Wall Effects on the Excitation and Propagation of Instabilities in Hall Thrusters

We present recent experimental results of the use of electrostatic probes to characterize the differences seen in the low frequency instabilities (<200 kHz) in a laboratory Hall discharge operating with two very different wall materials alumina and boron nitride. A wavelet decomposition analysis of the probe signals was used to determine the dispersion and turbulent broadening characteristics, which have a strong dependence on the axial location within the channel. The azimuthal direction of propagation is shown to be closely linked to the directions of the density and magnetic field gradient drifts. While the general behavior seen with the two wall materials is similar, a noticeable difference is in the location of the ionization zone, as determined by the location in the peak of the plasma density. The location of the ionization zone with alumina walls remains very close to where the magnetic field peaks, whereas with boron nitride walls, they are separated by 20-30 mm. As a result, the direction of the gradient driven electron drift in these two cases differ somewhat, leading to differences in the propagation characteristics of the disturbances. The possible impact that these disturbances in plasma density may have on electron transport properties is discussed.