June 2009); preprint UW-CPTC 09-4 at www.cptc.wisc.edu. [Preview Abstract] |
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