Device for reducing the electric charge of a space vehicle

Abstract

According to one aspect, the present disclosure relates to a device for reducing the electric charge of a space vehicle, the device comprising: a substrate (201) made of a conductor or semiconductor which is configured to be electrically connected to the structure of the space vehicle; a first layer (202) formed of a plurality of carbon nanotubes which are substantially oriented perpendicular to the substrate, this first layer (202) at least partially covering the substrate (201) and being in contact with the substrate (201); a second layer (203) at least partially covering the first layer (202) and comprising a dielectric material so as to acquire a more positive charge than the first layer (202) when the device is subjected to a stream of electrons, of ions and / or a stream of vacuum ultraviolet radiation (VUV).

Description

[0001] Device for reducing the electrical load of a space vehicle

[0002] Technical field of the invention

[0003] The present description relates to a device for reducing the electrical load of a space vehicle and to a space vehicle comprising such a device. In addition, the present description relates to a method for reducing the electrical load of a space vehicle. State of the art

[0004] Many space vehicles such as satellites, launch vehicles, landers, space stations, space probes and rovers are intended to operate in a space environment without an atmosphere, also known as a "loaded space environment".

[0005] The charging space environment includes electrically charged particles that can interact with the vehicle. In particular, satellites are generally very sensitive to electrons in the charging space environment, the energy of which is between a few tens of electron volts and a few hundred kiloelectron volts.

[0006] Due to the interaction with the charging space environment, vehicles acquire an electrical charge, in particular a negative electrical charge, which can lead to adverse consequences if not controlled, due to the occurrence of leakage currents and electric arcs which can damage the space vehicle and / or lead to failures. Space vehicles generally comprise a conductive structure and different coatings arranged on the structure comprising materials with different electrical properties and which can charge to different levels depending on their capacity to store and emit electrons when they are subjected to the impact of electrons and ions from the charging space environment.The electrical charge levels of the coatings also depend on the positions of the coatings on the spacecraft, with significant differences between coatings exposed to sunlight and coatings in the shade because coatings exposed to sunlight re-emit a significant portion of their negative charge to the charging space environment due to photoemission triggered by vacuum ultraviolet (VUV) radiation.

[0007] The balance between electrical currents on each coating and on the conductive structure of the satellite governs the evolution of the electrical potentials of each element. When the potential difference between two elements reaches a critical threshold, electrical energy can be released spontaneously and sporadically in the form of an electrostatic discharge (or "ESD"). The severity of the discharge and its possible consequences on the spacecraft depend on the amplitude of the discharge, its temporal dynamics, its duration and the proximity of sensitive elements of the spacecraft.

[0008] In some cases, the spacecraft structure and all connected conductive elements may be negatively charged relative to the space environment, while an external coating arranged on the outer surface of the spacecraft structure is negatively charged, but less negatively than the structure. In this particular configuration, called an inverse potential gradient, the risks of ESD are especially increased due to the appearance of an electric field between the external coating and the spacecraft structure, which lowers the potential barrier that electrons must cross to escape from the spacecraft. Thus, in an inverse potential gradient configuration, the emission of excess electrons is favored at the point where the conductive material, the insulator, and the charging space environment meet, called the triple point in astronautics.Reverse potential gradient configurations are particularly common for spacecraft solar panels, in which an external protective glass covering the solar cells generally charges less negatively than the spacecraft structure.

[0009] In order to limit ESD problems related to the acquisition of charges by a space vehicle, it is known to use an active electric charge emission device.

[0010] US patent 6,362,574 [REF1] describes for example a micro-tip type electrical charge emission device comprising a set of micro-tips made of conductive material configured to be in electrical contact with the structure of the space vehicle and a conductive grid. In this device, a voltage source imposes a potential difference between the grid and the set of micro-tips allowing the emission of a flow of electrons from the micro-tips towards the charging space environment. Such a device therefore makes it possible to reduce the negative electrical charge of the space vehicle.

[0011] However, such a microtip device has the disadvantage of being an active device that is complex to control and requires a voltage source that consumes electrical energy. In addition, when using such a device, the emission of electrons must be stopped when the potential of the spacecraft is reduced to zero in order to prevent said potential from reaching positive values ​​that could re-attract the electrons emitted by the system. In some cases, the return of the electrons may be in the form of a focused beam that could damage sensitive systems of the spacecraft. US Patent 8,511,616 [REF 2] discloses another type of microtip device in which the conductive grid is electrically connected to a large conductive plate exposed to the Sun and which emits a large quantity of electrons due to a photoemission effect induced by VUV radiation.In this configuration, the conductive grid does not need to be powered by a voltage generator. However, such a device only becomes effective if the conductive plate is sufficiently extended, which is bulky and takes up significant space on the spacecraft hosting the device, to the detriment of other installations such as photovoltaic cells.

[0012] On the other hand, US patent 8,014,121 [REF3] discloses a passive ESD prevention device comprising a conductive substrate electrically connected to the structure of a satellite, a conductive layer in electrical contact with the substrate, and a dielectric material disposed on the conductive layer. In such a device, when the charging space environment negatively charges the satellite, different electrical charge levels are acquired by the conductive layer and by the dielectric material. In the same way as for an inverse potential gradient, an electric field then appears between the conductive layer and the dielectric material so that the emission of electrons from the conductive layer to the charging space environment is favored at the triple point.

[0013] The applicant observed that this device, although passive, has disadvantages related to electron emission which depends on the asperities of the dielectric material and the conductive layer and therefore cannot be controlled, which can in particular lead to avalanche effects which can damage the device. The reverse gradient necessary to start the emission can indeed sometimes be too high, which can lead to high intensity ESD occurring on the device itself even before the device is put into operation, thus limiting the usefulness of the device in reducing the negative charge of a space vehicle.

