Cathode for a Hall effect ion thruster

Abstract

The present invention relates to a cathode for an ion propulsion device, said cathode having a cylindrical shape extending along a principal axis A-A' between a downstream end (10A) comprising an electron outlet (18) and an upstream end (10B) comprising a gas inlet (22).The cathode comprises a hollow cylindrical cathode body (14), an electron emitter (12) made of a thermoemissive material arranged inside the cathode body (14), a cylindrical internal electrode (21) coaxially surrounding the cathode body (14) and positioned at a distance from the cathode body (14), a hollow cylindrical dielectric support (17) coaxially surrounding said internal electrode (21) and positioned at a distance from said internal electrode (21), a heating filament (13) arranged in a spiral on an inner face of the dielectric support (17), the internal electrode (21) comprising a plurality of through-holes (25) configured to allow the passage of thermal radiation emitted by the heating filament (13), in order to heat the cathode body (14). Abstract figure: Figure 2.

Description

Title of the invention: Cathode for a Hall effect ion thruster technical field

[0001] The present disclosure falls within the field of ion ejection devices in particular for forming plasma thrusters or electric thrusters.

[0002] The present disclosure relates more particularly to a gas-fed cathode for a Hall effect ion propulsion device adapted to propel a spacecraft. Previous technique

[0003] In the field of space propulsion, it is well known that electric or plasma thrusters are used to maintain a satellite in geostationary orbit, to move a satellite between two orbits, to compensate for drag forces on satellites in low Earth orbit, or for missions requiring low thrust over very long periods during an interplanetary mission. Indeed, plasma thrusters generate a specific impulse that is generally higher than that of chemical thrusters, which implies a reduction in fuel consumption (also called propellant) and consequently an increase in the lifespan and / or payload of satellites.

[0004] The Hall effect propulsion device is based on accelerating ions from a plasma using an electric field directed towards the outlet of the ejection channel, induced by a magnetic barrier perpendicular to the channel. Generally, it comprises an annular main channel forming an ionization and acceleration chamber around a central axis, a magnetic circuit configured to create magnetic field lines at an open end of the main channel, an anode positioned inside the annular main channel, near the bottom of the channel, and a cathode located outside the channel. The cathode is oriented towards the open end of the main channel in order to eject electrons in the direction of the main axis.Electrons ejected from the cathode travel partly towards the anode and are initially trapped and confined by the intense radial magnetic field near the first end of the annular channel. These electrons then collide with the atoms or molecules constituting the propellant (injected as a gas from the bottom of the annular channel) and flowing upstream to the anode. These electrons thus achieve partial or total ionization of the gas, and the mixture of ions and electrons constitutes the plasma state. Furthermore, the electrons trapped by the radial magnetic field (or barrier) have a... low conductivity in the perpendicular direction, which induces an axial electric field responsible for accelerating the ions between the anode and the outlet of the channel, such that these ions are ejected at high speed from the channel, in an ejection direction mainly parallel to the longitudinal axis, thus generating a thrust directed upstream.

[0005] The cathode emits electrons when heated by a process called thermionic emission. The cathode is thus a crucial component responsible for electron emission and plasma generation in a Hall-effect propulsion device.

[0006] Fig. 1 represents a schematic longitudinal cross-sectional view of a prior art cathode 1.

[0007] The cathode 1 generally comprises a hollow cylindrical support body 6 and a hollow cathode body 4 extending along a principal longitudinal axis AA'. The hollow support body and the cathode body are arranged coaxially around the longitudinal axis AA'. The cathode body 4 comprises a downstream end with an electron outlet 7 and an upstream end with a gas inlet. The cathode 14 further comprises an electron emitter 2 made of a thermoemissive material capable of emitting electrons when brought to a certain operating temperature. In the example illustrated in [Fig. 1], the electron emitter 2 is in the form of a tube inserted into and held in contact with the inner wall of the cathode body. A heating filament 3 is located around the outer wall of the cathode body 4, and is primarily situated opposite the electron emitter 2.The ends of the heating filament 3 are connected to an electrical power supply. The heating filament is designed to heat the cathode ray tube and the emitter by thermal emission and conduction when an electric current flows through it. The power dissipation by the heating filament 3 causes the thermoemissive material 2 to heat up, which then induces the emission of electrons. Simultaneously, a flow of gas, usually xenon, is injected into the cathode ionization chamber 9, delimited by the wall of the cathode ray tube 4, via a gas injection orifice located at the upstream end of the cathode.

