Analysis of the Electrospray Plume from the EMI-Im Propellant Externally Wetted on a Tungsten Needle

Abstract

The room temperature ionic liquid propellant, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI-Im) is being tested for the NASA DRS-ST7 mission. A capillary thruster configuration is planned for ST7, and time-of-flight experiments have shown that the spray of EMI-Im produces a mixture of primarily droplets and low levels of ions, resulting in a low specific impulse. Recently, pure ion emission was achieved for EMI-Im in a wetted needle thruster, suggesting that this propellant, which has passed all space environmental exposure tests, may also be a candidate for high specific impulse missions. The use of wetted tips raises the question whether electrochemistry at the liquid-metal interface causes significant propellant fouling that -will ultimately result in performance degradation due to the significantly longer propellant metal interaction times in comparison with the capillary design and-the higher flow rates. Electrochemical fouling can be mitigated through a polarity alternation approach, which adds complexity to the power processing unit.