[0014] In this context, the present description describes devices for reducing the load of a space vehicle which solve all or part of the problems of the devices according to the state of the art.

[0015] Summary of the invention

[0016] In this specification, the term "include" means the same as "include" or "contain", and is inclusive or open and does not exclude other elements not described or shown. Furthermore, in this specification, the term "approximately" or "substantially" is synonymous with (means the same as) having a lower and / or upper margin of 10%, for example 5%, of the respective value.

[0017] According to a first aspect, the present description relates to a device for reducing the electrical charge of a space vehicle comprising: a substrate made of conductive or semi-conductive material configured to be electrically connected to a conductive structure of the space vehicle; a first layer formed of a plurality of carbon nanotubes substantially oriented perpendicular to a plane of the substrate, said first layer at least partially covering the substrate and being in contact with the substrate; a second layer at least partially covering the first layer and comprising a dielectric material configured to emit electrons when said dielectric material is subjected to an external flux (ER) comprising electrons, ions and / or vacuum ultraviolet radiation (VUV), so as to give the second layer a more positive charge than the first layer when the device is subjected to said external flux.

[0018] In the present description, the external flux (ER) may originate from a charging space environment.

[0019] In the present description, "vacuum ultraviolet" or VUV radiation refers to electromagnetic radiation comprising a wavelength spectrum whose central wavelength is between about 140 nm and about 200 nm.

[0020] The applicant has shown that in such a device, the synergy between the first layer and the second layer allows a reduction in the load of the spatial device with very good efficiency.

[0021] In particular, the presence of carbon nanotubes tends to increase the electric field at the upper surface of the first layer and therefore to reduce the potential barrier that the electrons present in the substrate must cross to be evacuated from the substrate, in particular by tunnel effect or by field effect.

[0022] Furthermore, the presence of the second layer tends to further increase the electric field at the upper surface of the first layer, and thus, again, to reduce the potential barrier that the electrons present in the substrate must cross to be evacuated from the substrate.

[0023] Furthermore, the carbon nanotubes in the first layer produce a multitude of peak effects promoting regular and homogeneous electron emission and adapting at each moment to the most favorable local conditions, so that avalanche effects are avoided.

[0024] Thus, such a device allows, under the influence of the charging space environment, an evacuation of electrons from the substrate, which results in a reduction of the negative electric charge of the substrate, and a reduction of the potential difference between the second layer and the substrate. The substrate being electrically connected to the conductive structure of a space vehicle, it has the same electric charge as the structure of the space vehicle. Thus, the emission of electrons from the substrate to the charging space environment tends to jointly reduce the negative charge of the substrate and the negative charge of the space vehicle to which the substrate is connected.

[0025] The applicant has shown that the technical effect results from a plurality of mechanisms. A first part of the electrons is emitted directly from the first layer to the charging spatial environment, for example via a field effect. A second part of the electrons is emitted indirectly from the first layer to the charging spatial environment, via the second layer. In the second case, an electron propagates from the first layer to the second layer, for example by tunneling. Said electron is then accelerated by the electric field from the junction between the first layer and the second layer to the second layer, which leads to ionizing collisions of said electron with molecules included in the second layer. This results in the emission of one or more electrons, called secondary electrons, from the second layer to the charging spatial environment.

[0026] Advantageously, such a device operates passively and therefore does not need to be powered by an energy source such as, for example, a voltage or current source. It is the interaction between the charging spatial environment and the device that produces the electric field promoting the emission of electrons from the substrate.

[0027] Moreover, such a device is efficient while being compact and the device does not require a conductive plate as may be the case in state-of-the-art passive microtip-type devices.

[0028] The carbon nanotubes of the plurality of carbon nanotubes are produced in a known manner by various methods described for example in [REF4]. The manufacturing methods include for example laser ablation, arc discharge or catalytic growth by chemical vapor deposition (or CVD). According to one or more examples, the substrate is made of metal, for example stainless steel, aluminum or copper.

[0029] The substrate may also be made of a semiconductor material, for example silicon, boron-doped silicon (p-type doping), germanium, gallium arsenide or silicon carbide.

[0030] According to one or more examples, a conductivity of the semiconductor material substrate is greater than 0.01 ohm. 1 . cm 1 .

[0031] According to one or more examples, the substrate is attached to the conductive structure of the spacecraft.

[0032] In other exemplary embodiments, the substrate is attached to an insulating coating and connected to the conductive structure of the spacecraft via an electrical connection, for example by an electrical wire or an electrically conductive cable.

[0033] According to one or more examples, the device further comprises a resistive component having an electrical resistance greater than or equal to 10 kΩ configured to electrically connect the substrate to the conductive structure of the spacecraft.

[0034] Such a resistor, called a protective resistor in this description, makes it possible to prevent an excessively high electron current from being emitted by the device and then damaging the device, for example in the case of a strong external flux.

[0035] According to one or more examples, the protective resistor comprises a sheet of dielectric materials.

[0036] In the case where the substrate is fixed on an insulating coating and connected to the conductive structure of the spacecraft via an electrical connection, the electrical connection may comprise said resistive component connected in series.

[0037] According to one or more examples, the first layer and / or the second layer is divided into several blocks.

[0038] According to one or more examples, the first layer is divided into several blocks, that is to say into several subsets of carbon nanotubes of the same nanotube density, but separated from each other laterally.

[0039] The applicant observed that the block structuring of the nanotubes makes it possible to promote the adhesion of the second layer to the assembly formed by the first layer and the substrate. In addition, structuring the plurality of CNTs into several blocks makes it possible to increase the number of side walls of the first layer, and therefore to increase the number of CNTs located on side walls. The applicants have shown that the emission of electrons by field effect in a device according to the present description results from the electric field generated by the individual tips of the CNTs and the electric field generated, on a larger scale, by the upper edges of the side walls of the first layer; such block structuring therefore improves the emission of electrons by field effect.

