Characterization of an iodine-based ionic liquid ion source and studies on ion fragmentation

Abstract

Electrosprays are a well studied source of charged droplets and ions. A specific subclass is the ionic liquid ion source (ILIS), which produce ion beams from the electrostatically stressed meniscus of ionic liquids. ILIS have several applications, two of which include space propulsion and focused ion beam technologies. An ILIS using the 1-butyl-3-methylimidazolium iodide (BMI-I) is characterized with a focus on those properties that would impact its performance in a focused ion beam (FIB) system. The currents obtained in the negative mode from this ILIS are on the order of 100 nA, and composed of nearly 50% pure iodine ions with the remainder composed of solvated ions. The energy deficit of the ion beam is seen to be on the order of 15 eV, with full width at half maximum of about 12 eV. The angular intensity is estimated to be 0.65 tA sr- 1 . The energy signal from the BMI-I ILIS contained several features that suggested a significant amount of ion fragmentation in the acceleration region. This prompted a secondary study using the the ionic liquid 1-methyl-3-methylimidazolium tetrafluoroborate (EMI-BF 4). The amount of fragmentation was seen to be up to 25% of the ion beam. The impact of such fragmentation on the performance of an ILIS thruster is examined. The effects are integrated into the definition of the polydispersive efficiency by characterizing the fragmentation products as two discrete populations of ions and neutrals. This adjusted polydispersive efficiency is seen to be about 88%, about 6% lower than values not taking into account the fragmentation.