Thruster employing electron cyclotron resonance

Abstract

The invention relates to a satellite thruster (10) that employs electron cyclotron resonance and a magnetic nozzle, that extends at least partially along a longitudinal axis (X), and that comprises an external conductor (22) and an ionisation chamber (8), the external conductor and the ionisation chamber extending longitudinally along this axis, the external conductor bounding the ionisation chamber transversely, and the ionisation chamber comprising a circular transverse aperture (24) having a diameter D, characterized in that it further comprises a screen (6) coaxial with the longitudinal axis (X), having a maximum transverse dimension D' such that the ratio of D' divided by D is between 3 and 15, said screen (6) comprising a portion made of dielectric material (61) and / or a portion made of electrically conductive material (62).

Description

ELECTRONIC CYCLOTRONIC RESONANCE PROPULSION

[0001] This description relates to an electron cyclotron resonance thruster.

[0002] Artificial satellites require thrusters to be placed in position, maintained in position, and to perform trajectory or attitude correction maneuvers. Similarly, space probes intended for exploring the solar system have thrusters that allow them to position themselves very precisely around the chosen planet, or even to land on an asteroid to collect material samples.

[0003] Typically, these small thrusters provide thrusts of a few newtons at most. Chemical thrusters use liquid propellants such as hydrazine (N2H2) or hydrogen peroxide (hydrogen peroxide). When these propellants decompose, the chemical energy is converted into heat and then into thrust during the expansion of the hot gases in a suitable nozzle. The material is ejected at high speed and creates a reaction force, called thrust. This thrust can modify the momentum of the spacecraft, probe or satellite, and influence its trajectory. A well-known limitation of these technologies is related to the ejection speed that can be reached by the gases. In addition, the mass of the propellants can reach a significant fraction of the total mass of the satellite, reducing the satellite's payload, which constitutes a second limitation.

[0004] However, it is possible to increase the ejection speed of the propellant gas by several orders of magnitude, by using ionized gas particles that are accelerated out of the vehicle by means of electric and / or magnetic fields. These so-called plasma thrusters make it possible to carry out missions with greater trajectory modifications for an equivalent quantity of material, or to carry out missions with equivalent trajectory modifications but with less material on board. This makes it possible to reduce the mass of the satellite or to increase its payload, at constant mass.

[0005] With the increasing number of new satellites equipped with only a plasma thruster, it is clear that these thrusters are of great application interest. However, they pose many technical challenges.

[0006] For example, the first ECR (Electron Cyclotron Resonance) plasma thrusters, i.e. electron cyclotron resonance thrusters, usable by space vehicles, had the disadvantage of not being miniaturizable. Their size was determined by the size of their cavity for coupling electromagnetic waves with the plasma, this coupling being necessary for initiating the plasma. Patent FR 11 62545 B1 proposes a miniaturized electron cyclotron resonance thruster, consisting of being able to initiate the plasma in a very small volume by the resonance of magnetized electrons (ECR). This is made possible thanks to a particular configuration of the arrival of the gas to be ionized, the magnetic field lines and the coupling of the electromagnetic wave.

[0007] However, it remains desirable to improve the performance of such thrusters. In addition, problems related to increasing service life persist. Problems of part erosion and wear related to plasma discharges are still relevant. Technical problem

[0008] From this situation, an aim of the present invention is to propose a new electron cyclotron resonance thruster which does not have the drawbacks cited above, or for which at least some of these drawbacks are reduced.

[0009] The invention aims in particular to propose a particularly high-performance electron cyclotron resonance thruster. One aspect of the invention thus relates to an electron cyclotron resonance thruster, extending at least partially along a longitudinal axis, comprising an external conductor and an ionization chamber, the external conductor and the ionization chamber extending longitudinally along this axis, the external conductor transversely delimiting the ionization chamber, the ionization chamber comprising a circular transverse opening 24 having a diameter D, characterized in that it further comprises a screen coaxial with the longitudinal axis, having a maximum transverse dimension D' such that the ratio of D' divided by D is between 3 and 15, said screen comprising a part made of dielectric material and / or a part made of electrically conductive material.

[0010] Thanks to the invention, it is possible to significantly improve the performance of the thruster. The thrust produced by such a thruster can be multiplied by a factor of 1.3 compared to a state-of-the-art plasma thruster. The total efficiency of the system, defined here by the ratio of the power contained in the plasma jet to the electrical power supplied to the thruster, is multiplied by a factor of 1.7 compared to a state-of-the-art plasma thruster. When the thruster is operating and a plasma comprising ions and electrons is ejected, the coaxial screen prevents the electrons trapped in the most divergent magnetic field lines from coming into contact with one of the parts of the thruster. Thus, instead of being collected by the thruster in the form of an electric current, these divergent electrons accumulate near the screen, locally forming a negative electric charge.The latter allows other divergent electrons to be repelled and therefore redirected into the beam of emitted particles. The flow of plasma ejected by the thruster is thus increased.

