Impact of Azimuthal Magnetic Field Inhomogeneity on Hall Thruster high-frequency azimuthal instability via 2D radial-azimuthal PIC simulations

TL;DR

This work addresses how inherent azimuthal magnetic field inhomogeneity in SPT-type Hall thrusters affects high-frequency azimuthal instabilities and cross-field electron transport. Using 2D radial-azimuthal PIC simulations, it maps the response over azimuthal length $L_z$ and inhomogeneity $\alpha_{inh}$, focusing on ECDI, MTSI, and a long-wavelength mode ($\lambda$-long). Key findings show that mobility is stable for $\alpha_{inh}\leq5\%$ but drops by about $15.7\%$ at $10\%$, while stronger inhomogeneity broadens the ECDI spectrum, reduces discrete instability peaks, and redistributes energy to broader wavenumbers, leading to a more turbulent state and reduced amplitudes of ECDI and $\lambda$-long in nonlinear saturation; gradient drifts contribute to azimuthal energy distribution with modest mobility impact. These results imply that azimuthal magnetic inhomogeneity can substantially reshape instability dynamics and transport in Hall thrusters, informing magnetic circuit design and motivating further study of alternative transport channels and near-wall effects.

Abstract

For the SPT-type Hall thrusters, the magnetic structure with magnetic conductive columns leads inherent to azimuthally inhomogeneous magnetic configurations. This azimuthal magnetic inhomogeneity may impact electron azimuthal closed-drift motion and cross-field transport characteristics. This study systematically investigates the effects of azimuthal magnetic field gradient on high frequency azimuthal instability and associated anomalous electron transport through 2D radial-azimuthal Particle-in-Cell (PIC) simulations. The results reveal dual mechanisms of magnetic inhomogeneity on electron cyclotron drift instability (ECDI) characteristics: (1) The azimuthal drift velocity distribution becomes modulated by the magnetic field inhomogeneity, with increased average drift velocity enhancing ECDI intensity under stronger inhomogeneity; (2) Simultaneously, the ECDI wavenumber spectrum broadens with elevated magnetic inhomogeneity, reducing discrete ECDI spectral peaks. Under the dual influence of magnetic field inhomogeneity, when the inhomogeneity level is below 5%, the ECDI saturation amplitude and electron cross field mobility remains largely unchanged. However, a notable reduction of 13.4% in ECDI saturation intensity and a 15.7% decrease in electron mobility are observed when magnetic field inhomogeneity reaches 10%.

Figures (19)

• Figure 1: (a) The SPT-type Hall thruster magnetic structure with four magnetic conductive columns, the red line indicates the the median circumference of the channel exit. (b) The radial magnetic field intensity distribution along the red line, obtained by magnetic field simulation software.
• Figure 2: Computational domain setup
• Figure 3: Comparisions between different azimuthal lengths, (a) ion number density evolving with time, and (b) radial electron temperature evolving with time.
• Figure 4: Comparison of time-averaged azimuthal electric field amplitudes at different normalized wavenumbers during 21-27 $\mu$s, $k_0$ = 7035 $\mathrm{m^{-1}}$. And this figure delineates the wavenumbers corresponding to the ECDI, MTSI and $\lambda$-long mode. The three distinct peaks observed between ECDI and MTSI arise from the resonant interaction of multiple instability modes at beat frequencies.
• Figure 5: Azimuthal magnetic field configuration of the four simulation cases.