WO2025177108A1

WO2025177108A1 - Spacecraft electric thruster

Info

Publication number: WO2025177108A1
Application number: PCT/IB2025/051250
Authority: WO
Inventors: Vittorio Giannetti; Eugenio Ferrato; Tommaso Andreussi
Original assignee: Celeste Srl
Current assignee: Celeste Srl
Priority date: 2024-02-19
Filing date: 2025-02-06
Publication date: 2025-08-28
Anticipated expiration: 2026-08-19

Abstract

The present invention relates to a spacecraft electric thruster particularly suitable for both propellant-fed and air-breathing electric space propulsion comprising an ionization chamber (11) for plasma generation which develops along a central development axis (X), wherein the ionization chamber (11) has a chamber inlet (12) configured to allow gas inflow and a chamber outlet (13) configured to allow extraction of ionized gas particles; a power injection assembly (30) configured to perform electron heating for plasma generation inside the ionization chamber (11); a magnetic assembly (20) configured to generate a magnetic field for plasma confinement in the ionization chamber (11); and an ion extraction and acceleration assembly (40) configured to extract ionized gas particles from the ionization chamber (11) through the chamber outlet (13) and accelerate them along an ion acceleration direction (Z), wherein the power injection assembly (30) comprises a microwave power delivery system (31) configured to deliver an electric field oscillating at a selected microwave frequency (f) and to inject the generated oscillating electric field in the ionization chamber (11), and wherein the ionization chamber (11) comprises a plurality of walls (11a, 15,16,41) closed to the passage of microwaves apart from a portion of at least one lateral wall (11a) of the plurality of walls (11a, 15,16,41) at which the power delivery system (31) is coupled to inject the generated oscillating electric field in the ionization chamber (11), and is resonant for electromagnetic fields oscillating at the selected microwave frequency.

Description

SPACECRAFT ELECTRIC THRUSTER

TECHNICAL FIELD

The present disclosure generally relates to the field of electric space propulsion particularly for satellite platforms. More in detail, the present disclosure relates to a spacecraft electric thruster particularly suitable for both propellant-fed and air-breathing electric space propulsion.

BACKGROUND

Nowadays, there is a high interest in operating spacecrafts in the so-called Very Low Earth Orbits (VLEOs), namely at orbital altitudes below 400 km, since it provides significant advantages.

First, operating closer to Earth's surface greatly eases communication missions due to the reduction of the latency and transmission power required for reaching a preset data link performance. Also, Earth observation missions are improved due to enhanced reconnaissance conditions. Moreover, lowering the spacecraft altitude allows improving the payload performance, thereby offering the possibility to decrease the number and size of satellite platforms. Also, launch costs can be reduced. In addition, by operating at VLEOs the spacecrafts are protected by the high atmosphere and face lower radiation levels. Finally, since mitigation measures to cope with increasing debris population are becoming urgent, operating at VLEOs offers the advantage of an automatic re-entry and disposal of an unpropelled spacecraft due to atmospheric drag.

In general, spacecraft electric thrusters use electrical power generated by solar panels to ionize and accelerate an onboard stored propellant by applying electromagnetic fields. In this way, thrust is generated that enables maneuvering the satellite in orbit. The high speeds at which the ionized propellant can be accelerated make the known thrusters particularly efficient, allowing for complex maneuvers with minimal consumption of the onboard stored propellant.

In any case, the lifetime of the spacecraft is bound to the amount of propellant that can be stored onboard which, on the other hand, is limited by the severe requirements that the propellant tank dimensions pose on the system design and size. This becomes even more relevant for satellites operating at VLEOs due to the significant atmospheric drag experienced at such orbital altitudes, the compensation of which considerably increases the propellant consumption.

For these and other reasons, there is currently a high interest in developing spacecraft electric thrusters capable of operating in the so-called “air-breathing” mode, namely of generating a sufficient thrust by ionizing and accelerating the atmospheric particles that can be collected at VLEOs. This allows untying the link between the lifetime of a spacecraft operating at VLEOs and the amount of stored propellant.

However, using the atmosphere as propellant for electric thrusters presents a unique set of technological and scientific challenges:

- the primary constituents of the atmosphere in the VLEOs are molecular nitrogen and atomic oxygen which are more difficult to ionize compared to the typical propellants for electric propulsion, namely xenon or krypton. Additionally, the presence of numerous different species in the flow impairs the efficient acceleration of the gas.

- the low gas densities expected in the VLEOs where drag compensation is feasible make the ionization of atmospheric gas particularly complex.

Accordingly, the critical issue of spacecraft electric thrusters operated in air-breathing mode resides in the difficulty in producing efficient ionization and acceleration of very low-density atmospheric gases.

Up to date, different research groups of the field of electric space propulsion have investigated the operation of a spacecraft electric thruster prototype in air-breathing mode by making use of a VLEO flow source. However, up to now, none of the investigated prototypes showed to be suitable to fully counteract the simulated drag level of realistic VLEO mission scenarios.

Accordingly, the present disclosure is directed to - at least in part - improve or overcome one or more aspects of prior spacecraft electric thrusters and particularly to a solution which can achieve an efficient ionization and acceleration of very low-density atmospheric gases.

Within the scope of the above problem, the Applicant considered the objective of devising a spacecraft electric thruster capable of fully counteracting the drag level of realistic VLEO mission scenarios when operated in air-breathing mode.

SUMMARY OF THE INVENTION

A first aspect of the present invention relates to a spacecraft electric thruster comprising:

- an ionization chamber for plasma generation which develops along a central development axis, wherein the ionization chamber has a chamber inlet configured to allow gas inflow and a chamber outlet configured to allow extraction of ionized gas particles;

- a power injection assembly configured to perform electron heating for plasma generation inside the ionization chamber, the power injection assembly comprising a microwave power delivery system configured to deliver an electric field oscillating at a selected microwave frequency to be injected in the ionization chamber;

- a magnetic assembly configured to generate a magnetic field in the ionization chamber to cause axial plasma confinement along the central development axis and radial plasma confinement around the central development axis in at least one common region along the central development axis; and

- an ion extraction and acceleration assembly configured to extract ionized gas particles from the ionization chamber through the chamber outlet and accelerate them along an ion acceleration direction.

According to the present invention, the magnetic assembly is configured to generate a magnetic field in the ionization chamber with a region in which the magnetic field has a minimum magnetic field intensity at a radial distance separated from lateral walls delimiting laterally the ionization chamber.

Moreover, the magnetic assembly is configured to generate a magnetic field in the ionization chamber characterized by a plurality of surfaces which surround the region wherein the magnetic field has the minimum magnetic field intensity and do not intercept the ionization chamber walls, each surface of the plurality of surfaces being at a constant magnetic field intensity and at a constant gyrofrequency, wherein the selected microwave frequency corresponds to a gyrofrequency of a surface of the plurality of surfaces of the magnetic field.

Applicant has identified that axial plasma confinement along the central development axis hinders electrons from leaving the chamber from the chamber inlet and outlet, and radial plasma confinement minimizes plasma losses at the lateral chamber walls. The combination of the two plasma confinement strategies advantageously allows strongly increasing the residence time of the electrons inside the ionization chamber, and consequently, also the probability of an ionization event. Accordingly, plasma generation is intensely boosted and reaches levels sufficient to generate a thrust which is capable to counteract the drag level of realistic VLEO mission scenarios, also when the spacecraft electric thruster is operated in very low-pressure environments, such as the one resulting from the air-breathing operation mode.

Moreover, having the region of minimum magnetic field intensity separated from the lateral walls of the ionization chambers allows creating a plasma confinement away from the chamber walls, thereby minimizing plasma losses. In fact, when an ion impinges on a wall, it is neutralized thereby reducing the overall efficiency of the system.

Not least, the surfaces at constant magnetic field intensity and at a constant gyrofrequency (isointensity and iso-gyrofrequency surfaces) which surround the region wherein the magnetic field has the minimum magnetic field intensity and characterize the magnetic field generated by the magnetic assembly further contribute to the plasma confinement distant from the lateral walls of the ionization chambers.

