A simple retarding-potential time-of-flight mass spectrometer for electrospray propulsion diagnostics

Abstract

The time-of-flight mass spectrometer (ToF-MS) is a useful tool for quantifying the performance of electrospray thrusters and characterizing their plumes. ToF-MS data can be used to calculate the mass-to-charge distribution in the plume, but the kinetic-energy-to-charge (i.e., the potential) distribution must be known first. Here we use a ToF-MS in tandem with a retarding potential (RP) analyzer. By sweeping the retarding potential through the range of potentials present in the plume, both the mass-to-charge distribution and the potential distribution can be measured independently. We demonstrate this technique in a case study using a capillary electrospray emitter and the ionic liquid propellant 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, abbreviated EMI-Im. We report a linear correlation between retarding potential and mass-to-charge ratio that agrees with published data from more complex orthogonal RP/ToF-MS instruments. Calculated values for the jet velocity and jet breakup potential match within 2% and 12%, respectively. Using conventional ToF-MS, we estimated the propellant flow rate and compared those estimates to direct flow rate measurements. For flow rates between 233 pL/s and 565 pL/s, the error in ToF-based flow rate estimates ranged from -16% to -13% when the plume potential was assumed to be a function of mass-to-charge. Assuming a constant plume potential yielded mixed results. However, using the average stopping potential measured by a retarding potential analyzer resulted in higher errors, ranging from -26% to -30%. Data and MATLAB code are included as supplemental materials so that readers can easily apply the techniques described here.

Energy-Resolving Time-of-Flight Mass Spectrometry for Bulk Plasma Analysis

“…The E/Q ratio can be scanned in variable steps from 0 to 125 V, allowing the E/Q ratio resolution (0.1 V - 1 eV) to be adapted to the application requirements. The E/Q ratio resolution of 0.15 V is significantly lower than retarding field systems and comparable to various cylindrical ESA systems described in the literature. The E/Q result is sufficient for thin film deposition applications where the widths of the ion energy distribution functions are in the single eV range or larger.…”

“…The E-TOFMS presented in this work can also be applied in the field of propulsion thrusters, where determining the energy distribution of ions in the propulsion plume is of utmost importance. The high mass resolution of the orthogonal TOFMS using a reflectron provides an advantage. , …”

“…27 This work presents an energy-resolved TOFMS (E-TOFMS) that combines well-established TOFMS with an ESA, to allow for simultaneous identification of plasma ion species (m/Q range deliberately limited to 250 Th, however, can be increased up to 6000 Th) including their energy-tocharge (E/Q) ratio. Other ion energy filter design concepts, such as mirror-type and deflector plate analysers, have been extensively discussed in literature 28 and used in combination with QMS 29 and TOFMS 30 systems. ESAs use radial or spherical electric fields to bend and partially focus ion beams.…”