Bulletin of the American Physical Society
70th Annual Gaseous Electronics Conference
Volume 62, Number 10
Monday–Friday, November 6–10, 2017; Pittsburgh, Pennsylvania
Session MW3: Modelling of Propulsion and ExB Plasmas |
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Chair: Salomon Janhunen, University of Saskatoon Room: Oakmont Junior Ballroom |
Wednesday, November 8, 2017 1:30PM - 1:45PM |
MW3.00001: Bidimensional particle-in-cell simulations : Impact of dielectric walls on electron drift instability in Hall effect thrusters Antoine Tavant, Vivien Croes, Trevor Lafleur, Anne Bourdon, Pascal Charbert Hall effect thrusters (HET) are one of the main technology used and studied for spacecraft electrical propulsion. Grid-less, they present net advantages. However, their operation characteristics are not understood yet, resulting in an increasing need for predictive models, and a better understanding of the plasma discharge complex behavior. HETs consist of an \textbf{E$\times$B} discharge in an annular ceramic channel. One of the main characteristic of the thruster is its lifetime, limited by the ceramic channel eroded by the plasma. A better understanding of wall erosion is necessary, however long experiments are costly, and erosion diagnostics and measurements are difficult to perform. A bidimensional $r-\theta$ particle-in-cell simulation is therefore developed to investigate the plasma interaction with the ceramic walls. The dielectric aspect is emphasized: studies are done with metallic walls as well as dielectrics with various geometries and characteristics. Moreover impact and use of parietal capacitive probes is studied. Then secondary electron emissions are implemented to better understand the material effects. [Preview Abstract] |
Wednesday, November 8, 2017 1:45PM - 2:00PM |
MW3.00002: Structures induced by external magnetic field in discharge plasma. Irina Schweigert, Michael Keidar Recently some methods to control the Hall effect thruster characteristics with applying the oblique magnetic field with respect to the channel walls are widely discussed. Nevertheless with increasing the inclination of the magnetic field, discharge plasma properties can essentially change. In this work, in kinetic simulations we consider the dc discharge plasma in the external oblique magnetic field at pressure, P$=$0.1 mTorr. Our purpose is to study the plasma structure modification with changing the electron temperature, magnetic field strength and obliqueness for the conditions similar to the Hall thruster ones. The plasma is embedded in a cylindrical chamber an confined by the magnetic field of 25-100 G. To describe the plasma in electro-magnetic field at low gas pressure we solve Boltzmann equations for the distribution functions for electrons and ions with 2D3V particle-in cell Monte Carlo collision method. The Poisson equation was solved to find the electrical potential and electrical field distributions. The periodical structure with ridges of ion and electron densities was found for larger obliqueness of magnetic field. The electron and ion ridges are shifted with respect to each other and double--layer structure appears across B-field and along the potential rise. [Preview Abstract] |
Wednesday, November 8, 2017 2:00PM - 2:15PM |
MW3.00003: Student Excellence Award Finalist: Entangled effects of electron drift instability and secondary electron emissions in Hall effect thrusters: Insights from 2D PIC computations Vivien Croes, Antoine Tavant, Trevor Lafleur, Anne Bourdon, Pascal Chabert Since Hall effect thrusters (HETs) are one of the most successful electric propulsion (EP) technologies, the need for improved predictive models is increasing. Yet HETs complexity makes it difficult to understand and predict the plasma discharge behavior. One of the topic is that electron mobility across the imposed magnetic field in the channel discharge is anomalously high in comparison to predictions from classical diffusion theories. Multiple mechanisms have been proposed: Secondary electron emissions, sheath instabilities, gradient driven instabilities, or electron drift instabilities. Effect of these drift instabilities on the electron mobility has been recently investigated theoretically, and compared to $r-\theta$ simulations using a simplified $2.5$D PIC simulation model. However in these simulations, walls were assumed to be metallic with no secondary electron emission. In this work we compare results obtained with metallic and dielectric walls, with and without secondary electron emissions (using various models). These improvements enable a deeper look into the behavior of the thruster operation, and allow us to differentiate the relative importance of the mechanisms producing enhanced electron transport. [Preview Abstract] |