Described herein are variations of the spacecraft electric thruster that allow reliable axial and radial plasma confinement and particularly, of its magnetic assembly.

In one variation, the region wherein the magnetic field has the minimum magnetic field intensity may be substantially at the central development axis of the ionization chamber.

In one variation of a spacecraft electric thruster described herein, the magnetic assembly may be configured to generate a magnetic field in the ionization chamber with a magnetic mirror topology characterized by a substantially axial magnetic field with higher intensity towards both the ionization chamber inlet and the ionization chamber outlet. The magnetic mirror topology may be an asymmetric magnetic mirror topology, such as, e.g., a magnetic mirror topology with a magnetic field intensity towards the chamber outlet (in the following, threshold magnetic field intensity) which is lower than a magnetic field intensity towards the chamber inlet (in the following, maximum magnetic field intensity) with respect to a development of the magnetic field intensity along the central development axis.

Favorably, the axial magnetic field with intensity peaks towards both the ionization chamber inlet and the ionization chamber outlet reduces the electron losses through the chamber inlet and delays the point in time in which the electrons leave the chamber from the chamber outlet. Moreover, the difference between the intensity levels of the high intensity regions at the chamber inlet and outlet makes sure that plasma diffusion takes prevalently place towards the acceleration assembly.

In some instances, the plurality of surfaces may comprise a reference surface characterized by a reference gyrofrequency and a reference magnetic field intensity, with the reference magnetic field intensity being lower than or equal to the threshold magnetic field intensity and higher than the minimum magnetic field intensity. The ionization chamber may be resonant for electromagnetic fields oscillating at a microwave frequency substantially equal to the reference gyrofrequency. Advantageously, the specific geometry and structure of the ionization chamber which makes it resonant for electromagnetic fields oscillating at a microwave frequency which corresponds to the gyrofrequency of the reference iso-intensity and iso-gyrofrequency surface allows amplifying the electric field oscillating at the said frequency and confining it inside the ionization chamber where the reference surface is generated, thereby increasing the probability that electrons experience an energy increase and become more likely to ionize gas particles.

In one variation of a spacecraft electric thruster described herein, the magnetic assembly may comprise a plurality of magnetic elements which are permanent magnets or electromagnets. In particular, the plurality of magnetic elements may comprise at least two annular magnetic elements placed at an axial position along the central development axis substantially at or in the proximity of the chamber inlet or chamber outlet, respectively. The annular magnetic elements may be placed coaxially with respect to the central development axis. The annular magnetic elements may be radially magnetized permanent magnets with respectively opposite magnetization.

In some variations, the plurality of permanent magnets may comprise a set of radially and/or tangentially magnetized magnetic bars placed in a multipole cylindrical arrangement. The set of magnetic bars may be placed with circumferentially alternating poles and in a side-by-side arrangement so as to form a hollow cylinder body. The set of magnetic bars may be placed in a Halbach configuration, e.g., in a Halbach hexapole configuration.

Expediently, the above defined structure of the magnetic assembly allows obtaining the desired magnetic field topology in a simple and cost-effective way.

In some instances, the plurality of magnetic elements may comprise a set of additional annular magnetic elements which are placed coaxially with respect to the central development axis. At least a first additional annular magnetic element of the set of additional annular magnetic elements may be placed in the proximity of the annular magnetic element located at or in the proximity of the ionization chamber inlet and defines at least one radial slot configured to allow for lateral injection of electromagnetic power. At least a second additional annular magnetic element of the set of additional annular magnetic elements may be placed at an axial position along the central development axis of the ionization chamber substantially corresponding to a center of an axial development of the ionization chamber.

Favorably, the additional magnetic elements allow to trim and optimize the magnetic field topology for improving its performances of radial and axial plasma confinement. Moreover, the additional magnetic elements also allow for lateral injection of electromagnetic power, namely for injection of electromagnetic power without disrupting the magnetic field or the axial gas flow.

Generally, in some variations, the ionization chamber may define a simply connected internal space.

Beneficially, the specific structure of the ionization chamber defining a simply connected internal space allows creating a zone of plasma confinement which is distant from the chamber walls.

In some implementations, the power injection assembly may comprise a microwave power delivery system configured to generate an electric field oscillating at a selected microwave frequency to be injected in the ionization chamber.

Advantageously, the inj ection of microwave power in the ionization chamber leads to a remarkable increase of the electrons energy which on its turn increases the probability that gas particles are ionized by impacting on the electrons, thereby generating plasma.

A second aspect of the present invention relates to a spacecraft electric thruster comprising:

- a power injection assembly configured to perform electron heating for plasma generation inside the ionization chamber;

- a magnetic assembly configured to generate a magnetic field for plasma confinement in the ionization chamber; and

According to the present invention, the power injection assembly comprises a microwave power delivery system configured to deliver an electric field oscillating at a selected microwave frequency and to inject the generated oscillating electric field in the ionization chamber.

Moreover, the ionization chamber comprises a plurality of walls closed to the passage of microwaves apart from a portion of at least one lateral wall of the plurality of walls at which the power delivery system is coupled. Also, the ionization chamber is resonant for electromagnetic fields oscillating at the selected microwave frequency.

Applicant has considered that electromagnetic energy generated via the microwave power delivery system increases the electrons energy based on the “stochastic Electron Cyclotron Resonance (ECR) heating” phenomenon. Electrons with increased energy, also called “heated electrons”, are more likely to ionize gas particles when they impact on them, thereby generating plasma. Moreover, Applicant has identified that the combined effects of the magnetic field for plasma confinement and the microwave power delivery, both localized in the ionization chamber, advantageously allow having an increased generation of plasma directly in a magnetically confined region inside the ionization chamber. Accordingly, plasma generation is intensely boosted and reaches levels sufficient to generate a thrust which is capable to counteract the drag level of realistic VLEO mission scenarios, also when the spacecraft electric thruster is operated in very low-pressure environments, such as the one resulting from the air-breathing operation mode.

In addition, tuning the ionization chamber and the microwave power delivery system so that the latter is configured to deliver inside the ionization chamber an electric field which oscillates at a selected frequency which corresponds to the frequency at which the ionization chamber is resonant, leads to a strongly increased probability that electrons experience an energy increase and become more likely to ionize gas particles

Described herein are variations of the spacecraft electric thruster that allow intensely promoting plasma generation.

In one variation, the magnetic assembly is configured to generate a magnetic field in the ionization chamber to cause axial plasma confinement along the central development axis and radial plasma confinement around the central development axis in at least one common region along the central development axis.

Expediently, the axial and radial plasma confinements strongly increase the residence time of the electrons inside the ionization chamber, and consequently, the probability of an impact event between the electrons and the gas particles. The increased probability that an impact event occurs combined with the increased likelihood that gas particles are ionized trough the impact because of the increased electron energy caused by the delivery of microwave power inside the ionization chamber leads to an effectively increased plasma generation,

Moreover, also ions are expected to be confined. In particular, the magnetic topology according to the invention electrostatically confines the ions due to the self-consistent potential profile arising in the plasma bulk following the motion of the magnetized electrons. This process makes it possible to reduce ion losses at the chamber walls. Therefore, in the spacecraft electric thruster according to the invention most ions produced are confined for a certain time inside the volume of the ionization chamber and then extracted through the ion extraction and acceleration assembly. Generally, in some variations, the at least one lateral wall may be a solid wall and the ionization chamber may have end walls with apertures closed to the passage of microwaves both at the chamber inlet and at the chamber outlet.

Preferably, the ionization chamber is made of an electrically conductive material.

Preferably, the ionization chamber defines a simply connected internal space.

In one variation of a spacecraft electric thruster described herein, the chamber inlet comprises a solid end wall with an injection hole with a size not exceeding a threshold size above which a passage of an injected electric field oscillating at the selected microwave frequency would be allowed.

In another variation of a spacecraft electric thruster described herein, the chamber inlet comprises a metallic mesh with holes smaller than a threshold size above which a passage of an injected electric field oscillating at the selected microwave frequency would be allowed.

Generally, in some variations, the chamber outlet comprises at least one grid with holes for ion extraction smaller than a threshold size above which a passage of an injected electric field oscillating at the selected microwave frequency would be allowed.

