Analysis of beam currents under an oscillating cone-jet mode for developing high-precision electrospray thrusters

Abstract

As an application of electrospray, electrospray thrusters are preferred to work in a steady electrohydrodynamic mode to output high-precision and adjustable thrust through controlling voltages and flow rates. However, voltages and flow rates can only be modified in a small range to avoid mode transiting, thereby limiting the thrust range. To increase the thrust, adding the number of emitters or more grids for beam acceleration is considered, but it inevitably increases mechanical complexities and risks of component failure. In this paper, electrosprays are investigated by analyzing the beam current over the time and frequency domains. Results suggest that the steady cone-jet transfers into an oscillating cone-jet with a frequency of several kHz as the flow rate increases. In the oscillating cone-jet mode, beam currents and flow rates still follow the power-law relation, which is derived for the steady cone-jet. The finding suggests that setting a desired thrust over the oscillating cone-jet mode is feasible, whose maximum thrust allowed is 2.3 times the one in the steady cone-jet mode. The calculated thrust noises of the oscillating cone-jet mode are lower than 0.1 μN/Hz0.5 in the millihertz band, which meets the requirement of most drag-free satellites. The oscillation frequency is also adjustable by changing voltages or flow rates, offering another control parameter for electrospray devices. Compared to other instable modes, the oscillating cone-jet mode demonstrates superior low-noise output and controllability, thereby making it another suitable operational mode for high-precision electrospray thrusters in addition to the steady cone-jet mode.