Spatial evolution of small wavelength fluctuations in a Hall Thruster

The spatial evolution of small wavelength (<1 cm) fluctuations in the Hall direction of a 9-kW class magnetically shielded Hall effect thruster is experimentally characterized. High-speed electrostatic probes are used to measure plasma density perturbations in the acceleration region and near field plume of this cross field device. Two types of waves are observed: broadband turbulence in the low hundreds of kilohertz and megahertz oscillations characterized by distinct peaks in the measured power spectrum. The lower frequency content is shown to be commensurate with linear ion acousticlike waves, while the higher frequency oscillations are indicative of cyclotron resonances. It is found that the higher frequency content dominates the power spectrum in the upstream acceleration zone and saturates at a fraction (<0.002) of the thermal energy. Downstream of this region, the lower frequency content becomes dominant, growing until the total wave energy again saturates. It is proposed that both the upstream and downstream modes are the same oscillation, the electron drift instability, but in different limits. The transition from upstream to downstream dispersion may be explained by the electron drift instability undergoing an inverse energy cascade as it forms and propagates with the ion drift in the discharge.