[0011] According to one aspect of the invention, the portion of dielectric material covers the portion of conductive material. According to one aspect of the invention, the portion of dielectric material completely covers the portion of conductive material. According to one aspect of the invention, the portion of dielectric material completely covers the portion of conductive material such that an outer surface of the screen is made of dielectric material.

[0012] According to another aspect of the invention, the shield is attached to the external conductor. According to another aspect of the invention, the thruster comprises a magnetic field source and the shield is attached to the magnetic field source. According to another aspect of the invention, the thruster comprises a body and the shield is attached to the body. According to another aspect of the invention, the thruster comprises a magnetic field source and a body and the shield is attached between the magnetic field source and the body. According to one aspect of the invention, the shield is attached by means of screws made of dielectric material.

[0013] According to one aspect of the invention, the dielectric material portion is configured to electrically insulate the conductive material portion from the external conductor. According to one aspect of the invention, the dielectric material portion is configured to electrically insulate the conductive material portion from the remainder of the thruster. In this variant, the remainder of the thruster comprises all the elements of the thruster except the screen.

[0014] According to another aspect of the invention, the thruster further comprises a dielectric material element configured to electrically insulate the screen from the external conductor.

[0015] According to one embodiment, the portion of conductive material covers the portion of dielectric material. According to one aspect of the invention, the portion of conductive material completely covers the portion of dielectric material. According to one aspect of the invention, the portion of conductive material completely covers the portion of dielectric material such that an outer surface of the screen is made of conductive material.

[0016] According to one aspect of the invention, the portion made of electrically conductive material has a polarization different from that of the external conductor or that of another element of the thruster. According to one aspect of the invention, the portion made of electrically conductive material has a polarization different from that of the external conductor or that of another element of the thruster other than the screen. According to one aspect of the invention, the portion made of electrically conductive material has a negative polarization relative to the external conductor or relative to another element of the thruster. According to one aspect of the invention, the portion made of electrically conductive material has a negative polarization relative to the external conductor or relative to another element of the thruster other than the screen.

[0017] Thanks to the invention, it is possible to significantly improve the performance of the thruster. The thrust produced by such a thruster can be multiplied by a factor greater than 1.3 compared to a state-of-the-art plasma thruster. The total efficiency of the system, defined here by the ratio of the power contained in the plasma jet to the electrical power supplied to the thruster, is multiplied by a factor greater than 1.7 compared to a state-of-the-art plasma thruster. When the thruster is operating and a plasma comprising ions and electrons is ejected, the negatively polarized coaxial screen repels the electrons trapped in the most divergent magnetic field lines and therefore allows these divergent electrons to be redirected into the emitted particle beam. The plasma flow ejected by the thruster is thus increased.

[0018] According to another aspect of the invention, the screen has a disc shape with a hollow in its center or has a flared shape, for example a truncated hollow cone or nozzle.

[0019] According to one aspect of the invention, the outer conductor comprises an inner wall made of graphite. According to one aspect of the invention, the outer conductor comprises an inner wall of generally cylindrical shape, said inner wall being made of graphite. According to one aspect of the invention, the outer conductor is entirely made of graphite. According to one aspect of the invention, the outer conductor is made of conductive material and comprises an inner wall made of graphite. According to one example, the outer conductor comprises a graphite deposit on its inner wall. According to another example, the outer conductor comprises a graphite sleeve on its inner wall.

[0020] Under the impacts of plasma ions, graphite erodes less than a state-of-the-art material such as aluminum. Thus, the external conductor has a longer lifetime. In addition, the plasma electrons that strike the graphite inner wall of the external conductor have a lower probability of re-emitting electrons by secondary emission, which allows good confinement of the plasma, and consequently, avoids energy losses from the plasma to the inner wall of the external conductor. The thruster performance is significantly improved. The thrust produced by such a thruster is multiplied by a factor of 1.6 compared to a state-of-the-art plasma thruster. The total efficiency of the system, defined here by the ratio of the power contained in the plasma jet to the electrical power supplied to the thruster, is multiplied by a factor of 2.5 compared to a state-of-the-art plasma thruster.

[0021] According to one aspect of the invention, the dielectric material portion is made of a mixture of fiberglass and epoxy resin. According to one aspect of the invention, the dielectric material portion is made of a mixture of carbon fiber and epoxy resin. This composition makes it possible to obtain a more rigid screen while being lighter and thinner.

