Manufacturing and testing electric propulsion systems can be very expensive both in economic and time terms. One of the main limiting factors is the manufacturing of ceramic components, such as alumina. However, the advent of 3D printing has made it possible to produce complex-shaped components made of technical ceramics at low cost and in reduced time. This paper illustrates the advancements made by the University of Pisa in Fused Filament Fabrication of alumina components for electric propulsion purposes. Rapid prototyping can help not only in manufacturing thrusters in a faster and cheaper way but also in producing more complex geometries, resulting in more efficient thrusters. The alumina produced through this method has achieved near-full density and a flexural strength of 430 MPa. Components manufactured with this technique successfully passed vibration and pyroshock tests. This manufacturing approach was already used in the framework of the CHEOPS-VHP-BB (Consortium for Hall Effect Orbital Propulsive System-Very High Power-Building Blocks) project to produce the insulating components of a high-current hollow cathode.

Plasma instabilities play a fundamental role in determining the lifetime of hollow cathodes and, consequently, the operative life of electric thrusters. Three main categories are identified: ionization instabilities, rotational instabilities, and turbulent ion acoustic instabilities (IAT). Ionization instabilities typically arise when the cathode operates in plume mode and decrease at lower discharge currents and mass flow rate ratios. Rotational instabilities occur in the presence of an externally applied axial magnetic field, even under spot-mode operation, while IAT develops at frequencies above 100 kHz. Several studies suggest that plasma instabilities are linked to the generation of high-energy ions, responsible for keeper surface erosion and the limit cathode lifetime. This paper presents results on the onset of plasma instabilities in a 25 A-class cathode, investigated using a fast-scanning axial probe in the cathode orifice region. Through Fast Fourier Transform (FFT) and Continuous Wavelet Transform (CWT) analysis, mode frequencies have been identified and their spatial localization along the centerline observed. Without an applied magnetic field, ionization instabilities have been detected at low mass flow rates, with characteristic frequencies increasing as flow decreases. In the presence of an axial magnetic field, rotational instabilities couple with turbulent ion acoustic instabilities, generating more energetic ions, and interact with ionization instabilities when the cathode operates in plume mode. CWT plots demonstrate that rotational bursts are stronger just downstream of the keeper orifice, where sputtering occurs. The dynamics of these rotational instabilities, tracked with the scanning probe, are consistent with kink-type dynamics in the plume, even at low discharge currents.

One-dimensional fluid Hall thruster models are useful tools for the analysis of the plasma evolution within the discharge, and the surge and onset of instabilities. However, this type of model contains a series of simplifications and assumptions that result in a lack of self-consistency, in particular regarding the cross-field electron transport, as well as other uncertainties regarding wall interaction and injection conditions. All this leads to the addition of free parameters, which require calibrating the model against experimental data. Proper calibration requires a systematic exploration of the multidimensional parameter space, involving numerous model runs and time-consuming operations. Moreover, a means for quantifying the goodness of a particular combination of parameter values is necessary. The present research introduces a novel strategy for the automated calibration of fluid plasma models which permits to combine data from different sources, such as discharge current signals, thrust and local plasma measurements, and to better explore the parameter space while greatly reducing the required time and effort with respect to manual calibration. The strategy is developed using a Bayesian approach, which permits to combine physical modelling with multiple types of data, while robustly keeping trace of uncertainties. Likelihood of current signals is evaluated using a Wasserstein distance, while Gaussian likelihoods are used to evaluate thrust and local plasma properties measurements. The present study uses a benchmark model to generate a set of synthetic data to assess the advantages and limitations of the approach, as well as the difficulties arising from the evaluation the uncertainty related to the current signal and to the interaction between the different calibrated parameters. Additionaly, the article shows the limitations of calibrating several parameters relying only on discharge current signals, a clear indication of limited identifiability in the inference process, eventually requiring the use of additional data to obtain reliable calibrations.

In electric propulsion for space applications, searching for alternative propellants is increasingly important due to limited resources and economic considerations. Among the candidates, iodine has emerged as promising thanks to its favorable chemical and physical properties for propulsion and its lower cost and simpler storage compared with xenon. However, its corrosive behavior is a drawback, as iodine reacts with many aerospace materials, and its condensable nature prevents using propellant management systems like those for noble gases. At the University of Pisa, activities on fluidics for iodine-fed electric propulsion systems and material compatibility studies led to the development of a mass flowmeter within the “iFACT-MP” Horizon EU project. The device is a spectrophotometric flow meter measuring instantaneous mass flow through a cell in series with the iodine feeding line, upstream of a thermal throttle. A beam splitter directs part of the light to a reference photodiode to compensate for laser intensity variations. Temperature and absorption measurements allow inferring iodine pressure in the cell, while the thermal throttle ensures sonic conditions, enabling correlation with instantaneous mass flow. The mass flow meter shows good behavior and repeatability, especially at low mass flow rates.