Overview of various mechanisms leading to oscillations and instabilities in hall plasmas: Applications to Hall thrusters

Summary form only given. Devices with stationary, externally applied, electric field which is perpendicular to a moderate amplitude magnetic field B0, are common in magnetically controlled plasmas. High interest applications involve Penning type plasma sources, magnetrons and magnetic filters, and electric space propulsion such as Hall thrusters. The electric field produces a stationary current due to the E₀×B₀ electron drift, while ions do not feel the magnetic field due to their large Larmor radius. Such plasmas, widely defined here as Hall plasmas, exhibit a large number of instabilities observed experimentally, which have been shown to affect the operation of these devices. Similar conditions occur in some space plasmas, e.g. in the lower ionosphere of Earth. Plasma fluctuations result in anomalous transport phenomena, e.g. in anomalous electron mobility and heating. Studies of these phenomena have long history in ionosphere physics and Hall thrusters development. The goal of this presentation is to overview various mechanisms leading to plasma fluctuations that might be responsible for anomalous transport in these conditions. We emphasize common physical principles between various types of Hall plasmas, describe some recent theoretical results and experimental observations.Stationary electron current is a robust and principal source of free energy in Hall plasmas in above applications. Interaction of this current with plasma inhomogeneities, e.g. gradients of plasma density, temperature and magnetic field result in the so called anti-drift type modes. The electron flow also result in instabilities of negative energy ion sound modes destabilized by dissipation due to collisions and feedback response of plasma sheath. Dissipation also result in destabilization of low-hybrid modes. The latter are also may become unstable due to the stationary flow of accelerated ions. Smaller scale modes may be destabilized due to the electron cyclot- on resonance. This variety of instabilities creates a complex picture of fluctuations of different frequencies and at different length scales. We describe the current state of the theoretical research in this area by presenting the quantitative characteristics of these instabilities and potential ramifications for Hall thrusters.