[0022] According to other embodiments, the dielectric material part is made of resin or polyimide of the Kapton® type or ceramic such as alumina or boron nitride or Macor® ceramic.

[0023] According to one aspect of the invention, the dielectric material part is made of polyetheretherketone (PEEK) or polyetherketoneketone (PEKK).

[0024] According to one aspect of the invention, the part made of conductive material is for example a metal frame.

[0025] According to one aspect of the invention, the conductive material part is made of stainless steel or graphite or aluminum.

[0026] According to one aspect of the invention, the dielectric material portion is a film of insulating material. For example, the dielectric material portion is a polyimide film of the Kapton® type.

[0027] According to one aspect of the invention, the dielectric material portion is a polymer-based paint layer, for example a polydimethylsiloxane (PDMS)-based paint layer.

[0028] According to one aspect of the invention, the dielectric material portion has a thickness of 200 nanometers to 1 millimeter. According to one aspect of the invention, the dielectric material portion has a thickness greater than 1 micrometer.

[0029] According to one aspect of the invention, the conductive material portion has a thickness of 200 nanometers to 1 millimeter. According to one aspect of the invention, the conductive material portion has a thickness greater than 1 micrometer.

[0030] A thickness greater than one micrometer allows for better resistance over time of the screen which is subject to erosion linked to the space environment.

[0031] According to one aspect of the invention, the dielectric material part comprises a resin frame and a polyimide film of the Kapton® type.

[0032] According to one aspect of the invention, the portion made of conductive material is bonded to the portion made of dielectric material.

[0033] According to one aspect of the invention, the part made of dielectric material is bonded to the part made of conductive material.

[0034] According to a variant, the external conductor comprises an internal wall and an external wall separated by a transverse thickness and a uniformization chamber hollowed out in said transverse thickness of the wall, said chamber extending longitudinally over a portion of the external conductor and having the overall shape of a hollow cylinder. The uniformization chamber makes it possible to inject the gas uniformly around the periphery of the ionization chamber. The uniformization chamber makes it possible to inject the propellant gas in a controlled manner.

[0035] According to another aspect of the invention, the propellant comprises gas injection means, said means comprising:- at least one injection channel, said channel radially piercing the outer wall of the external conductor and opening into the uniformization chamber,- the uniformization chamber,- communication means configured so that the uniformization chamber opens into the ionization chamber.

[0036] According to one aspect of the invention, the communication means comprise a transverse circular part partially closing the longitudinal end of the uniformization chamber, said transverse circular part comprising at least three radial grooves connecting the uniformization chamber with the ionization chamber, or the communication means comprise orifices radially piercing the inner wall of the external conductor and opening into the uniformization chamber.

[0037] According to a variant, the propellant further comprises a conductor called an “internal conductor” of solid cylindrical geometry, said internal conductor extending along the longitudinal axis in the ionization chamber. According to one aspect of the invention, said internal conductor comprises an external graphite surface.

[0038] According to one aspect of the invention, the internal conductor is made entirely of graphite. According to another aspect of the invention, the internal conductor is made of conductive material and has an external graphite coating. According to another aspect of the invention, the internal conductor is made of conductive material and has a graphite sleeve on its external surface. According to another aspect of the invention, the internal conductor is made of conductive material and has a graphite deposit on its external surface. Under the impacts of the plasma ions, the graphite erodes less than a state-of-the-art material such as aluminum. The graphite makes it possible to increase the service life of the propellant.

[0039] The invention also relates to a method using a thruster according to the invention, the method comprising:- electrically connecting the part made of electrically conductive material to the negative pole of an electrical power supply,- electrically connecting the external conductor or another element of the thruster other than the screen to the positive pole of the electrical power supply,- starting the electrical power supply and the thruster.

[0040] The invention also relates to a device comprising a propellant according to the invention and an electrical power supply.

[0041] The invention also relates to a device comprising:- an electron cyclotron resonance thruster, extending at least partially along a longitudinal axis, comprising an external conductor and an ionization chamber, the external conductor and the ionization chamber extending longitudinally along this axis, the external conductor transversely delimiting the ionization chamber, the ionization chamber comprising a circular transverse opening having a diameter D, characterized in that it further comprises a screen coaxial with the longitudinal axis, having a maximum transverse dimension D' such that the ratio of D' divided by D is between 3 and 15, said screen comprising a part made of dielectric material and a part made of electrically conductive material, the part made of dielectric material being configured to electrically insulate the part made of electrically conductive material from the rest of the thruster,- a power supply comprising a negative pole electrically connected to the part made of conductive material and a positive pole electrically connected to the external conductor.,

[0042] According to one aspect of the invention, the device comprises a thruster which comprises a magnetic field source and a body, the positive pole of the power supply being electrically connected to the magnetic field source or the body.

