Investigation of the ion transit time instability in a Hall thruster combining time-resolved LIF spectroscopy and analytical calculations

Abstract

The performance of Hall thrusters is greatly influenced by the discharge current oscillations. In order to characterize the oscillations of the plasma parameters, we have measured the time-varying Xe ion velocity distribution in the discharge channel and in the nearfield plume of a low-power permanent magnets 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 located in the plasma to achieve resonance with the breathing mode. The measurements are therefore local, non invasive and the temporal resolution is 100 ns. The discharge oscillations are consistent with those of a standard operating point. The axial ion dynamics in the [100 kHz;1 MHz] band, i.e. the so-called ion transit time instability, is examined. The high-frequency spectral properties of the ion mean velocity are compared with a perturbed fluid model of the plasma flow on the source axis. The wavelength of the high-frequency axial mode is similar to the electric field extent outside the cavity. Surprisingly the mode appears only in the area of negative magnetic field gradient.