ION PROPULSION DEVICE

Abstract

An ion propulsion device including emission modules in an emission plane, each module having an insulating support, an emission electrode on the support, and a conductive liquid with a microfluidic channel depositing conductive liquid on the electrode; an extraction electrode common to the emission modules and facing the modules; and a control unit, in which each module is configured to emit an ion beam when an electric field is applied to the liquid; each control unit controls an ion emission current emitted by applying a potential difference between each emission electrode and the extraction electrode; the emission electrodes are spaced apart by a linear distance that is greater than a distance between two adjacent emission electrodes separated by an empty space; and a length of the insulating support between the electrodes is greater than a propagation distance of an electric leakage current by charge jumping along the support between the electrodes.

Claims

1. An ion propulsion device, the device comprising: a plurality of emission modules arranged in an emission plane of the device, each emission module comprising an insulating support, an emission electrode arranged on the insulating support, and a tank of conductive liquid with a microfluidic channel arranged to deposit conductive liquid on the emission electrode; an extraction electrode common to the plurality of emission modules and arranged opposite the emission modules; and at least one control unit; wherein: each emission module is configured to emit an ion beam when an electric field is applied to the conductive liquid; the at least one control unit is configured to control an emission current of the ions emitted by the application of a potential difference between at least one emission electrode and the extraction electrode; the emission electrodes of the emission modules are spaced apart from one another by a linear distance 1 greater than a breakdown distance between two adjacent emission electrodes separated by an empty space; and a length L of the insulating support between the emission electrodes is greater than a propagation distance of an electric leakage current by hopping conduction along the insulating support between the emission electrodes, the linear distance l and the length L of the insulating support being chosen so as to maximize the number of emission modules in the emission plane.

2. The device according to claim 1, in which the linear distance 1 is less than the propagation distance of an electric leakage current by hopping conduction along the insulating support between the emission electrodes.

3. The device according to claim 1, characterized in that the emission modules are configured to be juxtaposed in order to form a propulsion surface of variable size.

4. The device according to claim 3, characterized in that the juxtaposed emission modules form an integrated or connected unit.

5. The device according to claim 3, characterized in that the size of the propulsion surface is comprised between 100 mm2 and several m2.

6. The device according to claim 1, characterized in that when the device is in operation, at least one emission module emits an ion beam and at least one emission module does not emit a beam.

7. The device according to claim 1, characterized in that the ion source comprises a tank for conductive liquid connected to the emission electrode.

8. The device according to claim 1, characterized in that each emission electrode comprises a planar substrate comprising a plurality of emitters.

9. The device according to claim 8, characterized in that the substrate is made from crystalline silicon, glass or alternate layers of these materials.

10. The device according to claim 8, characterized in that each emitter is constituted by a plurality of nanowires, the nanowires extending essentially towards the extraction electrode and covering the whole of the upper surface of the substrate.

11. The device according to claim 10, characterized in that an emitter is constituted by a plurality of nanowires forming a bundle.

12. The device according to claim 1, characterized in that the extraction electrode comprises a grid of plaited metal wires, or a metal plate comprising openings.

13. The device according to claim 1, characterized in that when the device is in operation, a part of the emission modules is configured to emit an ion beam of opposite polarity with respect to the polarity of the ion beam emitted by another part of the emission modules.

14. The device according to claim 13, characterized in that the device is configured to emit positively-charged ions and negatively-charged ions so that there is the same quantity of positive charges as negative charges.

15. The device according to claim 13, characterized in that the device is configured to emit positively-charged ions and negatively-charged ions so that the total of the currents emitted and collected by a spacecraft in which the device is implemented is equal to zero.

16. A satellite, in particular of the CubeSat type, comprising an ion propulsion device according to claim 1.

Description

DESCRIPTION OF THE FIGURES AND EMBODIMENTS

[0064] Other advantages and characteristics will become apparent on examination of the detailed description of examples that are in no way limitative, and from the attached drawings in which:

[0065] FIG. 1a is a diagrammatic representation of a non-limitative example embodiment of an emission module of a device according to the invention;

[0066] FIG. 1b is an enlargement of details in FIG. 1a;

[0067] FIG. 2a shows an example embodiment of an emission electrode of a device according to the invention;

[0068] FIG. 2b shows an enlargement of the emission electrode in FIG. 2a; and

[0069] FIG. 3 is a diagrammatic representation of a propulsion device according to an embodiment.

[0070] It is well understood that the embodiments that will be described hereinafter are in no way limitative. It is possible in particular to envisage variants of the invention comprising only a selection of the characteristics described hereinafter, in isolation from the other characteristics described, if this selection of characteristics is sufficient to confer a technical advantage or to differentiate the invention with respect to the state of the prior art. This selection comprises at least one, preferably functional, characteristic without structural details, or with only a part of the structural details if this part alone is sufficient to confer a technical advantage or to differentiate the invention with respect to the state of the prior art.

[0071] In particular, all the variants and all the embodiments described can be combined together if there is no objection to this combination from a technical point of view.

[0072] In the figures, the elements common to several figures retain the same reference.