Advantageously, the structure of the ionization chamber and the material in which it is made are suitable to support the injection of microwave power.

In one variation of a spacecraft electric thruster described herein, the microwave power delivery system is placed outside of the ionization chamber and the power injection assembly comprises a microwave launching group configured to inject the oscillating electric field in the ionization chamber. The microwave launching group may comprise a waveguide which ends in the ionization chamber at the portion of at least one lateral wall of the ionization chamber at which the power delivery system is coupled. The microwave launching group may comprise an input element configured to allow the passage of electromagnetic power and preventing gas leakages from the ionization chamber. The input element may be an RF window.

Favorably, injecting the oscillating electric field in the ionization chamber through the lateral wall of the ionization chamber avoids disrupting both the magnetic field and the axial gas flow, thereby preventing interference with the trajectories of the collected atmospheric particles.

In one variation of a spacecraft electric thruster described herein, the magnetic assembly is configured to generate a magnetic field in the ionization chamber, which is characterized by a magnetic mirror topology characterized by a substantially axial magnetic field with high intensity regions towards the ionization chamber inlet and the ionization chamber outlet, a first high intensity region towards the ionization chamber outlet showing a threshold magnetic field intensity; and a plurality of surfaces at a fixed magnetic field intensity and at a fixed gyrofrequency which surround a region wherein the magnetic field has a minimum magnetic field intensity and do not intercept lateral walls which delimit laterally the ionization chamber, the plurality of surfaces comprising a reference surface characterized by a reference gyrofrequency and a reference magnetic field intensity, with the reference magnetic field intensity being lower than or equal to the threshold magnetic field intensity and higher than the minimum magnetic field intensity. The ionization chamber may be resonant for electromagnetic fields oscillating at a selected microwave frequency substantially corresponding to the reference gyrofrequency. The microwave power delivery system may be configured to deliver an electric field oscillating at the selected microwave frequency substantially corresponding to the reference gyrofrequency.

Beneficially, tuning the magnetic assembly, the ionization chamber and the microwave power delivery system so that the latter is configured to deliver inside the ionization chamber an electric field which oscillates at a frequency which corresponds to both the reference gyrofrequency of the reference surface generated by the magnetic assembly and the frequency at which the ionization chamber is resonant, leads to a strongly increased probability that electrons experience an energy increase and become more likely to ionize gas particles. In some instances, the ionization chamber may be cylindrical, and the radius r of the cylindrical ionization chamber and the length L of the cylindrical ionization chamber may satisfy the following equation of resonant TEmnp modes some combinations of m, n and p where fs is the reference gyrofrequency, 6 is the permittivity, p is the magnetic permeability and X'mn is the n-th zero of the derivative of the Bessel function of order m, so that J'm(X'mn) = 0.

Advantageously, in case of a cylindrical ionization chamber, the above defined geometry relations assure that the chamber is resonant for electromagnetic fields oscillating at a selected microwave frequency substantially corresponding to the reference gyrofrequency.

A third aspect of the present invention relates to a method of operating a spacecraft electric thruster which comprises:

- a power injection assembly configured to perform electron heating for plasma generation inside the ionization chamber, which comprises a microwave power delivery system configured to deliver an electric field oscillating at a selected microwave frequency and to inject the generated oscillating electric field in the ionization chamber;

- an ion extraction and acceleration assembly configured to extract ionized gas particles from the ionization chamber through the chamber outlet and accelerate them along an ion acceleration direction; wherein the magnetic assembly is configured to generate a magnetic field in the ionization chamber, which is characterized by

- a magnetic mirror topology characterized by a substantially axial magnetic field with high intensity regions towards the ionization chamber inlet and the ionization chamber outlet, a first high intensity region towards the ionization chamber outlet showing a threshold magnetic field intensity; and

- a plurality of surfaces at a fixed magnetic field intensity and at a fixed gyrofrequency which surround a region wherein the magnetic field has a minimum magnetic field intensity and do not intercept lateral walls which delimit laterally the ionization chamber, the plurality of surfaces comprising a reference surface characterized by a reference gyrofrequency and a reference magnetic field intensity, with the reference magnetic field intensity being lower than or equal to the threshold magnetic field intensity and higher than the minimum magnetic field intensity; and wherein the ionization chamber is resonant for electromagnetic fields oscillating at a selected microwave frequency substantially corresponding to the reference gyrofrequency; the method comprising the step of delivering an electric field in the ionization chamber oscillating at the selected microwave frequency substantially corresponding to the reference gyrofrequency through the microwave power delivery system.

Expediently, the so configured method of operating a spacecraft electric thruster achieves the same advantages described with reference to the spacecraft electric thruster according to the invention. A fourth aspect of the present invention relates to a spacecraft electric thruster comprising:

- a magnetic assembly configured to generate a magnetic field for plasma confinement in the ionization chamber, wherein the magnetic assembly is configured to generate a magnetic field in the ionization chamber to cause axial plasma confinement along the central development axis and radial plasma confinement around the central development axis in at least one common region along the central development axis; and

According to the present invention, the power injection assembly comprises

- a microwave power delivery system configured to generate an electric field oscillating at a selected microwave frequency and to inject the generated oscillating electric field in the ionization chamber, the microwave power delivery system being placed outside of the ionization chamber; and

- a microwave launching group configured to inject the oscillating electric field in the ionization chamber, the microwave launching group comprising a waveguide ending in the ionization chamber at a portion of at least one lateral wall of the ionization chamber, which is transparent to microwaves.

Applicant has identified that injecting the oscillating electric field in the ionization chamber from a microwave transparent portion of the lateral wall of the ionization chamber avoids disrupting both the magnetic field and the axial gas flow, thereby preventing interference with the trajectories of the collected atmospheric particles. This reveals to be markedly important when the spacecraft electric thruster is operated in very low-pressure environments, such as the one resulting from the air-breathing operation mode. In detail, the solution offered by the invention reveals to be particularly suitable to support and not disturb the plasma generation inside the ionization chamber, thereby allowing the spacecraft thruster to be able to counteract the drag level of realistic VLEO mission scenarios, also when it is operated in very low-pressure environments, such as the one resulting from the air-breathing operation mode. Described herein are variations of the spacecraft electric thruster that achieve an optimized microwave power delivery in the ionization chamber without disrupting either the magnetic field or the axial gas flow inside the ionization chamber.

In one variation, the waveguide extends along an extension axis and wherein, at the portion of the lateral wall, which is transparent to microwaves, the extension axis of the waveguide is comprised in a plane inclined with respect to the central development axis. At the said portion of the lateral wall, the extension axis of the waveguide may be comprised in a plane which is orthogonal to the central development axis.

In some implementations, the waveguide may be a rectangular waveguide and/or a ridge waveguide and/or a double ridge waveguide, and/or may contain a stepped impedance transformer. In some variations, the portion at which the waveguide ends in the ionization chamber may be placed at an axial position along the central development axis where the magnetic field generated by the magnetic assembly shows in average a magnetic field intensity which is higher than a reference magnetic field intensity.

Generally, in some variations, the magnetic assembly is configured to generate a magnetic field in the ionization chamber characterized by a magnetic mirror topology characterized by a substantially axial magnetic field with at least two high intensity regions, respectively located towards the ionization chamber inlet and the ionization chamber outlet; and a plurality of surfaces at a fixed magnetic field intensity and at a fixed gyrofrequency which surround a region wherein the magnetic field has a minimum magnetic field intensity and do not intercept lateral walls which delimit laterally the ionization chamber, the plurality of surfaces comprising a reference surface characterized by a reference gyrofrequency and a reference magnetic field intensity. The portion at which the waveguide ends in the ionization chamber may be placed at an axial position along the central development axis at which the magnetic field generated by the magnetic assembly shows a high intensity region of the at least two high intensity regions and which is outside of the reference surface.

In some instances, the portion at which the waveguide ends in the ionization chamber is placed at an axial position along the central development axis proximal to the chamber inlet.

