Magnetic Field Simulation and Mapping Based on Zeeman-split Laser-induced Fluorescence Spectra of Xenon in the Discharge Channel of a 5-6 kW Co-Axial Stationary-Plasma Hall Thrusters.

Abstract

We studied the effect of the Hall current’s induced magnetic field on the vacuum field of coaxial Hall thrusters based on MagNet 6 simulation of a 5 kW thruster (P5) and laser-induced fluorescence spectra taken at the exit plane of a 6 kW thruster (H6).

MagNet 6 simulations were performed at power settings of 1.6 kW and 3 kW— each with a proper set coil currents and Hall current density distributions. The Hall current—simulated using a rectangular array of current-carrying coils —induced a significant reduction of the field strength . The radial component of the field strength shifted towards the anode by as much as 10 mm and its gradient lost its mononoticity as the Hall current’s magnitude increased. Additionally, an increase in the concavity of streamlines along the axial direction was noted in the channel.

MagNet simulations were compared to non-intrusive radial field strength measurements at the exit of the 6 kW thruster from Zeeman-split laser-induced fluorescence spectra of neutral xenon (6S′ [1/2] → 6P′ [3/2] about 834.912 nm-vacuum) by fitting measured spectra with Lorentz and Doppler-broadened Zeeman-split fluorescence lineshapes.

Spectra were modeled using a linear Zeeman theory of fine structure splitting of isotopes with zero nuclear spin and a non-linear Zeman model of the splitting of hyperfine lineshape components of isotopes with non-zero nuclear spin. The resulting isotopes’ line spectra were shifted, scaled (by natural abundance) and Lorentz- and Doppler-broadened (Voigt-profile) to yield neutrals’ absorption spectrum.

A commercial non-linear least-squares solver was then used to compute radial components of the magnetic field strength and axial kinetic temperatures that minimize the fitting error between experimental and simulated spectra. The solver was applied to laser-induced fluorescence spectra measured at the exit plane of a 6 kW Hall thruster (H6). The resulting magnetic field strength calculations revealed, as in the MagNet 6 simulation study of the P5, that the plasma induces a reduction of the radial component of the field strength. Axial kinetic temperatures of neutral xenon particles remained fairly constant near the channel’s centerline; closer to the inner wall, however, the temperature was found to be twice larger.