Bulletin of the American Physical Society
63rd Annual Gaseous Electronics Conference and 7th International Conference on Reactive Plasmas
Volume 55, Number 7
Monday–Friday, October 4–8, 2010; Paris, France
Session VF4: Electric Propulsion |
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Chair: Amnon Fruchtman, Holon Institute of Technology Room: 162 |
Friday, October 8, 2010 4:00PM - 4:30PM |
VF4.00001: Ion and atom flow in a Hall discharge: Impact of operating parameters Invited Speaker: A Hall effect thruster is a verstaile electric propulsion device that is based on a low-pressure magnetized crossed-field discharge. In this contribution, ion and atom transport phenomena within this type of discharge are examined for a broad range of operating conditions and thruster sizes. The local Velocity Distribution Function of metastable Xe I atoms and metastable Xe II ions (xenon being the usual propellant) is captured by means of continuous-wave Laser Induced Fluorescence spectroscopy at 823.16 nm and 834.72 nm, respectively. The on-axis profile of Xe atom velocity indicates the neutral fluid accelerates in the interior of the thruster and it slows down beyond the exhaust whatever the operating conditions. This unexpected behavior can be explained in terms of ionization probability, energy transfer to walls and ion beam invasion by the background gas. The Xe II ion mean velocity and dispersion profiles along the thruster discharge chamber axis were obtained as a function of applied voltage, gas mass flow rate and magnetic field strength for several thruster geometries. Experimental outcomes provide numerous insights into the physics at work as it will be shown. Besides, the electric field distribution can be accurately inferred from the high order moments of the ion VDF. Recent experiments show ions rotate at relatively low speed in the plume near field of a Hall thruster although they are weakly magnetized. Surprisingly, the way ions rotate seems a priori unaffected by the sign of the magnetic field. To conclude, it will be pointed out that VDF measurements are of great relevance for the development of numerical simulations of a Hall discharge. [Preview Abstract] |
Friday, October 8, 2010 4:30PM - 4:45PM |
VF4.00002: Origin of electrons entering the channel of a Closed Electron Drift Thruster (CEDT) Andr\'e Bouchoule, Titaina Gibert CEDT's involve a magnetized DC discharge between an external hollow cathode and an anode located at the bottom of an annular ceramic channel. An efficient ionization is achieved in the magnetized channel plasma. The outflow of Xe ions and the inflow of primary electrons are the contributions to the thruster's discharge current at channel exit. The electrons entering the channel are generally assumed to be a fraction of those delivered by the hollow cathode hole. This straightforward assumption induced various attempts to describe electron transport from cathode hole to channel entrance. A different representation of the electron flow entering the channel is suggested in this contribution, the primary electron entering the channel being delivered by the thruster's plume. This result, derived from pulsed cathode polarization experiments, explains observations of the CEDT's insensitivity to the cathode location. These polarization experiments and the physical interpretation of experimental data will be presented in the ICRP extended abstract. [Preview Abstract] |
Friday, October 8, 2010 4:45PM - 5:00PM |
VF4.00003: A Monte Carlo Hall Thruster Electron Trajectory Model Michael McDonald, Alec Gallimore, Richard Hofer, Dan Goebel Predictive modeling of Hall thruster plasma properties and discharge channel erosion is hampered by poor understanding of electron transport across the thruster magnetic field, particularly in the near-field plasma plume between the cathode and thruster exit plane. An electron transport model has been developed to estimate the level of transport in the near-field plume of a 6-kW Hall thruster in the absence of turbulent transport mechanisms, considering only the ExB drift and collisions with neutrals, ions and exposed thruster surfaces. Instead of empirically determined electron mobility coefficients, the model integrates the electron equations of motion through experimentally measured electric and magnetic fields over timesteps proportional to the local electron Larmor frequency. A Monte Carlo treatment of neutral and ion collisions is employed, and approximately 10\^{}7 electrons are seeded to resolve the electron energy and angular emission distribution at the cathode. By comparison with observed cross-field electron current, this technique permits evaluation of the current deficit that must be attributed to turbulent or other anomalous transport processes. We find that classical transport occurs only at small levels, generally less than a percent of observed transport, and that what transport does occur from the cathode to thruster exit plane is more often attributable to thruster surface collisions than plume collisions. [Preview Abstract] |
Friday, October 8, 2010 5:00PM - 5:15PM |
