Cathode for a hall-effect ion thruster

Abstract

The present invention relates to a cathode for an ion thrust device, the cathode having a cylindrical shape extending along a main axis A-A' between a downstream end (10A) comprising an electron outlet (18) and an upstream end (10B) comprising a gas admission inlet (22). The cathode comprises a hollow cylindrical cathode body (14), an electron emitter (12) made of heat-emissive 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 the internal electrode (21) and positioned at a distance from the internal electrode (21), a heating filament (13) arranged in spiral form on an internal face of the dielectric support (17), the internal electrode (21) comprising a plurality of through-openings (25) configured to allow the passage of the thermal radiation emitted by the heating filament (13), in order to heat the cathode body (14).

Description

Description Title: Cathode for a Hall Effect Ion Thruster Technical field

[0001] The present disclosure relates to 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 suitable for propelling a spacecraft. Prior art

[0003] In the field of space propulsion, it is well known to use electric or plasma thrusters to maintain a satellite in geostationary orbit, or to move a satellite between two orbits, or to compensate for drag forces on satellites placed in a so-called low orbit, or for missions requiring low thrusts over very long times during an interplanetary mission. Indeed, plasma thrusters can generate a specific impulse that is generally higher than chemical thrusters, which implies a reduction in fuel consumption (also called propellant) with the consequence of increasing the lifetime and / or payload of satellites.

[0004] The Hall effect propulsion device is based on the acceleration of ions from a plasma using an electric field directed towards the exit of the ejection channel, and this, induced by a magnetic barrier perpendicular to the latter. 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, a cathode which is 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. The electrons ejected by the cathode are partly directed towards the anode and are first trapped and confined by the intense radial magnetic field in the vicinity of the first end of the annular channel.The electrons then collide with the atoms or molecules constituting the propellant (injected in gaseous form from the bottom of the annular channel) and flowing from downstream to upstream of the channel to reach the anode. These electrons thus achieve a partial or total ionization of the gas, and the mixture of ions and electrons then constitutes the plasma state. On the other hand, 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 the acceleration of the ions between the anode and the channel outlet, 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 through 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] Figure 1 shows a schematic longitudinal cross-sectional view of a cathode 1 of the prior art.

[0007] The cathode 1 generally comprises a cylindrical hollow support body 6 and a hollow cathode body 4 extending along a longitudinal main 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 provided with an electron outlet orifice 7 and an upstream end provided with a gas inlet orifice. The cathode 14 further comprises an electron emitter 2 made of thermoemissive material which is capable of emitting electrons when it is 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 and held in contact with the internal wall of the cathode body. A heating filament 3 is located around the external wall of the cathode body 4, and mainly located opposite the electron emitter 2.The ends of the heating filament 3 are connected to an electric current power source. The heating filament is intended to heat the cathode body and the emitter by emission and thermal conduction when it is traversed by an electric current. The dissipation of power by the heating filament 3 causes the heating of the thermoemissive material 2 which then induces the emission of electrons. Simultaneously, a flow of gas, generally xenon, is injected into the cathode ionization chamber 9 delimited by the wall of the cathode body 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 orifice 7 of the hollow cathode body 4, then the electron outlet orifice 8 of the support body 6, towards a zone located downstream of the end of the anode discharge channel of the Hall effect thruster. Furthermore, a portion 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, causing ionization of this gas and generating a plasma which makes it possible to maintain a high temperature within the cathode ionization chamber to continue heating the electron thermoemissive material. In order to reduce the temperature loss, a heat shield 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 operation of the cathode in self-sustained mode in which the high temperature is maintained by the plasma, the filament is then no longer powered.

[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 the emission of electrons, the thermoemissive material forming the electron emitter of the cathode must be heated to a temperature high enough to initiate the emission of electrons 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 helps maintain the high temperature within the cathode ray tube, the heating filament 3 is no longer powered and the cathode is said to be self-heated. It is thus used only for the start-up 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. Thus the diameter of the filament decreases with the cycles of use, and becomes more and more resistant, and the sublimation phenomenon is accentuated after each cycle of use. It is therefore essential to limit the power of the electric current injected into the filament to limit its sublimation.

