Enrique Bello-Benítez and Eduardo Ahedo published the Manuscript “Axial-azimuthal, high-frequency modes from global linear-stability model of a Hall thruster” (DOI: 10.1088/1361-6595/abde21) in Plasma Sources Science and Technology.
Axial-azimuthal instabilities of a Hall-thruster plasma discharge are investigated using fluid model and a linear global stability approach, appropriate to the large axial inhomogeneity of the equilibrium solution. Electron pressure and electron inertia are considered in both the equilibrium and perturbed solutions. Fourier transform in time and azimuth are taken and the dispersion relation for the resultant Sturm-Liouville problem governing the axial behavior of the modes is numerically obtained. The analysis, focused in mid-to-high frequencies and large wavenumbers identifies two main instability types. The dominant mode develops in the near plume at 1-5 MHz and azimuthal mode numbers ~ 10-50, has a weak ion response and seems to be triggered by negative gradients of the magnetic field. The subdominant mode develops near the anode at 100-300 kHz and azimuthal mode numbers ~ 1-10, and seems of the rotating-spoke type. Both instabilities are well characterized by investigating their oblique propagation, the influence of design and operation parameters, and the effects of anode-cathode electric connection, electron inertia, and temperature perturbations. At high frequencies, the dominant mode presents significant non-neutral effects. The possible impact of these instabilities on electron cross-field transport is estimated through a quasilinear approach, which yields a spatially-rippled turbulent force.