Study on the current stability and performances of electrospray thruster by coaxial capillary emitters of hybrid highly conductive ionic liquids

Electrospray technology has been widely applied in many fields, including materials science, chemical engineering, pharmaceuticals, and aerospace exploration. In particular, with the development of commercial space exploration and the demand for miniaturization of micro-nano satellites, capillary electrospray thrusters using ionic liquids as propellants have received extensive attention. However, the use of a single capillary emitter limits the overall performance of the thruster. To further enhance the emission performance of the thruster, the coaxial capillaries emitter with hybrid ionic liquids has been proposed. In this paper, the experimental observation was conducted on the situation where the same ionic liquid was injected into the inner and outer capillaries at different flow rates. It was observed that the outer cone-jet gradually envelops the inner cone-jet to evolute a coaxial cone-jet. Single ionic liquid coaxial electrospray has a limited effect on the stability of the cone-jet. Therefore, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF₄) and 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF₆) were separately supplied into the inner and outer capillaries to form a coaxial cone-jet. The results revealed a significant improvement in the current noise spectrum, and it was capable of stable emission current even at higher flow rates. Further analysis of the plume composition was conducted using the time-of-flight method. The results indicated that EMIM-BF₄ and BMIM-PF₆ supplied in outer and inner capillaries respectively as coaxial electrospray can provide the best specific impulse and thrust performances for the electrospray thruster in the conducted experiments. This greatly improves the thrust provided by a single emitter position of the thruster, which expands its application prospects.