The structure of direct-current microdischarges is investigated using a detailed
two-dimensional multi-species continuum model. Microdischarges are directcurrent
discharges that operate at a relatively high pressure of about 100 Torr
and geometric dimensions in the 10-100 micrometer range. Our motivation for
the study of microdischarges comes from a potential application of these devices in
microthrusters for small satellite propulsion. The Micro Plasma Thruster (MPT)
concept consists of a direct-current microdischarge in a geometry comprising a constant
area flow section followed by a diverging exit nozzle. A detailed description
of the plasma dynamics inside the MPT including power deposition, ionization,
coupling of the plasma phenomena with high-speed flow, and propulsion system
performance is reported in this study. A two-dimensional model is developed as part of this study. The model
consists of a plasma module coupled to a flow module and is solved on a hybrid
unstructured mesh framework. The plasma module provides a self-consistent, multispecies,
multi-temperature description of the microdischarge phenomena while the
flow module provides a description of the low Reynolds number compressible flow
through the system. The plasma module solves conservation equations for plasma
species continuity and electron energy, and Poisson’s equation for the self-consistent
electric field. The flow module solves mass, bulk gas momentum and energy equations.
The coupling of energy from the electrostatic field to the plasma species is
modeled by the Joule heating term which appears in the electron and heavy species
energy equations. Discretization of the Joule heating term on unstructured meshes
requires special attention. We propose a new robust method for the numerical discretization
of the Joule heating term on such meshes using a cell-centered, finite
volume approach.
A prototypical microhollow cathode discharge (MHCD) is studied to guide
and validate the modeling effort for theMPT. Computational results for the impedance
characteristics as well as electrodynamic and chemical features of the discharge are
reported and compared to experimental results. At low current (< 0.1 mA), the
plasma activity is localized inside the cylindrical hollow region of the discharge
operating in the so-called “abnormal regime”. For larger currents, the discharge
expands over the outer flat surface of the cathode and operates in the “normal
regime”. Transient relaxation oscillations are predicted in the plasma properties for
intermediate discharge currents ranging from 0.1 mA to 0.3 mA; a phenomenon
that is reported in experiments.
The MPT, in its present configuration, is found to operate as an electrothermal,
rather than as an electrostatic thruster. A significant increase in specific impulse,
compared to the cold gas micronozzle, is obtained from the power deposition
into the expanding gas. For a discharge voltage of 750 V, a power input of 650
mW, and an argon mass flow rate of 5 sccm, the specific impulse of the device is increased by a factor of 1.5 to a value of 74 s. The microdischarge remains mostly
confined inside the micronozzle and operates in an abnormal regime. Gas heating,
primarily due to ion Joule heating, is found to have a strong influence on the overall
discharge behavior. The study provides crucial understanding to aid in the design
of direct-current microdischarge based thrusters. / text
| Identifer | oai:union.ndltd.org:UTEXAS/oai:repositories.lib.utexas.edu:2152/7518 |
| Date | 27 May 2010 |
| Creators | Deconinck, Thomas Dominique, 1982- |
| Source Sets | University of Texas |
| Language | English |
| Detected Language | English |
| Format | electronic |
| Rights | Copyright is held by the author. Presentation of this material on the Libraries' web site by University Libraries, The University of Texas at Austin was made possible under a limited license grant from the author who has retained all copyrights in the works. |
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