Beneficially if the waveguide ends in a region of high magnetic field intensity, i.e., a region corresponding to or in the proximity of the area where the magnetic field shows an intensity peak, the microwaves can propagate without reflection towards the reference surface.

Moreover, the ending position proximal to the chamber inlet is particularly suited to the specific magnetic assembly architecture configured to generate the above defined field topology.

The microwave launching group of the spacecraft electric thruster generally comprises an input element configured to allow the passage of electromagnetic power and preventing gas leakages from the ionization chamber. The input element being e.g., an RF window. The input element may be configured to electrically insulate the ionization chamber. The input element may be configured to implement a DC break. Favorably, electromagnetic power can be delivered inside the ionization chamber in absence of gas leakages and without disturbing the axial gas flow.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and constitute a part of the specification, illustrate exemplary embodiments of the disclosure and, together with the description, explain the principles of the disclosure. It should be understood, however, that there is no intention to limit the invention to the specific embodiments illustrated in the figures, but, on the contrary, the invention is intended to cover all modifications, constructions, variations, alternatives and equivalents, which fall within the scope of the invention as defined in the appended claims.

FIG. l is a perspective view of an exemplary spacecraft electric thruster according to a preferred embodiment of the present invention;

FIG. 2 is a front view of the spacecraft electric thruster of Fig. 1;

FIG. 3 is a schematic depiction of a section of the spacecraft electric thruster of Fig. 1;

FIG. 4 is a perspective view of a magnetic assembly used in the spacecraft electric thruster of Fig. 1;

FIG. 5 is a schematic depiction of the magnetic field generated by the magnetic assembly of Fig. 4 inside a ionization chamber of the spacecraft electric thruster of Fig. 1;

FIG. 6 is a graph of the magnetic flux density generated by the magnetic assembly of Fig. 4 inside the ionization chamber of the spacecraft electric thruster of Fig. 1;

FIGS. 7 A to 7F are graphs of the magnetic field intensity overlaid to the magnetic streamlines with respect to different reference planes;

FIG. 8 is a schematic depiction of the oscillating electric field intensity generated by a microwave injection assembly inside the ionization chamber of the spacecraft electric thruster of Fig. 1;

FIG. 9 is a schematic depiction of the set-up of the spacecraft electric thruster of Fig. 1 when operated in air-breathing mode.

DETAILED DESCRIPTION

The following is a detailed description of exemplary embodiments of the present disclosure. The exemplary embodiments described therein and illustrated in the drawings are intended to teach the principles of the present disclosure, enabling those of ordinary skill in the art to implement and use the present disclosure in many different environments and for many different applications. Therefore, the exemplary embodiments are not intended to be, and should not be considered as, a limiting description of the scope of patent protection. Rather, the scope of patent protection shall be defined by the appended claims.

Unless otherwise indicated, the use of "e.g.," "etc.," "or" means non-exclusive alternatives, i.e. without limitation to exclusive alternatives.

Unless otherwise indicated, the terms "comprising," "having," "including," and "containing" are intended to be open terms (i.e., the meaning "comprising, but not limited to") and are also to be understood as forming a disclosure for expressions such as "consisting essentially of or "consisting of.

Unless otherwise indicated, all terms specific of the technical field, notations, and other scientific terms used herein are intended to have the meanings commonly understood by those skilled in the field to which this description belongs.

Figs. 1 and 2 schematically illustrate a spacecraft electric thruster 10 according to a preferred embodiment of the present invention. The spacecraft electric thruster 10 comprises an ionization chamber 11 for plasma generation which develops along a central development axis X and defines a simply connected internal space. In the depicted embodiment, the ionization chamber 11 has a substantially cylindrical shape with the central development axis X being the symmetry axis of the cylinder. The ionization chamber 11 has a chamber inlet 12 configured to allow gas intake and a chamber outlet 13 which allows the extraction of ionized gas particles.

The spacecraft electric thruster 10 also comprises a magnetic assembly 20 configured to generate a magnetic field for plasma confinement in the ionization chamber 11, a power injection assembly 30 configured to perform electron heating (of the electrons usually present in the inflowing gas) for plasma creation inside the ionization chamber 11 and an ion extraction and acceleration assembly 40 configured to extract ionized gas particles from the ionization chamber 11 through the chamber outlet 13 and accelerate them along an ion acceleration direction Z.

In the present description and in the appended claims, with the expression “plasma confinement” it is intended to refer to the phenomenon according to which magnetic fields are able to constrain the motion of charged particles, first of all in the plane perpendicular to the magnetic field, in which charged particles are induced to move in a circular motion around a guiding center. Moreover, depending on its orthogonal velocity component, a particle moving parallel to a local magnetic field line towards increasing magnetic field intensities is repelled due to the conservation of kinetic energy and magnetic moment. This phenomenon allows confining particles in magnetic mirrors and magnetic cusps.

The magnetic assembly 20 comprises a plurality of magnetic elements 21-25 (shown in Figs. 3 and 4) which may be implemented as permanent magnets and/or electromagnets. In detail, the plurality of magnetic elements 21-25 is chosen to realize both an axial plasma confinement along the central development axis X of the ionization chamber 11 and a radial plasma confinement around the central development axis X in at least one common region within the ionization chamber 11. In other words, there is at least one region within the ionization chamber 11 where the magnetic assembly 20 implements two overlapping plasma confinements, i.e., an axial plasma confinement along the central development axis X of the ionization chamber 11 and a radial plasma confinement around the central development axis X.

The axial confinement along the central development axis X is preferably implemented so as to provide a region of maximum magnetic field intensity towards the chamber inlet 12 in order to prevent electrons from leaving the chamber therefrom. The radial confinement around the central development axis X is preferably implemented so as to provide a region of minimum magnetic field intensity at a radial distance far from ionization chamber 11 lateral walls I la to minimize lateral walls plasma losses.

Particularly, the plurality of magnetic elements 21-25 comprises at least two annular magnetic elements 21 ,22 configured to generate a magnetic mirror topology, namely a mostly axial magnetic field with higher intensity towards the ionization chamber inlet 12 and, preferably, also towards the ionization chamber outlet 13, to provide the axial magnetic confinement.

In the depicted embodiment, the magnetic mirror topology is obtained by means of two annular magnetic elements 21,22 which are radially magnetized with opposite magnetization. Each of the annular magnetic elements 21,22 is preferably located at or in the proximity of the ionization chamber inlet 12 or outlet 13, respectively, and coaxially with respect to the central development axis X.

Preferably, the magnetic mirror topology is asymmetric; particularly, the magnetic field intensity shows a first high intensity region 27a located towards the chamber outlet 13 at a threshold magnetic field intensity Bth which is lower than a maximum magnetic field intensity B_max of a second high intensity region 27b located towards the chamber inlet 12. This difference between the intensity levels of the high intensity regions 27a, 27b makes sure that plasma diffusion takes place towards the acceleration assembly 40.

The plurality of magnetic elements 21-25 also comprises a set of magnetic bars 23 placed between the two annular magnetic elements 21,22 in a multipole cylindrical arrangement. In detail, the set of magnetic bars are radially and/or tangentially magnetized and placed with circumferentially alternating poles and in a side-by-side arrangement so as to form a hollow cylinder body. By way of an example, the set of magnetic bars may be placed in a Halbach configuration, preferably in a Halbach hexapole configuration.

In the depicted embodiment, the set of magnetic bars 23 are placed so as to generate six radial cusps to provide radial confinement and a minimum of the magnetic field intensity at a radial position distant from the ionization chamber 11 walls. Specifically, the depicted embodiment comprises twelve magnetic bars 23 in a Halbach hexapole configuration.

The plurality of magnetic elements 21-25 preferably also comprises a set of additional annular magnetic elements 24,25 configured to trim the magnetic field topology. In detail, at least one first additional annular magnetic element 25 of the set of additional annular magnetic elements 24,25 defines at least one radial slot 25a configured to allow for the lateral injection of electromagnetic power. The first additional annular magnetic element 25 is preferably placed in the proximity of the annular magnetic element 21 located at or in the proximity of the ionization chamber inlet 12. Moreover, the first additional annular magnetic element 25 is radially magnetized with magnetization corresponding to the annular magnetic element 21 located at or in the proximity of the ionization chamber inlet 12. In the depicted embodiment, at least one second additional annular magnetic element 24 of the set of additional annular magnetic elements 24,25 is placed at an axial position substantially corresponding to the center of the axial development of the ionization chamber 11. The second additional annular magnetic element 24 is preferably magnetized with magnetization corresponding to the annular magnetic element 22 located at or in the proximity of the ionization chamber outlet 13.

