In-plume acceleration of leading-edge ions from a pulsed plasma thruster

Pulsed plasma thrusters are a form of electric propulsion for spacecraft. Their research has undergone recent growth due to an increasing interest in small satellites. However, their plasma plumes are transient and challenging to study due to the extremely short discharge time that lasts in the order of microseconds. The plasma plume is accelerated by a combination of electrothermal and electromagnetic forces. With electromagnetic acceleration, the ions are typically thought to be only accelerated within the thruster by the Lorentz force. Estimated downstream exhaust velocities have ranged widely from 5 to 60 km s^-1, with values on the lower end corresponding to electrothermal acceleration. However, we show here that the fastest leading-edge ions continue to accelerate in the plume even in the absence of electromagnetic forces. Pulsed plasma thruster plumes are composed of electrons and a mixture of ion species with different ionization levels. Due to this quasi-neutrality of the plasma plume with electrons featuring much larger velocities than ions, we suggest that the acceleration might be ambipolar in nature. The leading-edge ions eventually reach velocities in excess of 100 km s^-1, far greater than values typically associated with electrothermal acceleration. This indicates that care must be taken in the accurate measurement of the ion velocity in the plume. The ion plume may also be more energetic than previously believed. Furthermore, we also propose that simple extrapolation can be used to determine the actual exit plane ion velocity; this value can be used as accurate input data for future simulations.