High current ion emission in electrohydrodynamic spraying

Abstract

The electrohydrodynamic spraying of ions from conducting liquids at a capillary tip has traditionally been explained as field evaporation at room temperature from a very small liquid tip, of the order of 10 -20 i. Consideration of the effect of space charge on the fields and force balance at the tip indicates that stable emission of the typical observed currents of 50 -100 ~ cannot occur from an area smaller than several microns in diameter on a naturally formed liquid tip. We have conducted experiments with EHD spraying of the GaIn eutectic alloy. These experiments provide considerable new information on the nature of the emission process. Quantities measured include the number of neutral atoms emitted with the ions, the spectrum of the light which accompanies ion emission, the energy and mass spectra of the emitted ions, and the effect on the emission of secondary electrons and residual gas in the vacuum chamber. Some results for spraying from a negative polarity emitter and also for spraying of sodium iodide-glycerol are also reported. The experiments confirm the large effective size of the emitting area predicted by the space charge calculations, and imply that it is caused by the movement of a small emitting tip over a much larger area. The experimental evidence also indicates that ion emission may occur by field ionization of neutral atoms which are emitted thermally from a tip at high temperature. We show that such a process could be self-sustaining.