[0043] According to one aspect of the invention, the device comprises a thruster which comprises an internal conductor, the positive pole of the power supply being electrically connected to the internal conductor.

[0044] Such a device makes it possible to apply a potential difference to the part made of conductive material and to obtain a part made of electrically conductive material negatively polarized with respect to the external conductor, or to the internal conductor, or to the magnetic field source or to the body of the thruster. When the device is operating and a plasma comprising ions and electrons is ejected, the negatively polarized coaxial screen repels the electrons trapped in the most divergent magnetic field lines and therefore makes it possible to redirect these divergent electrons into the beam of emitted particles. The plasma flow ejected by the thruster is thus increased.

[0045] According to one aspect of the invention, the device further comprises a measuring unit and an electronic processing circuit configured to control a potential difference applied between the conductive material portion and the external conductor or the magnetic field source or the body or the internal conductor.

[0046] According to the invention, the external conductor or the internal conductor or the magnetic field source or the body are called the reference.

[0047] The invention also relates to a method for controlling the potential difference applied between the part made of conductive material and the reference by means of a device according to the invention, the method comprising:- a measurement of the potential difference between the part made of conductive material and the reference, called the effective potential difference; when the power supply 9 does not apply a potential difference, this potential difference is called the floating potential; when the power supply applies a potential difference, the potential difference is called,- a calculation of the difference between the effective potential difference and a predetermined setpoint value,- if the difference exceeds a predetermined threshold, the application of a potential difference to the part made of electrically conductive material of a value equal to the setpoint value by the power supply, the method then resumes at the first step;if the deviation measurement does not exceed a predetermined threshold, the process resumes at the first step.;

[0048] According to one aspect of the invention, the setpoint value is between 10 volts and 1 kilovolt, preferably between 100 volts and 500 volts, in absolute values. Brief description of the figures

[0049] Other characteristics and advantages of the invention will emerge from the following description, which is purely illustrative and non-limiting, and must be read in conjunction with the appended drawings in which:

[0050] represents, partially and schematically, a longitudinal sectional view of a thruster according to the invention;

[0051] ,,and represent, partially and schematically, a longitudinal sectional view of an open coaxial conductor of a thruster according to the invention;

[0052] and represent, partially and schematically, a longitudinal sectional view of an external conductor and an internal conductor of a thruster according to the invention;

[0053] and represent, partially and schematically, a longitudinal sectional view of a thruster according to the invention;

[0054] represents, partially and schematically, a longitudinal sectional view of a thruster according to the invention;

[0055] represents, partially and schematically, a longitudinal sectional view of a thruster according to the invention;

[0056] represents, partially and schematically, a longitudinal sectional view of a thruster according to the invention;

[0057] and represent, partially and schematically, a longitudinal sectional view of an open coaxial conductor and a screen of a thruster according to the invention;

[0058] represents a diagram of a device according to the invention. Detailed description of the invention

[0059] For the sake of clarity, the dimensions of the elements shown in these figures do not correspond to actual dimensions or to actual dimensional ratios. In addition, some of these elements are represented only symbolically, and identical references indicated in different figures designate identical elements or those having identical functions.

[0060] There is shown, partially and schematically, an electron cyclotron resonance thruster according to the invention. This thruster 10 extends at least partially along a longitudinal axis X, and comprises: an open coaxial conductor 2, an ionization chamber 8, gas injection means 3, a microwave power injection device 4, a magnetic field source 1, a coaxial screen 6.

[0061] The principle of the electron cyclotron resonance satellite thruster is briefly recalled. The gas injection means 3 allow a gas to be introduced into the ionization chamber 8, in which the gas is ionized by the supply of microwaves. The microwaves provide the electrical power necessary for the ionization of the gas. They are generated, conveyed and deposited by the microwave power injection device 4. The frequency of the signal of the deposited electrical power is between 600 MegaHertz and 50 GigaHertz. The electrical power is transmitted to the propellant gas according to a coaxial geometry described later, or else by means of a waveguide. The magnetic field source 1 allows a magnetic field to be applied to the plasma and therefore to its electrons. The electrons of the plasma have a gyration movement in the plane perpendicular to the magnetic field, that is to say in a quasi-transverse plane.The frequency of this gyration movement is fixed by the value of the magnetic field. The value of the magnetic field and the frequency of the microwave signal are chosen such that the frequency of gyration of the electrons is equal to the frequency of the microwave signal. The power provided by the microwave signal will thus be efficiently deposited in the population of electrons of the plasma by a resonance phenomenon. The alternating electric field of the microwave signal will induce the acceleration of the electrons during their gyration movement and the increase of their kinetic energy in the plane perpendicular to the magnetic field, quasi-transverse. The plasma is then maintained with excellent energy efficiency. The electrons created are then ejected at high speeds under the effect of the magnetic force and thermal expansion. The ejection of the electrons generates an electric field which accelerates the ions of the plasma.These are ejected at high speeds, generating the thrust force.