[0008] The electrons thus emitted by the emitter 2 are partly ejected via the outlet 7 of the hollow cathode body 4, then through the electron outlet 8 of the support body 6, towards a region located downstream of the end of the anodic discharge channel of the Hall effect thruster. Furthermore, some of the electrons present in the ionization chamber of the cathode body collide with the atoms or molecules of the gas flowing from upstream to downstream in the cathode chamber, ionizing this gas and generating a plasma which maintains a high temperature within the cathode ionization chamber to continue heating the a thermoemissive electron material. In order to reduce the temperature loss, a thermal screen 5 in the form of a hollow cylinder is interposed in the space delimited by the wall of the support body 6 and the cathode body 4. In this phase of cathode operation in self-sustaining mode in which the high temperature is maintained by the plasma, the filament is no longer supplied.

[0009] Such a cathode has many advantages making it particularly suitable for the operation of low power Hall effect thrusters in the propulsion of small satellites.

[0010] However, the cathodes currently proposed do not offer an optimal architecture in terms of lifespan, reliability, stability and energy consumption savings.

[0011] Indeed, to initiate electron emission, the thermoemissive material forming the cathode's electron emitter must be heated to a sufficiently high temperature to initiate electron emission using the heating element, which is a filament wound around the wall of the cathode ray tube. When the plasma is generated within the cathode ray tube and maintains the high temperature, the heating filament 3 is no longer powered, and the cathode is said to be self-heated. It is thus used only for the startup phase of the Hall-effect propulsion device. However, due to the power of the electric current injected into the filament, the latter undergoes slight sublimation. Therefore, the filament's diameter decreases with each use cycle, becoming increasingly resistant, and the sublimation phenomenon is accentuated after each use cycle.It is therefore essential to limit the power of the electric current injected into the filament to limit its sublimation.

[0012] However, another factor in its sublimation is directly related to its arrangement within the cathode and the heating process implemented in currently used cathodes. Indeed, as illustrated in [Fig. 1], the filament 3 is wound on the outer wall of the cathode ray tube and, in thermal contact with the cathode ray tube wall, heats the cathode ray tube wall by thermal conduction, and consequently the thermoemissive emitter 2. Also, in self-sustaining mode when it is no longer supplied with electrical current, it continues to be heated by thermal conduction from the high temperature generated by the plasma within the ionization chamber of the cathode ray tube. Therefore, even if the filament is no longer supplied with electrical current, it can undergo sublimation over time due to the high-temperature operation of the cathode.It is therefore essential to provide effective thermal protection to the heating filament so that it is not heated by thermal conduction during cathode operation in self-sustaining mode, while maintaining its function of heating the emitter to initiate the initial phase. electron emission.

[0013] To improve the lifespan and reliability of the cathode's operation, it is therefore crucial to limit the deterioration of the heating filament by reducing the power of the electric current injected into the filament and to protect it effectively against the high operating temperature of the cathode.

[0014] As described above, to obtain an efficient propulsion device with lower energy consumption, the cathodes must operate in a so-called self-sustaining or self-heating mode, in which the heating power applied to the filament is zero during operation and the emitter is heated solely by the plasma. In conventional cathodes, the emitter is not thermally confined with respect to the rest of the cathode since it is also in thermal contact with the inner wall of the cathode tube, leading to heat dissipation by thermal conduction. Therefore, it is also important to optimize the thermal confinement of the emitter to limit heat dissipation from the cathode tube to the periphery of the cathode.In other words, the emitter must be thermally decoupled from the rest of the cathode, while ensuring the mechanical robustness of the cathode to withstand significant mechanical vibrations from the environment.

[0015] One objective of the present disclosure is therefore to propose an electron-emitting cathode with a new architecture that makes it possible to increase the lifetime of the heating filament to improve the lifetime of the cathode.

[0016] Another objective of the present disclosure is to reduce heat dissipation outwards from the cathode by radiation and thermal conduction in order to improve temperature maintenance performance within the cathode, thereby ensuring stable and reliable operation of the cathode, while improving its resistance to mechanical vibration. Summary

[0017] This disclosure improves the situation.