[0040] According to one or more examples, the first layer is divided into a number of blocks between 2 and 9.

[0041] According to one or more examples, the second layer is divided into several blocks, i.e. several parts separated from each other laterally.

[0042] The applicant observed that a structuring of the second layer makes it possible to produce a greater number of triple points on the side walls of the second layer, at the junction between the first layer, the second layer and the charging environment. Thus, the electric field is increased, which increases the emission of electrons by field effect at the level of said side walls.

[0043] According to one or more examples, the second layer is divided into a number of blocks between 2 and 9.

[0044] According to one or more examples, the second layer completely covers the first layer and comes into contact with the substrate.

[0045] The applicant observed that such an arrangement makes it possible to promote the adhesion of the second layer to the assembly formed by the first layer and the substrate. Such an arrangement is particularly advantageous when the second layer is deposited by a physical vapor deposition technique.

[0046] According to one or more examples, the dielectric material of the second layer is one of: alumina, magnesium oxide (MgO), silica, quartz, polydimethylsiloxane (PDMS), polyetheretherketone (PEEK).

[0047] The applicant observed that such dielectric materials are particularly advantageous for acquiring a more positive charge than the first layer, when the device is exposed to an external flux.

[0048] In particular, it has been observed that such dielectric materials have a good capacity to generate multiple electrons following the impact of a photon or electron coming from the charging space environment and / or following an impact of an electron coming from the first layer.

[0049] Thus, on the one hand, said second layer tends, under the effect of the charging spatial environment, to become positively charged on the surface exposed to said charging spatial environment. This leads to the creation of an advantageously high electric field between the second layer and the first layer, which thus promotes the emission of electrons from the first layer to the second layer, for example by tunnel effect.

[0050] On the other hand, when the second layer tends to generate multiple secondary electrons when electrons propagate in the second layer. Each secondary electron can then in turn lead to the generation of other electrons. This therefore leads to self-sustaining electron emission, even in the case where the external conditions are no longer conducive to positively charging the second layer, for example in the case where the device is sheltered from a large part of the radiation of the charging space environment.

[0051] According to one or more examples, the second layer has a thickness of about 4 micrometers to about 6 micrometers.

[0052] According to one or more examples, the second layer has a thickness of approximately 5 micrometers.

[0053] According to one or more examples, the second layer has a thickness strictly greater than the thickness of the first layer.

[0054] According to one or more examples, the second layer comprises at least one sheet of conductive materials.

[0055] According to one or more examples, said at least one sheet of conductive materials is arranged to separate the second layer into two superimposed parts.

[0056] According to one or more examples, the second layer comprises a plurality of conductive particles and / or at least one sheet of conductive materials, said at least one sheet of conductive materials being arranged to separate the second layer into two superimposed parts.

[0057] According to one or more examples, the sheet of conductive materials comprises one or more materials chosen from: gold, silver, copper, aluminum.

[0058] According to one or more examples, the second layer comprises a plurality of conductive particles.

[0059] According to one or more examples, the conductive particles are arranged in the second layer to form a two-dimensional mesh parallel to the surface of the second layer. According to one or more examples, the conductive particles are arranged in the second layer to form a three-dimensional mesh. The density of such a three-dimensional mesh may be uniform or non-uniform within the volume of the second layer.

[0060] According to one or more examples, the conductive particles comprise one or more materials chosen from: gold, silver, copper, aluminum.

[0061] According to one or more examples, the device further comprises: a measurement unit configured to measure a current output by the device, i.e. a current of electrons emitted from the device to the charging space environment. Such an arrangement makes it possible to know the current due to the emission of electrons by the device. The applicant has observed that this makes it possible to verify that the current remains within value ranges ensuring that the elements of the device such as the first layer and the second layer are operational and do not suffer irreversible damage. Furthermore, this device makes it possible to inform and alert on the appearance of a significant electrostatic charge on the surface of the satellite due, for example, to the charging space environment or to the activation of a system of the satellite.The device may, for example, be used to alert satellite operators to the risk incurred by satellites in the vicinity of said satellite due to the charging space environment or to adapt the use of the satellite systems. According to one or more examples, the device further comprises: a voltage generator configured to bring, in operation, the substrate to a negative electrical potential relative to the conductive structure of the space vehicle; a measurement unit configured to determine the electrical potential of the space vehicle; and a control unit configured to activate the voltage generator when the electrical potential determined by the measurement unit is, in absolute value, greater than or equal to a set value.

[0062] Such an optional arrangement, which makes the device active, allows for improved reduction of the negative electrical charge of the spacecraft by bringing the substrate to a negative electrical potential relative to the structure of the spacecraft, which promotes the emission of electrons from the substrate.

[0063] According to a second aspect, the present description relates to a space vehicle comprising a conductive structure and a device for reducing the electrical charge of a space vehicle according to the first aspect, in which the substrate of the device is electrically connected to the conductive structure of the space vehicle.

[0064] Such a space vehicle subjected to radiation present in the charging space environment has its negative electric charge reduced via the emission of electrons by the device, which avoids the destructive effects of electrostatic discharges.

[0065] According to one or more examples, the spacecraft further comprises: a light source adapted to emit vacuum ultraviolet radiation; wherein the light source is configured to positively charge the second layer of the device relative to the substrate. Such a light source replaces or acts as a supplement to the charging space environment so that the negative charge reduction device of the spacecraft continues to operate when the charging space environment does not sufficiently interact with the device to positively charge the second layer relative to the substrate. Such an eventuality may occur, for example, when the device is in the shadow of the spacecraft or a celestial object.

[0066] According to one or more examples, the light source is configured to be activated continuously or on demand.

