Stationary profiles and axial mode oscillations in hall thrusters

The axial modes (and breathing mode in particular) are among the most violent oscillation modes in Hall thruster. Breathing mode is manifested by strong fluctuations of discharge current and other plasma parameters. It is generally understood as the instability associated with ionization and neutral depletion processes in the thruster channel, but a number of other effects, such as temperature evolution and boundary conditions are also important. Despite long history of studies, exact physical mechanisms and conditions for the instability are not well understood. Within one-dimensional model we have proceeded to study the general case of axial mode instabilities starting from of stationary (time-independent) solutions. The nature and typical characteristics of stationary solutions are established with analytical analysis. It is shown that the existence of the singular sonic point results in “stiff” profiles of the accelerating electric field and other plasma parameters. Such “stiff” profiles have global nature, in particular, show strong dependence on boundary conditions on the anode. Using fluid and hybrid (fluid electrons and kinetic ions/neutrals) simulations we have studied the axial stability of such global profiles. It is shown that axial instabilities are sensitive to the electron/mobility and magnetic field as well as the electron temperature evolution model and losses.