[0018] A cathode for an ion propulsion device is proposed, said cathode having a cylindrical shape extending along a principal axis A-A' between a downstream end comprising an electron outlet and an upstream end comprising a gas inlet, the upstream end of the cathode being coupled to a base, said cathode comprising: - a hollow cylindrical cathode body extending along the main axis A-A' between the downstream end and the upstream end, the wall of said cylindrical cathode body delimiting an ionization cavity; - an electron emitter made of thermoemissive material arranged inside the cathode ray tube, said ionization cavity being located downstream of the emitter of electrons; - a cylindrical internal electrode coaxially surrounding the cathode body and positioned at a distance from the cathode body, said internal electrode extending parallel to the main axis A-A' between the downstream end and the upstream end, a part of the wall of said internal electrode being opposite the electron emitter; - a hollow cylindrical dielectric support coaxially surrounding said internal electrode and positioned at a distance from said internal electrode; - a heating filament arranged in a spiral on an inner face of the dielectric support, at least a portion of said filament being opposite the electron emitter, the filament comprising a downstream end connected to a downstream end of the internal electrode, the internal electrode comprising a plurality of through-openings configured to permit the passage of thermal radiation emitted by the heating filament, in order to heat the cathode body; - a cylindrical cathode envelope extending between the upstream end and the downstream end along the main axis A-A' and coaxially surrounding the dielectric support, the downstream end of the envelope being provided with an opening forming the electron exit of the cathode.

[0019] According to one embodiment, the part of the wall of the internal electrode that is not opposite the electron emitter comprises a plurality of through openings configured to limit heat transfer by thermal conduction to the base.

[0020] According to another embodiment, the cathode further comprises a cylindrical external electrode coaxially surrounding a lower part of the internal electrode and positioned at a distance from said internal electrode, the heating filament comprising an upstream end connected to a downstream end of the external electrode.

[0021] Preferably, the external electrode comprises a plurality of through-openings configured to limit heat transfer by thermal conduction to the base.

[0022] According to one embodiment, the external electrode is positioned between the internal electrode and the dielectric support so that its downstream end is in line with the upstream end of the heating filament.

[0023] According to one embodiment, the dielectric support comprises at least one groove formed along a helical generatrix along said principal axis A-A' on at least a portion of its inner face, the heating filament being received and held in position in said groove.

[0024] The downstream end of the cathode body is free and is provided with an opening opposite the electron exit of the envelope.

[0025] According to one embodiment, the electron emitter is in the form of a solid cylindrical body extending along the main axis A-A' and is held in position in a central area of ​​the ionization cavity so as to form a passage for the circulation of the gas injected into the ionization cavity.

[0026] According to another embodiment, the electron emitter is in the form of a tubular body extending along the main axis A-A' and is held in position in a central area of ​​the ionization cavity so as to form a passage for the circulation of the gas injected into the ionization cavity.

[0027] The features described in the following paragraphs may optionally be implemented independently of each other or in combination with each other:

[0028] According to an advantageous embodiment, the wall of the hollow cathode body has at least one radial deformation towards the inside of the ionization cavity so as to hold the electron emitter in position by crimping.

[0029] Preferably, the electron emitter has a length less than the length of the cathode body and is positioned closer to the downstream end of the cathode body than to the upstream end of the cathode body.

[0030] Preferably, the internal electrode is positioned closer to the heating filament than to the cathode body.

[0031] According to one embodiment, the openings of the internal electrode and the openings of the external electrode are regularly spaced from each other, forming a network.

[0032] Preferably, the heating filament has a cross-section smaller than the axial cross-section of the internal electrode and / or the axial cross-section of the external electrode so that the heating filament is electrically more resistive than the internal and external electrodes.

[0033] According to one embodiment, the cathode further comprises a hollow cylindrical thermal screen coaxially surrounding the dielectric support and interposed between the cathode envelope and the dielectric support.

[0034] According to another aspect of the invention, a Hall effect ion propulsion device is proposed comprising at least one cathode as described above. Brief description of the drawings

[0035] Other features, details and advantages will become apparent from reading the detailed description below and from analyzing the accompanying drawings, in which: Fig. 1

[0036] [Fig.1] Fig.1 shows a schematic cross-sectional view of a conventional cathode. Fig. 2

[0037] [Fig.2] Fig.2 shows a schematic cross-sectional view of a cathode along a method of implementation. Fig. 3

[0038] [Fig.3] The [Fig.3] shows a sectional view along line B-B' of the [Fig.2]. Fig. 4

