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

S. Mazouffre, L. Grimaud, S. Tsikata, K. Matyash and R. Schneider
Plasma Sources Sci. Technol. 28, 054002 (2019)

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 photon counting
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 × B drift, in agreement
with numerical simulations performed with a 3D-3V PIC code.