Non-linear response of Hall thruster plasma to modulation of the discharge voltage.

For cylindrical Hall thrusters, naturally occurred breathing oscillations have a characteristic frequency of \textasciitilde 13 kHz. The external modulation of the anode potential was applied to make this mode coherent. To determine the driving frequency, a set of natural frequencies was defined from the Fourier transform of the discharge current and the ion current in the plume. By varying driving frequency in the range of 5-20 kHz and monitoring the power spectra of the currents, we found that the coupling of the driving frequency to the intrinsic breathing mode is at frequency of this mode \textasciitilde 13 kHz rather than at the driving frequency. More than that, with the increase of the driving voltage, there is a non-linear response of the discharge current and the ion current as well as a shift of the breathing mode to lower frequencies. We will discuss this interesting non-linear behavior of breathing oscillations in response to driving signal.