[0039] [Fig.4] Fig.4 represents an enlarged view of a radiative heat transfer zone between the heating filament and the cathode body of the cathode of Fig.2, showing on the diagram (a) the radiative heat transfer from a portion of the heating filament to a surface of the cathode body through an opening made on the internal electrode and on the diagram (b) the radiative heat transfer from a surface of the heated cathode body to a portion of the heating filament. Fig. 5

[0040] [Fig.5] The [Fig.5] shows on the left diagram (a) a schematic longitudinal section view of a cathode body comprising an electron emitter according to one embodiment and on the right diagram (b) a section view along line C-C' of the left diagram. Fig. 6

[0041] [Fig.6] The [Fig.6] shows on the right diagram (a) a network of openings forming a honeycomb and on the left diagram (b) a network of openings having a rectangular geometric shape. Description of the implementation methods

[0042] The drawings and the description below contain, essentially, elements of a definite nature. They can therefore not only serve to better understand the present invention, but also contribute to its definition, if necessary.

[0043] First of all, it should be noted that the figures are not to scale.

[0044] With reference to Figures 2 and 3, a cathode 10 according to an embodiment of the The present invention is described below. This electron-emitting cathode is particularly suitable for use in an ion propulsion device, in particular a Hall effect propulsion device.

[0045] The cathode 10 has a cylindrical shape extending along a principal axis A-A' between an upstream end 10B and a downstream end 10A.

[0046] Fig. 2 is a longitudinal cross-sectional view along the axial direction of the cathode. The downstream end 10A of the cathode is configured to allow the emission of electrons through an electron outlet 18 and the upstream end 10B is configured to define a gas inlet 22.

[0047] Fig. 3 is a cross-sectional view perpendicular to the main axis A-A' along the line B-B' of Fig. 2.

[0048] The cathode 10 comprises a hollow cylindrical body 14 defining a cavity ionization chamber 19 into which a gas suitable for ionization is injected, an electron emitter 12 arranged in the cavity 19 and capable of emitting electrons when brought to a certain temperature, a heating filament 13 capable of emitting thermal radiation to heat the emitter 12, an internal electrode 21 and an external electrode 20 configured to connect the ends of the heating filament 13 to a source to circulate an electric current in the filament, a thermal barrier 15 to limit heat dissipation to the outside of the cathode and a casing 16 which covers all the elements of the cathode, commonly called by the English name "Keeper".

[0049] The hollow cylindrical cathode body 14 extends along the main axis A-A' between an upstream end 30 and a downstream end 29. According to one embodiment, the cylindrical cathode body 14 has a circular cross-section and thus forms a cathode tube.

[0050] The downstream end 29 of the cathode body 14 includes an opening 27 to allow electrons to exit. The opening 27 is opposite the electron exit 18 of the cathode. The upstream end 30 of the cathode body 14 also includes an opening 28 to allow the injection of a gas into the ionization cavity 19. The upstream end 30 of the cathode body 14 is closed by an injection system. The opening 28 of the upstream end of the cathode body is in fluidic connection with a supply line 40 for ionizable gas. The direction of gas injection is represented by an arrow at the inlet of the line 40 in [Fig. 2]. The wall of the cathode ray tube 14 has an outer face 14A and an inner face 14B which defines the ionization cavity 19 into which the gas is injected. The gas can be, for example, xenon, argon, krypton, or any gas capable of being ionized.

[0051] According to one embodiment, the electron emitter 12 is preferably positioned closer to the downstream end 29 of the cathode body 14, that is to say closer to the opening 27 so that the thermal conduction from the emitter 14 to the cathode body is reduced during its operation.

[0052] Generally, the cathode body 14 is made of a material chosen from a group including tungsten W, Tantalum Ta, molybdenum Mo, or any mechanically robust, electrically conductive material with a melting (or sublimation) point higher than the operating temperature of the emitter.

[0053] For the purposes of the present invention, the terms "upstream" and "downstream" are defined with respect to the normal flow direction of the injected gas, which is in the same direction as the ejection of electrons, from upstream to downstream through the ionization cavity 19.

[0054] In the following description, the term "internal" refers to a part close to the main axis A-A' while the term "external" refers to a part far from the main axis A-A'.

[0055] The electron emitter 12 is made of a thermoemissive material capable of emitting electrons when heated to a certain temperature. It is arranged inside the cathode body 14, within the ionization cavity 19.