[0067] According to a third aspect, the present disclosure relates to a method of reducing the electrical charge of a spacecraft using a device according to the first aspect, the method comprising: electrically connecting the substrate of the device to the conductive structure of the spacecraft. exposing the spacecraft to an external flux (ER) of a charging space environment comprising electrons, and / or ions, and / or vacuum ultraviolet radiation (VUV).

[0068] According to one or more examples, the method further comprises emitting an alert signal by the device when the electrical charge of the spacecraft reaches a predetermined threshold value.

[0069] Brief description of the figures

[0070] Other advantages and characteristics of the invention will appear on reading the description, illustrated by the following figures:

[0071] [Fig. 1], represents a diagram of a satellite including examples of a device for reducing the electrical load of a space vehicle according to the present description;

[0072] [Fig. 2A], represents a first embodiment of a device for reducing the electrical load of a space vehicle according to the present description.

[0073] [Fig. 2B], represents a second embodiment of a device for reducing the electrical load of a space vehicle according to the present description;

[0074] [Fig. 3A], represents a third embodiment of a device for reducing the electrical load of a space vehicle according to the present description;

[0075] [Fig. 3B], represents a fourth embodiment of a device for reducing the electrical load of a space vehicle according to the present description;

[0076] [Fig. 4], represents a fifth embodiment of a device for reducing the electrical load of a space vehicle according to the present description; [Fig. 5], represents a diagram illustrating a principle of operation of a device for reducing the electrical load of a space vehicle according to the present description;

[0077] [Fig. 6], shows several configurations of a device for reducing the electrical load of a space vehicle according to the present description.

[0078] The attached figures are schematic and are not necessarily to scale; they are intended primarily to illustrate the principles of the invention. From one figure (fig.) to another, identical elements (or parts of elements) are identified, where possible, by the same reference signs.

[0079] Detailed description of the invention

[0080] Fig. 1 shows a spacecraft of the satellite type 100 subjected to an external flux (ER) originating from a space environment charging surrounding the satellite 100. The satellite 100 has a main body 103 and two solar panels 101, 102 and comprises an internal conductive structure. Each of the solar panels has two faces 110, 111. The body 103 and the faces of the solar panels 101, 102 of the satellite 100 are covered with various coatings. When the satellite 100 is subjected to the external flux (ER), the coatings tend to charge positively or negatively with respect to the internal conductive structure of the satellite which is not exposed to the external flux (ER); however, electrostatic discharges are avoided because the excess electrons of the conductive structure are evacuated by devices 200 for reducing the electrical charge arranged on the satellite and in accordance with the present description.

[0081] The location of the electrical load reduction devices 200 may be adapted to the specifics of the space vehicle, and is shown solely by way of example in Fig. 1. For example, the devices 200 may be arranged on masts deployed from the body of the space vehicle.

[0082] Although in the example illustrated in Fig. 1 the devices 200 for reducing the electrical load are only visible on the visible faces of the solar panels, one or more devices may be arranged on the hidden faces of the solar panels.

[0083] Although in the example illustrated in Fig. 1 several devices 200 for reducing the electrical load are arranged on the body 103 and on the solar panels 101, 102, according to other examples the devices may be arranged differently. In particular, according to certain examples, the body 103 and / or the solar panels 101, 102 may comprise only a single device 200.

[0084] Fig. 2A, Fig. 2B, Fig. 3A, Fig. 3B and Fig. 4 show devices for reducing the load of a space vehicle according to several embodiments. In all these embodiments, the device comprises a substrate 201, a first layer 202 arranged on the substrate and a second layer 203 arranged on the first layer 202. The second layer 203 may be at least partially in contact with the substrate 201 (see in particular Fig. 4 described later).

[0085] The substrate 201 comprises a conductive or semiconductive material and is configured to be electrically connected to a conductive structure of a spacecraft. In particular, the substrate may comprise a metal plate, for example a stainless steel, aluminum or copper plate, or a semiconductive plate, for example a silicon, boron-doped silicon (p-type doping), germanium, gallium arsenide or silicon carbide plate.

[0086] In exemplary embodiments, the substrate, for example a plate made of a material as described above, is fixed to the conductive structure of the space vehicle.

[0087] In other exemplary embodiments, the substrate is attached to an insulating coating and connected to the conductive structure of the spacecraft via an electrical connection, for example by an electrical wire or an electrically conductive cable.

[0088] Such an electrical connection may include a resistive component having an electrical resistance greater than or equal to 10 kΩ and connected in series to limit the current output by the device. An electrical cable connection also makes it possible to connect a unit for measuring the current output by the device.

[0089] The first layer 202 comprises a set of carbon nanotubes (CNTs), said set preferentially occupying a surface substantially centered relative to the substrate.

[0090] The set of CNTs, also called CNT forest in the present description, comprises carbon nanotubes substantially oriented perpendicular to a plane of the substrate 201.

[0091] In exemplary embodiments, the CNTs are produced in a known manner by laser ablation, arc discharge or catalytic growth by chemical vapor deposition (or CVD).

[0092] [REF5] describes for example a procedure for catalytic growth on an n-doped silicon substrate. The precursor layer comprises a 10 nm thick aluminum layer, deposited by sputtering on the substrate and on which is deposited a 3 nm thick iron layer. The substrate with the catalytic layers thus deposited is then introduced into an HFCVD reactor (acronym for "Hot Filament Chemical Vapor Deposition", i.e. a chemical vapor deposition assisted by hot filament), into which a mixture of argon and hydrogen at a pressure of 3 Torr is introduced. A plasma treatment is then carried out for 2 minutes at 400 °C, in order to form the catalytic particles from which the CNTs will grow. For the growth of the CNTs, a mixture of methane and hydrogen at a pressure of 28 Torr is used. For example, the filament temperature is 2050°C (degrees Celsius), and the substrate temperature is 750°C (degrees Celsius).