The superposition of the magnetic fields generated by the plurality of magnetic elements 21-25 allows obtaining a so-called minimum-B magnetic field topology, namely a magnetic field topology which ensures the locally concurrent axial and radial magnetic confinement of the plasma, and that the magnetic field intensity has a minimum B_min inside the ionization chamber 11, namely away from the chamber walls. The so obtained magnetic field topology is characterized by surfaces at fixed magnetic field intensity (iso-intensity surfaces) and at a fixed gyrofrequency (iso-gyrofrequency surfaces), which surround the region of minimum magnetic field intensity Bmin and do not intercept the ionization chamber 11 walls. Preferably, the surfaces at fixed magnetic field intensity and at a fixed gyrofrequency are closed surfaces.

Moreover, the so obtained magnetic field topology is characterized by the first high intensity region 27a located towards the chamber outlet 13 at the threshold magnetic field intensity Bth which is substantially lower than the maximum magnetic field intensity B_max of the second high intensity region 27b which is located towards the chamber inlet 12. The region of minimum magnetic field intensity B_min is located between the two high intensity regions 27a, 27b.

Fig. 5 schematically shows the magnetic field intensity generated by the magnetic assembly 20 of Fig. 4 inside the ionization chamber 11. Particularly, a closed surface 26 at a specific magnetic field intensity and gyrofrequency is plotted inside the ionization chamber 11. As can be appreciated from Fig. 5, the closed surface 26 is far from the ionization chamber 11 walls. Thereby, a confinement of the plasma is achieved in a central region of the ionization chamber 11.

Fig. 6 schematically shows the development 27 of the magnetic field intensity along the central development axis X overlaid with the closed surface 26 at a specific magnetic field intensity and gyrofrequency plotted inside the ionization chamber 11. Fig. 6 shows the first 27a and second 27b high intensity regions as peaks of the development 27 of the magnetic field intensity along the central development axis X. Fig. 6 also shows the minimum magnetic field intensity Bmin as the minimum value of the development 27 of the magnetic field intensity along the central development axis X.

Figs. 7a to 7f show the magnetic field intensity generated by the magnetic assembly 20 inside the ionization chamber 11 at specific planes, with darker regions corresponding to lower magnetic field intensity. In detail, Fig. 7a shows the magnetic field intensity in a first longitudinal plane A- A which intercepts the hexapole cusps and contains the central development axis X; Fig. 7b shows the magnetic field intensity in a second longitudinal plane B-B which passes between two hexapole cusps and contains the central development axis X; Fig. 7c shows the magnetic field intensity in a first transversal plane C-C orthogonal to the central development axis X and placed in the proximity of the chamber inlet 12; Fig. 7d shows the magnetic field intensity in a second transversal plane D-D orthogonal to the central development axis X and placed further downstream with respect to the first transversal plane C-C along the ion acceleration direction Z; Fig. 7e shows the magnetic field intensity in a third transversal plane E-E orthogonal to the central development axis X and placed further downstream with respect to the second transversal plane D-D along the ion acceleration direction Z; and Fig. 7f shows the magnetic field intensity in a fourth transversal plane F-F orthogonal to the central development axis X and placed in the proximity of the chamber outlet 13.

The power injection assembly 30 is schematically shown in Fig. 8 and comprises a microwave power delivery system 31 configured to deliver an oscillating electric field to be injected in the ionization chamber 11 and preferably resonate within the ionization chamber 11.

The power injection assembly 30 is configured to receive microwave power and inject an oscillating electric field at a selected microwave frequency corresponding to a gyrofrequency fs of a reference closed surface 26 of the magnetic field generated by the magnetic assembly 20.

The reference closed surface 26 is characterized by a reference magnetic field intensity Bs which is preferably lower than or equal to the threshold magnetic field intensity Bth and higher than the minimum magnetic field intensity B_min. More preferably, the-reference magnetic field intensity Bs is lower than the threshold magnetic field intensity Bthand higher than a magnetic field intensity value which corresponds to 1.1 B_min or, even more preferably, to 1.2 Bmin.

By way of an example, Fig. 6 schematically shows the intensity distribution of the targeted electric field oscillating at a microwave frequency corresponding to the gyrofrequency fs of the reference closed surface 26, namely of the closed surface at the reference magnetic field intensity B_s plotted inside the ionization chamber 11.

It has to be considered that when an electron performing a cyclotron motion around a magnetic field line is exposed to an electric field oscillating at a frequency equal to the local gyrofrequency it increases its energy. This phenomenon is called “stochastic Electron Cyclotron Resonance (ECR) heating”. Electrons with increased energy, also called “heated electrons”, are more likely to ionize gas particles when they impact on them, thereby generating plasma.

Thus, the electromagnetic energy generated via an external microwave generator (not shown) which is injected in the ionization chamber 11 through the microwave power delivery system 31 of the power injection assembly 30 at the selected microwave frequency corresponding to the gyrofrequency fs of the reference closed surface 26 will reach and heat the electrons along the said surface 26. Accordingly, the plasma will be generated by the heated electrons directly in a magnetically confined region inside the reference closed surface 26.

In the depicted embodiment, the power injection assembly 30 also comprises a microwave launching group 32,33 configured to inject the oscillating electric field in the ionization chamber 11. Advantageously, the microwave launching group 32,33 comprises a waveguide 32, e.g., a rectangular waveguide, which may end in the ionization chamber 11 at a portion of lateral walls I la of the ionization chamber 11, which is transparent to microwaves, e.g., through an opening in the lateral walls 1 la of the ionization chamber 11 which may be open or closed through a closure (not shown) made of a material which is transparent to microwaves.

In detail, the waveguide 32 extends along an extension axis Y. At the portion of the lateral wall I la transparent to microwaves, the extension axis Y of the waveguide 32 may be comprised in a plane inclined with respect to the central development axis X, i.e. a plane not comprising the central development axis. In the depicted embodiment, at the portion of the lateral wall I la transparent to microwaves, the extension axis Y of the waveguide 32 is comprised in a plane which is orthogonal to the central development axis X. Broadly speaking, at the ionization chamber 11 the waveguide 32 develops along an extension axis Y which is comprised in a plane inclined with respect to the central development axis X, e.g., orthogonal to the central development axis X. This avoids disrupting the axial gas flow.

Particularly, the ending position of the lateral waveguide is chosen at an ionization chamber position of high magnetic field intensity and low plasma density, e.g., at a position proximal to the second high intensity region 27b generated by the magnetic assembly 20 and outside of the reference closed surface 26, such at a position proximal to the chamber inlet 12.

Preferably, the portion of the lateral wall I la at which the waveguide 32 ends in the ionization chamber 11 is placed at an axial position along the central development axis X where the magnetic field generated by the magnetic assembly 20 shows in average a magnetic field intensity which is higher than the reference magnetic field intensity Bs.

The lateral waveguide 32 interfaces with an input element 33 configured to allow for passing of electromagnetic power while preventing gas leakages from the ionization chamber 11 like e.g., an RF window. The input element 33 is also configured to insulate the (high voltage) ionization chamber 11 from the (grounded) spacecraft (not shown). By way of an example, the RF window may implement a DC break. Also, a grounded element (not shown) is provided to interface the waveguide 32 with driving means (not shown), like e.g., the external microwave generator or a launcher connected to the waveguide 32, e.g., through a coaxial cable.

In different embodiments (not shown), the power injection assembly 30 may comprise a microwave power delivery system 31 directly injecting the microwaves in the ionization chamber 11 like e.g., an antenna placed inside the ionization chamber 11.

