Reduced-order particle-in-cell simulations of a high-power magnetically shielded Hall thruster

High-power magnetically shielded Hall thrusters have emerged in recent years to meet the needs of the next-generation on-orbit servicing and exploration missions. Even though a few such thrusters are currently undergoing their late-stage development and qualification campaigns, many unanswered questions yet exist concerning the behavior and evolution of the plasma in these large-size thrusters that feature an unconventional magnetic field topology. Noting the complex, multi-dimensional nature of plasma processes in Hall thrusters, high-fidelity particle-in-cell (PIC) simulations are optimal tools to study the intricate plasma behavior. Nonetheless, the significant computational cost of traditional multi-dimensional PIC schemes renders simulating the high-power thrusters without any physics-altering speed-up factors unfeasible. The novel reduced-order “quasi-2D” PIC scheme enables a significant reduction in the computational cost requirement of the PIC simulations. Thus, in this article, we demonstrate the applicability of the reduced-order PIC for a cost-efficient, self-consistent study of the physics in high-power Hall thrusters by performing simulations of a 20 kW-class magnetically shielded Hall thruster along the axial-azimuthal and radial-azimuthal coordinates. The axial-azimuthal quasi-2D simulations are performed for three operating conditions in a rather simplified representation of the thruster’s inherently 3D configuration. Nevertheless, we have resolved self-consistently an unprecedented 650 µs of the discharge evolution without any ad-hoc electron mobility model, capturing several breathing cycles and approximating the experimental performance parameters with an accuracy of 70 to 80 % across the operating conditions. The radial-azimuthal simulations, carried out at three cross-sections corresponding to different axial locations within the discharge channel, have casted further light on the evolution of the azimuthal instabilities and the resulting variations in the electrons’ cross-field mobility and the plasma-wall interactions. Particularly, we observed the development of a long-wavelength, relatively low-frequency wave mode near the exit plane of the thruster’s channel that induces a notable electron transport and a significant ion heating

On the onset of breathing mode in Hall thrusters and the role of electron mobility fluctuations

Breathing mode is an ionization instability which is observed ubiquitously in the operation of Hall thrusters. It is recognized as a relatively low frequency (10-30 kHz) longitudinal oscillation of the discharge current and the plasma parameters. Although breathing instability is widely studied in the literature, the conditions for its origin are not fully understood. In this work we investigate the mechanisms responsible for the origin of the breathing mode in Hall thrusters by using a numerical model, allowing us to highlight the importance of electron mobility fluctuations for the onset and self-sustenance of the instability. Our one-dimensional, fully fluid model of the thruster channel is calibrated against the measured discharge current signal for a 5 kW-class Hall thruster operating in a condition where breathing mode is fully developed. The corresponding steady, unstable configuration (base state) is numerically computed by applying the Selective Frequency Damping (SFD) method. Then, a series of numerical tests is performed to show the existence of a feedback loop involving fluctuations around the base state of the neutral density, electron mobility, and electric field. We show that oscillations of the electron mobility are mainly caused by variations of the neutral density and are in phase with them; this, in turn, induces oscillations of the electric field, which are in phase opposition. The electric field acts simultaneously on the electron temperature and on the ion dynamics, promoting the depletion and replenishment of neutrals in the chamber

Design for test and qualification through activity-based modelling in product architecture design

Test and qualification (T&Q) phases take a significant portion of the time to market for critical products in the space industry, especially when introducing new technologies. Since T&Q are treated as standard procedures, they tend to be independent of the architectural design phases and kept away from design decisions. However, when introducing new technologies, qualification procedures may differ from those established in regular design scenarios, and the estimation of qualification costs and duration is problematic. There is a lack of design for qualification methods capable of modelling these activities in early phases and use those models to support the architecture design of products with affordable test and qualification phases. In this article, a computer-assisted, model-based design method to model T&Q activities concerning early product architecture designs is proposed. Product architecture alternatives, test schedules and cost are connected through the quantification of T&Q drivers and driver rates. The design method is demonstrated using a case study about electric propulsion for satellites. The method is applicable for design situations where the choice of technology has a strong dependence on the qualification procedure

The selection of an electric propulsion subsystem architecture for high-power space missions

The arise of high-power electric propulsion is paving the way towards new horizons of space exploration. Hall thrusters represent a promising propulsion concept, able to fulfil challenging mission requirements for both commercial and exploration applications. This technology offers several benefits in terms of flexibility of operation, extensive lifetime and high reliability. However, the design of a high-power electric propulsion subsystem (E-PROP) still presents challenges to address. Filling the corresponding technological gaps will open new market opportunities, owing mainly to the extension of mission capabilities and the reduction of the overall mission costs. Therefore, investigations of innovative technology alternatives will allow to identify the most promising E-PROP architectures for various high-power mission scenarios. One of the most critical trade-off to perform is between a high-power monolithic thruster and a cluster of thrusters of lower power. Another criticality is the amount of propellant necessary to perform high delta-v missions. The high price of xenon prompted the investigation on alternative propellants, such as krypton. The propellant selection should consider the impact on different aspects of the platform design, including performance, system complexity and mission costs. Last, due to the high-power levels that the E-PROP shall manage, a different architecture can be implemented by adopting the direct-drive approach, i.e. a direct and non-isolated connection between the solar array and the thruster. However, even if the disruptive direct-drive technology allows a significant reduction in the EP system mass and cost, its implementation rises additional challenges to the design of the spacecraft power subsystem. This paper analyses the impact of innovative architecture solutions on the design of a high-power E-PROP. In the framework of this research, we first carried out an extensive investigation of possible mission scenarios and we derived corresponding mission requirements and constrains. Then, we performed three technological trade-offs: monolithic 20 kW vs 5 kW cluster configuration, Xe vs Kr propellant and direct-drive vs standard PPU. All the analysis are based on the experimental data obtained during the 5 kW and 20 kW thrusters development and characterisation at SITAEL. We characterized each design option through several figures of merit, evaluating them for each identified mission scenario. We exploited an Analytical Hierarchy Process for the trade-off analyses and a Monte Carlo method to perform the preliminary evaluation of the trade-off weights. The analyses are based on the research activities that are currently ongoing at SITAEL and PoliTo in the framework of 20 kW E-PROP development programmes. The results of the work highlight the effects of each architecture alternative on both platform design and mission performance