[0093] The CNT forest preferably comprises several CNT sheets forming a layer having a substantially uniform height of between about 5 micrometers and about 150 micrometers. The substantially uniform nature of the height means that the distribution of the heights of the CNTs of the CNT forest has a dispersion less than or equal to about 10%.

[0094] In some examples, the forest of CNTs can be divided laterally into several blocks or subsets of carbon nanotubes, separated from each other laterally, in order to promote the adhesion of the second layer 203 to the assembly formed by the first layer 202 and the substrate 201. For example, the forest of CNTs can be divided into 9 blocks arranged in 3 rows of 3 blocks.

[0095] The blocks occupy, on the substrate 201, surfaces included in square or circular areas having sides or diameters between approximately 1 mm and approximately 2 mm, for example square or circular areas with sides or diameters equal to approximately 1.5 mm. When the forest of CNTs is divided into blocks, said blocks are preferably separated by distances between approximately 1 mm and approximately 10 mm, for example distances equal to 5 mm.

[0096] The second layer 203 is an insulating layer at least partially covering the first layer 202. The second layer 203 may comprise an alumina deposit carried out for example by physical vapor deposition or “PVD” according to the English expression “Physical Vapor Deposition”.

[0097] In some examples, the second layer 203 may also be divided into multiple blocks.

[0098] Thus, in the examples illustrated in Fig. 2A and in Fig. 2B, the forest of CNTs of the first layer 202 comprises only a single block, i.e. the first layer 202 is “monoblock”.

[0099] In the example illustrated in Fig. 2A, the second layer 203 completely covers the upper surface of the first layer 202 (i.e. the surface opposite the substrate 201) whereas in the example illustrated in Fig. 2B, the second layer 203 comprises two blocks 203a, 203b each covering a distinct part of the first layer 202.

[0100] The configuration of Fig. 2A allows the first layer to be completely covered by the second layer and thus to protect the first layer from the external environment, including the charging space environment. In addition, the first layer covers all or almost all of the substrate, which increases the average density of carbon nanotubes per unit area and therefore, the electric field is increased by the presence of the carbon nanotubes over a larger area.

[0101] The configuration of Fig. 2B allows for triple points located at the junction between the first layer, the second layer and the external environment, which locally increases the electric field.

[0102] In the examples illustrated in Fig. 3A and Fig. 3B, the CNT forest of the first layer 202 comprises two blocks 202a, 202b separated by a predetermined distance. Said predetermined distance is between approximately 1 millimeter and approximately 10 millimeters, for example distances equal to 5 mm.

[0103] In the example illustrated in Fig. 3A, the two blocks 202a, 202b of the first layer 202 are covered by a single second layer 203 which comprises a single block.

[0104] The configuration of Fig. 3A has the advantage of protecting the first layer from the external environment and of locally increasing the electric field at the periphery of the blocks of the first layer.

[0105] In the example illustrated in Fig. 3B, the two blocks 202a, 202b of the first layer 202 are each covered by one of the two blocks 203a, 203b of the second layer 203, respectively.

[0106] The configuration of Fig. 3B has the advantage of increasing the electric field at the periphery of the first layer bocs and at the triple points located at the junction between the first layer, the second layer and the external environment, which locally increases the electric field.

[0107] In the example illustrated in Fig. 4, the CNT forest of the first layer 202 comprises two blocks 202a, 202b. In addition, the second layer 203 is arranged to cover both the upper surface of the two blocks 202a, 202b and the side walls of the two blocks 202a, 202b, so as to come into contact with the substrate 201. In this example, the second layer 203 may also be nested in the first layer 202 so that the CNTs are separated from each other by the dielectric material of the second layer 203. In the example illustrated in Fig. 4, the second layer is preferably an alumina layer deposited, for example, by physical vapor deposition. The alumina layer preferably has a thickness of approximately 5 micrometers. The applicant observed that such a thickness makes it possible to obtain good performance in terms of electronic emission.

[0108] In the example illustrated in Fig. 4, the second layer 203 may also be a layer of PDMS deposited, for example by means of a syringe, on the substrate 201 and the first layer 202.

[0109] The applicant observed that an alumina layer with a thickness of about 5 micrometers is also advantageous in the exemplary devices illustrated in Figs. 2A-2B and Figs. 3A-3B.

[0110] The configuration of Fig. 4 makes it possible, in particular, to facilitate the adhesion of the second layer to the arrangement formed by the substrate and the second layer and to increase the electric field at the periphery of the blocks of the first layer, i.e. at the level of the side walls of the first layer.

[0111] Additionally, in examples not shown, it is possible to have a spacecraft load reduction device comprising a first monobloc layer and a second layer that is arranged to cover both the top surface of the first layer and the side walls of the first monobloc layer, so as to contact the substrate to increase the surface area of ​​the substrate that is covered by the first layer, thereby increasing the average density of CNTs per unit area of ​​the substrate.

[0112] In these examples, similarly to the device illustrated in Fig. 4, the second layer may also be nested within the first layer such that the CNTs are separated from each other by the dielectric material of the second layer.

[0113] Fig. 5 shows, according to an example, the operating principle of a device for reducing the electrical load of a space vehicle within the framework of the exemplary device illustrated in Fig. 2B, i.e. a device comprising a first layer 202 with a single block and a second layer with two blocks 203a, 203b. The operating principle described nevertheless applies to the other exemplary devices described in the present description.

[0114] In the example illustrated in Fig. 5, external fluxes (ER) from a charging spatial environment interact with the first layer 202 and the second layer 203 of the device to cause, by impact of electrons or photons, the emission of impact electrons (IE) and, by field effect, the emission of field effect electrons (F- E).

