Thomson Scattering Measurements of Anomalous Electron Transport in a Hall Thruster*

Hall thrusters are efficient, moderate-thrust electric propulsion devices which accelerate ions through an annular, crossed-field discharge to achieve high specific impulse (>2000 s). While these devices are commonly used as in-space propulsion systems for both Earth satellites and interplanetary missions, the rapid design and development of this technology is dependent on expensive vacuum testing. This is because the fundamental operation of Hall thrusters is governed by the transport of electrons across a magnetic field. This cross-field electron mobility is experimentally found to be far higher than predicted by classical collisions alone. Experimental and theoretical studies point to a growing consensus that high-frequency, drift-driven, azimuthal plasma turbulence is a primary cause of non-classical momentum transfer to the electrons. However, the precise nature of these wave interactions are not fully understood, precluding high-fidelity modeling of the Hall thruster without experimental inputs. The recent application of incoherent Thomson scattering (ITS) on low-density plasmas such as Hall thrusters has enabled the direct measurement of the electron velocity distribution, including information regarding the transport of energy and momentum via the electrons in the acceleration region of the Hall thruster.  This technique enables characterization of the electron transport within the Hall thruster. We carry out Thomson scattering measurements to infer the effective electron Hall parameter in a laboratory Hall thruster channel. This is accomplished using a simplified Ohm’s law electron force balance. With this result, we provide an avenue for directly informing simulations of Hall thruster physics with experimental transport profiles, paving the way for first-principles modeling investigations of the sources of anomalous electron resistivity in these devices.