Plasma instabilities in Hall thrusters
Date
2016-02-24
Authors
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Journal ISSN
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ORCID
Type
Degree Level
Doctoral
Abstract
Plasmas involving strong electron drift in crossed electric and magnetic
fields are of great interest for a number of applications such as space
propulsion and material processing plasma sources. Specific applications include Hall thrusters, which are
high efficiency, low thrust propulsion systems used on many missions for satellite orbit corrections and for future planned
interplanetary missions, as well as magnetrons of various configurations used in plasma deposition devices. Similar conditions also exist in the E-layer of the ionosphere and on the Sun.
Despite many successful applications of Hall thrusters and other Hall plasma
sources, some aspects of their operation are still poorly understood. A
particularly important problem is the anomalous electron transport, which greatly exceeds
classical collisional values. Hall plasma devices exhibit numerous turbulent
fluctuations in a wide frequency range and it is believed that fluctuations resulting from plasma
instabilities are likely one of the main causes of the observed anomalous transport. Plasma turbulence also affects
many other important processes such as electron injection, location of the ionization region and wall erosion among others
that influence the operation and efficiency of Hall thrusters.
In Hall thrusters, the E0xB0 flow is made unstable due to gradients in the plasma density,
temperature and magnetic field. The gradient drift instabilities are long wavelength instabilities that propagate
in the azimuthal direction. A fluid theory of these unstable modes is proposed. It is shown that a full account of the compressibility of the
electron flow in inhomogeneous magnetic field leads to quantitative modifications of the previously obtained instability criteria and
characteristics of the unstable modes.
The ExB drift also drives ion sound type instabilities in Hall thrusters. The reactive/dissipative response
of the closure current to the thruster walls drives these negative energy modes. A model for this type of instabilities is proposed and
analyzed for typical Hall thruster conditions. It is shown how wall parameters modify the characteristic growth rate and frequency of the unstable modes and the related anomalous transport.
Nonlinear phenomena are important to understand different aspects of the Hall thruster plasma dynamics. A nonlinear fluid model for the
typical Hall thruster plasma is proposed. The model takes into account electron inertia, electron collisions with neutrals, density gradients as well as various nonlinear terms that arise from the electron drift and nonlinear polarization that were included via the gyroviscous cancellation. The proposed model includes the long wavelength and the low hybrid modes destabilized by density gradients and collisions. This system of fluid equations was implemented using the computational framework BOUT++ from which a set of nonlinear simulations of plasma turbulence was performed. It is shown from these first principles nonlinear simulations that small scale low hybrid oscillations result in an anomalous electron current significantly exceeding the classical collisional current.
Description
Keywords
Plasma physics, instabilities, Hall thrusters, computer simulations
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Physics and Engineering Physics
Program
Physics