[0012] However, another factor of its sublimation is directly linked to its arrangement within the cathode and to the heating process implemented in the cathodes currently used. Indeed, and as illustrated in Figure 1, the filament 3 is wound on the external wall of the cathode ray tube and in thermal contact with the wall of the cathode ray tube, it heats by thermal conduction the wall of the cathode ray tube and thereby the thermoemissive emitter 2. Also, in self-sustained mode when it is no longer powered by electric current, it continues to be heated by thermal conduction by the high temperature generated by the plasma within the ionization chamber of the cathode ray tube. Consequently, even if the filament is no longer powered by electric current, it can undergo sublimation over the course of its use 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-sustained mode, while maintaining its function of heating the emitter to initiate the initial phase of electron emission.

[0013] To improve the lifespan and reliability of the cathode 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 achieve an efficient propulsion device with lower energy consumption, the cathodes must operate in a so-called self-sustained or self-heated mode, in which the heating power applied in the filament is zero during operation and the emitter is heated only 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 CRT, 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 CRT 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 be able to cope with significant mechanical vibrations from the environment.

[0015] An objective of the present disclosure is therefore to provide an electron-emitting cathode with a novel architecture that allows the heating filament to be extended to improve the cathode life.

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

[0017] This disclosure improves the situation.

[0018] A cathode for an ion propulsion device is provided, said cathode having a cylindrical shape extending along a main 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 body, said ionization cavity being located downstream of the electron emitter;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 portion of the wall of said internal electrode facing 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 the form of a spiral on an internal face of the dielectric support, at least a portion of said filament facing 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 allow 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 outlet of the cathode.;

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

[0020] According to another embodiment, the cathode further comprises a cylindrical external electrode coaxially surrounding a lower portion of the internal electrode and positioned at 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 an exemplary 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 an exemplary embodiment, the dielectric support comprises at least one groove formed along a helical generatrix following said main axis A-A' on at least a portion of its internal 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 outlet from the envelope.

[0025] According to an exemplary 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 zone 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 exemplary 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 zone of the ionization cavity so as to form a passage for the circulation of the gas injected into the ionization cavity.

[0027] The features set out 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 an exemplary 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 section smaller than the axial section of the inner electrode and / or the axial section of the outer electrode so that the heating filament is electrically more resistive than the inner electrode and the outer electrode.

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

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

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

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

[0037] [Fig. 2] Figure 2 shows a schematic sectional view of a cathode according to one embodiment. Fig. 3

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

[0039] [Fig. 4] Figure 4 represents an enlarged view of a radiative heat transfer zone between the heating filament and the cathode body of the cathode of Figure 2, showing in diagram (a) the radiative heat transfer from a portion of the heating filament to a surface of the cathode body through an opening provided on the internal electrode and in 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] Figure 5 shows on the left diagram (a) a schematic view in longitudinal section of a cathode body comprising an electron emitter according to one embodiment and on the right diagram (b) a section view along the line C-C' of the left diagram. Fig. 6

[0041] [Fig. 6] Figure 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 embodiments

[0042] The drawings and the description below contain, for the most part, elements of a certain character. They may therefore not only serve to better understand the present invention, but also to contribute to its definition, if necessary.

[0043] First of all, note that the figures are not to scale.

[0044] With reference to Figures 2 and 3, a cathode 10 according to one embodiment of 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 main axis A-A' between an upstream end 10B and a downstream end 10A.

[0046] Figure 2 is a longitudinal 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] Figure 3 is a cross-sectional view perpendicular to the main axis A-A' along the line B-B' of Figure 2.

[0048] The cathode 10 comprises a hollow cylindrical body 14 defining an ionization cavity 19 into which a gas capable of being ionized is injected, an electron emitter 12 arranged in the cavity 19 and capable of emitting electrons when it is 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 for circulating an electric current in the filament, a thermal barrier 15 to limit heat dissipation towards the outside of the cathode and an envelope 16 which covers all of the elements of the cathode, commonly called by the English term "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 comprises an opening 27 to allow the electrons to exit. The opening 27 is opposite the electron outlet 18 of the cathode. The upstream end 30 of the cathode body 14 also comprises 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 fluid connection with a pipe 40 for supplying ionizable gas. The direction of injection of the gas is represented by an arrow at the inlet of the pipe 40 in FIG. 2. The wall of the cathode body 14 has an external face 14A and an internal face 14B which delimits the ionization cavity 19 into which the gas is injected. The gas can be, for example, xenon, argon, krypton, any gas capable of being ionized.

[0051] According to an exemplary embodiment, the electron emitter 12 is preferably positioned closer to the downstream end 29 of the cathode body 14, i.e. closer to the opening 27 in order that the thermal conduction from the emitter 14 to the body of the cathode is reduced, during its operation.