EPS architecture analysis for future highpower missions

The space tug can represent a valid solution to provide transportation capabilities for future space missions. In particular, the tug can be effectively adopted for different applications such as electric orbit raising for commercial satellites and cargo transfer to resupply space infrastructures. The adoption of high-power electric propulsion is a fundamental enabler for these mission scenarios, owing to its advantages in terms of long lifetime, high performance and operational flexibility. However, further investigation should be performed in order to optimize the design of the space tug considering different architecture alternatives. We defined two sets of thruster operative points for a more representative comparison of EPS architecture cases. In particular, we analyzed three aspects: the adoption of a cluster of thrusters vs the monolithic approach; the implementation of a direct drive power supply vs the traditional power processing unit; the selection of krypton propellant vs xenon. The design of the space tug is performed with MAGNETO tool, a software developed in a collaboration between SITAEL and Politecnico di Torino in the framework of an ESA GSTP project. The results are compared by means of the analytical hierarchy process to identify the optimal design solution for the spacecraft design

Hybrid plasma simulations of a magnetically shielded Hall thruste

Numerical simulations of a magnetically shielded Hall effect thruster with a centrally mounted cathode are performed with an axisymmetric hybrid particle-in-cell/fluid code and are partially validated with experimental data. A full description of the plasma discharge inside the thruster chamber and in the near plume is presented and discussed, with the aim of highlighting those features most dependent on the magnetic configuration and the central cathode. Compared to traditional magnetic configurations, the acceleration region is mainly outside the thruster, whereas high plasma densities and low temperatures are found inside the thruster. Thus, magnetic shielding does not decrease plasma currents to the walls, but reduces significantly the energy fluxes, yielding low heat loads and practically no wall erosion. The injection of neutrals at the central cathode generates a secondary plasma plume that merges with the main one and facilitates much the drift of elec- trons toward the chamber. Once inside, the magnetic topology is efficient in channeling electron current away from lateral walls. Current and power balances are analyzed to assess performances in detail.This work has been supported by the EDDA project, funded by the European Union’s Horizon 2020 Research and Innovation Program, under Grant Agreement No. 87047

Testicular Histomorphometric Evaluation of Zebu Bull Breeds.

The objective of this study was to evaluate the quantitative histology and testicular biometrics in zebu bulls of different breeds. Testicular fragments of Nelore (n=10), Polled Nelore (n=6), Gir (n=5), Guzerat (n=5) and Tabapuã bulls (n=5) were used. The fragments were perfusion-fixed in Karnovsky solution, embedded in glycol methacrylate and stained with toluidine blue-1% sodium borate. The Nelore animals had a higher tubular volumetric proportion (85.2%) and greater height of the seminiferous epithelium (73.2 ?m) than the Gir, Guzerat and Tabapuã breeds. The Nelore animals also had a higher volumetric proportion of Leydig cells (5.2%) than the Guzerat and Tabapuã breeds. There was no significant difference for any of these parameters between the Nelore and Polled Nelore breeds. The gonadosomatic index, seminiferous tubule diameter, cross-sectional area of the seminiferous tubule and tubule length (total length and length per gram of testicular parenchyma) did not vary among the breeds studied. The morphometric parameters evaluated suggested that the genetic selection applied to the Nelore and Polled Nelore breeds improved the efficiency of spermatogenesis in these breeders

Progress on the Development of an Iodine-fed Hall Effect Thruster

The paper deals with the results of an ongoing activity carried out by the Department of Civil and Industrial Engineering (DICI) and the Department of Chemistry and Industrial Chemistry (DCCI) of the University of Pisa (UniPi) in collaboration with SITAEL SpA, aimed at the development of technologies for Iodine-fed Hall Effect Thrusters. A feeding system architecture is described and the results of reduced order numerical models of the feeding system are illustrated, in both steady and unsteady state conditions. An activity for iodine interaction with materials is in progress. The experimental setups for material characterization tests are described. Material samples can be heated from room temperature up to 300 °C and exposed to iodine at high (soakage test) or low (flow test) concentration, simulating the condition at which the materials will undergo in the propulsion system, in the vacuum facility or in the spacecraft. Calibration and preliminary soakage test results are illustrated. On the thruster unit side, the candidate thruster and cathode are presented along with the modifications needed to operate them on iodine. Finally, a description of the foreseen test campaign and associated facilities is presented