Instability mechanism and discharge regime diagnosis of microthrusters based on plasma properties

In order to make it possible to control the plasma state and predict the regime transitions via coupling optical and electrical diagnosis in aerospace engineering, we have experimentally investigated the regime transitions under 0.1-15 kPa with an input discharge power of 0-25 W in a parallel-plate electrode configuration. An abnormal glow discharge (AGD), filamentary discharge (FD), and arc discharge (AD) are distinguished using the voltage-current characteristics under different gas pressures. The electron excitation temperature (T_e), electron density (N_e), spatial resolutions of T_e and N_e, and ionization degree are obtained via optical emission spectroscopy to reveal the transition mechanisms. Thermal instability, characterized by T_e, plays a dominant role during the transition from an AGD to an FD. The conclusions are supported by analysis of ionization degree, whereas electronic instability becomes the dominant mechanism in the transition from an FD to an AD. This is related to collision kinetics because of an observed drop in N_e, which is verified by the spatial resolution as well. Moreover, planar laser-induced fluorescence provides further insight into the instantaneous location and relative number variation of Ar 1s₅ metastable atoms, which agrees well with the plasma properties mentioned above. In addition, a pressure of 1 kPa with a maximum input power of 17.5 W are specified as suitable working parameters for further study when applied to microthrusters due to its higher N_e and better stability.