Analysis of ion emitting jet structures during ionic liquid electrospraying

The ionic liquid [Bmim][DCA] is a propellant candidate for a standalone electrospray thruster or a dual-mode propulsion system. Characterization of positive polarity ions produced by [Bmim][DCA] capillary emitters with a nominal extraction voltage of 2.0 kV within a quadrupole and time-of-flight mass spectrometers is presented along with the predictions of propulsion performance. Flow rates from 0.05 to 2.18 nL/s are used to investigate the impact variations in the flow parameter have on the electrospray plume. The retarding potential analysis reveals ions emitted from the capillary are formed below the emitter potential of 500 eV. Angular distributions indicate broadening of both the beam current and mass distribution for increasing flow rates. Derived thrust and specific impulse change from 0.84 micro-Newtons and 200 s to 2.90 micro-Newtons and 80 s, respectively. Time-of-flight measurements delineate two distinct droplet distributions at approximately 2,000 to 40,000 amu/q and 50,000 to 300,000 amu/q. These multiply-charged droplet species, with wide mass-to-charge distributions, are due to the electric field conditions and associated charge range available within the parent jet. Additionally, the data show a transition to higher mass ions and droplets with increasing flow rate. The combination of two data sources allow for the assertion that ionic liquid electrospray from 0.05 to 2.18 nL/s does not conform to the traditional view of emission from the electrospray cone-jet. The mixed droplet and ion emission suggest that the primary jet disintegrates into secondary structures that are responsible for the emission species detected by the TOF instrument. --Abstract, page iv