This paper presents a linear analysis of gradient plasma instabilities in Hall thrusters. The study obtains and analyzes the dispersion equation of high-frequency electromagnetic waves based on the two-fluid model of a cold plasma. The regions of parameters corresponding to unstable high frequency modes are determined and the dependence of the increments and intrinsic frequencies on plasma parameters is obtained. The obtained results agree with those of previously published studies.
The problem of the anomalous electron transport in crossed electric and magnetic fields in Hall plasma thrusters is investigated. Two mechanisms of turbulent transport are considered: a gradient-drift mechanism and a resistive one. A strong connection between these mechanisms and the corresponding instabilities is established. Our analysis shows that the gradient-drift transport prevails over the resistive one in the most turbulent domain of the channel. The gradient-drift transport organizes plasma in such a way to minimize the increment of the corresponding instability. This allows one to avoid explicit calculations of the fluctuation intensity while modeling transport phenomena. The modification of the dispersion relation for the gradient-drift instability, offered by Frias et al. (Phys. Plasmas, 19 (2012) 072112) to account for the current-free (vacuum) nature of a two-dimensional magnetic field, is discussed.
Location of the ionization and acceleration regions determines erosion belt location and thruster's maximum throughput consequently. For Hall thrusters with wide throttle ratio the location of the ionization and acceleration regions in each operating mode could vary significantly, which makes it difficult to provide necessary throughput for multi-mode operation. In this article, the shift of the ionization and acceleration region location caused by the change of operating mode is studied with the help of numerical method. Numerical investigation was conducted with 1D3V hybrid-PIC simulations. According to the results, the ionization and acceleration regions could move upstream significantly with gas flow rate increase and magnetic field decrease. In addition, it was proved that an increase in the magnetic field gradient shifts the ionization and acceleration region location outside of the channel significantly. The trends obtained in numerical simulations were experimentally testified. The acceleration region trend validation was carried out with electrical probe measurements on a 1.5 kW laboratory Hall thruster. The ionization region shift was validated with the shift of the maximum light intensity of the xenon inside the channel of a 2.3 kW laboratory Hall thruster. Moreover, experimental investigations indicate that the ionization and acceleration regions could move significantly when oscillation mode changes.
According to present knowledge, countless numerical simulations of the discharge plasma in Hall thrusters were conducted. However, on the one hand, adequate two-dimensional (2D) models require a lot of time to carry out numerical research of the breathing mode oscillations or the discharge structure. On the other hand, existing one-dimensional (1D) models are usually too simplistic and do not take into consideration such important phenomena as neutral-wall collisions, magnetic field induced by Hall current and double, secondary, and stepwise ionizations together. In this paper a one-dimensional with three-dimensional velocity space (1D3V) hybrid-PIC model is presented. The model is able to incorporate all the phenomena mentioned above. A new method of neutral-wall collisions simulation in described space was developed and validated. Simulation results obtained for KM-88 and KM-60 thrusters are in a good agreement with experimental data. The Bohm collision coefficient was the same for both thrusters. Neutral-wall collisions, doubly charged ions, and induced magnetic field were proved to stabilize the breathing mode oscillations in a Hall thruster under some circumstances.
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