Study on the plasma characteristics in a pulsed plasma thruster with a non-volatile liquid propellant by optical emission spectroscopy

In previous studies, the non-volatile liquid perfluoropolyether (PFPE) has shown potential to suppress carbon deposition when used as a propellant for pulsed plasma thrusters (PPTs). However, those studies did not focus on the plasma characteristics within the discharge channel under PFPE operation. This study investigates the plasma behavior in both side and front views of the discharge channel using a spectroscopic measurement system, comparing PFPE with the conventional solid propellant polytetrafluoroethylene (PTFE). The time-resolved distributions of C⁺, C²⁺, F⁺, electron temperature, and electron density were analyzed to understand ion population dynamics and ionization levels. The results reveal that due to the differing material properties of solid PTFE and liquid PFPE, using PTFE as the propellant tends to form an inclined current sheet, whereas PFPE leads to more uniform ion generation across the surface. When using PTFE as the propellant, ions generated in the upper region exhibit downward motion, which may reduce thruster performance. In contrast, the use of PFPE produces a vertical current sheet that causes the ion populations to eject horizontally. The distributions of electron temperature and density indicate that the ionization degree may be slightly higher when PTFE is used. However, the use of PTFE also leads to localized regions of low ionization and insufficient ablation, particularly on the lower half of the surface. Additionally, ions produced in the upper-right region of the PTFE surface are more likely to escape the discharge channel, potentially due to the non-uniform distribution of arcs across the surface. By comparison, the use of PFPE enables more uniform ablation and ionization, with ion populations diffusing more evenly and at lower velocities during discharge.