[0062] For the following, the front of the thruster is defined as the longitudinal end of the thruster through which the propellant gases escape. Thus, in the embodiment of the, the coaxial screen is positioned at the front of the thruster. Similarly, the rear of the thruster is defined as the opposite longitudinal end. Thus, the microwave power generator 41 is located at the rear of the thruster.

[0063] The thruster comprises an open coaxial conductor 2 comprising an outer conductor 22 and an inner conductor 21. The outer conductor 22 has an overall shape of a hollow cylinder, coaxial with the longitudinal axis X and which extends longitudinally along this axis. The inner conductor 21 has a solid cylindrical geometry, extending longitudinally along the axis X, within the outer conductor 22. The thruster comprises a transverse plate 23 called a rear plate having an overall disc geometry. The rear plate 23 is in contact with the rear longitudinal end of the outer conductor 22.

[0064] The ionization chamber 8 consists of the volume between the external conductor 22, the internal conductor 21 and the rear plate 23. In other words, the ionization chamber is delimited transversely by the external conductor 22 and it is delimited axially at its rear longitudinal end by the rear plate 23. The ionization chamber has a transverse opening 24 at its front longitudinal end. This transverse opening 24 allows the propellant gases to escape. The transverse opening 24 is circular and has a diameter D.

[0065] The external conductor 22 comprises an internal wall 221. As illustrated in the set, the internal wall comprises a cylindrical geometry. Alternatively, the internal wall 221 comprises a truncated cone geometry as illustrated in. Alternatively, and as illustrated in the, the internal wall 221 comprises a first portion 2211 of cylindrical geometry at the rear of the ionization chamber 8 and a second portion 2212 of truncated cone geometry, the second portion being located in the extension of the first portion and the axes of revolution of the first and second portions being merged with the longitudinal axis X. In the embodiments previously described, the external conductor is machined from a single block of material. In an embodiment illustrated in the, the external conductor is manufactured from two separate pieces.The outer conductor comprises a second part of truncated cone geometry 225 fixed to a first part of overall cylindrical geometry 224, which extends longitudinally beyond the free longitudinal end 213 of the inner conductor 21. Alternatively, the geometry of the inner wall is defined by any surface of revolution. The outer conductor makes it possible to contain the propellant gas radially, in order to constrain its ejection to the axial direction, through the opening 24.

[0066] The external conductor 22 is made of a conductive material. More specifically, according to the invention, the external conductor is made entirely of graphite. Alternatively, the external conductor is made of a conductive material and has a graphite deposit on its internal wall 221. The graphite deposition is carried out using the PVD technique, an acronym for "physical vapor deposition". For example, the graphite deposition is carried out by plasma or magnetron sputtering of a graphite target. A thickness of 10 micrometers of graphite can be obtained by this technique and makes it possible to significantly improve the performance of the thruster. Alternatively, a graphite sleeve is inserted into the external conductor made of conductive material.

[0067] The internal conductor 21 of solid cylindrical geometry extends longitudinally along the X axis. The internal conductor comprises an external surface made of graphite. The internal conductor 21 is for example made entirely of graphite, as shown in the. Alternatively, the internal conductor is made of a conductive material comprising an external coating of graphite. For example, the conductive material is covered with a graphite deposit, carried out for example by magnetron or plasma sputtering of a graphite target. A thickness of 10 micrometers of graphite can be obtained by this technique and makes it possible to significantly improve the performance of the thruster. In another example and as shown in the, the internal conductor comprises a first part 211 made of conductive material, and a second part 212 made of graphite.The first part 211 comprises a first solid cylinder, which extends along the longitudinal axis X, extended axially by a cylindrical head whose diameter is greater than the diameter of the first cylinder. The second part 212 comprises a sleeve complementary to the first part, so that the assembly of the first and second parts forms a solid cylinder extending along the axis X in the ionization chamber. In this example, the cylindrical head is covered with a graphite deposit, not shown in the.

[0068] The outer conductor 22 and the inner conductor 21 form the open coaxial conductor 2 which allows microwaves to be deposited in the ionization chamber. The microwaves are generated and supplied by the microwave power generation device 4 which will be described later.

[0069] The external conductor 22 comprises an external wall 222, separated from the internal wall 221 by a transverse thickness 223. The external conductor comprises a uniformization chamber 32 hollowed out in said transverse thickness 223. The uniformization chamber 32 extends longitudinally over a portion of the external conductor 22. The uniformization chamber has the overall shape of a hollow cylinder.

