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Kinetic Simulation of Ion Propulsion Systems

  • Ion thrusters are Electric Propulsion systems used for satellites and space missions. Within this work, the High Efficient Multistage Plasma Thruster (HEMP-T), patented by the THALES group, is investigated. It relies on plasma production by magnetised electrons. Since the confined plasma in the thruster channel is non-Maxwellian, the near-field plume plasma is as well. Therefore, the Particle-In-Cell method combined with a Monte-Carlo Collision model (PIC-MCC) is used to model both regions. In order to increase the sim- ulated near-field plume region, a non-equidistant grid is utilised, motivated by the lower plasma density in the plume. To minimise artificial self-forces at grid points bordered by cells of different size a modified method for the electric field calculation was developed in this thesis. In order to investigate the outer plume region, where electric field and collisions are negligible, a ray-tracing Monte-Carlo model is used. With these simulation methods, two main questions are addressed in this work. What are the basic mechanisms for plasma confinement, plasma-wall-interaction and thrust generation? For the HEMP-T the plasma is confined by magnetic fields in the thruster channel, generated by cylindrical permanent magnets with opposite polarity. Due to different Hall parameters, electrons are magnetised, while ions are not. Therefore, the dominating electron transport is parallel to the magnetic field lines. In the narrow cusp regions, the magnetic mirror effect reduces the electron flux towards the wall and confines the electrons like in a magnetic bottle. At the anode, propellant gas streams into the thruster channel, which gets ionised by the electrons creating the plasma. As a result of the electron oscillation between the two cusp regions, ionisation of the propellant gas is efficient. The magnetic field configuration of the HEMP-T also influences the plasma potential inside the thruster channel. Close to the symmetry axis, the mainly axial magnetic field results in a flat potential. At the inner wall, the field configuration reduces the plasma wall interaction to only the narrow cusp regions. Here, the floating potential of the dielectric channel wall and its plasma sheath result in a rather low radial potential drop compared to the applied anode potential. As a result, the electric potential is rather flat and impinging ions at the thruster channel wall have energies below the sputter threshold energy of the wall material. Therefore, no sputtering appears at the dielectric wall. At the thruster exit the confinement by the magnetic field is weakened and the potential drops with nearly the full anode voltage. The resulting electric field accelerates the generated ions into the plume and generate the thrust, but they are also able to sputter surfaces. During terrestrial testing, sputteringat vacuum vessel walls leads to the production of impurities. The amount of back-flux towards the channel exit is determined by the sputter yield of the vacuum chamber wall. A large distance between thruster exit and vessel wall reduces the back-flux and smooths the pattern of deposition inside the thruster channel. Dependent on their material, the evolving deposited layers can get conductive, modify by this the potential distribution and reduce the thrust. For the HEMP-T, ions are mainly generated at high potential close to the applied anode potential. Therefore, the accelerated ions producing the thrust gain the maximum energy as observed in experiment. Ions emitted from the thruster into different angles in the plume contribute mainly to the ion current at angles between 30 ◦ and 90 ◦ . They mainly originate from ionisation at the thruster exit. The resulting angular distribution of the ejected ion current is close to the one of the experiment, slightly shifted by about ten degrees to higher emission angles. In front of the thruster exit, electrons are trapped by electrostatics forces. This enhanced density allows ionisation and an additional electron density structure establishes. What are possible physics based ideas for optimisation of an ion thruster? An optimised thruster should have a high ionisation rate inside the thruster channel, low erosion and an ion angular distribution with small contributions at high angles for min- imised thruster satellite interactions. In experiments, the HEMP-T satisfies already quite nicely these requests. In the simulations, low erosion inside the thruster channel and angular ion distributions close to the experimental data are demonstrated. However, the ionisation efficiency is lower and radial ion losses are larger than in experiment. A possible explanation of these differences is an underestimated transport perpendicular to the magnetic field lines, well known for magnetised plasmas. A successful example for an optimisation using numerical simulations is the reduction of back-flux of sputtered impurities during terrestrial experiments by an improved set-up of the vacuum vessel. The implementation of baffles reduces the back-flux towards the thruster exit and therefore deposition inside the channel. These improvements were successfully im- plemented in the experiment and showed a reduction of artefacts during long time measure- ments. This leads to a stable performance, as it is expected in space.
  • Ionenantriebe sind elektrische Antriebe, welche für Satelliten und Raumfahrtmissionen genutzt werden. Die vorliegende Arbeit untersucht im Speziellen den High Efficient Multistage Plasma Antrieb (HEMP-T) der Firma THALES. Er basiert auf magnetisierten Elektronen, welche im Inneren des Antriebskanals ein Plasma erzeugen. Die angelegten magnetischen und elektrischen Felder schließen das Plasma im Inneren des Kanals ein und beschleunigen die generierten Ionen am Ausgang aus dem Kanal hinaus, wodurch der Satellit einen Schub erfährt. Da das Plasma im Antriebskanal als auch in der Abgasfahne (Plume) eine nicht Maxwellsche Geschwindigkeitsverteilung aufweist, wird die Particle-in-Cell Methode in Kombination mit einem Monte-Carlo Stoßmodell (PIC-MCC) verwendet, um beide Regionen zu simulieren. Um die Simulationsdomäne auf einen größeren Plumebereich auszuweiten, wurde ein nicht-äquidistantes Gitter verwendet und die numerische Berechnung des elektrischen Feldes verbessert. In den äußeren Regionen des Plumes können elektrische Felder und Stöße mit Restgasatomen vernachlässigt werden. Daher wurde zur Simulation dieses Bereichs eine Raytracing Monte-Carlo Methode verwendet. Mit Hilfe dieser Simulationsmethoden wurden in der vorliegenden Arbeit folgende zwei Hauptfragen beantwortet: Welches sind die zugrundeliegenden physikalischen Mechanismen für den Plasmaeinschluß, die Plasma-Wand-Wechselwirkung und die Produktion des Schubes? Welche physikbasierten Ideen zur Optimierung von Ionenantrieben gibt es?

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Author: Julia Duras
Title Additional (German):Kinetische Simulation von Ionenantrieben
Referee:Prof. Dr. Akiyoshi Hatayama
Advisor:Prof. Dr. Ralf Schneider
Document Type:Doctoral Thesis
Year of Completion:2018
Date of first Publication:2018/09/05
Granting Institution:Universität Greifswald, Mathematisch-Naturwissenschaftliche Fakultät
Date of final exam:2018/04/05
Release Date:2018/09/05
Tag:Ionthruster; Kinetic simulation; Low temperature plasma
GND Keyword:Plasmaphysik, Simulation
Page Number:139
Faculties:Mathematisch-Naturwissenschaftliche Fakultät / Institut für Physik
DDC class:500 Naturwissenschaften und Mathematik / 530 Physik