Giulia Becatti

In 1996, when the danger of sideways forces was first recognised, the operational window of the JET tokamak was significantly restricted. Limits were placed on the maximum magnetic field and plasma current to prevent excessive stresses on the vacuum vessel. Despite three decades of research, predictions for the sideways force on the ITER wall remain highly uncertain, ranging from 2 to 60 MN with the upper estimate exceeding the design value of 48 MN, which would result in operational restrictions (operation with I_p<15 MA potentially affecting ITER's ultimate fusion performance). To address this issue, the ITPA community launched the dedicated joint experiment MDC-25 in 2018. While substantial progress has been made, recent findings highlight the need for complementary studies on a specialised linear device to investigate plasma-wall currents, key contributors to sideways forces. Here, we propose the first linear device specifically designed to investigate plasma-wall interactions in the presence of kink modes, with the aim of resolving uncertainties related to plasma-wall currents and associated sideways forces. It is expected that the findings obtained with the new device will guide the design of vacuum vessels and plasma facing components for future tokamaks.

Hollow cathodes (HCs) are a widely used technology in space propulsion, representing the standard for electric thrusters both as neutralizers and plasma heating sources nowadays. HC discharges have been proven to be a highly efficient and reliable plasma generation technology with demonstrated lifetimes of tens of thousands of hours without any maintenance, making these devices a valuable alternative for negative ion sources. Consorzio RFX in Padua and University of Pisa have been testing this technology on the ATHENIS facility, an innovative concept of a new neutral beam injector (NBI) source based on hall thruster designs, both in argon and with a mix of argon and hydrogen as feed gas, with plasma parameters characterized via a movable Langmuir probe. The preliminary results, described in this article, show good plasma densities (above 5 × 10¹⁷ /m³) even at low injected powers (around 400 kW/m³), both in argon and with the argon–hydrogen mix. Furthermore, in the mixed fuel gas case, the Langmuir probe characteristics indicate the presence of asecond electron population, well described by a temperature similar to the accelerating potential of the HC-extracted electrons. The electron temperature remains around 2eV in all tested configurations, indicating that the radial magnetic field of the Hall thruster acted efficiently also as a magnetic filter, representing a promising feature toward negative ion production.

Obtaining the electron energy distribution function (EEDF) with intrusive diagnostics such as Langmuir probe (LP) is in general very challenging. Typically, this is done through double numerical differentiation of the probe current–voltage characteristic, which poses significant difficulties due to noise amplification. Traditional filtering and smoothing techniques often introduce arbitrary assumptions about the EEDF, which may unpredictably affect the results. This paper presents an innovative Bayesian deconvolution method that reconstructs the EEDF from LP data without requiring numerical or analog double differentiation. This approach allows for the estimation of plasma parameters while consistently accounting for the uncertainties in the inference. Additionally, it highlights correlations between model parameters which can, in turn, improve the quality of the experiments, by identifying quantities that require more accurate measurement. The methodology was validated with both synthetic data and real experimental measurements. In both cases, the method proved effective in reconstructing the EEDF, even in the presence of high noise levels in the probe characteristic. The probability distributions of the calculated plasma parameters were also consistent with the true values. Overall, the proposed Bayesian deconvolution method offers a fast and robust approach to reconstructing the EEDF from LP data. However, a more flexible parametrization of the unknown EEDFs is needed to capture all the features of the real distributions.

Applied-field magnetoplasmadynamic thruster (AF-MPDT) is a type of spacecraft electric propulsion (EP) with input powers of up to several 100 kW, which uses an external magnetic field to accelerate a propellant and to generate thrust. They reach a high-thrust density for an EP and medium to high specific impulses (Isp), which enables their usage in interplanetary space missions and for space tugs, for example, to the Moon. At the Institute of Space Systems, the water-cooled 100 kW steady state AF-MPDT SX3 is investigated. Magnetic field measurements with a Hall-probe were performed for a wide variation of thruster operating conditions, in order to determine the local current densities and, hence, the current densities’ distribution in the plume of the AF-MPDT SX3 for thruster distances between 140 and 1000 mm. Up to 70%–80% of the discharge current was measured as current in the plume at a thruster distance of 1000 mm. At a thruster distance of 300 mm, the fraction was 90% and more for the same operation conditions. An azimuthal current, with a current density of 7.9±2.2mA/mm2, was measured with an applied field of 100 mT at a thruster distance of 403 mm. Increasing applied magnetic fields moved it further downstream. Lorentz forces, calculated from the currents in the plume, are of the order of 10% of the thrust, which was measured with a thrust balance before.

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.

The past two decades have seen an increasing interest in Hall thrusters in space propulsion, thanks to their favorable performance characteristics with respect to a wide variety of missions of current and future interest and to the significant extension in operational life potential achievable with magnetic shielding. Nevertheless, the physics underlying their behavior is complex and not yet fully understood, limiting the practical applications of models based on first principles due to their inability to self-consistently predict the device performance. Fortunately, modern Hall thrusters were developed through a lengthy process of gradual refinement, and thus they represent convenient reference devices to design new thrusters using appropriately defined scaling criteria. The objective of this work is to propose a new scaling methodology, especially intended for magnetically shielded high-power Hall thrusters. To this purpose, a novel phenomenological model for shielded thrusters is presented and discussed. This model includes free coefficients, whose values are chosen based on the agreement with the empirical data collected in a specially created high-power Hall thruster database. The proposed methodology features a new reference thruster and aims at keeping unchanged its main plasma intensive parameters in a scaling transformation. The possibility of creating performance maps at constant discharge power, which show how the scaling results vary with the channel dimensions, is also proposed as a preliminary design tool.