In this work, a force measurement system is proposed to measure the thrust of plasma microthruster with thrust magnitude ranging from sub-micro-Newtons to hundreds micro-Newtons. The thrust measurement system uses an elastic torsional pendulum structure with a capacitance sensor to measure the displacement, which can reflect the position change caused by the applied force perpendicular to the pendulum axis. In the open-loop mode, the steady-state thrust or the impulse of the plasma micro-thruster can be obtained from the swing of the pendulum, and in the closed-loop mode the steady-state thrust can be obtained from the feedback force that keeps the pendulum at a specific position. The thrust respond of the system was calibrated using an electrostatic weak force generation device. Experimental results show that the system can measure a thrust range from 0 to 200 μN in both open-loop mode and closed-loop mode with a thrust resolution of 0.1 μN, and the system can response to a pulse bit at the magnitude of 0.1 mN s generated by a micro cathode arc thruster. The background noise of the closed-loop mode is lower than that of the open-loop mode, both less than 0.1 / mN Hz in the range of 10 mHz to 5 Hz.

Pulsed plasma thrusters (PPTs) have been widely used in space flight applications due to their low cost and robustness. Recently, it has been observed that during the operation of a PPT, the plasma plume exhibits an in-plume acceleration phenomenon where the leading-edge ions continue to accelerate in the plume even in the absence of electromagnetic forces. The physical mechanism behind the phenomenon may be ambipolar diffusion in nature. However, details regarding the propagation of electrons within the plume are still unknown. To better understand the complete plasma diffusion process, it is important to understand the behavior of both ions and electrons. To investigate this mechanism, a diagnostic method using neutral gas injection is proposed to estimate the velocity of fast electrons within the plasma plume. This is a new method where neutral gas is injected at the plasma plume edge to increase the local visible light emission and ionization rate of electrons. In addition to this, a triple Langmuir probe, bandpass filter, and spectrometer were also used to study the plume characteristics. From the experimental results, we estimate that the exhaust velocity of the electrons is over 200 km s −1 , roughly an order of magnitude higher than the ion velocity (26 km s −1 ). The acceleration of ions and the corresponding deceleration of electrons with downstream distance was observed using time-of-flight probe data. Furthermore, the spectral data was also used to identify electron deceleration within the plume. These results further suggest that ambipolar diffusion of electrons and ions may exist in the PPT plume, and that the in-plume acceleration of ions is due to the ambipolar electric field in the plume. Another implication is that depending on the length scales, the plasma plume of the PPT can be non-quasineutrally distributed during the initial period of plume formation.

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