Hollow Cathode Materials in an Iodine Plasma Enviroment

Current electric propulsion devices have proven their worth for large, conventional satellites. Trade studies using iodine as a propellant show superior system-level performance when compared to existing xenon-based systems. As a halogen, iodine introduces chemical reactivity issues not present in xenon plasmas, which are especially relevant for the extreme environment of hollow cathodes. An RF inductively coupled plasma source was designed, built, and characterized to simulate the environment found in a hollow cathode. Langmuir probe measurements were taken for a range of plasma conditions in iodine and argon plasmas. The iodine plasma required measurement with a double probe due to the occurrence of significant negative ionization. A combination of low-resolution survey spectra and high-resolution spectra were measured to measure plasma composition.Material samples of tungsten, tantalum, and molybdenum were tested for a range of temperatures in argon plasma, iodine plasma, and iodine vapor; their associated erosion ranges were measured. Molybdenum's erosion rate showed a roughly exponential relationship with respect to temperature, and a maximum erosion rate at 2000 K in iodine plasma of 70.1 µm/hr. Tungsten's erosion rate showed two distinct linear relationships with respect to temperature, and a maximum erosion rate at 2000 K in iodine plasma of 36.8 µm/hr. Tantalum experienced significant oxidation and expanded in size but had heavy cracking present which weakened the samples. Molybdenum was found to be the best material in iodine vapor, but tungsten was superior in iodine plasma. Tantalum demonstrated very unfavorable changes in surface structure and is not recommended for a system that contains any iodine or oxygen/water contamination. A significant challenge of this experiment was eliminating oxygen due to the oxygen and water presence in the iodine source. The primary improvements suggested for future studies are careful design of the iodine source to minimize moisture and oxygen contamination, a larger plasma source and sample loading area, and use of a differential vacuum system.