High-Speed Characteristics of Ion Emission Onset in Passively-fed Ionic Liquid Ion Sources

The development of high specific impulse propulsion devices operating at low power using the electrospraying process has faced challenges in operating lifetime and thrust efficiency. To improve performance, a more complete understanding of the emission process is necessary. High-speed ion current on the extractor electrode and downstream beam detector electrode of a passively-fed ionic liquid ion source was measured. The voltage applied to the emitter was rapidly alternated in a square-wave pattern at several frequencies, amplitudes, and between positive polarity to ground, negative polarity to ground, and positive to negative polarity. The time delay between full-scale voltage on the emitter and ion current on the beam detector decreased with increasing frequency and increasing voltage, and was eliminated when emitter voltage was switched directly from positive polarity to negative polarity. The standard deviation in delay time for repeated switches with constant operating parameters was also evaluated and shown to decrease with increasing voltage. Multiple steps in the ion current during onset were observed, this is attributed to the formation and emission from discrete sites across the surface of the emitter. An analytical model of the time delay is used to support the hypothesis that the measured variance in emission onset time at fixed emitter voltage is driven by differences in the number of pores feeding the Taylor cone between repeated tests.