Time-resolved measurement of plasma parameters in the far-field plume of a low-power Hall effect thruster

Time-resolved measurements using electrostatic probes are performed in the far-field plume of a low-power permanent magnet Hall effect thruster. These measurements are necessary in order to account for the non-stationary behavior of the discharge. The plasma potential is measured by means of a cylindrical Langmuir and a sufficiently heated emissive probe, the electron temperature and density are measured with a cylindrical Langmuir probe. The thruster is maintained in a periodic quasi-harmonic oscillation regime by applying a sinusoidal modulation to a floating electrode in the vicinity of the cathode in order to guarantee repeatable conditions for all measurements. The modulation depth of the discharge current does not exceed approximately 10%. In order to achieve synchronism, the frequency of the modulation has to be close to the natural frequency of the observed phenomena. It is different depending on whether the discharge current or the plasma potential is selected as a reference. The measurements show that the fluctuations of the electron density follow the discharge current fluctuations. The time evolution of the plasma potential and the electron temperature is similar. The time-averaged properties of the discharge remain almost uninfluenced by the modulation. Measurements of the plasma potential with the two different probes are in good agreement. The observed phenomena are similar for Xe and Kr used as propellant gases. (paper)