Wednesday, November 8, 2017 2:15PM - 2:30PM |
MW3.00004: The role of instability-enhanced friction on electron transport in ExB discharges Trevor Lafleur, Pascal Chabert The applied discharge voltage and magnetic field in many ExB discharges produces large electron drift velocities that can drive plasma instabilities leading to increased particle transport. Here we present self-consistent 2D particle-in-cell (PIC) simulations investigating such instabilities in typical ExB discharges such as Hall-effect thrusters. The PIC simulations preserve fundamental plasma spatial and temporal scales and do not include any artificial geometric or parametric scaling factors. Short-wavelength, high-frequency, oscillations are observed to form just a few microseconds after the discharge begins and with a Fourier spectrum that matches that for an ion acoustic-type instability (in agreement with kinetic theory). Correlated with the presence of this instability is an increased electron cross-field transport that cannot be explained by standard electron-neutral or electron-ion Coulomb collisions. By taking velocity moments of the electron distribution function in the PIC simulations, we reconstruct each term in the electron momentum conservation equation and demonstrate that "anomalous" electron transport in such discharges can be explained entirely due to an instability-enhanced friction force between electrons and ions. This friction force acts as an additional momentum loss mechanism aiding electron transport, and as an ion acceleration mechanism causing both rotation and heating. [Preview Abstract] |
Wednesday, November 8, 2017 2:30PM - 3:00PM |
MW3.00005: Multi-dimensional PIC modelling of crossed-fields low temperature plasma devices Invited Speaker: Francesco Taccogna Different plasma devices are based on the partial magnetisation of electrons in a ExB configuration. In Hall thrusters, a quasi-radial magnetic field creates a strong impedance to the axial electron transport, making a path from the external cathode to the internal anode-gas distributor longer. This increases the ionisation event chances and at the same time it creates a large electron azimuthal drift and virtual cathode for axial ion acceleration. In negative ion source, the tandem-magnetic barrier concept is often used and it consists of using a perpendicular to the flow magnetic field trapping electrons in order to reduce their density and temperature towards the extraction grid region. This reduces the negative ion destruction by electron detachment and the co-extracted electron current. In all the cases, a larger electron current across the magnetic field is measured and often ascribed to an anomalous (non-collisional) character of the transport. By means of multi-dimensional Particle-in-Cell / Monte Carlo Collision (PIC-MCC) models, we have shown how plasma uses all the different dimensions to create self-organised potential structures to increase the limited transport across B field lines. Results show the non-ambipolar character of the transport driven by electron magnetic drifts. Finally, the walls of the device play the role of short-circuiting the electron flow that exhibits a quite complex distribution. [Preview Abstract] |
Wednesday, November 8, 2017 3:00PM - 3:15PM |
MW3.00006: Magnetohydrodynamic simulation study of plasma jets and plasma-surface interactions in coaxial plasma accelerators Vivek Subramaniam, Laxminarayan Raja Coaxial plasma accelerators belong to a class of electromagnetic acceleration devices that utilize the Lorentz force generated by self-induced magnetic fields to accelerate high density thermal plasmas to large velocities (10Km/s). A MHD simulation study of the coaxial plasma accelerator is performed to elucidate the physical mechanisms responsible for the formation of these hypervelocity plasma jets. Distinct modes of jet-formation are identified based on the prefill conditions in the accelerator. The plasma jet is used as a high energy density source to mimic the extreme stagnation conditions generated on the confining walls of fusion reactors during Edge Localized Mode (ELM) type disruption events. This is achieved by impinging the jet on a target material surface placed normal to the jet trajectory. The MHD simulations are used to resolve the transient shock structure that develops on the target surface during the course of the plasma-surface interaction event. The jet-target impact studies indicate the existence of two distinct stages involved in the plasma-surface interaction. A fast transient stage characterized by a thin normal shock that transitions into a pseudo-steady stage that exhibits an extended oblique shock structure. [Preview Abstract] |
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