• Breathing Mode
• Forced Oscillations
• Hall Thruster
• Plasma

This thesis investigates the dynamic response of a Hall Effect Thruster (HET) to
externally imposed oscillations in the anode voltage, with the aim of understanding and
potentially controlling the natural low-frequency breathing mode instability. A one-
dimensional unsteady plasma model—previously calibrated to reproduce the nominal
behavior of Sitael’s HT5k thruster—is used as the simulation framework. Sinusoidal
voltage perturbations of varying amplitude and frequency are superimposed on the
baseline discharge voltage, and the resulting system behavior is analyzed in terms of
discharge current, plasma density, and spectral properties.
Key metrics such as the average current, oscillation amplitude, dominant frequency,
and peak spectral density are extracted for each operating point. A classification of
the responses is proposed, distinguishing between lock-in, suppression, amplification,
and mixed regimes. Correlation analysis reveals strong coupling between spectral and
current-based indicators, particularly under lock-in conditions. An empirical lock-in
boundary is derived as a function of forcing amplitude and frequency offset.
The physical mechanisms behind these phenomena are interpreted through energy
balance considerations and the predator–prey model of ionization instability. Notably,
suppression is linked to the disruption of the ionization–replenishment feedback loop
by externally driven electron dynamics. The findings offer insight into future control
strategies for Hall thrusters, including suggestions for feedback-based modulation of
propellant or magnetic field inputs.