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

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. Mass spectrometric experiments have the ability to identify electrochemical byproducts among the electrospray plume ions. We have conducted mass spectrometric, retarding potential, and angular distribution measurements for ions emitted from EMI-Im when sprayed from a wetted tungsten needle at nominal extraction voltages of ~1 kV. The angularly resolved measurements indicate that the spray comprises a mixture of droplets and ions, with the droplets concentrated in the center of the spray. The major ionic species identified are EMI + (EMI-Im)n and Im - (EMI-Im)n, with n = 0,1,2 in the positive and negative polarities, respectively. The retarding potential analysis indicates that all major ions are formed at or near the needle potential. A small amount of fragment ions is observed that may be attributed to electrochemical degradation. We will present the time evolution of these fragment ions when operated in a continuous polarity mode in comparison with an alternating polarity approach.