[0056] Generally, emitters are made of ceramic, usually lanthanum boride (LaB6) due to its robustness, high current density, and long lifespan. LaB6 electron emitters, for example, can operate in a temperature range between 1000 °C and 1700 °C. Other ceramic emitters, such as C12A7, also known as mayenite, have the advantage of operating at lower temperatures (between 900 °C and 1200 °C). Finally, these emitters can also be made of pure metals (such as tungsten W, molybdenum Mo or tantalum Ta), or have increased ther-moionic emissivity when mixed with various oxides such as barium oxide BaO, scandium oxide Sc2O3, strontium oxide SrO, yttrium oxide Y2O3, hafnium oxide HfO2, zirconium oxide ZrO.

[0057] The electron emitter 12 can be in different geometric shapes. It can be formed by a hollow body or a solid body.

[0058] In [Fig.2], the electron emitter 12 is for example in the form of a solid body elongated along the main axis A-A' and is held fixed on an area of ​​the inner face 14B of the wall of the cathode body 14 by any suitable means of fixing.

[0059] The electron emitter 12 can, for example, be in the form of a solid cylindrical body extending along the main axis A-A' and is held in position in a central area of ​​the ionization cavity of the cathode body 14 so as to form a passage for the circulation of the gas injected into the ionization cavity 19.

[0060] According to another embodiment, the electron emitter 12 can be in the form of a tubular body extending along the main axis A-A' and is held in position in a central area of ​​the ionization cavity of the cathode body 14 so as to form a passage for the circulation of the gas injected into the ionization cavity 19.

[0061] According to one embodiment, the emitter 12 has a length less than the length of the cathode body 14 and is positioned closer to the downstream end 29 of the cathode body than to the upstream end 30 of the cathode body.

[0062] Preferably, and with reference to [Fig. 5], the emitter is positioned at the center of the ionization cavity 19 of the cathode body 140, and the wall of the hollow cathode body 14 has radial deformations towards the interior of the ionization cavity 19 so as to hold the emitter in position by crimping. The crimping ensures good mechanical retention and good electrical contact between the cathode body and the emitter, while allowing the gas flow around the emitter to pass through. from upstream to downstream in the ionization cavity. The crimping also helps to limit the thermal contact between the cathode body and the emitter in order to limit the heat dissipation of the emitter to the outside by thermal conduction when the cathode is in self-sustaining mode.

[0063] Schematic (a) of [Fig.5] is a longitudinal cross-sectional view of the cathode body 140, showing the presence of four radial deformations 140, 141, 143, 144 which maintain the emitter 12 positioned in the center of the cavity by crimping.

[0064] Scheme (b) of [Fig.5] is a cross-sectional view along line C-C' of Scheme (a), showing two points of contact (thermal and mechanical) between the emitter 12 and the inner face of the wall of the cathode body.

[0065] Unlike prior art cathodes as illustrated in [Fig. 1], the heating filament 13 is held away from the inner wall of the cathode ray tube 14 by means of a dielectric support 17 and positioned opposite the electron emitter 12. More specifically, and as illustrated in [Fig. 2], the dielectric support 17 is in the form of a hollow cylinder arranged to coaxially surround the cathode ray tube 14 and at a distance from it. The dielectric support 17 is made of an electrically insulating material with a high melting or sublimation point, for example, alumina Al₂O₃, boron nitride BN, or zirconium dioxide ZrO₂. The heating filament is wound in a spiral around the main axis A-A' on the inner wall of the dielectric support 17.The heating filament 13 is held on the dielectric support 17 in such a way that the turns are separated to prevent any short circuit of the turns.

[0066] According to one embodiment, the heating filament 13 is arranged in a meander shape on an inner face of the dielectric support 17.

[0067] According to another embodiment, the heating filament 13 is wound axially in a serpentine shape on the inner wall of the dielectric support 17.

[0068] In general, the heating filament 13 can be wound in different geometric shapes on the inner wall of the dielectric support 17.

[0069] According to one embodiment, the dielectric support 17 comprises a groove formed along a helical generatrix about the principal axis A-A' on a portion of its inner face 17B. The filament 13 is received and held in position in the groove. The filament 13 comprises a downstream end 13A connected to the downstream end 21A of the inner electrode 21 and an upstream end 13B connected to the downstream end 20A of the outer electrode 20.

[0070] It is noted that in the embodiment shown in [Fig. 2], the length of the heating filament 13 corresponds approximately to the length of the emitter 12. The filament is preferably positioned closer to the downstream end 10A of the cathode than to the upstream end 10B of the cathode so that the filament circumfers conference across the entire surface of the transmitter.

