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Tesi etd-09062017-134256

Thesis type
Tesi di laurea magistrale
Investigation and Modeling of Turbulence-Induced Electron Cross-Field Transport in Hall Thrusters
Corso di studi
relatore Prof. Andrenucci, Mariano
relatore Prof. Andreussi, Tommaso
Parole chiave
  • instability
  • fluctuations
  • anomalous transport
  • electric propulsion
  • anomalous collision frequency
  • electron cyclotron drift
  • beam-plasma
  • rotating spoke
  • mechanisms
  • waves
  • oscillations
Data inizio appello
Riassunto analitico
The problem of electron anomalous cross-field transport in Hall thrusters has been extensively studied and a variety of mechanisms have been proposed from the early days of Hall thruster research in order to explain the higher-than-classically-predicted axial current. One aim was to understand the physics of Hall thrusters operation in a way that the simulation models can become self-consistent and no more dependent on experimental results. In addition, considering the impact of turbulence on overall thruster performance, it was felt necessary to characterize the most influential mechanisms so as to try to identify possible mitigation techniques.
Being proved to be of large contribution, the turbulence-induced cross-field transport of electrons became an interesting topic studied theoretically, numerically and experimentally for more than a decade. All these efforts, although yielding lots of insights into the nature of various instabilities in Hall devices, their physics and contributions to momentum and energy transport, never reached a conclusive point.
The present thesis is based on the fact that several experimental and numerical studies elucidated that the Hall thruster channel and near-plume can be divided into at least three regions of different electron mobility. Accordingly, it is considered in the present effort that this variable conductivity can be due to different turbulent mechanisms, each present in a region where the plasma properties there justify its excitation.
The relevant turbulent mechanisms have been identified. Implementing the relevant physical characteristics of the selected instabilities, their contribution to cross-field electron transport have been modeled resulting in the final model, named "Unified Anomalous Transport Code (UATC)". The code was found to produce results consistent with those in the available literature.
The UATC serves as complementing a baseline Quasi-2D fully-fluid simulation code developed at SITAEL by providing the code with axial profile of the spatial variations of the effective collision frequency. The converged solution after the iteration of the two models together under different operating conditions showed improved predictions of the baseline model regarding the intensive plasma parameters and obviated the need to introduce non-physical tunable parameters into the baseline.
Finally, an effort was made to experimentally analyze the oscillations in the SITAEL HT-5k Hall thruster in different magnetic field topologies, changing from a conventional to a magnetically-shielded configuration. As a result, the Breathing mode and some interesting phenomena regarding the Secondary Electron Emission from the channel walls were characterized.