Theory and Experimental Evaluation of Electrodynamic Tether Systems and Related Technologies.

Abstract

The unique work presented in this thesis will first focus on integration of the latest theoretical and experimental electrodynamic aspects of an electrodynamic tether (EDT) into a time-independent simulation tool. Numerous elements have then be compared on a system level, including passive electron collection (or active ion emission) technologies, active electron emission technologies, bare versus insulated tether scenarios, boosting and de-boosting conditions, and various system element configurations. These results indicate that in many cases bare tether anodes are the optimal electron collection mechanism. In addition, it was shown that while hollow cathodes may be the best active electron emission technique, field emitter arrays result in less than 1% difference in system thrusting and use no consumables. This is based on the assumption that several-amp field emitter arrays can be built eventually.

Issues that have troubled previous systems are the efficiency at which the tether collects current, the total surface area, and the bare tether geometry. Experimental work was conducted to compare the effects of porous flat-tape tether geometries to those of slotted and solid geometries. The experiment investigated these different tether configurations to better understand the physics involved and how to apply the different tether geometries to an EDT system. This work has resulted in evidence showing that, regardless of the orientation of the probe with respect to the flowing plasma, equivalent mass holed tapes outperform that of slotted tapes. These slotted tapes, in turn, outperform solid tapes on an equivalent mass basis.

Modeling of hollow cathodes and other ion emission technologies has been a key concern to EDT technology and will have great implications to EDT systems. As tether systems venture outside of the ionosphere, there will likely need to be an alternate method for collecting electrons. An initial investigation using a hollow cathode as an electron collection source in the momentum exchange electrodynamic reboost (MXER) system was conducted. Results indicated that although this technology may produce a slight enhancement in thrust over a bare tether in altitudes over 1000 km, however, it requires too much consumable mass to be feasible.