Hybrid study on the mechanisms, characteristics, and parametric dependence of ion transit-time instability in Hall thrusters

Abstract During the discharge process of Hall thrusters, various plasma instabilities occur across frequency bands. The ion transit-time instability (ITTI), with frequencies of 100–500 kHz and periods matching ion transit times through the acceleration region, significantly impacts ion energy distribution and thruster performance. While influenced by channel structure and discharge conditions, its mechanisms need clarification. This study develops a one-dimensional axial hybrid model, treating ions and neutrals kinetically via particle-in-cell methods and electrons as a fluid, to investigate ITTI generation and effects of channel length, gas mass flow rate, and magnetic field strength. Results show the ITTI originates from ion acoustic waves driven by electron pressure gradient, inducing density perturbations and electric field oscillations that propagate downstream at ion-like phase velocities. This causes periodic variations in discharge current, ion velocity and density, generating ions into low- and high-energy groups, with low-energy ion packet dominating ITTI propagation. Stronger magnetic fields intensify the electric field and amplify ITTI oscillations. Longer channels modify magnetic field distribution, increasing electron mobility and reducing ITTI amplitude and frequency. Higher mass flow rates enhance ion flux, amplify electric field oscillations and raise the ITTI amplitude and frequency. Findings validate the dispersion relation theory, with growth rate inversely proportional to the square root of electron mobility and frequency correlated with ionization rate and atom density.