Bulletin of the American Physical Society
72nd Annual Gaseous Electronics Conference
Volume 64, Number 10
Monday–Friday, October 28–November 1 2019; College Station, Texas
Session HW1: Positron Transport and Applications |
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Chair: Sandra Ward, University of North Texas Room: Century I |
Wednesday, October 30, 2019 8:00AM - 8:30AM |
HW1.00001: Positron transport, thermalization and annihilation in atomic noble gases and liquids and metal vapours gases Invited Speaker: Ron White A transport theory that explicitly incorporates loss of flux due to annihilating collisions is developed and applied to low energy positron drift, diffusion and annihilation. The use of more complete momentum transfer and annihilation cross sections for helium and argon have resulted in improved descriptions of the time dependence of Zeff for positrons injected into these gaseous systems. Similarly, the variation of Zeff versus reduced electric field for experiments where the annihilation region is immersed in an electric field is in closer agreement with experimental data. In this work we highlight new results for positrons in xenon gas and compare with existing experimental results to highlight the need for careful interpretation of these experiments. We also highlight new results for positrons in various metal vapours including Zn, Be and Mg using new cross-section calculations and how the results compare with the electron counterpart. Extension to include dense gas and liquid phase effects are also considered. [Preview Abstract] |
Wednesday, October 30, 2019 8:30AM - 8:45AM |
HW1.00002: Student Excellence Award Finalist: Control of ion energy distribution functions in intermediate pressure plasmas Scott Doyle, Andrew Gibson, Roderick Boswell, Christine Charles, James Dedrick Ion energy distribution functions (IEDFs) incident upon material surfaces in radio-frequency capacitively coupled plasmas (rf CCPs) are coupled to the spatial and temporal sheath dynamics. Tailoring the ion energy distribution function within intermediate-pressure plasmas ($\approx$~133~Pa, 1~Torr), finding application in surface modification and aerospace industries, is challenging due to the collisional conditions. In this work, experimentally benchmarked fluid/Monte-Carlo simulations are employed to demonstrate control of the shape of IEDFs in a collisional (200~Pa 1.5~Torr argon) rf hollow cathode discharge through the application of high frequency ($\geq$~13.56~MHz) voltage waveforms. Two distinct transitions in the shape of the IEDF are observed at 450~V, corresponding to the formation of mid-energy (60~-~180~eV) structures between 40.68~-~54.24~MHz and additional high energy ($\gtrsim$~180~eV) structures between 81.36~-~94.92~MHz. Transitions between these energy ranges occurred at lower applied voltages for increased applied voltage frequencies, providing increased control of the mean and modal ion energy, varying by 106~eV and 280~eV, respectively. Structured IEDFs are of interest to applications requiring control of ion-bombardment energy under collisional conditions. [Preview Abstract] |
Wednesday, October 30, 2019 8:45AM - 9:00AM |
HW1.00003: Towards a self-consistent approach to model cool hydrogen plasma emission Mark Zammit, James Colgan, Jeremy Savage, Dmitry Fursa, Igor Bray, Christopher Fontes, David Kilcrease, Peter Hakel, Jeffery Leiding, Eddy Timmermans Cool (molecular) plasmas are ubiquitous throughout the Universe. As far as we are aware, all opacity and emissivity studies of molecular plasmas are conducted utilizing data or codes taken from several different sources. To this end, we are developing a fully generalizable self-consistent approach to model cool hydrogen (H$_2$ and H$_2^+$) plasmas opacity and emissivity. Here we present results of cool hydrogen plasmas emission, and a preliminary investigation of the plasma effects in low-temperature hydrogen plasmas using an equation of state model. [Preview Abstract] |
Wednesday, October 30, 2019 9:00AM - 9:30AM |
HW1.00004: Ab initio modelling of the transport and solvation of positrons in dense gases and liquids Invited Speaker: Daniel Cocks I will contrast the modelling of the transport of electrons and positrons through dilute gases with the formalism required for modelling the same transport through dense fluids. In dilute gases, this is well described by the Boltzmann equation and easily accessible to particle-track Monte Carlo simulations. The major impediment to accurate simulation predictions lies in the determination of accurate cross section sets. In contrast, the accurate simulation of dense fluids, including liquids and dense gases, provide challenges for ab initio calculations in the form of screening effects, interference, self-trapping and collective excitations. This talk will overview our progress towards a complete ab initio model for transport in dense fluids, and describe the requirements to include solvation as a Monte Carlo process. A key parameter, $V_0$, the minimum of the conduction band, lies at the heart of understanding transport in liquids. While experiments to determine $V_0$ for electrons in dense fluids can be implemented through photo- or field-ionization, these processes are unavailable to positrons and so direct measurement $V_0$ is unlikely to be performed. Hence, theoretical models are crucial and I present an ab-initio prediction of $V_0(n_0)$, where $n_0$ is the fluid density, for positrons in noble-gas gases and liquids. I will show how this result can be applied in transport calculations to determine lifetimes. Finally, I will show how the extension to address dense fluids of polar molecules can easily be described, by formally including molecule orientation. However, the implementation of this extension causes considerable difficulty in both the identification of natural fluctuations in the bulk and the use of appropriate structure information of the fluid. I will discuss different techniques for addressing these issues. [Preview Abstract] |
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