To be resonating, the ionization chamber 11 is advantageously made in an electrically conductive material, e.g., metallic, with lateral walls I la and end walls 15,16,41 closed to the passage of microwaves, apart from the portion of the lateral wall 1 la at which the power delivery system 31 ends and/or is coupled to inject the generated oscillating electric field into the ionization chamber 11. Moreover, the ionization chamber 11 is also closed to the passage of microwaves both at the chamber inlet 12 and at the chamber outlet 13.

Within the present description and in the appended claims with “wall closed to the passage of microwaves” is intended to indicate an electrically conductive solid wall or comprising one or more openings with a size (e.g., in terms of diameter of a circumference circumscribed around the opening) not exceeding a threshold size above which the passage of the injected microwaves would be allowed.

In detail, the chamber inlet 12 may comprise a solid end wall 16 with an injection hole 16a (shown in Fig. 8) with a characteristic size not exceeding a threshold size above which the passage of the injected microwaves would be allowed, for the case that the thruster is fed with stored propellant. Differently, the chamber inlet 12 may comprise a fine metallic mesh 15 with holes much smaller than the threshold size characteristic of the injected microwaves (shown in Fig. 9) in case the thruster is operated in air-breathing mode. In this second case, the spacecraft electric thruster 10 comprises an intake 14 (see Fig. 9) which may be placed at the chamber inlet 12 outside of the ionization chamber 11. The gas collected by means of the intake 14 can diffuse in the chamber 11 but the electromagnetic fields are confined in the ionization chamber 11 to resonate.

In a different embodiment (not shown) the intake 14 may be comprised in the ionization chamber 11 with the fine metallic mesh 15 being placed at the intake inlet.

Moreover, the chamber outlet 13 may comprise a first grid 41 with holes for ion extraction much smaller than the threshold size characteristic for the injected electromagnetic field.

The ionization chamber 11 is designed with geometric characteristics that make it resonate at the frequency selected for ECR heating, i.e., at the gyrofrequency fs of the reference closed surface 26. In detail, the geometric characteristics of the ionization chamber 11 are chosen so that the selected microwave frequency fs is equal to a resonant TE mode of the microwaves in the chamber or to a superposition of resonant modes, to maximize the electric field oscillating at the selected microwave frequency fs in an area inside the ionization chamber 11 where the reference closed surface 26 is generated. In this way, the electrons can absorb the injected power.

By way of an example, in case of a cylindrical ionization chamber 11, the radius r of the chamber and the length L of the chamber are chosen so that the selected microwave frequency fs is equal to the resonant TEmnp modes: some combinations of m, n and p ’ ¹ where € is the permittivity, p is the magnetic permeability and X'mn is the n-th zero of the derivative of the Bessel function of order m, so that J'm(X'mn) = 0.

A schematic representation of the electric field intensity 34 for a resonant ionization chamber 11 is shown in Fig. 8, where electromagnetic power at the microwave frequency corresponding to the gyrofrequency fs of the depicted reference closed surface 26 of the magnetic field is injected from the lateral waveguide 32.

The ion extraction and acceleration assembly 40 is configured to extract ions from the plasma proximal to the chamber outlet 13 and accelerate them electrostatically to high speeds through a set of apertures of the ion extraction and acceleration assembly 40 itself along the ion acceleration direction Z. The ion extraction and acceleration assembly 40 of the depicted embodiment comprises a plurality of electrostatic parallel grids 41,42 or perforated plates biased to different potentials. However, in different embodiments the ion extraction and acceleration assembly 40 may be implemented differently, such as through magnetic nozzles or ExB acceleration system.

In the present embodiment, each of the grids or perforated plates comprises a set of apertures through which ions are extracted and accelerated, while electrons are repelled and kept inside the ionization chamber 11.

Preferably, the plurality of electrostatic grids or perforated plates comprises two or three grids or perforated plates.

In detail, the depicted embodiment comprises a two-grid assembly. In this case, the ion extraction and acceleration assembly 40 comprises a first grid 41 which is called “screen grid” and a second grid 42 which is called “acceleration grid” (both indicated in Fig. 8).

The acceleration grid 42 is placed downstream of the screen grid 41 with respect to the ion acceleration direction Z. Both the screen 41 and the acceleration 42 grid feature a series of apertures of a size comparable to the Child-Langmuir length of the plasma generated in the ionization chamber 11 (see 5.2-2, page 197 of Fundamentals of Electric Propulsion: Ion and Hall Thrusters, Dan M. Goebel and Ira Katz, March 2008, https://descanso.ipl.nasa.gov/ SciTechBook/seriesl/Goebel cmprsd_opt.pdf).

The screen 41 and acceleration 42 grid are positioned so that the apertures of the acceleration grid 42 are concentric with the apertures in the screen grid 41. The acceleration grid 42 is biased negatively with respect to the screen grid 41 to impart an electrostatic acceleration to the ions that flow through the apertures, expelling them at high speeds.

Since the ion extraction and acceleration assembly 40 is typically configured to emit high speed positive ions only, the spacecraft electric thruster 10 may also comprise an electron emitting element 50 (shown in Fig. 9) which is configured to emit a current of electrons suitable to neutralize an ion beam emitted through the ion extraction and acceleration assembly 40. The electron emitting element 50 may be positioned downstream of the ion extraction and acceleration assembly 40 with respect to the ion acceleration direction Z. The electron emitting element 50 may comprise an electrode thermionically emitting electrons.

The operation of the spacecraft electric thruster 10 is illustrated with reference to Fig. 9 and to the air-breathing operation mode.

When orbiting at VLEOs, the spacecraft electric thruster 10 experiences a flux of rarefied atmospheric gas which impinges on the intake 14 at orbital speed (usually about 7,8 km/s) towards the ionization chamber 11.

The intake 14 collects the impinging gas and directs it to the inside of the ionization chamber 11. The atmospheric particles of the impinging gas are slowed down and compressed via collisions with the intake 14 and ionization chamber 11 walls I la, 16, 15, 41 resulting in a higher gas pressure level in the ionization chamber 11 with respect to asymptotic flow conditions impinging on the intake 14 at orbital speed.

During operation, the power injection assembly 30 injects microwave power through the waveguide 32 at a selected microwave frequency. Due to the geometry of the waveguide 32 and the selected microwave frequency, the power is not dispersed and traverses the input element 33 to reach the ionization chamber 11.

The ionization chamber 11 whose geometry and material are configured to make the chamber resonant in a suitable mode at the selected microwave frequency allows generating a strong oscillating transverse electric field substantially at the center of the chamber 11.

In parallel, the magnetic assembly 20 generates an intense static minimum-B magnetic field characterized by closed surfaces at a fixed magnetic field intensity and at a fixed gyrofrequency. Particularly, the microwave frequency of the oscillating electric field is selected to coincide with a gyrofrequency of a closed surface 26 of the magnetic field which is generated near the central development axis X of the ionization chamber 11 (i.e., away from the chamber walls) and substantially at the center of the ionization chamber 11, where the oscillating electric field is strong.

In an example, the microwave frequency is selected to be substantially equal to 5.6 GHz, which is the gyrofrequency of the electrons performing their cyclotron motion when crossing the isointensity closed surface of about 0.2 T (closed surface 26 of Fig. 5). In this case, the geometrical features of the ionization chamber 11 are configured so that the chamber is resonating at the selected microwave frequency in the TE111 mode.

When the free electrons present in the impinging gas cross this specific closed surface 26 characterized by a gyrofrequency fs corresponding to the selected microwave frequency of the oscillating electric field, they undergo ECR heating, absorbing the microwave power and increasing their energy.

The energized electrons can ionize neutral atoms when impacting on them, thereby generating ions and additional electrons, which undergo the same process in a cascade effect.

Additionally, the electrons are confined by the static minimum-B magnetic field. The cusps of field intensity generated by the magnetic bars 23 provide for a radial confinement which significantly impedes the motion of the electrons toward the ionization chamber 11 lateral walls I la. The mirror effect provided by the annular magnetic elements 21,22 provides for axial confinement not allowing the electrons to leave the chamber from the chamber inlet 12.