[0071] The internal electrode 21 is in the form of a hollow cylinder. It is arranged to coaxially surround the cathode body 14. It is positioned between the cathode body 14 and the dielectric support 17. It is not in contact with the external face 14A of the wall of the cathode body 14 nor with the internal face 17B of the wall of the dielectric support 17 on which the filament 13 is wound. The internal electrode 21 extends parallel to the main axis A-A' between the downstream end and the upstream end along a portion that corresponds substantially to the length of the hollow cathode body 14.

[0072] According to a particularly advantageous technical feature, the internal electrode 21 is configured to form a thermal wall which allows thermal radiation from the filament 13 to pass through to heat the emitter 12 by means of a radiative heating process and limits the passage of thermal radiation from the cathode body 14 to the filament 13. To this end, a portion of the internal electrode which is opposite the electron emitter 12 comprises a plurality of through-holes 25 configured to allow the passage of thermal radiation to heat the cathode body 14 and the electron emitter 12. When the plasma is generated within the ionization cavity 19 which maintains the temperature within the ionization cavity to continue heating the emitter to a temperature sufficient for electron emission, the filament is then no longer supplied with current.The thermal radiation emitted by the heated cathode body 14 towards the filament 13 is partially blocked by the solid part of the wall of the internal electrode 21, thus helping to maintain the temperature within the ionization cavity.

[0073] Thus, the perforated wall of the internal electrode 21 allows the cathode body to be heated even though the filament and the wall of the cathode body are not in physical contact. During the cathode heating phase, the cathode body 14 is primarily heated radiatively by the filament 13 through the openings 25. This type of heating allows for better direction of the heat flow, i.e., from the filament 13 to the emitter 12, and reduces the power injected into the filament, which is necessary for heating the emitter. Reducing this power then lowers the filament's operating temperature and therefore the thermal stresses that could induce degradation, deformation, or evaporation of the material constituting it. For this reason, it has a longer lifespan than the filaments used in conventional cathodes, thus increasing the number of start-up cycles and the reliability of the cathode.During cathode operation in self-sustaining mode, the heat transmitted from the emitter 12 to the cathode body 14 is transmitted by thermal conduction along the cathode body 14. to a channel 40 located at the upstream end of the cathode and radiatively evacuated from the cathode body 14 to the filament 13 where it is partially blocked by the internal electrode 21. In addition, the filament no longer being in physical contact with the cathode body, the heat dissipation to the outside is limited, allowing to improve the performance of maintaining at high temperature within the cavity.

[0074] With reference to [Fig.4], the process of radiative heat transfer through an opening 25 of the internal electrode is described below.

[0075] Fig. 4 represents an enlarged view of a radiative heat transfer zone between the heating filament 13 and the cathode body 14 of the cathode of Fig. 2.

[0076] A portion of the filament 13 is positioned opposite the outer face 14A of the cathode body. The wall of the inner electrode 21, which has through-holes 25, is positioned between the outer surface 14A and the filament 13.

[0077] In the diagram (a), when an electric current flows through the filament 13, the thermal radiation emitted by the filament 13 passes through the aperture 25 to heat a surface SI of the external surface 14A of the cathode ray tube. The direction of radiation emission is represented by an arrow.

[0078] In the diagram (b), when the cathode body 14 is brought to a certain temperature by the generated plasma, it also emits thermal radiation which passes partly through the opening 25 to heat a surface S2 of the filament.

[0079] According to one embodiment, in order to limit the surface area S2 to limit the heating of the filament and to maximize the surface area SI to heat the cathode body, the internal electrode 21 is positioned closer to the filament 13 than to the cathode body 14. The distance L1 between the wall of the internal electrode 21 and the wall of the cathode body is advantageously greater than the distance L2 between the wall of the internal electrode 21 and the filament 13.

[0080] The wall of the internal electrode 21 not only ensures electrical conduction to power the filament 13, but also constitutes a thermal wall to allow direct radiative transfer from the heating filament 13 to the cathode body 14 containing the electron emitter by allowing thermal radiation to pass through the through openings to heat the cathode body and a thermal screen to block the radiation emitted by the cathode body when it is heated towards the heating filament which is no longer powered.