[0115] As illustrated in Fig. 5, under the influence of the external flux (ER), the second layer 203a, 203b can emit one or more impact electrons (IE), represented by solid arrows. The second layer comprises for this purpose a dielectric material having electrical properties giving it a strong capacity to emit electrons after impact of electrons or ions or photons coming from a charging spatial environment. Thus, the external flux (ER) tends to produce a large quantity of positive charges on the part or parts of the second layer most exposed to the external flux (ER), in particular the upper surface of the second layer 203a, 203b, which causes said part or parts to become more positively charged than the other parts of the second layer and than the first layer.

[0116] The charge difference between the different parts of the second layer and the first layer then causes the formation of an electric field (E) directed from the upper surface of the second layer 203a, 203b towards the first layer 202 and which promotes the emission of electrons by field effect (FE), represented by dotted arrows. As illustrated in Fig. 5, the second layer of the device according to the invention comprises materials capable of generating impact electrons (IE) in its own volume. In particular, when a field effect electron (FE) emitted due to the electric field (E) propagates in the second layer 203a, 203b, it is accelerated by the electric field (E) and generates, by ionizing collisions, positive (holes) and negative (IE impact electrons) free charge carriers.The electrons thus formed then move in turn into the second layer 203a, 203b under the effect of the electric field by carrying out new ionizing collisions or by recombining with the holes. When the ionizing collisions are not compensated by the recombination of the charges, the density of the electric charge carriers increases.

[0117] The second layer 203 of the device according to the invention comprises materials chosen to have a free charge carrier generation rate greater than the recombination rate such that a large portion of the electrons generated following the ionizing collisions can reach the upper surface of the second layer 203 and be evacuated towards the charging space environment, thus making the charge of the second layer 203 more positive and increasing the electric field (E) even more.

[0118] This phenomenon allows self-maintenance of charge generation within the second layer even when external conditions no longer favor charge generation. Such a specificity of the device can be understood by the Malter effect [REF6] which explains the significant emission of electrons from certain materials appearing for example in the form of a very thin sheet (a few nanometers to several micrometers), under irradiation by a beam of electrons, photons or ions and this even after the beam has stopped. The phenomenon is therefore maintained as long as the charge of the first layer 202 (and of the substrate 201) remains more negative than the charging spatial environment surrounding the device.

[0119] The applicant observed that alumina is a particularly advantageous material which makes it possible to obtain the three effects mentioned above. Magnesium oxide is also a particularly advantageous material based on its known properties.

[0120] Furthermore, in Fig. 5, FE electrons emitted to the external environment by the first layer near the triple points located at the junction between the first layer, the second layer and the external environment collide with the surface of the second layer, which generates electron emission by impact IE, which makes said impact surfaces more positive, especially when a material having a high electron emission power under electron impacts is chosen to constitute the second layer. This increases the electric field in the vicinity of the triple point and then increases the electron emission by field effect.

[0121] Furthermore, the applicant has shown that the presence of carbon nanotubes tends to increase the electric field (E) at the surface of the first layer, and therefore cooperates with the second layer to further reduce the potential barrier that the electrons present in the substrate must cross to be evacuated from the substrate, in particular by tunnel effect or by field effect.

[0122] Furthermore, the carbon nanotubes in the first layer produce a multitude of peak effects promoting regular and homogeneous electron emission and adapting at each moment to the most favorable local conditions, so that avalanche effects are avoided.

[0123] Fig. 6 is a diagram illustrating more precisely three examples of configurations (306, 307, 308) for implementing a device 200 for reducing the load of a space vehicle 300 according to the present description. In all configurations, the device comprises a substrate 201, a first layer 202 formed from a plurality of carbon nanotubes, and a second layer 203 comprising a dielectric material in accordance with the present description, and examples of which have been described above. These three configurations present in particular three ways of connecting the load reduction device to the space vehicle. Depending on the intended application and the technical constraints resulting therefrom, one or other of the configurations may be preferred.

[0124] In the example of Fig. 6, all three configurations are possible with a single load reduction device arranged on a given spacecraft, for example a satellite, and a user can select one or other of the configurations by means of an electrical circuit comprising a switch. In the example of Fig. 6, the switch is in a position selecting configuration 306 but can, in the general case, be switched to select one of configurations 306, 307 or 308.

[0125] In some examples, the load reduction device allows only one of the three configurations, which is chosen according to the desired application.

[0126] In a first configuration 306, as illustrated in Fig. 6, the substrate 201 is directly connected to the conductive structure 301 of the spacecraft 300. In this configuration, the device operates passively, and emits electrons (e-) under the action of the external flux ER without external power supply as described previously, for example by means of Fig. 5. This configuration makes it possible to return electrons collected by the satellite to the external environment ER as soon as the satellite reaches a sufficiently negative potential. This configuration therefore limits the negative charge of the satellite. This configuration does not include a measurement unit and is therefore particularly simple. This configuration is therefore particularly advantageous in the case where it is desired to limit the energy consumption of the device.

[0127] In a second configuration 307, the device comprises a substrate 201 which is connected to the conductive structure 301 of the spacecraft 300 via a measuring unit 305 configured to measure the electron current delivered by the device 200. In this second configuration, the device 200 also operates passively, and emits electrons (e-) under the action of the external flux ER. In addition, the current delivered by the device 200 is measured. The second configuration therefore makes it possible to have knowledge of the current due to the emission of electrons by the device, which makes it possible to verify that the current remains within value ranges ensuring that the elements of the device such as the first layer and the second layer are operational and do not suffer irreversible damage.The second configuration 307 further makes it possible to inform a user about the possible occurrence of a significant electrostatic charge of the space vehicle, for example due to the charging space environment or the activation of a system on the space vehicle. Informing the user about the significant electrostatic charge can for example be carried out via the display of an alert signal by the measurement unit when the electrical charge of the space vehicle reaches a predetermined threshold value.

