Comparing Direct and Indirect Thrust Measurements from Passively Fed Ionic Electrospray Thrusters

Abstract

Highly ionic beams of several hundred μA per cm 2 have been measured from porous glass ionic liquid electrospray sources fabricated using a conventional mill. The thrust output from three prototype devices, two emitting the ionic liquid EMI-Im and one emitting EMI-BF 4, was measured directly using a precise balance. Thrusts up to 50 μN were measured when emitting EMI-Im in a bipolar, alternating potential configuration at less than 0.8 W input power and with propellant supplied from an internal reservoir. Measurements of mass spectra via Time of Flight spectrometry, angle resolved current distributions, ion fragmentation and energy deficits have been applied to accurately calculate thrust and mass flow rates indirectly from the same devices. For two of three cases, calculated and directly measured thrusts were in agreement to within a few μN at input powers from 0.1 W to 0.8 W . Emissions of EMI-BF4 are shown to yield nearly purely ionic beams supporting high propulsive efficiencies and specific impulses, ∼ 65 % and > 3200 s respectively at 0.5 W . Conversely, greater polydispersity was observed in EMI-Im emissions, contributing to reduced specific performance, ∼ 50 % propulsive efficiency and ∼ 1500 s specific impulse at 0.5 W .

A 10 nN resolution thrust-stand for micro-propulsion devices

“…[6][7][8][9] Direct measurement of thrust has also been performed for some electrospray propulsion devices. 6,8,19 If an electrospray emitter emits a current I of particles with uniform kinetic energy qV em , the normal thrust on the device can be written as 6 where m 0 and q are mass and charge of a monomer emitted from the ionic liquid. γ 0 is a factor that takes into account the emission of different charge species together and is obtained from ToF traces of the emission.…”

“…6 β 0 is a factor that corrects for the angular spread of the emission into the thrust normal to the axis of emission and is obtained from measurement of current density as a function of angle from the axis of emission. 6 The values of γ 0 and β 0 are measured before the thrust measurements and are, γ 0 = (1.45 ± 0.03) and (1.63 ± 0.07), and β 0 = (0.93 ± 0.01) and (0.93 ± 0.01) in positive and negative polarities of emission, respectively. 32 The thrust-stand is mounted on a flange of a vacuum chamber and installed inside the vacuum chamber with the sensing plate placed 28 mm away from the emitter.…”

“…The ionic-liquid electrospray device used for thrust measurement is a porous borosilicate emitter strip array, emitting from ionic liquid EMI-BF 4 and capable of up to 10's of µA beam current, details of which can be found elsewhere. 6 For electrospray devices, indirect thrust measurement by time-of-flight mass spectrometry (ToF-MS) has been the most widely used method, but it requires a good knowledge of the beam composition. [6][7][8][9] Direct measurement of thrust has also been performed for some electrospray propulsion devices.…”

“…6 For electrospray devices, indirect thrust measurement by time-of-flight mass spectrometry (ToF-MS) has been the most widely used method, but it requires a good knowledge of the beam composition. [6][7][8][9] Direct measurement of thrust has also been performed for some electrospray propulsion devices. 6,8,19 If an electrospray emitter emits a current I of particles with uniform kinetic energy qV em , the normal thrust on the device can be written as 6 where m 0 and q are mass and charge of a monomer emitted from the ionic liquid.…”

“…1,2 Several miniature thruster technologies, such as, cold-gas thrusters, 1,3 electrospray propulsion using liquid metals 4 or ionic liquids, [5][6][7][8][9][10][11] micro resisojets, 12,13 and micro pulsed plasma thrusters 13 are under investigation as prospective candidates targeting stable thrusts in the range of a few µN to several hundred µN. Several thrust-stands have been reported over the last few years for measuring thrust from these microthrusters with sub-µN resolutions.…”