Mass Spectrometric Analysis of Colloid Thruster Ion Emission from Selected Propellants

Abstract

Retarding potential, time-of-flight (TOF), and quadrupole mass spectrometric methods are applied to study the charged species emitted from a colloid thruster operated in a cone-jet mode at high-specific-impulse operating conditions. Measurements are conducted for two propellants, a 28.4 wt% NaI/formamide electrolyte solution and the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]). When operated at a volume flow rate of 2‐6 × × 10 −14 m 3 /s and a positive polarity, the electrolyte solution yields an average specific charge, q/m ,o f ∼11,000 C/kg with an ion/droplet current ratio of 9 to 1. Fully angularly integrated TOF measurements determine an average mass-to-charge ratio of 253 atomic mass units. The axial quadrupole mass spectrometric measurement identifies solvated sodium ions, Na + (HCONH2)n (n =1 ,..., 10), with a distribution peaked at n =4 .Stopping potential and retarding potential measurements suggest that most charged particles are produced with energies ∼200 V below the capillary bias potential. The inability to operate this propellant in a pure ion emission mode makes the thruster inefficient when operated in the ion-evaporation, high-specific-impulse conditions. The ionic liquid, [EMIM][BF4], is operated in a pure ion emission mode for both positive and negative polarity, yielding mean specific charges exceeding 250,000 C/kg. X([EMIM][BF4])n (n =0 , 1, 2) ions, where X = EMIM + or BF − ,a re identified. Contrary to the electrolyte solution, the energy analysis of the field-evaporated ions reveals that the ions are produced at the capillary potential with a narrower energy width of 50 V. For an acceleration voltage of 1500 V, a specific impulse of ∼4000 s at an efficiency exceeding 90% is recorded.