VF4.00004: Two-dimensional particle-in-cell simulation of a Hall thruster for the investigation of the secondary-electron effect In Cheol Song, Hyo-Won Bae, Won Ho Choe, Jongho Seon, Hae June Lee Several studies have focused on the instability of sheath plasma caused by secondary electron emissions (SEEs) from dielectric material [D. Sydorenko \textit{et al.} Phys. Rev. Lett. \textbf{103} 145004 (2009)], but these 1D model considers only the magnetic field transverse to the wall. To observe the sheath plasma instability in a Hall thruster, a 2D axisymmetric particle-in-cell code is used in this study. SEE from the wall and electron collisions are treated with a Monte Carlo method. Magnetic mirror motion is observed for the electrons in the acceleration channel, and the electron oscillation between dielectric walls is also closely related to SEE. The sheath plasma instability is triggered when the number of SEE per incident electrons to the wall exceeds a critical value. The secondary electrons injected from the wall move toward the bulk region, and collide with neutral particles or reach opposite side wall. This process continues until the trapped electrons make electron-rich sheath and reduce the potential difference from the wall to the center. After the electron-rich sheath is formed, both average electron kinetic energy and SEE decrease, and the instability is turned off. This phenomenon is shown periodically, and is related to the oscillation observed in a Hall thruster experiment. [Preview Abstract] |
Friday, October 8, 2010 5:15PM - 5:30PM |
VF4.00005: Direct Emission of a 200W Hall Thruster Plume David Liu, Richard Branam Fundamental frequencies within the plasma plume of a 200-Watt Hall thruster have been measured by optical means. Although there are several studies which have tackled this area of research, none have been able to visualize the entire flow field while providing information up to 500 kHz. Time and space resolved data was determined by using a high-speed CCD camera capturing the direct emission of the plasma plume. Observations reveal the breathing mode exhibiting the temporary depletion and replenishment of excited neutrals near the exit plane of the thruster. At less than optimal operating conditions, the plasma plume exhibits azimuthal plasma structures not periodic in nature as previously believed. Moreover, the azimuthal plasma structures are not singular; rather they exist as multiple structures in the plasma plume having different angular velocities allowing them to collide with one another. This result suggests when the thruster is operating at certain conditions the azimuthal plasma structures are a combination of several modes and not just due to ionization potential of the fuel. [Preview Abstract] |
Friday, October 8, 2010 5:30PM - 5:45PM |
VF4.00006: Diagnostics of a cold gas thruster with electron beams Denis Packan, Jean Bonnet, Paul-Quentin Elias In this study, we demonstrate the possibility of measuring the number density and the velocity in the plume of a cold gas thruster, using electron beams and optical diagnostics. The electron beam is generated by a secondary-emission electron gun. For number density measurements, the electron beam fluorescence technique is used, in a nitrogen plume. The intensity of the fluorescence signal, once calibrated, gives the absolute number density in the plume. For velocity measurement, done in an Argon plume, the electron beam is used to populate excited states of Argon. Laser Induced Fluorescence measurements at 772 nm are then done from one of this excited state to yield the velocity in the plume. These two techniques could be used together to map the plume, and thus measure mass flow rate, beam divergence or thrust vector. [Preview Abstract] |
Friday, October 8, 2010 5:45PM - 6:00PM |
VF4.00007: Axially-propagating millimetric electron density fluctuations in the Hall thruster plasma Sedina Tsikata, Cyrille Honore, Nicolas Lemoine, Dominique Gresillon The application of the collective light scattering diagnostic to the study of electron density fluctuations has so far provided information on unstable mode characteristics in the Hall thruster plasma. An azimuthally-propagating mode, believed to contribute to anomalous electron transport, has been observed experimentally at the thruster exit [1], confirming some key theoretical predictions as to its frequency and length scale [2], and revealing new mode features such as directivity of propagation, angular extent, dispersion relation and amplitude. This work focuses on the second key mode observed experimentally, which propagates axially in the thruster plume. Its characteristics (group velocity, directivity and angular extent) appear to be strongly linked to those of the axially-directed ions. It may serve as a new means of remotely measuring large-scale features of the ion plume, such as its divergence. Experimental and theoretical aspects concerning this mode are discussed. \\[4pt] [1] Tsikata et. al., Phys. Plasmas, vol. 16, p. 033506 (2009) \\[0pt] [2] Adam et. al., Phys. Plasmas, vol. 11, p. 295 (2004) [Preview Abstract] |
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