[0081] According to another particularly advantageous feature, the lower part of the wall of the internal electrode 21 that is not opposite the electron emitter 12 also comprises a plurality of through-holes 23. These openings 23 have a different function from the openings 25 of the upper part of the electrode wall opposite the emitter. These openings are configured to limit the thermal transfer by thermal conduction to the base 32 which is thermally coupled to the upstream end 10B of the cathode 10.

[0082] The external electrode 20 of the cathode 10 is in the form of a hollow cylinder extending along the principal axis A-A'. It is arranged to coaxially surround a lower portion of the internal electrode 21 and at a distance from the internal electrode. It is positioned between the internal electrode 21 and the dielectric support 17. The external electrode 20 comprises a downstream end 20A connected to the upstream end 13b of the filament 13 and an upstream end 20B connected to the base 32.

[0083] In the embodiment shown in [Fig.2], the external electrode 20 is positioned between the internal electrode 21 and the dielectric support 17 so that its downstream end 20A is in line with the upstream end 13B of the filament 13. In other words, the diameter of the external electrode 20 is equal to the diameter of the loop formed by the filament 13 and the external electrode 20 is positioned against an internal face 17B of the lower part of the wall of the dielectric support 17 which is devoid of filament.

[0084] According to another particularly advantageous feature, the wall of the external electrode 20 also includes a plurality of through openings 26 which are configured to limit heat transfer by thermal conduction to the base 32.

[0085] The openings 25 of the upper part of the internal electrode 21 opposite the emitter 12, the openings 23 of the lower part of the internal electrode 21 and the openings 26 of the external electrode 20 can have different geometric shapes in the plane of the wall of the electrodes.

[0086] According to one embodiment, the internal electrode 21 and external electrode 20 have a larger electrically conductive cross-section than that of the heating filament 13. The thickness of each wall and the size of the through openings 23, 25, 26 are configured so that the electrical resistance along the internal electrode 21 and external electrode 20 is lower than that along the heating filament 13. As a result, the energy deposition will be mostly localized at the filament during the heating phase.

[0087] The through openings can have a hexagonal shape as in diagram (a) of [Fig.6] forming a honeycomb or a rectangular shape in the diagram on the left (b) of [Fig.6].

[0088] They can be regularly spaced from each other. Of course, the openings can alternatively be arranged in a staggered pattern.

[0089] The density of the openings 23, 25 and 26 can be determined beforehand. Of course, the density of the openings made may differ depending on their function in the cathode.

[0090] According to one embodiment, the through-holes 23 of the lower part of the internal electrode 21 are arranged in a staggered pattern with respect to the through-holes slopes 26 of the external electrode 20. Thus the wall of the external electrode 20 obstructs the through openings of the internal electrode 21 so as to limit the thermal radiation of the cathode body outwards.

[0091] According to one embodiment, the downstream end 29 of the cathode body is free and is not connected to the cathode structure, thus limiting thermal dissipation.

[0092] According to one embodiment, the cathode 10 further comprises a hollow cylindrical heat shield 15 which surrounds the dielectric support 17. It is formed for example of one or more metallic films which limit the dissipation of heat to the outside of the cathode.

[0093] The entire set of components of the cathode is covered by the cylindrical cathode envelope 16 which extends between the upstream end 10B and the downstream end 10A along the main axis A-A'. It is arranged so as to surround the thermal screen 15. The downstream end of the envelope is closed by a plate 31 which is provided with an opening forming the electron exit 18 of the cathode and the upstream end of the envelope is coupled to the base 32.

[0094] Advantageously, this envelope also forms a thermal barrier to limit heat dissipation to the outside of the cathode.

[0095] The cathode proposed in the present invention is particularly suitable for use in an ion propulsion device, in particular a Hall effect ion propulsion device. By way of example, the proposed cathode can be arranged in a central cavity of the propulsion head of the device.

[0096] However, the proposed technical solution can be applied to other types of propulsion systems.

[0097] This disclosure is not limited to the examples described above, but encompasses all the variants that a person skilled in the art may consider in the context of the protection sought.

