Rotating spoke instabilities in a wall-less Hall thruster: experiments

Properties of rotating spokes have been investigated in the plasma discharge of the 200 W-class permanent magnet ISCT200 Hall thruster (HT) in wall-less (WL) configuration under various operating conditions. For comparison purposes, the HT was also operated in the standard configuration. High-speed camera imaging combined with sophisticated image processing allowed the observation of the plasma inhomogeneity dynamics and the determination of characteristics such as mode number, sizes, rotation frequency and propagation velocity. The time varying Xe+ ion azimuthal velocity distribution function has been measured in front of the anode of the WL thruster by means of laser-induced fluorescence spectroscopy in photoncounting regime. Experiments show ions move at the thermal speed on average, yet some ions have a negative velocity, i.e. they rotate in the direction opposite to the E x B drift, in agreement with numerical simulations performed with a 3D-3V PIC code.