This implies that the residence time of the electrons in the ionization chamber 11 is very high. Also, the probability to cross the specific closed surface 26 multiple times increases, thereby leading to a further increase of the electron’s energy. On its turn, this also substantially increases the probability of an ionization event, allowing for plasma generation despite a very low-pressure environment, such as the one resulting from the compression level offered by the intake 14 and the relatively high operating altitude needed for drag compensation feasibility, which is very limited. Because of the high magnetic field, ions as well result to be confined, even if this phenomenon is less relevant than for electrons because of the higher ion mass. However, ions are also confined due to the ambipolar electric field (charge separation) generated by the confinement of the electron population.

At an ionization steady state, a sufficiently dense plasma, roughly at the cut-off density of the selected microwave frequency, is generated in the ionization chamber 11 and magnetically confined near its center, wherein the cut-off density is given by ^ecut:off ~ where e is the electron charge, m_e is the electron mass, /is the selected microwave frequency and Eo is the vacuum permittivity.

Since the mirror effect of the static magnetic field is asymmetric, and particularly weaker towards the chamber outlet 13, the plasma in the confined region preferably escapes from the chamber outlet 13.

After a statistically suitable time to sustain ionization, electrons can thus scatter in the loss cone of the mirror and can diffuse towards the chamber outlet 13, leaving the confinement together with ions.

At the chamber outlet 13, there is the screen grid 41 of the ion extraction and acceleration assembly 40. The apertures of the screen grid 41 allow for the extraction of ions towards the downstream acceleration grid 42, which is at a more negative bias with respect to the bias of the screen grid 41 (e.g., a few thousands of volts below the screen grid 41). The ions feel the strong static electric field between the grids 41,42 and are accelerated through the apertures to high speeds (e.g., 10-50 km/s) exiting the system and generating thrust.

Finally, if it is necessary to maintain the charge balance in the thruster 10, the electron emitting element 50 placed downstream of the acceleration grid 42 emits electrons, which are attracted by the outbound ion beam and neutralize it.

In conclusion, all details can be substituted by other technically equivalent elements; the materials used, as well as the shapes and sizes, may be any according to specific implementation needs without departing from the scope of protection of the following claims.

Claims

1. A spacecraft electric thruster (10) comprising

- an ionization chamber (11) for plasma generation which develops along a central development axis (X), wherein the ionization chamber (11) has a chamber inlet (12) configured to allow gas inflow and a chamber outlet (13) configured to allow extraction of ionized gas particles;

- a power injection assembly (30) configured to perform electron heating for plasma generation inside the ionization chamber (11);

- a magnetic assembly (20) configured to generate a magnetic field for plasma confinement in the ionization chamber (11); and

- an ion extraction and acceleration assembly (40) configured to extract ionized gas particles from the ionization chamber (11) through the chamber outlet (13) and accelerate them along an ion acceleration direction (Z) wherein the power injection assembly (30) comprises a microwave power delivery system (31) configured to deliver an electric field oscillating at a selected microwave frequency (f) and to inject the generated oscillating electric field in the ionization chamber (11), and wherein the ionization chamber (11) comprises a plurality of walls (I la, 15,16,41) closed to the passage of microwaves apart from a portion of at least one lateral wall (I la) of the plurality of walls (I la, 15,16,41) at which the power delivery system (31) is coupled to inject the generated oscillating electric field in the ionization chamber (11), and wherein the ionization chamber (11) is resonant for electromagnetic fields oscillating at the selected microwave frequency.

2. The spacecraft electric thruster (10) of claim 1, wherein the magnetic assembly (20) is configured to generate a magnetic field in the ionization chamber (11) to cause axial plasma confinement along the central development axis (X) and radial plasma confinement around the central development axis (X) in at least one common region along the central development axis (X).

3. The spacecraft electric thruster (10) of claim 1 or 2, wherein the at least one lateral wall (I la) is a solid wall and wherein the ionization chamber (11) has end walls with apertures (15,16,41) closed to the passage of microwaves both at the chamber inlet (12) and at the chamber outlet (13).

4. The spacecraft electric thruster (10) of any one of claims 1 to 3, wherein the ionization chamber (11) is made of an electrically conductive material; and/or wherein the ionization chamber (11) defines a simply connected internal space.

5. The spacecraft electric thruster (10) of any one of the preceding claims, wherein the chamber inlet (12) comprises a solid end wall (16) with an injection hole (16a) with a size not exceeding a threshold size above which a passage of an injected electric field oscillating at the selected microwave frequency (f) would be allowed; or wherein the chamber inlet (12) comprises a metallic mesh (15) with holes smaller than a threshold size above which a passage of an injected electric field oscillating at the selected microwave frequency (f) would be allowed; and/or wherein the chamber outlet (13) comprises at least one grid (41) with holes for ion extraction smaller than a threshold size above which a passage of an injected electric field oscillating at the selected microwave frequency (f) would be allowed.

6. The spacecraft electric thruster (10) of any one of the preceding claims, wherein the microwave power delivery system (31) is placed outside of the ionization chamber (11) and the power injection assembly (30) comprises a microwave launching group (32,33) configured to inject the oscillating electric field in the ionization chamber (11), preferably wherein the microwave launching group (32,33) comprises a waveguide (32) which ends in the ionization chamber (11) at the portion of at least one lateral wall (I la) of the ionization chamber (11) at which the power delivery system (31) is coupled.

7. The spacecraft electric thruster (10) of claims 6, wherein the microwave launching group (32,33) comprises an input element (33) configured to allow the passage of electromagnetic power and preventing gas leakages from the ionization chamber (11), the input element (33) being preferably an RF window.

8. The spacecraft electric thruster (10) of any one of the preceding claims, wherein the magnetic assembly (20) is configured to generate a magnetic field in the ionization chamber (11) characterized by

- a magnetic mirror topology characterized by a substantially axial magnetic field with at least two high intensity regions (27a, 27b), respectively located towards the ionization chamber inlet (12) and the ionization chamber outlet (13), a first high intensity region (27a) of the at least two high intensity regions (27a, 27b) being located towards the ionization chamber outlet (13) and showing a threshold magnetic field intensity (B_th); and

- a plurality of surfaces (26) at a fixed magnetic field intensity and at a fixed gyrofrequency which surround a region wherein the magnetic field has a minimum magnetic field intensity (B_min) and do not intercept lateral walls (I la) which delimit laterally the ionization chamber (11), the plurality of surfaces (26) comprising a reference surface (26) characterized by a reference gyrofrequency (fs) and a reference magnetic field intensity (Bs), with the reference magnetic field intensity (Bs) being lower than or equal to the threshold magnetic field intensity (B_th) and higher than the minimum magnetic field intensity (Bmin); and wherein the selected microwave frequency substantially corresponds to the reference gyrofrequency (fs) and wherein the microwave power delivery system (31) is configured to deliver an electric field oscillating at the selected microwave frequency substantially corresponding to the reference gyrofrequency (fs).

9. The spacecraft electric thruster (10) of claim 8, wherein the ionization chamber (11) is cylindrical, and the radius (r) of the cylindrical ionization chamber (11) and the length (L) of the cylindrical ionization chamber (11) satisfy the following equation of resonant TEmnp modes some combinations of m, n and p where fs is the reference gyrofrequency, € is the permittivity, p is the magnetic permeability and X'mn is the n-th zero of the derivative of the Bessel function of order m, so that J'm(X'mn) = 0.

10. Method of operating a spacecraft electric thruster (10) according to claim 8 or 9, comprising the step of delivering an electric field in the ionization chamber (11) oscillating at the microwave frequency substantially corresponding to the reference gyrofrequency (fs) through the microwave power delivery system (31).

11. A spacecraft electric thruster (10) comprising

- a magnetic assembly (20) configured to generate a magnetic field for plasma confinement in the ionization chamber (11), wherein the magnetic assembly (20) is configured to generate a magnetic field in the ionization chamber (11) to cause axial plasma confinement along the central development axis (X) and radial plasma confinement around the central development axis (X) in at least one common region along the central development axis (X); and

- an ion extraction and acceleration assembly (40) configured to extract ionized gas particles from the ionization chamber (11) through the chamber outlet (13) and accelerate them along an ion acceleration direction (Z) wherein the power injection assembly (30) comprises

- a microwave power delivery system (31) configured to generate an electric field oscillating at a selected microwave frequency (f) and to inject the generated oscillating electric field in the ionization chamber (11), the microwave power delivery system (31) being placed outside of the ionization chamber (11); and

- a microwave launching group (32,33) configured to inject the oscillating electric field in the ionization chamber (11), the microwave launching group (32,33) comprising a waveguide (32) ending in the ionization chamber (11) at a portion of at least one lateral wall (I la) of the ionization chamber (11), which is transparent to microwaves.

