Parametric investigation of azimuthal instabilities and electron transport in a radial-azimuthal E x B plasma configuration

Partially magnetized low-temperature plasmas (LTP) in an E x B configuration, where the applied magnetic field is perpendicular to the self-consistent electric field, have become increasingly relevant in industrial applications. Hall thrusters, a type of electrostatic plasma propulsion, are one of the main LTP technologies whose advancement is hindered by the not-fully-understood underlying physics of operation, particularly, with respect to the plasma instabilities and the associated electron cross field transport. The development of Hall thrusters with unconventional magnetic field topologies has imposed further questions regarding the instabilities' characteristics and the electrons' dynamics in these modern cross field configurations. Accordingly, we present in this effort a detailed parametric study of the influence of three factors on the plasma processes in the radial-azimuthal coordinates of a Hall thruster, namely, the magnetic field gradient, secondary electron emission, and plasma number density. The studies are carried out using the reduced-order particle-in-cell code developed by the authors. The setup of the radial-azimuthal simulations largely follows a well-defined benchmark case from the literature in which the magnetic field is oriented along the radius, and a constant axial electric field is applied perpendicular to the simulation plane. The salient finding from our investigations is that, in the studied cases corresponding to elevated plasma densities, a long-wavelength azimuthal mode with the frequency of about 1 MHz is developed. Moreover, in the presence of strong magnetic field gradients, this mode results from an inverse energy cascade and induces a significant electron cross field transport as well as a notable heating of the ions.

Automated summary

Partially magnetized low-temperature plasmas in an E x B configuration have become important in industrial applications, but the underlying physics of operation, such as plasma instabilities and electron cross field transport, are not fully understood. This study investigates the influence of three factors (magnetic field gradient, secondary electron emission, and plasma number density) on the plasma processes in a Hall thruster using particle-in-cell simulations. The findings show the development of a long-wavelength azimuthal mode with a frequency of about 1 MHz in high-density plasma cases, induced by an inverse energy cascade and resulting in significant electron cross field transport and ion heating.