Observation of fast Alfvén wave in the plasma plume of the KAERI Applied-Field MagnetoPlasmaDynamic thruster*

We observed electromagnetic fluctuations between 300 and 500 kHz in both ion saturation current and magnetic field measurements in the plume of the KAERI Applied-Field MagnetoPlasmaDynamic (AF-MPD) thruster. Plasmas were ignited with D₂ gas in this study and the plasma current was varied between 55 A and 155 A. The electron temperature and density measured by Langmuir probe downstream of the MPD thruster plasma were ∼5 eV and (2–4) × 10¹² cm⁻³, respectively. In order to identify the fluctuations, the dispersion relation and the phase velocity were experimentally obtained using the Beall method and were compared to the theoretical values with warm plasma assumption. It was found that the observed fluctuations were compressional, fast Alfvén waves with a right-handed circularly polarized magnetic field. In addition to the fast Alfvén waves, we also observed electrostatic fluctuations at a higher frequency that was similar to the lower-hybrid frequency and we conjectured that they are the lower-hybrid drift instability which are widely observed in many MPD thrusters.