Ph.D Student | Abraham Cohen-Zur |
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Subject | Theoretical Study of the Hall Thruster and its Operating Characteristics |

Department | Department of Aerospace Engineering |

Supervisors | Professor Emeritus Gany Alon |

Full Professor Amnon Fruchtman | |

Full Thesis text |

The Hall thruster is an electric space propulsion device, which utilizes a radial magnetic field, and an axial electric field, for the acceleration of high density, quasi-neutral plasma, without space charge limitations.

The purpose of the study was to enhance the understanding of the basic physical processes governing the Hall thruster, in order to improve its performance or configuration. The models developed describe the plasma behavior in a simplified manner, depicting the most significant physical processes, yet are clear enough to improve the understanding of the underlying physics. The study focused on the two major regimes of plasma flow in the Hall thruster: The plasma generation and acceleration in the channel, and the divergence of the plasma plume beyond the thruster exit.

For the first regime a one dimensional steady state fluid model of the plasma was developed. In the ideal case, neglecting losses and heat conduction, the flow is characterized by intense full ionization. For this case, employing momentum and energy balance equations, yields analytical relations between the flow parameters, and analytical expressions for the thruster performance. These relations describe the basic operating concept in a compact and comprehensive way, help in performing a fast parametric study of the thruster performance, and help in guiding more elaborate models.

For the study of the Hall thruster plume a quasi-one-dimensional envelope model was developed, investigating the effect of the electron pressure on the plume divergence. The model includes heat conduction in order to describe the evolution of the electron temperature and pressure, and the divergence. The model also takes into consideration the effect of the radial magnetic field in the near field plume. The magnetic field has two opposing effects on the plume divergence as it both inhibits the conducted heat flow and, on the other hand, heats up the electrons by impeding their propagation across the magnetic field. These opposing effects may be used to find optimal configurations.

Employing the envelope model for the general problem of supersonic plasma beams expanding into vacuum, describes the significant effect of the beam geometry. In a slab geometry, the conducted heat is transferred into convected thermal energy which turns into radial kinetic energy i.e. expansion. Larger heat conductivity, in this case, results in larger divergence. In a cylindrical beam, the conducted heat is approximately constant throughout the beam, thus increasing the conducted heat flow does not result in an increased expansion.