[0052] Generally, the cathode body 14 is made of a material chosen from a group comprising 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 relative 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 remainder of the description, the term "internal" designates a part close to the main axis A-A' while the term "external" designates a part far from the main axis A-A'.

[0055] The electron emitter 12 is made of a thermoemissive material which is 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 (LaBe) because of its robustness, high current density and long lifetime. LaBe electron emitters can, for example, operate in a temperature range between 1000 °C and 1700 °C. Non-exhaustively, there may also be ceramic emitters based on C12A7, also called mayenite, which has 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 an increased thermionic emissivity when mixed with various oxides such as barium oxide BaO, scandium Sc2O3, strontium SrO, yttrium Y2O3, hafnium HfO2, zirconium 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 Figure 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 internal face 14B of the wall of the cathode body 14 by any suitable fixing means.

[0059] The electron emitter 12 may 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 zone 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 exemplary embodiment, the electron emitter 12 may be in the form of a tubular body extending along the main axis A-A' and is held in position in a central zone 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 in 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 inside of the ionization cavity 19 so as to hold the emitter in position by crimping. The crimping ensures good mechanical support and good electrical contact between the cathode body and the emitter while allowing the gas flow around the emitter to pass from upstream to downstream in the ionization cavity. The crimping also limits 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] Diagram (a) of Figure 5 is a longitudinal sectional view of the cathode body 140, showing the presence of four radial deformations 140, 141, 143, 144 which hold the emitter 12 positioned in the center of the cavity by crimping.

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

[0065] Unlike the cathodes of the prior art as illustrated in Figure 1, the heating filament 13 is kept at a distance from the inner wall of the cathode body 14 by means of a dielectric support 17 and positioned opposite the electron emitter 12. More precisely and as illustrated in Figure 2, the dielectric support 17 is in the form of a hollow cylinder which is arranged so as to coaxially surround the cathode body 14 and at a distance from the cathode body. The dielectric support 17 is made of an electrically insulating material which has a high melting or sublimation point, for example alumina AI2O3, boron nitride BN or zirconium dioxide ZrO2. The heating filament is wound in a spiral shape 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 so that the turns are separated to avoid any short circuit of the turns.

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

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

[0068] Generally, 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 according to a helical generatrix along the main axis A-A' on a portion of its internal face 17B. The filament 13 is received and held in position in the groove. The filament 13 comprises an end downstream 13A connected to the downstream end 21A of the internal electrode 21 and an upstream end 13B connected to the downstream end 20A of the external electrode 20.

[0070] It is noted that in the exemplary embodiment of FIG. 2, the length of the heating filament 13 corresponds substantially 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 circumferentially surrounds the entire surface of the emitter.

[0071] The internal electrode 21 is in the form of a hollow cylinder. It is arranged so as to coaxially surround the cathode body 14. It is positioned between the cathode body 14 and the dielectric support 17. It is neither in contact with the external face 14A of the wall of the cathode body 14 nor in contact 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 which corresponds substantially to the length of the hollow cathode body 14.

[0072] According to a particularly advantageous technical characteristic, the internal electrode 21 is configured to form a thermal wall which allows the thermal radiation coming from the filament 13 to pass through in order to heat the emitter 12 according to a radiative heating process and limit the passage of the thermal radiation coming from the cathode body 14 towards the filament 13. To do this, a part of the internal electrode which is opposite the electron emitter 12 comprises a plurality of through openings 25 configured to allow the passage of the thermal radiation making it possible to heat the cathode body 14 and the electron emitter 12. When the plasma is generated within the ionization cavity 19 which makes it possible to maintain the temperature within the ionization cavity to continue to heat the emitter to a temperature sufficient for the emission of electrons, the filament is then no longer supplied with current.The thermal radiation emitted by the cathode body 14 heated towards the filament 13 is partly blocked by the solid part of the wall of the internal electrode 21, thus contributing to maintaining the temperature within the ionization cavity.

[0073] Thus, the perforated wall of the internal electrode 21 makes it possible to heat the cathode body although the filament and the wall of the cathode body are not in physical contact. During the cathode heating phase, the cathode body 14 is mainly heated radiatively by the filament 13 through the openings 25. This type of heating thus makes it possible to better direct the heat flow, i.e. from the filament 13 to the emitter 12 and to reduce the power injected into the filament, necessary for heating the emitter. The reduction of this power then makes it possible to reduce the operating temperature of the filament and therefore the thermal stresses which can induce degradation, deformation, or evaporation of the material constituting it. For this reason, it has a longer service life than the filaments used in conventional cathodes, thus making it possible to increase the number of start-up cycles and the reliability of the cathode.When the cathode is operating 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 pipe 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 towards the outside is limited, making it possible to improve the high temperature maintenance performance within the cavity.

