Arising from the myriad of electromagnetic interactions between excited propellant particles, electrons, and the imposed magnetic and electric fields, HET plasmas (as well as cathodes, ion thrusters, pulsed plasma discharges, etc.) are rich with oscillation in a wide band ranging from 1 kHz to 20 GHz.

The existence of plasma oscillations in the near- and far-field discharge of a Hall effect thruster alters the conventionally held view of their operation as steady electrostatic propulsion devices. Indeed, the consequences from fluctuations in ionized propellant density, temperature, and potential may include increased thrust, exacerbated engine erosion, and spacecraft interference. In this research project, the unsteady nature of a Hall effect thruster discharge has been investigated via two-dimensional, time-resolved plasma measurements. A novel dual Langmuir probe diagnostic (HDLP, see ref. 2) was developed to enable an unprecedented temporal resolution for electrostatically acquired plasma properties near the upper theoretical limits (1-10 MHz, see ref. 3). Observations of large amplitude transient oscillations caused by the Hall thruster breathing mode were seen for all thruster conditions at all spatial locations and in all measured plasma properties including: discharge current, electron density, electron temperature, plasma potential, and visible light emission. A unique method of spatiotemporal data fusion (see ref. 4) facilitates visualization of two-dimensional time-resolved planar plasma density contour maps of discrete turbulent bursts of plasma ejected as the thruster exhales breaths of ionized propellant at velocities in excess of 12 km/s (see fig. 1). An initial time-resolved investigation of the plasma downstream from a Hall thruster unveiled an environment rich in oscillatory behavior dominated by the Hall thruster breathing mode. These insights emphasize the importance of time-resolved plasma measurements and, through enhanced understanding of the discharge process, may ultimately lead to improved thruster designs that work in concert with plasma fluctuations to achieve enhanced performance. (From ref. 1)

The initial 100-kHz HDLP diagnostic developed for this research provided measurements of unprecedented spatial and temporal detail within a HET plume. However, an examination of the temporal limitations of the probe (from ref. 3) suggests that an upper probing rate of 1-10 MHz is possible. Future development of this capability is already underway, and the finer temporal detail (coupled with a faster 1-Mfps high-speed camera) will enable the examination of higher-frequency plasma oscillations such as the azimuthal modes (ionization, transit-time, drift instability, etc.) common in the very-near-field of HETs. To that end, future measurements using this advanced HDLP to perform internal time-resolved measurements would be tremendously insightful, and are thus part of ongoing research. The strong magnetic fields in this region are known to exhibit anomalous electron transport, and with detailed time-resolved measurements, one could examine the nature of turbulent transport. Additionally, the use of a HDLP for obtaining time-resolved electron energy distribution functions was shown to be promising in early work and there is ongoing effort to obtain time-resolved EEDFs at a rate of 1 MHz. Measurements of the unsteady state of electron equilibrium could help identify transient plasma phenomena that could be tailored to enhance the efficiency of ionization and particle acceleration in Hall thrusters and other plasma devices.