[0073] FIG. 1a is a diagrammatic representation of an example of an ion propulsion device, shown in cross section, according to an embodiment of the invention.

[0074] The view of the device 1 as illustrated in FIG. 1a shows seven emission modules 10. For example, this may concern an example device 1 having 7×7 emission modules 10. Of course, the device 1 can comprise a different number of emission modules.

[0075] Each emission module 10 comprises an emission electrode 11 comprising emitters in the form of tips (not shown). Advantageously, the emission electrode comprises a planar substrate or a plate.

[0076] An example embodiment of an emission electrode is shown in FIG. 2a. According to this example, the emission electrode comprises a substrate 110 or a plate comprising a plurality of emitters 111.

[0077] The substrate of the emission electrode 11 is made from an electrically insulating or semiconductive material. This is for example a plate containing crystalline silicon. Of course, other materials suitable for growing nanowires, such as the silica glasses, the borosilicate glasses or combinations of layers of these materials, can be used for the substrate.

[0078] The device 1 according to the invention also comprises an extraction electrode 14. The extraction electrode 14 is common to all the emission modules 10 of the device 1 according to the invention.

[0079] Each module 10 also comprises a tank 12 of conductive liquid on which the emission electrode 11 is arranged. The tank 12 of conductive liquid constitutes an ion source. It can contain a determined amount of conductive liquid. The conductive liquid can be, for example, an ionic liquid, a liquid rendered conductive, or a liquid or molten metal. The liquid is passively diffused on the emission electrode 11, for example through openings in the substrate 110, even when there is no potential applied between the electrodes 11, 14.

[0080] The tank 12 is made from a dielectric material, for example an epoxide polymer material, optionally reinforced by glass fabric, or polyether ether ketone (PEEK). The set of tanks 12 of a device 1 can be constituted by a dielectric plate in which cavities are provided. Alternatively, as illustrated for the embodiment in FIG. 1a, the single tanks 12 can be juxtaposed in the device 1.

[0081] The tank 12 thus constitutes an insulating support 17 for the emission electrode 11.

[0082] For each emission module 10, the tank 12 is connected to the emission electrode 11. Assembly of the reservoir 12 with the emission electrode 11 can, for example, be carried out by bonding, screwing or welding.

[0083] The extraction electrode 14 is made from an electrically conductive material. This material can be, for example, a metal such as tungsten, stainless steel, molybdenum or tantalum.

[0084] In the example embodiment shown in FIG. 1a, the extraction electrode 14 is formed by a grid of plaited metal wires. The grid can have a mesh constituted by wires of approximately 50 to 80 μm diameter, spaced apart by approximately 100 μm. Alternatively, the extraction electrode can be a metal plate having openings.

[0085] In both cases, it is not necessary for an opening of the extraction electrode to be positioned exactly facing an emitter of the emission electrode.

[0086] The conductive liquid forms a pool of liquid on the upper surface of the emission electrode 11. In order to polarize this pool, an immersed polarization electrode 18 can be provided in each tank, as illustrated in FIG. 1a. When an electric field is generated between the two electrodes 11, 14 by application of an electric potential difference, a very strong local electric field (of the order of 10.sup.9 V/m) is generated at the emitters, which causes the conductive liquid to form a Taylor cone situated over a plurality of tips of the emission electrode 11. Ions are then emitted at the apex of each cone, forming an ion beam 15 for each module 10 in operation. The charged particles are then accelerated at high speeds of the order of several tens of kilometres per second by the applied electric field. The polarity of the ion beam is determined by the sign of the electric field created between the electrodes 11, 14. FIG. 1a illustrates two ion beams 15 of opposite polarity.

[0087] The operation of a propulsion device can be characterized by a function I(V) where I is the emission current of the emitted ions and V is the potential difference applied between the electrodes. On this curve, the point of stable operation is reached after a certain stabilization time.

[0088] The ion propulsion device 1 also comprises a control unit 13. The control unit 13 can be common to several modules 10 or even to all the modules 10 of the device 1. Alternatively, each emission module 10 can have its own control unit 13.

[0089] The control unit 13 is configured to apply the potential difference between the emission electrodes 11 and the extraction electrode 14. To this end, the control unit 13 comprises in particular a high-voltage electricity generator, shown in FIG. 1a by an electrical diagram for two of the modules. The electricity generator is configured to generate the voltage necessary for the potential difference and to allow a polarity inversion between its terminals.

[0090] The control unit 13 also comprises an electronic module configured to control ion emission such as the flux and the velocity of the particles emitted, and to monitor the chemical characteristics of the conductive liquid. The electronic module can comprise, for example, an on-board platform, such as a microcomputer, a digital electronic circuit and/or software means. The electronic module can also comprise a communication means.

[0091] One or more emission modules 10 can be fixed on a control unit 13 by epoxy bonding, or by any other suitable means. In the embodiment shown in FIG. 1a, the control unit 13 is represented by an integrated unit.

[0092] The device 1 comprises an outer case 16 (or a housing) in which the emission modules 10, the extraction electrode 14 and, optionally, the control unit 13 are arranged. The case 16 is at the chassis potential (0 V).

