Diagnostics of ion and electron diffusion in pulsed plasma thrusters using neutral gas injection

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.