Non-Invasive Measurements of the Electron Mobility in a Hall Effect Accelerator

The anomalously high mobility of electrons across magnetic field lines in crossed-field, Hall effect accelerators is one the most dominant but least understood processes in these low-temperature plasmas. This lack of understanding stems largely in part from the difficulty of making direct experimental measurements of this process. Indeed, the types of direct physical probing that traditionally would yield this measurement have been shown to be highly perturbative in these devices [1]. In this work, we demonstrate that the cross-field transport can be inferred non-invasively by measuring the ion dynamics in the Hall direction of the discharge. This technique hinges on the assumption that the mechanism that drives the electron transport is the result of a momentum-conserving interaction with the ions. The process thus works to drive electrons across field lines while also imparting momentum to the ions in the Hall direction. By measuring this component of the ion momentum non-invasively with Laser Induced Fluorescence (LIF) and employing a quasilinear approximation to relate the ion and electron dynamics, it is possible to infer the effective electron mobility in the discharge.