Claims

Demands

1. Cathode (10) for an ion propulsion device, said cathode having a cylindrical shape extending along a principal axis A-A' between a downstream end (10A) comprising an electron outlet (18) and an upstream end (10B) comprising a gas inlet (22), the upstream end (10B) of the cathode being coupled to a base (32), said cathode comprising: - a hollow cylindrical cathode body (14) extending along the main axis A-A' between the downstream end and the upstream end, the wall of said cylindrical cathode body delimiting an ionization cavity (19); - an electron emitter (12) made of thermoemissive material arranged inside the cathode body (14), said ionization cavity being located downstream of the electron emitter; - a cylindrical internal electrode (21) coaxially surrounding the cathode body (14) and positioned at a distance from the cathode body (14), said internal electrode (21) extending parallel to the main axis A-A' between the downstream end and the upstream end, a part of the wall of said internal electrode being opposite the electron emitter (12); - a hollow cylindrical dielectric support (17) coaxially surrounding said internal electrode (21) and positioned at a distance from said internal electrode (21); - a heating filament (13) arranged in a spiral on an inner face of the dielectric support (17), at least a portion of said filament being opposite the electron emitter (12), the filament (13) comprising a downstream end (13A) connected to a downstream end (21A) of the internal electrode (21), the internal electrode (21) comprising a plurality of through-openings (25) configured to permit the passage of thermal radiation emitted by the heating filament (13), in order to heat the cathode body (14); - a cylindrical cathode envelope (16) extending between the upstream end and the downstream end along the main axis A-A' and coaxially surrounding the dielectric support (17), the downstream end (31) of the envelope being provided with an opening forming the electron exit (18) of the cathode.

2. Cathode according to claim 1, wherein the portion of the wall of the internal electrode (21) that is not opposite the electron emitter (12) comprises a plurality of through-openings (23) configured to limit heat transfer by thermal conduction to the base (32).

3. Cathode according to claim 1 or 2, further comprising an external cylindrical electrode (20) coaxially surrounding a lower portion of the internal electrode (21) and positioned at a distance from said internal electrode, the heating filament (13) comprising an upstream end (13B) connected to a downstream end (20A) of the external electrode.

4. Cathode according to claim 3, wherein said external electrode (20) comprises a plurality of through-openings (26) configured to limit heat transfer by thermal conduction to the base (32).

5. Cathode according to claim 3 or 4, wherein the external electrode (20) is positioned between the internal electrode (21) and the dielectric support (17) such that its downstream end (20A) is in line with the upstream end (13B) of the heating filament (13).

6. Cathode according to any one of claims 1 to 5, wherein the dielectric support (17) comprises at least one groove formed along a helical generatrix along said principal axis A-A' on at least a portion of its inner face, the heating filament (13) being received and held in position in said groove.

7. Cathode according to any one of the preceding claims, wherein the downstream end (29) of the cathode body is free and is provided with an opening (27) opposite the electron exit (18) of the envelope (16).

8. Cathode according to any one of the preceding claims, wherein the electron emitter (12) is in the form of a solid cylindrical body extending along the principal axis A-A' and is held in position in a central area of ​​the ionization cavity so as to form a passage for the circulation of the gas injected into the ionization cavity (19).

9. Cathode according to any one of the preceding claims, wherein the electron emitter (12) is in the form of a body tubular extending along the main axis A-A' and is held in position in a central area of ​​the ionization cavity so as to form a passage for the circulation of the gas injected into the ionization cavity (19).

10. Cathode according to claim 8 or 9, wherein the wall of the hollow cathode body (14) has at least one radial deformation (141, 142, 143, 144) towards the interior of the ionization cavity (19) so as to maintain in position by crimping the electron emitter (12).

11. Cathode according to any one of claims 1 to 10, wherein the electron emitter (12) has a length less than the length of the cathode body (14) and is positioned closer to the downstream end (29) of the cathode body than to the upstream end (30) of the cathode body.

12. Cathode according to any one of claims 1 to 11, wherein the internal electrode (21) is positioned closer to the heating filament (13) than to the cathode body (14).

13. Cathode according to any one of claims 1 to 12 and claim 3, wherein the openings (25, 23) of the internal electrode (21) and the openings (26) of the external electrode (20) are regularly spaced from each other, forming a network.

14. Cathode according to any one of claims 1 to 13 and claim 3, wherein the heating filament (13) has a cross-section smaller than the axial cross-section of the internal electrode (21) and / or the axial cross-section of the external electrode (20) so that the heating filament is electrically more resistive than the internal and external electrodes.

15. Cathode according to any one of claims 1 to 14, further comprising a hollow cylindrical heat shield (15) coaxially surrounding the dielectric support (17) and interposed between the cathode envelope (16) and the dielectric support (17).

16. Hall effect ion propulsion device comprising at least one cathode according to any one of claims 1 to 15.

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