Characterization of Momentum and Heat Flow in Hall Thrusters with Laser Scattering

Abstract

Non-invasive measurement techniques are used to investigate electron and ion flows in Hall thruster and hollow cathode discharges to improve predictive models for performance and lifetime. This study combines recent advancements in incoherent Thomson scattering for low-density plasmas with laser-induced fluorescence (LIF) velocimetry to obtain direct, kinetic, and non-intrusive measurements of electron and ion properties in electric propulsion plasmas.

For the first time, Thomson scattering is used to directly measure the axial Mach number of current-carrying electrons in a hollow cathode plume. An empirical relationship between this Mach number and background plasma properties is established, consistent with marginally stable ion acoustic turbulence. This provides a constraint for modeling hollow cathode turbulence from first principles using a fluid framework for electron particle, momentum, and energy conservation.

These diagnostics are applied to spatially resolve the sparser electron population in the near-field plume of a 9-kW magnetically shielded Hall thruster. A theoretical method is developed to infer the anomalous transport profile, representing turbulent electron scattering forces essential to Hall thruster operation. This technique is validated against transport estimations from models, and parametric studies are conducted on transport variation with propellant type, discharge voltage, and current. The results confirm that electrons cross field lines with Bohm scaling downstream of the ion acceleration region but become trapped in a high-confinement region, increasing the electric field and temperature.

Analysis of the electron temperature profile in the Hall thruster plume reveals that peak electron temperature, measured via Thomson scattering, scales with discharge voltage but at a factor of 2.5 higher than traditionally expected. This aligns with prior findings in low-power Hall thrusters and suggests that pressure effects have been underestimated. To understand the heat flow leading to these high temperatures, a two-dimensional mapping of electron and ion properties is performed using Thomson scattering and LIF. The widely held assumption of constant electron temperature along magnetic field lines is found to be inaccurate, with significant variation along the field and a central hot spot of 40-100 eV electrons in the Hall thruster channel. A collisional argument explains this gradient in terms of heat flux restrictions not captured in current fluid models. The effective polytropic index for thermodynamic electron expansion is examined, showing that in the far-field plume, where plasma turbulence drives Bohm-like transport, electron heat flux transitions from adiabatic to isothermal scaling.

Beyond time-averaged studies, time-resolved extensions of the LIF and Thomson scattering diagnostics are explored. A nonlinear algorithm, “Shadow Manifold Interpolation,” is introduced to map noisy pulsed diagnostics like Thomson scattering to a noise-free reference signal, such as discharge current, representing the thruster’s state as a dynamical system. This method is validated against standard transfer function estimation. Preliminary time-resolved Thomson scattering data suggest small fluctuations in the electron velocity distribution, though more signal is needed to resolve plasma oscillations. Time-resolved LIF measurements investigate azimuthal ion waves near the cathode and their potential to enhance erosion by energizing ions.

These findings provide new insights into electron and ion flow physics in Hall thrusters, contributing to fluid-based modeling efforts. Comparisons between Thomson scattering and other electron temperature estimation methods, such as Langmuir probes, highlight discrepancies that warrant further study.