Quantifying Secondary Species Emission from Surfaces Bombarded by an Ionic Liquid Ion Source

Plume-surface interactions are an important field of study for electric propulsion devices of all types. High-energy (keV) plume species impacting a surface can result in many possible interactions, including sputtering, secondary electron emission, and reflection of the incident particle. Plume-surface interactions are especially complex for ionic liquid electrospray plumes, which contain a mixture of molecular ions and ion clusters. A specialized diagnostic known as a "Secondary Species Emission probe" (SSE probe) can be used to quantify the emission of charged secondary species and correct for the associated measurement errors. Here we demonstrate, for the first time, how time-of-flight mass spectra can be corrected for SSE effects and how that correction affects the calculated plume species fractions. Further, we demonstrate how an SSE probe can be used in tandem with an energy analyzer and mass spectrometer to measure the SSE yields of specific species at specific impact energies, providing the first experimental method do to so. Correcting time-of-flight data for SSE effects, we find that the error in plume current density is -10% in the raw measurements. However, SSE correction had a negligible effect on the calculated monomer and dimer fractions. When SSE correction was applied to a time-of-flight dataset representing a narrow range of species energies (1772 + < q( % < 1820 + ), we found that the raw (uncorrected) data underestimate the dimer fraction, resulting in a -6% error in the calculated average mass-to-charge and a -14% error in the corresponding mass flux. These two novel applications of SSE measurements have the potential to improve accuracy and reduce uncertainty in measurements of electrospray plume properties and the associated performance estimates.