Investigation of MHD Instabilities in an Applied Field MPD Thruster by means of Measurements of Magnetic Field Fluctuations

The paper describes the main results of an experimental activity, carried out on a gas fed, applied field MPD thruster. The thruster has operated in a pulsed, quasi-steady mode (pulse length: 5 ms). Magnetic fluctuations in the plasma jet have been measured by means of two non-intrusive sets of magnetic probes. The first set consists of two flux probes, turned around the thruster cylindrical insulator. The probes allow the fluctuations of the magnetic field on the entire length of the thruster to be detected and the axial periodicity in axial direction of the instabilities (mode n) to be obtained. The second set consists of an array of five biaxial probes, placed inside the cylindrical insulator of the thruster. By measuring the magnetic field fluctuations, the array allows the azimuthal periodicity (mode m) to be obtained. Tests have been carried out on the thruster with and without the insertion of an insulating plate in the discharge chamber for passive instability suppression. Magnetic field fluctuations and current-voltage electrical characteristics have been measured for both the thruster configurations at 100 mg/s of argon, external applied magnetic fields Bext= 0, 50, 100 mT on the axis, overall thruster power ranging from 60 and 700 kW. Data gathered have shown the onset of instabilities beyond critical current values, having spatial property of a m/n=1/1 kink, with frequencies close to 400 Hz. The insertion of the plate has proved to effectively reduce fluctuations and arc voltage drop at Bext=100 mT. No significant effects have been observed at lower external magnetic field.