[0093] FIG. 1b shows an enlargement of a part of the device 1 shown in FIG. 1a. FIG. 1b displays the distances between two emission electrodes, which will be described hereinafter.

[0094] The emission electrodes 11 of the emission modules 10 are arranged in relation to one another so that the length L of their insulating support 17 between two adjacent emission electrodes is greater than the propagation path of an electric leakage current between these adjacent electrodes. In other words, the modules are spaced apart so as to avoid breakdowns and leakage currents between their respective emission electrodes.

[0095] This can be carried out, for example, by providing for a groove or even a series of corrugations of the insulating surface between two adjacent emission modules, as illustrated for the embodiment shown in FIG. 1a. The linear distance l between two emission electrodes 11 can thus be chosen to be less than the distance travelled by electrons on the dielectric material through the hopping conduction mechanism, and thus less than the extent L of the insulating surface between two adjacent modules. The length of the path to be travelled by the electrons in the dielectric material can be increased by increasing the depth of the groove between two emission modules.

[0096] FIG. 2b shows an enlargement of the surface of an emission electrode, such as that in FIG. 2a, comprising a plurality of nanowires 113. No conductive liquid is shown on this photograph taken with an electron microscope.

[0097] The nanowires 113 extend essentially towards the extraction electrode and cover the whole of the microstructured substrate.

[0098] Like the substrate 110, the nanowires 113 are made from an electrically insulating or semiconductive material. This material can be for example crystalline gallium nitride (GaN), or any other suitable material.

[0099] The nanowires can be produced by molecular epitaxy on the substrate. For a given method, the diameters of the nanowires vary very little. The diameter of the nanowires can be, for example, between 40 and 80 nm. A material addition production process such as epitaxy makes it possible to obtain densities of nanowires of the order of 10.sup.9/cm.sup.2.

[0100] An emitter site can be constituted by a plurality of nanowires, for example by 5 to 10 nanowires, this number varying according to the aspect ratio (diameter/length of the wires), the density of the wires and the Young's modulus (mechanical stiffness) of the material constituting the nanowires.

[0101] In the example shown in FIG. 2b, the nanowires form groups in the form of bundles 20. These groups can form when the pool of ionic liquid is deposited on the emission electrode or when a potential difference is applied between the emission electrode and the extraction electrode.

[0102] Each bundle 20 can constitute an emitter. Each bundle 20 can contain several nanowires 113 or several tens of nanowires 113, as shown in the photo in FIG. 2c. The bundles 20 allow renewal of the conductive liquid at the ends of the nanowires 113. The conductive liquid flows between the nanowires by capillarity. Thus, there is no counterion accumulation effect at the Taylor cones, which promotes prolonged operation, for example over several hours, without the need to invert the polarity of the emission module and without degrading the conductive liquid.

[0103] According to a preferred embodiment, the device 1 according to the invention comprises a plurality of emission modules 10. The device 1 also comprises an extraction electrode 14 common to all the emission modules 10 as well as one or more control units 13. The emission modules 10 are configured to be juxtaposed beside one another. The emission modules 10 can for example have a square cross section in the plane of the extraction electrode 14. Of course, the cross section can also be rectangular.

[0104] The emission modules 10 are preferably arranged so as to form flat elements, such as slabs. These slabs can easily be combined with an extraction electrode as well as one or more control units. Such an assembled device has a very small bulk.

[0105] An example of a propulsion device according to the invention is illustrated in FIG. 3. In this example, the device comprises an assembly of 7×7 emission modules 10, arranged on a control unit 13 and placed opposite a single extraction electrode 14.

[0106] According to an example, the size of the propulsion surface formed by the juxtaposed emission modules can be comprised between 100 mm.sup.2 and several m.sup.2. The device can be integrated into spacecraft of all sizes.

[0107] According to an embodiment, the juxtaposed emission modules can form an integrated or connected unit. To this end, it is for example possible to combine them with a single common control unit, or to form emission units with a single substrate.

[0108] Alternatively, the emission modules can be independent modules. Thus, it is possible to easily extract one or more modules from the device, for example for replacing them in the event of failure.

[0109] When the ion propulsion device is in operation, at least one emission module emits an ion beam and at least one emission module does not emit a beam. The module or modules that are not operating can thus be reserved for the circumstances where another module is unable to operate, for example when the local reserve of conductive liquid is degraded. Modules can then be stopped and others can be started without the propulsion device having down time when it is in operation. This makes it possible to emit ions during an extended period.

[0110] Similarly, when the ion propulsion device is in operation, a part of the emission modules can be configured to emit an ion beam of opposite polarity with respect to the polarity of the ion beam emitted by another part of the emission modules.

[0111] According to a first example, the device can be configured to emit positively-charged ions and negatively-charged ions at the same time so that the ion plume emitted by the thruster resulting from all the emitted ion beams is electrically neutral.

[0112] According to a second example, by making the emission modules operate with opposite polarities at the same time, it is also possible to produce the charge neutrality of the spacecraft bearing the propulsion device. To this end, the total of the currents emitted and collected by the spacecraft must be equal to zero.

[0113] Of course, the invention is not limited to the examples that have just been described and numerous adjustments can be made to these examples without departing from the scope of the invention.