Description
Hall thrusters are plasma-discharge-based propulsion devices for spacecraft, where ions are accelerated by an axial electric field E to generate thrust. A radial magnetic field B is applied perpendicular to the electron current, reducing electron mobility across the field lines. This sustains the axial electric field E and confines electrons, creating a complex magnetized plasma environment. Such environments are inherently prone to instabilities and turbulence due to the disparate magnetization and temperature of ions and electrons, electron drifts, and plasma gradients (leading to velocity stratification).
The system exhibits a rich spectrum of instabilities, from a few kHz to GHz, which have been extensively studied since the 1970s [Esipchuk et al., Technical Soviet Physics, 1973]. Two low-frequency instabilities are particularly notable: the breathing mode (BM), a global low-frequency (1–30 kHz) oscillation of the plasma characterized by axial displacement of the ionization front, and ion transit time oscillations (ITTO), an axial electrostatic instability occurring at 100–500 kHz. Previous research has identified potential interactions between these low-frequency instabilities, as the ITTO emerge periodically with BM oscillations [Delaviere-Delion et al., Phys. Plasmas, 2024].
In this study, we investigate the interactions among low-frequency instabilities in partially magnetized Hall thrusters, focusing on nonlinear coupling and energy transfer mechanisms. Our approach combines time-resolved experimental diagnostics with advanced hybrid numerical simulations, enabling a detailed exploration of instability dynamics. Using high-order spectral analysis on both experimental and simulated data, we identify clear evidence of nonlinear coupling between the BM, ITTO, and their harmonics. Our preliminary results indicate that energy is transferred from the BM and an intermediate frequency to drive the ITTO, which exhibits characteristics reminiscent of cascade turbulence. We further analyze several macroscopic quantities to identify the physical mechanisms driving instability propagation and interaction. Beyond advancing the understanding of Hall thruster dynamics, these findings offer general insights into the behavior of partially magnetized cross-field plasmas characterized by multi-scale instabilities.