The Role of Secondary Species Emission in Vacuum Facility Effects for Electrospray Thrusters

The role of secondary particle emission (SPE) in the context of electrospray thruster performance, life, and measure- ment uncertainty is explored through discussions on theory, analytical modelling, and experimental investigations. A review of SPE mechanisms is presented, along with an analysis of onset thresholds contextualized for electrospray propulsion, which indicate that SPE should always be expected when testing electrospray thrusters in vacuum facilities. We present arguments that glow discharges observed in electrospray thrusters are a consequence of secondary particles backstreaming to the emission site. An analytical heat and mass flux model is presented to support the hypothesis that SPE-induced Ohmic dissipation may cause performance limitations in electrospray thrusters. For a typical ionic liquid ion source, the results of our model suggest that 7pA to 5.6nA may cause property variation in the liquid. Apply- ing the model to published testing results on the Air Force Electrospray Thruster Series 2 (AFET-2), backstreaming current that contributes to 0.057−23% of the measured emitter current may cause the 0.86μgs−1 neutral mass flow rate observed when testing the AFET-2 thruster. Experimental results of a voltage biasing experiment demonstrate the thruster-to-facility current coupling caused by SPE, which has a substantial effect on measurement uncertainty. Beam target biasing sweeps show that SPE can be effectively mitigated with an appropriate beam target geometry and biasing scheme. The Electrospray SPE Control-volume Analysis for Resolving Ground Operation of Thrusters (ESCARGOT) model is applied to the presented experimental data to determine secondary particle yield and temperature. For cationic species, we found a yield of γ c = 0.131 ions per incident particle and temperature of T c =30 eV. Anionic species have a yield of γ a = 0.29 ions per incident particle, and temperature of T a =88 eV. Our work concludes with suggestions for mitigating SPE when testing electrospray thrusters in vacuum facilities.