Emission measurements and in-situ observation of ionic liquid electrospray thrusters with longitudinally grooved emitters

Abstract

An externally wetted emitter array with longitudinally grooved structures for ionic liquid electrospray thrusters was fabricated to improve ionic liquid transport to the emitter tips. Two grooved emitter shapes with different groove depths were successfully fabricated using microelectromechanical system processing techniques. We evaluated the current–voltage characteristics, measured the mass spectra using time-of-flight (ToF) spectrometry, and conducted in-situ observations using a high-speed microscope. The experimental results of ion emission show that the absolute emission current increases compared with that of our previous emitter without grooves. This tendency is strengthened with deeper grooves. Moreover, the slope of the current–voltage curve for the grooved emitters does not decrease even when high voltages are applied, indicating that the grooved structure improves the ionic liquid transport to the emitter tips. This improvement is attributed to the low hydraulic impedance of the emitter structure. However, deeper grooving also increases the percentage of current intercepted by the extractor electrode, and electrochemical reactions are not avoided at an alternation frequency of 1 Hz. Although the first current–voltage measurement tended to have unstable characteristics, the ToF results indicated that the emission in the center line was in the pure-ion regime, composed mostly of monomer and dimer ions, under all the measured conditions. High-speed microscope observations showed that too much ionic liquid deposited on the extractor causes ion emission from the extractor to the emitter, known as backspray, and implies that no large droplets are emitted for either grooved emitter structure, which is consistent with the ToF results.