[0070] The gas injection means 3 comprise: - at least one injection channel 31, said channel radially piercing the external wall 222 and partially the transverse thickness 223 of the external conductor and opening into the uniformization chamber 32, - the uniformization chamber 32, - communication means configured so that the uniformization chamber opens into the ionization chamber.

[0071] The communication means comprise, for example, a transverse circular part 331 pressed against the rear plate 23. The transverse circular part 331 partially closes the rear longitudinal end of the uniformization chamber 32. The transverse circular part 331 comprises at least three radial grooves connecting the uniformization chamber 32 with the ionization chamber 2. Alternatively, orifices radially piercing the internal wall 221 of the external conductor and opening into the uniformization chamber are made. In this second embodiment, the rear end of the uniformization chamber is closed directly by the rear plate 23 or by the transverse circular part 331 which does not comprise a groove in this case.

[0072] The propellant gas is injected into the uniformization chamber via at least one injection channel 31. The gas then propagates longitudinally and transversely in the uniformization chamber 32, then in the ionization chamber via the radial grooves of the transverse circular part 331. The uniformization chamber allows the gas to be injected in a controlled manner into the ionization chamber.

[0073] The back plate 23 comprises a dielectric material capable of accepting high thermal loads while being relatively transparent to microwaves. For example, the back plate is made of ceramic or quartz. The back plate allows the propellant gas to diffuse towards the opening 23 of the thruster. It also receives a significant portion of the heat flux from the plasma. The thruster comprises a body 5 in which the back plate 23 is integrated.

[0074] The body 5 of the thruster is made of conductive material. The body 5 ensures the mechanical fixing of the various elements of the thruster. It also allows the electrical connection to be made between the microwave power injection device 4 and the open coaxial conductor 2.

[0075] The microwave power injection device 4 comprises a microwave power generator 41 and a coaxial transmission conductor 42. In an alternative not described here, the microwave power injection device comprises a waveguide 42.

[0076] The microwave power generator 41 produces the microwaves required to operate the thruster. It produces a signal of a few tens of W (Watt) to a few kW (kiloWatt) at a frequency of 2.45 GHz (Giga Hertz). This signal is produced from a direct voltage of a few tens of V (Volt) and using microwave oscillator technology. The signal is amplified either by a solid-state amplifier or by a field-effect amplifier (traveling wave tube, klystron).

[0077] The coaxial transmission conductor 42 transmits the microwave signal from the generator 41 to the open coaxial conductor 2. The coaxial transmission conductor 42 has an impedance of 50 Ohms. It comprises a concentric inner conductor 421 and an outer conductor 422. They are separated from each other by a concentric insulating layer 423, for example made of polytetrafluoroethylene (PTFE) or boron nitride (BN).

[0078] The magnetic field source 1 comprises a coil 12 or a set of coils, supplied with electric current. Alternatively, the magnetic field source comprises a permanent magnet or a set of permanent magnets. For a microwave signal of frequency 2.45 GHz, the magnetic field intensity is adjusted to 875 Gauss, so that the electron cyclotron resonance region is located in the first two centimeters downstream of the back plate 23. The magnetic field is of strong intensity in the source 1 but decreases rapidly at the exit of the thruster, creating a longitudinal magnetic field gradient. In addition, the magnetic field is locally divergent at the exit of the ionization chamber, creating a magnetic nozzle contributing to the acceleration of the plasma ions.

[0079] The thruster 10 according to the invention comprises a coaxial screen 6. As shown in the, the coaxial screen 6 extends transversely to the axis X. The coaxial screen 6 is directly fixed to the front longitudinal end of the external conductor 22 by means of three screws 63. Alternatively, the coaxial screen 6 is fixed at another axial position on the external conductor 22. For example, the coaxial screen 6 is advantageously fixed to the external conductor 22 at the shoulder 226 that the external conductor 22 has, as illustrated by the. Alternatively and as illustrated in the, the coaxial screen 6 is fixed between the body 5 and the magnetic field source 1. In this exemplary embodiment, the screen is held by three screws 63 which pierce it and securely hold the magnetic field source 1 to the body of the thruster 5. Alternatively, and as illustrated in the, the coaxial screen 6 is fixed to the magnetic field source 1.In this embodiment, the screen is fixed to the cover 13 by means of screws 63. In another example not shown, the screen is inserted between the two covers 13 and 14 which hold the coil 12, the covers 13, 14 and the screen 6 being screwed to each other in the longitudinal direction.

[0080] The coaxial screen 6 has a hollow disc shape in its center in order to leave free the opening 24 of the ionization chamber, which allows the ejection of the propellant gas. Alternatively and as illustrated by the, the coaxial screen has a truncated hollow cone shape in its center.

