  The low-frequency rotating plasma instability (spoke) in the ISCT200 thruster
operating in the wall-less configuration was simulated with a 3 dimensional PIC
MCC code. In the simulations an m = 1 spoke rotating with a velocity of 6.5
km/s in the ExB direction was observed. The rotating electron density structure
in the spoke is accompanied by a strongly depleted region of the neutral gas,
which clearly shows that the spoke instability is of an ionization nature,
similar to the axial breathing mode oscillations. In the simulation the
electron cross-field transport through the spoke core was caused by diffusion
in the high-frequency (4-10 MHz), short-scale (3 mm) electric field
oscillations. These short-scale oscillations play a crucial role in the
thruster discharge as over 70% of the electron current to the anode originates
from the spoke core. The rest of the current originates from the spoke front
where the electron cross-field transport toward the anode is due to the ExB
drift in the spoke macroscopic azimuthal electric field.