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

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

[0076] A portion of the filament 13 is positioned opposite the external face 14A of the cathode body. The wall of the internal electrode 21 provided with through openings 25 is positioned between the external surface 14A and the filament 13.

[0077] In diagram (a), when the filament 13 is traversed by an electric current, the thermal radiation emitted by the filament 13 passes through the opening 25 to heat a surface S1 of the external surface 14A of the cathode body. The direction of emission of the radiation is represented by an arrow.

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

[0079] According to an exemplary embodiment, in order to limit the surface S2 to limit the heating of the filament and to maximize the surface S1 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 provides 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 supplied with current.

[0081] According to another particularly advantageous characteristic, the lower part of the wall of the internal electrode 21 which is not opposite the electron emitter 12 also comprises a plurality of through openings 23. These openings 23 have a different function from the openings 25 of the upper part of the wall of the electrode opposite the emitter. These openings are configured to limit the heat transfer by thermal conduction towards 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 main axis A-A'. It is arranged so as 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 exemplary embodiment of Figure 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 the extension of 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 turn 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 without filament.

[0084] According to another particularly advantageous characteristic, the wall of the external electrode 20 also comprises a plurality of through openings 26 which are configured to limit the heat transfer by thermal conduction towards 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 may have different geometric shapes in the plane of the wall of the electrodes.

[0086] According to one embodiment, the internal 21 and external 20 electrodes have a larger electrically conductive 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 21 and external 20 electrodes is lower than that along the heating filament 13. As a result, the energy deposition will be mainly localized at the filament during the heating phase.

[0087] The through openings can have a hexagonal shape as in diagram (a) of figure 6 forming a honeycomb or a rectangular shape in the diagram on the left (b) of figure 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 openings 23, 25 and 26 can be determined beforehand. Of course, the density of the openings made can be different depending on their function in the cathode.

[0090] According to one embodiment, the through openings 23 of the lower part of the internal electrode 21 are arranged in a staggered manner relative to the through openings 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 towards the outside.

[0091] According to one embodiment, the downstream end 29 of the cathode body is free and is not connected to the structure of the cathode, thus making it possible to limit heat 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 metal films which make it possible to limit the dissipation of heat towards the outside of the cathode.

[0093] All of the constituent elements of the cathode are 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 heat shield 15. The downstream end of the envelope is closed by a plate 31 which is provided with an opening forming the electron outlet 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 towards the outside of the cathode.

[0095] The cathode provided in the present invention is particularly suitable for use in an ion propulsion device, in particular a Hall effect ion propulsion device. For example, the proposed cathode may 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 thrusters.

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

Claims

Claims

1. Cathode (10) for an ion propulsion device, said cathode having a cylindrical shape extending along a main 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 facing 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 the form of a spiral on an internal 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 allow the passage of the 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 outlet (18) of the cathode.;

2. The cathode of claim 1, wherein the portion of the wall of the inner electrode (21) that is not facing 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 a cylindrical external 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. The cathode of 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, in which the external electrode (20) is positioned between the internal electrode (21) and the dielectric support (17) so that its downstream end (20A) is in the extension of the upstream end (13B) of the heating filament (13).

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

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

8. Cathode according to one of the preceding claims, in which the electron emitter (12) is in the form of a solid cylindrical body extending along the main axis A-A' and is held in position in a central zone 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 one of the preceding claims, in which the electron emitter (12) is in the form of a tubular body extending along the main axis A-A' and is held in position in a central zone 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, in which the wall of the hollow cathode body (14) has at least one radial deformation (141, 142, 143, 144) towards the inside of the ionization cavity (19) so as to hold the electron emitter (12) in position by crimping.

11. A 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 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 one of claims 1 to 12 and claim 3, in which 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 one of claims 1 to 13 and claim 3, in which the heating filament (13) has a section smaller than the axial section of the internal electrode (21) and / or the axial section of the external electrode (20) such that the heating filament is electrically more resistive than the internal electrode and the external electrode.

15. Cathode according to 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 one of claims 1 to 15.

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