[0081] The coaxial screen 6 has a maximum transverse dimension D'. In the exemplary embodiment of the, this maximum transverse dimension is the diameter D'. In the exemplary embodiment of the, the maximum transverse dimension D' is the dimension obtained by the projection of the screen in the transverse direction. The maximum transverse dimension D' is chosen such that the ratio D' / D is between 3 and 15. In the exemplary embodiment of the, the ratio D' / D is 9. The drawings do not quantitatively represent this ratio. They are illustrations from which no dimensions or dimensional ratios can be extracted.

[0082] According to the embodiment of the, the coaxial screen 6 is entirely made of dielectric material, here a mixture of fiberglass and epoxy resin. Alternatively, the screen is made of polyimide of the Kapton® type, or even of ceramic such as alumina or boron nitride. Alternatively, as illustrated in the, the screen comprises a portion of conductive material 62 which is covered with dielectric material 61. The portion of conductive material 62 is for example a metal frame, onto which a resin 61 is cast. Alternatively, a film of insulating material 61 is affixed to the frame or the metal structure 62. The film 61 is for example made of polyimide of the Kapton® type. The screws 63 may be made of dielectric material, for example polyetheretherketone (PEEK) or polyetherketoneketone (PEKK).

[0083] Alternatively, and as shown in 1a and 1a, the screen comprises a part made of dielectric material 61 and a part made of electrically conductive material 62. The part made of dielectric material 61 is configured to electrically insulate the part made of conductive material 62 from the other elements of the thruster. The part made of dielectric material 61 is for example in the form of a ring onto which the part made of conductive material 62 is threaded and fixed. Alternatively and according to the embodiment of 1a, the part made of dielectric material is of conical geometry, according to a truncated cone, and is fixed in the extension of the external conductor 22. The axes of revolution of the part made of dielectric material 61 and of the external conductor 22 are merged with the longitudinal axis X. The part made of dielectric material extends longitudinally beyond the free longitudinal end of the internal conductor 21.The conductive material portion 62 is attached to the dielectric material portion 61. The conductive material portion 62 is, for example, made of stainless steel, graphite, or aluminum. Dielectric screws may be used to attach the screen 6 to the external conductor 22 or to any other element of the thruster 10. For example, if the attachment screws pierce the conductive material portion 62 and the dielectric material portion 61 to fit into the external conductor 22, polyetheretherketone (PEEK) or polyetherketoneketone (PEKK) screws are used. The conductive material portion 62 is electrically connected to the negative pole of a power supply 9, and the positive pole of the power supply is connected to the external conductor 22. When the power supply is turned on, the conductive material portion becomes negatively polarized.The reference is taken from a ground which is common to the external conductor 22, to the magnetic field source 1, and to the body of the thruster 5 which are electrically connected to each other. Alternatively, the reference is taken from any element of the thruster, on the internal conductor 21 for example.

[0084] In an embodiment illustrated in the, the conductive material portion 62 covers the dielectric material portion 61. The dielectric material portion 61 is for example a frame, which comprises an external coating of conductive material, the latter constituting the conductive material portion. The coating is for example obtained by a deposition of conductive material according to the PVD technique. The conductive material portion 62 can also be bonded to the dielectric material portion 61. For these exemplary embodiments, the electrostatic potential of the conductive material portion may not be controlled and be left floating, or be fixed according to the same characteristics described in the preceding paragraph, with reference to the.

[0085] The invention also relates to a device 113 comprising a propellant 10 according to the invention and an electrical power supply 9, as shown in.

[0086] The device 113 may comprise more elements, as illustrated in. Thus, the device 113 comprises:- an electron cyclotron resonance thruster 10, extending at least partially along a longitudinal axis, comprising an external conductor 22 and an ionization chamber 8, the external conductor 22 and the ionization chamber 8 extending longitudinally along this axis, the external conductor 22 transversely delimiting the ionization chamber 8, the ionization chamber comprising a circular transverse opening 24 having a diameter D, characterized in that it further comprises a coaxial screen 6 with the longitudinal axis X, having a maximum transverse dimension D' such that the ratio of D' divided by D is between 3 and 15, said screen 6 comprising a part made of dielectric material 61 and a part made of electrically conductive material 62,the part made of dielectric material 61 being configured to electrically isolate the part made of electrically conductive material 62 from the rest of the thruster, - an electrical power supply 9 comprising a negative pole electrically connected to the part made of conductive material 62 and a positive pole electrically connected to the external conductor 22.,

[0087] Alternatively, the device 113 comprises a thruster which comprises a magnetic field source 1 and a body 5, the positive pole of the power supply 9 being electrically connected to the magnetic field source 1 or to the body 5.