[0128] In a third configuration 308, the device comprises a substrate 201 which is connected to the conductive structure 301 of the space vehicle 300 via a measuring unit 304 configured to measure a current output by the device 200 and an electrical potential of the space vehicle 301. Although, in Fig. 6, the measuring unit 304 is shown as a current measuring unit, it is also suitable for measuring a voltage.

[0129] Additionally, the substrate 201 is connected to the conductive structure 301 via a voltage generator 303 configured to, when in operation, bring the substrate 201 to a negative electrical potential relative to the structure 301 of the spacecraft.

[0130] The voltage generator 303 is connected to a control unit 302 configured to activate the voltage generator 302 when the electrical potential determined by the measurement unit 304 is, in absolute value, greater than or equal to a set value.

[0131] The third configuration 308 improves the reduction of the negative electrical charge of the spacecraft by negatively charging the substrate relative to the structure of the spacecraft, which promotes the emission of electrons from the substrate.

[0132] Fig. 7A, Fig. 7B and Fig. 8 are curves illustrating experimental results obtained during tests of two electrical load reduction devices according to the present description.

[0133] For these tests, the devices include a polished stainless steel substrate with dimensions of 23 mm in diameter and 3 mm in thickness. The first layer is obtained by catalytic growth of 9 discs of 1.4 mm in diameter separated by 5 mm from each other and forming a matrix of 3x3 discs. 9 blocks of vertically aligned carbon nanotubes are thus obtained by catalytic growth, with an average height of the carbon nanotubes of 10 pm to 50 pm. The second layer includes a 5 pm thick alumina layer deposited over the entire surface of the substrate by PVD (acronym for "physical vapor deposition").

[0134] In the following, we consider two devices (device 1 and device 2) for which the carbon nanotubes were produced by two substantially different processes but within the specifications mentioned above.

[0135] The device is mounted on a conductive structure of a nanosatellite model. Electrical contact with the substrate is made by means of conductive cables. The nanosatellite model is equipped with an electronic system allowing the electrical connection between the device and the conductive structure of the satellite to be activated or deactivated by opening / closing relays. The nanosatellite model is placed in a large vacuum chamber (3.4 mx 2 m cylinder).

[0136] In a first test, the nanosatellite model is connected to a negative voltage generator simulating a constant negative charge of the satellite from 0 to -150 V. Each device 1 and 2 is connected in turn to the conductive structure of the nanosatellite model. A plasma source composed of Argon ions and electrons is used to positively charge the second layer of the device relative to the nanosatellite model, thanks to a plasma density, temperature and velocity of 10 A l 1 m A -3, 1 eV and 10 km / s respectively. These characteristics reproduce an ionospheric type plasma. After a few seconds under these conditions, each device emits a current of a few tens of microamperes.

[0137] Fig. 7A illustrates the evolution of the current emitted by the device 1 for a voltage applied to the conductive structure of the nanosatellite model of -100 V.

[0138] A typical example of a 5-minute test is given in this figure 7A. At t = 0, a voltage of -100 V is applied. At t = 60s, the plasma source is activated. Within a few seconds, the current emitted by the device 1 reaches more than 40 pA. At t = 240 s, the plasma source is stopped. The current emitted by the device 1 decreases to 30 pA and then stabilizes again. This is the remanence effect described in the invention and linked to the Malter effect, which continues after the source causing the phenomenon has been removed, as long as the electrical connection between the device and the conductive structure of the nanosatellite model is activated and the conductive structure of the nanosatellite model is negatively polarized.

[0139] Fig. 7B shows the current emitted by devices 1 and 2 after a few seconds of activation of the plasma source for voltages applied to the conductive structure of the nanosatellite model varying from 0 V to - 150 V. The absolute value of the applied voltage is represented on the abscissa. The electrical connection between the conductive structure of the nanosatellite model and each of the two devices 1 and 2 is activated in turn. The two devices 1 and 2 each emit a current of more than 100 pA for a voltage applied to the conductive structure of the nanosatellite model, electrically connected in turn to each device, more negative than -110 V. This shows that the devices are effective in removing electrons from a negatively charged satellite immersed in an ionospheric plasma.

[0140] It appears in Fig. 7B that devices 1 and 2 gave very good results (respectively curves 71, 72). The emission is exponential as a function of the voltage applied to the nanosatellite model for both devices, which is predicted by the field effect emission theory. Even if the emission starts a little later for device 1, it should be noted that it is capable of emitting 100 pA for a satellite potential of -110 V. Device 2 emits 100 pA for a satellite potential of -90 V. The difference is tiny compared to the potentials of several hundred negative volts that a satellite can acquire in space. Both devices are efficient. The technology is robust.

[0141] In a second test, the nanosatellite model is isolated from the electrical ground of the laboratory.

[0142] Fig. 8 shows the potential of the conductive structure of the nanosatellite model as a function of time.

