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2D simulation of heat flux distribution in space-relevant applications including electromagnetic fields in partially-ionized Argon plasmas

  • Manipulating and utilizing plasmas becomes a more and more important task in various research fields of physics and in industrial developments. Especially in nowadays spacerelevant applications there are different ideas to modify plasmas concerning particular tasks. One major point of interest is the ability to influence plasmas using magnetic fields. To study the underlying physical effects that were achieved by these magnetic fields for both scenarios Particle-in-Cell simulations were done. Two examples are discussed in this thesis. The first example originates from an experiment performed by the European Space Agency ESA in collaboration with the German Space Agency DLR. To verify the possibility of heat-flux reduction by magnetic fields onto the thermal protection system of a space vehicle a simplified experiment on earth was developed. Most of the heat that is created during re-entry comes from compression of the air ahead of the hypersonic vehicle, as a result of the basic thermodynamic relation between temperature and pressure. The shock front, which builds up in front of the vehicle deflects most of the heat and prohibits the surface of the space vehicle from direct contact with the maximum flux. State of the art spacecrafts use highly developed materials like ceramics to handle the enormous heat. An attractive approach to reduce costs is to use magnetic fields for heat-flux reduction. This would allow the use of cheaper materials and thus reduce costs for the whole space mission. A partially-ionized Argon beam was used to create a certain heat-flux onto a target. The main finding of the experimental campaign was a large mitigation of heat-flux by applying a dipole-like magnetic field. The Particle-in-Cell method was able to reproduce experimental observations like the heat-flux reduction. An additionally implemented optical diagnostics module allowed to confirm the results of the spectroscopy done during the experiment. The underlying effect that is responsible for the heat-flux reduction was identified as a coupling between the modified plasma and the dominating neutral flux component. The plasma, that is guided towards the target, act as a shield in front of the target surface for arriving neutrals. These neutrals are slowed down by charge-exchange collisions. Furthermore the magnetic field induces an increased turbulent transport that is also needed to reach a reduction in heat-ux. The turbulent transport was also obtained by three-dimensional Direct Simulation Monte Carlo simulations. Unfortunately, such source driven turbulence can not be expected in space, so that a heat flux reduction in real space applications is questionable. Nevertheless, other effects like the induced turbulence by the rotating vehicle can compensate the missing source driven effect. The second scenario in which a magnetic field is used to modify the heat flux of a plasma is the operation of the pulsed cathodic arc thruster. The same Particle-in-Cell code was used to simulate a typical pulse of this newly developed thruster of Neumann Space Pty Ltd. The typical behavior of the thruster could be reproduced numerically. The thrust is mainly produced by fast electrons. These electrons are accelerated by electric fields as a result of a plasma-beam instability. This plasma-beam instability was verified by a phase space diagnostics for the electrons. To demonstrate the influence of the magnetic field a simulation of the cathodic arc thruster without magnetic field and one with magnetic field were compared. It was shown that the use of a magnetic field leads to a ten times larger thrust by directing the heat ux. The resulting narrow plume is an additional Advantage of the particle guiding magnetic field. This narrowness of the plume reduces the danger of interaction with other components of the space vehicle. Both scenarios demonstrate the different capabilities for electromagnetic fields to manipulate plasmas and especially the corresponding heat-flux with respect to certain tasks. The possibilities range from reducing the heat-flux onto a target to maximizing the thrust by directing the heat-ux. This thesis demonstrates that simulations are a great tool to support experiments and to deliver an improved physics understanding. They help to identify the basic physics principles in the different systems, because they can deliver information not accessible to experiments. In particular, a better understanding of the influence of electromagnetic fields on the heat-flux distribution in space-relevant applications was obtained. This can be the basis for further simulation-guided optimization, e.g. for the design of more effective cathodic arc thrusters. Here, the goal is to minimize costs for prototypes by replacing the hardware by virtual prototypes in the simulations. This allows to test basic design ideas in advance and get more highly-optimized designs at a fraction of time and costs.
  • Plasmen zu manipulieren und zu nutzen ist eine immer wichtiger werdende Aufgabe in unterschiedlichsten Forschungsbereichen der Physik und Industrie. Besonders in raumfahrtrelevanten Anwendungen gibt es verschiedene Konzepte Plasmen für bestimmte Aufgaben zu nutzen. Ein Hauptaugenmerk liegt dabei auf der Möglichkeit Plasmen mittels magnetischer Felder zu beeinflussen. Um die grundlegenden physikalischen Effekte, die durch magnetische Felder hervorgerufen werden, zu untersuchen wurden Particle-in-Cell Simulationen genutzt. Die beiden in der Arbeit untersuchten Beispiele zeigen die Möglichkeiten elektromagnetische Felder zur Manipulation von Plasmen zu nutzen. Im ersten untersuchten Fall wurde experimentell nachgewiesen, dass es möglich ist, den Wärmefluss auf das Hitzeschild eines Raumfahrzeugs mittels magnetischer Felder zu reduzieren. Dieses Experiment wurde in der vorliegenden Arbeit mit Hilfe von Computersimulationen nachgestellt, was tiefere Einblicke in die zu zugrundeliegenden Prozesse ermöglicht. Es konnte nachgewiesen werden, dass der Haupteffekt zur Reduktion des Wärmeflusses eine Kopplung von Neutralen und Plasma über Ladungsaustauschstöße ist. Im zweiten Teil der Arbeit wurde der entwickelte Gepulste Kathodenbogenentladungsantrieb der Firma Neumann Space Pty Ltd. untersucht. Im Gegensatz zu herkömmlichen Ionenantrieben nutzt dieser Antrieb hauptsächlich Elektronen zur Erzeugung des Schubs. Die Simulation konnte mit Hilfe einer Phasenraumanalyse zeigen, dass sehr schnelle Elektronen das Ergebnis einer Plasma-Beam-Instabilität sind. Der Antrieb nutzt ein magnetisches Feld zur Maximierung des Schubs und um einen gerichteten Ausstoßstrahl zu erzeugen. Beide Beispiele zeigen, dass magnetische Felder mit unterschiedlichen Zielen zur Manipulation von Plasmen eingesetzt werden können. Die Arbeit weist außerdem nach, dass Simulationen eine Möglichkeit darstellen, tiefere Einblicke in zugrundeliegende physikalische Prozesse und Anregungen für zukünftige Entwicklungen der Industrie zu liefern.

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Author: Karl Felix Lüskow
Title Additional (German):2D Simulation von Wärmeflussverteilung in raumfahrtrelevanten Anwendungen einschließlich elektromagnetischer Felder in teilweise ionisierten Argonplasmen
Referee:Prof. Dr. Ralf Schneider, Dr. David Tskhakaya
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:2018/07/20
Release Date:2018/09/05
GND Keyword:Heat-flux, Plasma, Simulation
Faculties:Mathematisch-Naturwissenschaftliche Fakultät / Institut für Physik
DDC class:500 Naturwissenschaften und Mathematik / 530 Physik