[0088] Alternatively, the device 113 comprises a thruster 10 which comprises an internal conductor 21, the positive pole of the power supply 9 being electrically connected to the internal conductor 21.

[0089] The device 113 further comprises a measuring unit 111 and an electronic processing circuit 112 configured to control a potential difference applied between the conductive material portion 62 and the external conductor 22 or the magnetic field source 1 or the body 5 or the internal conductor 21.

[0090] The outer conductor 22 or the inner conductor 21 or the magnetic field source 1 or the body 5 are called the reference.

[0091] The invention also relates to a method for controlling the potential difference applied between the part made of conductive material 62 and the reference by means of a device 113 according to the invention, the method comprising: - a measurement Vmes of the potential difference between the part made of conductive material 62 and the reference, called the effective potential difference Veff; when the power supply 9 does not apply a potential difference, this potential difference is called the floating potential Vflott;when the power supply applies a potential difference, the potential difference is called Vappl,- a calculation of the difference ε between the effective potential difference Veff and a predetermined set value Vcons,- if the difference ε exceeds a predetermined threshold, the application of a potential difference Vappl to the part made of electrically conductive material 62 of a value equal to the set value Vcons by the power supply 9, the method then resumes at the first step; if the difference measurement ε does not exceed a predetermined threshold, the method resumes at the first step.;

[0092] The setpoint value Vcons is between 10 volts and 1 kilovolt, preferably between 100 volts and 500 volts, in absolute values.

Claims

Satellite thruster (10), with electron cyclotron resonance and magnetic nozzle, extending at least partially along a longitudinal axis (X), comprising an external conductor (22) and an ionization chamber (8), the external conductor and the ionization chamber extending longitudinally along this axis, the external conductor transversely delimiting the ionization chamber, the ionization chamber comprising a circular transverse opening (24) having a diameter D, characterized in that it further comprises a screen (6) coaxial with the longitudinal axis (X), having a maximum transverse dimension D' such that the ratio of D' divided by D is between 3 and 15, said screen (6) comprising a part made of dielectric material (61) and a part made of electrically conductive material (62), the part made of dielectric material covering the part made of conductive material, or said screen being entirely made of dielectric material. Propellant (10) according to claim 1, characterized in that the part of dielectric material (61) completely covers the part of conductive material (62) so that an external surface of the screen (6) is made of dielectric material. Propellant according to any one of the preceding claims, characterized in that the screen (6) is fixed to the external conductor (22). A thruster according to any one of claims 1 or 2, characterized in that the thruster comprises a magnetic field source (1) and a body (5), the screen (6) being fixed to the magnetic field source (1), or to the body (5), or between the magnetic field source (1) and the body (5). Propellant according to any one of the preceding claims, characterized in that the part made of dielectric material (61) or the screen (6) is made of a mixture of glass fiber and epoxy resin or carbon fiber and epoxy resin or resin or polyimide of the Kapton® type or ceramic. Propellant according to any one of the preceding claims, characterized in that the screen (6) comprises a resin frame and a polyimide film of the Kapton® type. Propellant according to any one of claims 1 to 5, characterized in that the part made of conductive material is a metal frame. Propellant according to any one of the preceding claims, characterized in that the screen (6) has a hollow disc shape in its center or has a flared shape, for example a truncated hollow cone or nozzle. Propellant according to any one of the preceding claims, characterized in that the external conductor (22) comprises an internal wall (221) made of graphite. Propellant according to the preceding claim, characterized in that the external conductor (22) comprises an internal wall (221) and an external wall (222) separated by a transverse thickness (223) and comprises a uniformization chamber (32) hollowed out in said transverse thickness (223), said uniformization chamber extending longitudinally over a portion of the external conductor and having the overall shape of a hollow cylinder. Propellant according to the preceding claim, characterized in that it comprises gas injection means (3), said means comprising:- at least one injection channel (31), said channel radially piercing the external wall (222) and partially piercing the transverse thickness (223) of the external conductor and opening into the uniformization chamber (32),- the uniformization chamber (32),- communication means configured so that the uniformization chamber (32) opens into the ionization chamber (8). Propellant according to the preceding claim, characterized in that the communication means comprise a transverse circular part (331) partially closing the longitudinal end of the uniformization chamber, said transverse circular part comprising at least three radial grooves connecting the uniformization chamber with the ionization chamber, or the communication means comprise orifices radially piercing the internal wall (221) of the external conductor and opening into the uniformization chamber. Propellant according to any one of the preceding claims, characterized in that it further comprises a conductor called an internal conductor (21) of solid cylindrical geometry, said internal conductor extending along the longitudinal axis (X) in the ionization chamber, said internal conductor comprising an external graphite surface.

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