12. The spacecraft electric thruster (10) of claim 11, wherein the waveguide (32) extends along an extension axis (E) and wherein, at the portion of the lateral wall (I la), which is transparent to microwaves, the extension axis (E) of the waveguide (32) is comprised in a plane inclined with respect to the central development axis (X), preferably at the portion of the lateral wall (I la), the extension axis (E) of the waveguide (32) is comprised in a plane which is orthogonal to the central development axis (X).

13. The spacecraft electric thruster (10) of claim 11 or 12, wherein the waveguide (32) is a rectangular waveguide and/or a ridge waveguide and/or a double ridge waveguide, and/or contains a stepped impedance transformer.

14. The spacecraft electric thruster (10) of any one of claims 11 to 13, wherein the portion at which the waveguide (32) ends in the ionization chamber (11) is placed at an axial position along the central development axis (X) where the magnetic field generated by the magnetic assembly (20) shows in average a magnetic field intensity which is higher than a reference magnetic field intensity (Bs).

15. The spacecraft electric thruster (10) of any one of claims 11 to 14, wherein the magnetic assembly (20) is configured to generate a magnetic field in the ionization chamber (11) characterized by

- a plurality of surfaces (26) at a fixed magnetic field intensity and at a fixed gyrofrequency which surround a region wherein the magnetic field has a minimum magnetic field intensity (B_min) and do not intercept lateral walls (I la) which delimit laterally the ionization chamber (11), the plurality of surfaces (26) comprising a reference surface (26) characterized by a reference gyrofrequency (fs) and a reference magnetic field intensity (Bs), with the reference magnetic field intensity (Bs) being lower than or equal to the threshold magnetic field intensity (B_th) and higher than the minimum magnetic field intensity (Bmin); and wherein the microwave power delivery system (31) is configured to deliver an electric field oscillating at the selected microwave frequency substantially corresponding to the reference gyrofrequency (fs).

16. The spacecraft electric thruster (10) of claim 15, wherein the portion at which the waveguide (32) ends in the ionization chamber (11) is placed at an axial position along the central development axis (X) at which the magnetic field generated by the magnetic assembly (20) shows a high intensity region (27b) of the at least two high intensity regions (27a, 27b) and which is outside of the reference surface (26).

17. The spacecraft electric thruster (10) of any one of claims 11 to 16, wherein the portion at which the waveguide (32) ends in the ionization chamber (11) is placed at an axial position along the central development axis (X) proximal to the chamber inlet (12).

18. The spacecraft electric thruster (10) of any one of claims 11 to 17, wherein the microwave launching group (32,33) comprises an input element (33) configured to allow the passage of electromagnetic power and preventing gas leakages from the ionization chamber (11), the input element (33) being preferably an RF window.

19. The spacecraft electric thruster (10) of claim 18, wherein the input element (33) is configured to electrically insulate the ionization chamber (11), by preferably implementing a DC break.

20. A spacecraft electric thruster (10) comprising

- a power injection assembly (30) configured to perform electron heating for plasma generation inside the ionization chamber (11), the power injection assembly (30) comprising a microwave power delivery system (31) configured to deliver an electric field oscillating at a selected microwave frequency (f) to be injected in the ionization chamber (11);

- a magnetic assembly (20) configured to generate a magnetic field in the ionization chamber (11) to cause axial plasma confinement along the central development axis (X) and radial plasma confinement around the central development axis (X) in at least one common region along the central development axis (X); and

- an ion extraction and acceleration assembly (40) configured to extract ionized gas particles from the ionization chamber (11) through the chamber outlet (13) and accelerate them along an ion acceleration direction (Z). wherein the magnetic assembly (20) is configured to generate a magnetic field in the ionization chamber (11) with a region in which the magnetic field has a minimum magnetic field intensity (Bmin) at a radial distance separated from lateral walls (I la) delimiting laterally the ionization chamber (11), and characterized by a plurality of surfaces (26) which surround the region in which the magnetic field has the minimum magnetic field intensity (Bmin) and do not intercept the ionization chamber (11) walls, each surface (26) of the plurality of surfaces (26) being at a constant magnetic field intensity and at a constant gyrofrequency, and wherein the selected microwave frequency (f) corresponds to a gyrofrequency of a surface (26) of the plurality of surfaces (26) of the magnetic field.

21. The spacecraft electric thruster (10) of claim 20, wherein the region in which the magnetic field has the minimum magnetic field intensity (Bmin) is located substantially at the central development axis (X) of the ionization chamber (11).

22. The spacecraft electric thruster (10) of claim 20 or 21, wherein the magnetic assembly (20) is configured to generate a magnetic field in the ionization chamber (11) with a magnetic mirror topology characterized by a substantially axial magnetic field with higher intensity towards both the ionization chamber inlet (12) and the ionization chamber outlet (13), preferably with an asymmetric magnetic mirror topology, more preferably with a threshold magnetic field intensity (Bth) towards the chamber outlet (13) which is lower than a maximum magnetic field intensity (Bmax) towards the chamber inlet (12) with respect to a development of the magnetic field intensity along the central development axis (X).

23. The spacecraft electric thruster (10) of any one of claims 20 to 22, wherein the plurality of surfaces (26) comprises a reference surface (26) characterized by a reference gyrofrequency (fs) and a reference magnetic field intensity (Bs), the reference magnetic field intensity (Bs) being lower than or equal to the threshold magnetic field intensity (Bth) and higher than the minimum magnetic field intensity (B_min), and wherein the ionization chamber (11) is resonant for electromagnetic fields oscillating at a microwave frequency substantially equal to the reference gyrofrequency (fs).

24. The spacecraft electric thruster (10) of any one of claims 20 to 23, wherein the magnetic assembly (20) comprises a plurality of magnetic elements (21-25) which are permanent magnets or electromagnets, preferably wherein the plurality of magnetic elements (21-25) comprises:

- at least two annular magnetic elements (21,22) placed at an axial position along the central development axis (X) substantially at or in the proximity of the chamber inlet (12) or chamber outlet (13), respectively, and coaxially with respect to the central development axis (X); and/or

- a set of radially and/or tangentially magnetized magnetic bars (23) placed in a multipole cylindrical arrangement.

25. The spacecraft electric thruster (10) of claim 24, wherein the set of magnetic bars (23) are placed with circumferentially alternating poles and in a side-by-side arrangement so as to form a hollow cylinder body, preferably wherein the set of magnetic bars (23) are placed in a Halbach configuration, more preferably wherein the set of magnetic bars (23) are placed in a Halbach hexapole configuration.

26. The spacecraft electric thruster (10) of claim 24 or 25, wherein the two annular magnetic elements (21,22) are radially magnetized permanent magnets with respectively opposite magnetization.

27. The spacecraft electric thruster (10) of any one of claims 24 to 26, wherein the plurality of magnetic elements (21-25) comprises a set of additional annular magnetic elements (24,25) which are placed coaxially with respect to the central development axis (X), preferably wherein

- at least a first additional annular magnetic element (25) of the set of additional annular magnetic elements (24,25) is placed in the proximity of the annular magnetic element (21) located at or in the proximity of the ionization chamber inlet (12) and defines at least one radial slot (25a) configured to allow for lateral injection of electromagnetic power; and/or

- at least a second additional annular magnetic element (24) of the set of additional annular magnetic elements (24,25) is placed at an axial position along the central development axis (X) of the ionization chamber (11) substantially corresponding to a center of an axial development of the ionization chamber (11).

28. The spacecraft electric thruster (10) of any one of claims 20 to 27, wherein the ionization chamber (11) defines a simply connected internal space.