Spatio-temporal characteristics of ion velocity in a Hall thruster discharge

The time evolution of the axial velocity of a metastable Xe+ ion was examined in the crossed-field discharge of a PPS100-ML Hall thruster fired at 250V by means of laser-induced fluorescence spectroscopy at 834.72 nm. A pulse-counting detection technique was employed to achieve a time resolution of 0.1 mu s. A periodic break of 10 mu s duration of the anode current is used to stabilize the discharge and allow the investigation of both forced and natural plasma oscillations. Measurements were carried out along the channel axis throughout the region of large magnetic field strength. The mean ion flow velocity was found to oscillate at the discharge breathing mode frequency of about 21 kHz. By contrast, the ion velocity dispersion appears not to depend on time, which suggests a strong correlation between ionization and acceleration processes. The spatio-temporal behavior of the electric field was computed from experimental data using a Lagrangian description of the ion fluid motion. As expected, the field amplitude varies significantly at 21 kHz. More surprisingly, an electric field front seems to propagate periodically from the exterior toward the interior of the discharge chamber with a speed close to the thermal speed of the Xe atom.