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.
The problem of determining the electron anomalous conductivity profile in Hall thruster, when its operating parameters are known from the experiment, is considered. To solve the problem, we suggest varying the parametrically set anomalous conductivity profile until the calculated operating parameters match with the experimentally measured ones in the best way. The axial 1D3V hybrid model was used to calculate the operating parameters with parametrically set conductivity. Variation of the conductivity profile was performed using Bayesian optimization with a Gaussian process (machine learning method), which can resolve all local minima, even for noisy functions. The calculated solution corresponding to the measured operating parameters of Hall thruster in the best way proved to be unique for the studied operating modes of KM-88. The local plasma parameters were calculated and compared with the measured ones for four different operating modes. The results show the qualitative agreement. An agreement between calculated and measured local parameters can be improved with a more accurate model of plasma-wall interaction
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