Direct and Indirect Mass Flow Rate Measurements for Ionic Liquid Ion Sources

Abstract

The dominant performance loss in ionic liquid ion sources is thought to be the mass utilization efficiency, where an electrospray source appears to shed neutral propellant mass that does not appear its exhaust. The underlying cause of this phenomenon is presently unclear. Investigating and characterizing potential utilization losses requires accurate measurements of electrospray mass flow rates, which is difficult due to the extremely small flow rates that are processed by individual sources, particularly those that operate in the pureion regime. In this work, we present an experimental platform that allows for simultaneous, rapid, and in-situ measurements of both supply and exhaust mass flow rates, allowing for measurements of the mass utilization efficiency for single electrospray emitters. Supply flow rates are measured directly using an optical approach that provides ng/s level resolution. Exhaust flow rates are measured indirectly using a time-of-flight mass spectrometer. This platform is employed to measure mass flow rates for a 3 µm internally fed emitter using the ionic liquid EMI-BF4 at emission currents ranging from 100 to 500 nA. At all currents, there is a major discrepancy between the direct and indirect flow rates, with the direct value being greater in almost all cases. Component efficiency estimates confirm that the mass utilization is the most significant performance loss at low flow rates when the source is working in the pure-ion regime.