Time evolution of the electric field in a Hall thruster

In order to characterize the natural oscillations of the electric field, the time-dependent Xe+ ion velocity distribution function is measured in the near-field plume of a low power permanent magnet Hall thruster. The distribution functions are obtained by means of a time-resolved laser-induced fluorescence technique based on a photon-counting method, while time coherence is ensured by applying a sinusoidal potential modulation on a floating electrode. The temporal resolution is 100 ns. The electric field profile is composed of two peaks. This peculiar shape is not yet understood. The electric field is shown to oscillate outside the thruster discharge channel. The results do not reveal the propagation of an ionization front.