Compatibility of field emission cathode and electric propulsion technologies.

Field emission (FE) cathodes are being considered to replace hollow cathodes in Hall and ion thrusters because they do not require propellant and operate cold, unlike hollow cathodes. In addition to higher operating efficiency these cathodes have much smaller dimensions, therefore they can be used with meso- and microscale propulsion systems. Field Emission Array (FEA) cathodes are typically used in flat panel display applications in a close-spaced triode configuration where pressures are 10-9--10 -7 Torr. The environment of a Hall thruster is much more hostile with xenon pressures of 10-5 Torr, plasma ions, and a plasma virtual anode to collect the emitted electron current. In this environment, ions bombarding the cathode will damage them to shorten their lifetime and affect their performance, and the current emitted from the cathode will be space-charge limited to possibly prohibitively small currents. This thesis work addressed some of the compatibility issues of FE cathodes and electric propulsion systems. A hall thruster was built, and the cathode environment was characterized with ion current density and neutral particle pressure measurements. Experiments were performed with Si and Mo field emission array (FEA) cathodes and carbon film FE cathodes in xenon environments similar to the environment of a Hall thruster. It was shown that the performance of these cathodes is very sensitive to operating voltage in xenon environments. Performance degradation was observed with the FEA cathodes due to sputtering of the microtip structures if the gate voltage was above a threshold value. Si and Mo FEA and carbon film cathodes were operated in 2 x 10 -5 Torr of xenon without performance degradation during a few hours. The FEA cathode performance degradation process was modeled, and the results agreed with experimental observations. The experimental and modeling results were used together to estimate that the energy threshold for sputtering Mo with Xe ions id 39 eV and the energy threshold for sputtering Si by Xe ions is 49 eV. Considering ions created near the surface of the tips and the charge-exchange ions created near the thruster, the cathode operating voltages are limited to 19 eV (Mo) and 29 eV (Si) to achieve lifetimes greater than 100 hours in Hall thruster environments. Space-charge limited emission from FE cathodes emitting into virtual plasma anodes was also modeled. Current limitations were determined for different cathode environments, dimensions, and operating voltages. These models were used to optimize the cathode configurations for Hall thruster and tether applications to improve the compatibility of these systems. The results are presented in the thesis.PhDAerospace engineeringApplied SciencesElectrical engineeringElectromagneticsPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/131946/2/9938487.pd