[0143] The following sequence is performed. At t < 0 s, an electron gun is used to charge the conductive structure of the nanosatellite model negatively relative to the vacuum box, using an electron beam with an energy of 5 keV and a current density of the order of 1 nA / cm A2. These characteristics are representative of the electron flows generating the negative charge of satellites in orbit. Being located opposite the face of the nanosatellite model irradiated by the electron beam, device 1 is not subjected to the electron beam. Under these conditions the conductive structure of the model charges to -1100 V. At t = 0 s, the VUV beam is used to irradiate the face of the nanosatellite model on which device 1 is located. The VUV beam slightly reduces the potential of the conductive structure of the nanosatellite model from which it extracts electrons by photoemission with a current of the order of 1 nA / cm A2. This flux is representative of space conditions. Under these conditions, the conductive structure of the nanosatellite mockup charges to -900 V. The VUV beam is used to positively charge the second layer of Device 1 relative to the nanosatellite mockup. At t = 180 s, Device 1 is electrically connected to the electrical ground of the conductive structure of the nanosatellite mockup. The charge of the conductive structure of the nanosatellite mockup decreases to -350 V + / - 100 V after about 30 seconds. At t = 540 s, the VUV source is stopped. The potential of the conductive structure of the nanosatellite model is established at -600 V + / - 100 V. It is observed that the reduction of the potential continues despite the stopping of the VUV source having served to initiate the emission of electrons by device 1. At t = 840 s, device 1 is electrically disconnected from the ground of the conductive structure of the nanosatellite model.The potential of the conductive structure of the nanosatellite model instantly rises to -1100 V under the effect of the electron beam which always irradiates the face of the nanosatellite model not equipped with the device. This demonstrates the efficiency of the system when the first layer of the device is electrically connected to the conductive structure of the satellite.

[0144] Although described through a certain number of exemplary embodiments, the methods and devices for reducing the electrical load of a space vehicle according to the present description include various variations, modifications and improvements which will be obvious to those skilled in the art, it being understood that these various variations, modifications and improvements are part of the scope of the invention as defined by the following claims.

[0145] References

[0146] [REFI] AGUERO, Victor Manuel et ADAMO, Richard Cosmo. System for emitting electrical charge from a space object in a space plasma environment using micro-fabricated gated charge emission devices. U.S. Patent No 6,362,574, 26 mars 2002.

[0147] [REF2] ADAMO, Richard Cosmo Solar Powered Excess Electron Emission Device. U.S. Patent 8,511,616, 20 août 2013.

[0148] [REF3] CHO, Mengu, SANMARU, Yuya, HOSODA, Satoshi, et al. Electrical discharge countermeasure device. U.S. Patent No 8,014,121, 6 sept. 2011.

[0149] [REF4] Bonard, J. M., Kind, H., Stôckli, T., & Nilsson, L. O. (2001). Field emission from carbon nanotubes: the first five years. Solid-State Electronics, 45(6), 893-914.

[0150] [REF5] Nguyen Tuan Hong, Ken Ha Koh, and Soonil Lee, Fast Growth of Millimeter-Long Vertically-Aligned Carbon Nanotubes via Hot Filament Chemical Vapor Deposition, J.

[0151] Korean Phys. Soc. 53, 3603 (2008).

[0152] [REF6] Louis Malter, “Thin Film Field Emission” Phys. Rev. 50, 48 - Published 1 July 1936.

Claims

CLAIMS 1. Device (200) for reducing the electrical charge of a space vehicle (100) comprising: a substrate (201) made of conductive or semiconductive material configured to be electrically connected to a conductive structure of the space vehicle; a first layer (202) formed of a plurality of carbon nanotubes substantially oriented perpendicular to a plane of the substrate (201), said first layer (202) at least partially covering the substrate (201) and being in contact with the substrate (202); and a second layer (203) at least partially covering the first layer (202) and comprising a dielectric material configured to emit electrons when said dielectric material is subjected to an external flux (ER) comprising electrons, ions and / or vacuum ultraviolet radiation (VUV), so as to give the second layer a more positive charge than the first layer when the device is subjected to said external flux.

2. Device according to claim 1, further comprising a resistive component having an electrical resistance greater than or equal to 10 kΩ configured to electrically connect the substrate (201) to the conductive structure of the space vehicle.

3. Device according to any one of claims 1 to 2, wherein the first layer (202) and / or the second layer (203) is divided into several blocks.

4. Device according to any one of claims 1 to 3, in which the second layer (203) completely covers the first layer (202) and comes into contact with the substrate (201).

5. Device according to any one of claims 1 to 4, in which the substrate is made of metal, for example stainless steel, aluminum or copper, or of semiconductor material, for example silicon, boron-doped silicon (p-type doping), germanium, gallium arsenide or silicon carbide.

6. Device according to any one of claims 1 to 5, wherein the dielectric material of the second layer (203) is one of: alumina, magnesium oxide (MgO), silica, quartz, polydimethylsiloxane (PDMS), polyetheretherketone (PEEK).

7. Device according to any one of claims 1 to 6, wherein the second layer (203) comprises a plurality of conductive particles and / or at least one sheet of conductive materials, said at least one sheet of conductive materials being arranged to separate the second layer (203) into two superimposed parts.

8. A device according to any one of claims 1 to 7, further comprising: a measuring unit (305) configured to measure a current output by the device.

9. A device according to any one of claims 1 to 8, further comprising: a voltage generator (303) configured to carry, in operation, the substrate (201) at a negative electrical potential relative to the conductive structure (301) of the space vehicle; a measuring unit (304) configured to determine the electrical potential of the space vehicle; and a control unit (302) configured to activate the voltage generator (303) when the electrical potential determined by the measuring unit (304) is, in absolute value, greater than or equal to a set value.

10. A space vehicle comprising a conductive structure (301) and a device (200) for reducing the electrical charge of a space vehicle according to any one of the preceding claims, wherein the substrate (201) of the device is electrically connected to the conductive structure (301) of the space vehicle.

11. Space vehicle according to claim 10, comprising: a light source capable of emitting vacuum ultraviolet radiation; wherein the light source is configured to positively charge the second layer (203) of the device (200) relative to the substrate (201).

12. A method of reducing the electrical charge of a space vehicle using a device according to any one of claims 1 to 9, the method comprising: electrically connecting the substrate (201) of the device (200) to the conductive structure (301) of the space vehicle; and exposing the space vehicle to an external flux (ER) of a charging space environment comprising electrons, and / or ions, and / or vacuum ultraviolet radiation (VUV).

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