Fullerene propellant research for electric propulsion

Abstract

The large mass, low first ionization potential and large electron impact ionization cross-section make Buckminsterfullerene (Cgo) potentially attractive as an ion engine propellant It has the potential for significant increases in engine efficiency over that obtained with xenon at specific impulses less than 3,000 s. One problem encountered in fullercne ion engines has been dissociation of the propellant. Previously this was attributed to thermal decomposition due to operation of the ion engine at temperatures greater than 1073 K. However, during tests conducted at temperatures lower than 1073 K fullerene fragmentation was still observed. This prompted an investigation to detennine if the dissociation was still due to thermal effects or if it was due to collisional processes in the discharge chamber. An ExB probe designed to discriminate between C60 and C5g was used to determine the composition of positively charged particles extracted from a filament cathode ion engine. Ideally only Cgo* ions would be extracted from the ion engine; however, in addition to Cgo large quantities of fullerene fragment ions were observed. During these tests the ion engine was operating at temperatures below 910 K and fullerene fragmentation was not detected in the vaporizer used to supply ions to the discharge chamber. However after running the engine, dissociated fullerene residue was found in the discharge chamber. Typically this residue accounted for up to 2/3 of the Oft mass supplied to the discharge chamber during an experiment. From these results it is evident that fullerene dissociation is caused by processes inherent to plasma production and not due to thermal effects, provided the ion engine temperature is below 1073 K. In electron impact cross-section experiments appearing in the literature, the appearance energy for fullerene fragment ions is greater than 45 eV. In those experiments single electron impact collisions form a metastable C60 which has an energy dependent half-life for dissociation. In cross-section experiments ions are accelerated into a mass spectrometer within a few (is of the time at which they were ionized. In contrast, the average residence time for C60 in an ion engine is ~1 ms during which time it will undergo several collisions. As a result, more dissociation will be observed in an ion engine even with lower electron energies. Therefore, if fullerenes are to be a useful propellant, methods must be devised to ionize and accelerate large amounts of CgQ on much shorter time scales than those typical of conventional ion engines.