METHOD FOR CONTROLLING AN ION DRIVE AND ION DRIVE

Description

technical field

[0001] The present invention relates to a method for controlling an ion propulsion system. It also relates to an ion propulsion system implementing such a method.

[0002] The field of the invention is, without limitation, that of the propulsion of spacecraft. State of the art

[0003] For spacecraft, such as satellites, various propulsion technologies are known, such as chemical propulsion, cold gas propulsion, or electric propulsion.

[0004] Miniaturized satellites, such as CubeSats, are increasingly used for information transmission and space exploration.

[0005] Unlike conventional satellites, miniaturized satellites significantly reduce deployment costs. These satellites also offer the advantage of greater maneuverability.

[0006] For these small satellites, appropriate propulsion and control systems are required. However, existing propulsion technologies have the drawback of not being easily adaptable to small satellites for technical or efficiency reasons. For example, the exhaust velocity of these chemical propellants is limited by the inherent specific energy released during combustion. Furthermore, the fuel tanks and power systems for chemical or ionized gas propellants are bulky and heavy, making them unsuitable for propelling miniaturized satellites and small spacecraft.

[0007] Propulsors based on electrosprays, called ion thrusters, have been proposed. Electrospray technology is a form of electric propulsion that generates thrust from an ionic liquid by ejecting and accelerating ions in an electrostatic field on the order of a billion volts per meter. An example is field-emitting electric propulsion (FEP) thrusters. This type of thruster is particularly well-suited for applications requiring thrust values ​​in the micronewton to millinewton range to control the orientation and position of spacecraft with masses between 1 kg and 300 kg.

[0008] Ion thrusters essentially consist of an emitting electrode comprising an array of emitters, an extraction electrode, a propellant reservoir and, in some cases, an accelerating electrode.

[0009] An example of such a system is described in PC Lozano, "Less in Space", American Scientist, Volume 104, Page 270 (2016). The emitting electrode has a plurality of emitters aligned in the form of porous tips infused with ionic liquid, as well as an extraction electrode and an acceleration electrode whose respective openings are aligned with the tips.

[0010] Another example of an ion propulsion system is described in S. Dandavino et al., "Microfabricated electrospray emitter arrays with integrated extractor and accelerator electrodes for the propulsion of small spacecraft", J. Micromech. Microeng., 24, 075011 (2014), in which the emitters include a microcapillary tube to bring the propellant liquid to its end from where it is ionized.

[0011] Another example of an ion propulsion system is described in I. Vasiljevich et al., “Development of an Indium mN-FEEP Thruster”, 44th AIAA / ASME / SAE / ASEE Joint Propulsion Conference & Exhibit, 21-23 July 2008, Hartford, CT, in which a liquid indium film brought to its melting point by heating diffuses into an array of microporous structures from which it is ionized by the application of an electric field.

[0012] A potential difference of approximately 1–10 kV is applied to generate a strong local electric field at the tip of the emitter. This electric field deforms the liquid propellant film into a conical structure known as a Taylor cone, located at the emitter tip, and extracts charged particles from the apex of the cone. The charged particles are then accelerated to high speeds, on the order of tens of kilometers per second, by the applied electric field.

[0013] The thrust of an ion thruster is a function of the flux, or emission current, and the velocity of the ions ejected by it. For such a thruster to operate efficiently, the flux and velocity of the ejected ions must be controlled. Description of the invention

[0014] One aim of the present invention is to propose a method for controlling an ionic propellant that allows the speed of the ejected ions to be controlled independently of the emission current.

[0015] Another objective of the present invention is to propose a method for controlling an ionic thruster that allows the thruster's power to be controlled independently of the polarity of the emitted ions.

[0016] Another objective of the present invention is to propose a method for controlling an ionic propellant that allows prolonged operation without depletion of the emitted ionic species and therefore without an accumulation of counter-ions in the propellant.

[0017] It is another objective of the present invention to propose a method for controlling an ionic thruster which has high efficiency and which is suitable for use in nanosatellites (about 1 kg - 50 kg) and microsatellites (about 50 kg - 300 kg).

[0018] At least one of these goals is achieved with a method for controlling an ion thruster, the ion thruster comprising an emitting electrode, an extraction electrode and a conductive liquid deposited on the emitting electrode, the ion thruster being adapted to emit an ion beam when an electric field is applied to the conductive liquid, the ion beam being adapted to provide thrust to the thruster, the thrust depending on an emission current I em and an ion emission rate, characterized in that the process comprises the following steps: setting the emission current to a setpoint value I cby applying a threshold emission potential V threshold at the emitting electrode by means of a current generator; and when the setpoint value I c Once the emission current is reached, the emission speed is adjusted by applying an extraction potential. V ext to the extraction electrode by means of a voltage generator, so as to raise the emission potential V em to a predetermined value V em = V threshold + V ext.

[0019] The thrust of the propulsion system, called thrust In English, and expressed in Newtons, it is defined as: T N = 2 1 q m I em × V em , with respectively q the charge of the ejected particle, m its mass, I the current or ejected flow and V em the potential of the issuer.

[0020] The method for controlling an ionic propellant according to the invention proposes a step of adjusting the emission current, corresponding to an activation phase of the propellant, and a step of adjusting the emission speed, corresponding to a phase of increasing the speed of the ejected ions.

[0021] During the activation phase, ion ejection is initiated by applying an emission potential to the emitting electrode using a current source. This creates a local electric field at the emitting electrode, for example, on the order of 10⁹ V / m, necessary for ion ejection by field evaporation. The flux, or the number of ions ejected, is determined by a setpoint for the emission current, which is supplied and regulated by the current source. The setpoint for the emission current is reached at a threshold emission potential. The current source is, for example, a high-voltage power supply operated in "current source" mode, adapted to rapidly deliver the desired ion current.

[0022] During the ion velocity ramp-up phase, the velocity, and therefore the energy, of the ejected ions is selected by applying a voltage source to the extraction electrode. Adjusting the extraction potential thus allows the velocity, and therefore the beam potential, of the ions to be selected, adjusted, and maintained at a desired level. Here, the term beam potential refers to the potential resulting from the difference between the emission and extraction potentials applied to the emission and extraction electrodes, respectively.

[0023] When the emission potential threshold changes, the beam potential can be adjusted to maintain proper thruster operation over extended periods. For example, the threshold value may increase due to the depletion of species in the conductive fluid, and simultaneously, there may be an accumulation of counterions that will affect the mobility of the ionic species of interest. It is therefore possible to monitor the degradation of the conductive fluid by observing the evolution of the emission potential threshold value or the adjusted beam potential value. Furthermore, it is possible to automatically implement countermeasures to prevent exceeding a critical depletion threshold for the emitted ionic species, as well as the corollary of counterion accumulation, thus protecting the thruster from degrading chemical reactions resulting from these combined mechanisms of cumulative accumulation and depletion.

[0024] Thanks to the control method according to the invention, the emission and acceleration of ions are controlled independently of each other, allowing for improved and prolonged operation of the ion propellant. Furthermore, the control method according to the invention allows for the ejection and acceleration of ions independently, regardless of their polarity.

[0025] Preferably, the step of adjusting the emission current can be carried out by progressively increasing the value of the potential applied to the emission electrode from 0 to the threshold value. V threshold for which the emission current I em reaches the setpoint value I c.

[0026] According to another embodiment of the invention, the step of adjusting the emission speed can be carried out by one of the following steps: application of a potential of 0 V, application of a potential of the same sign as the emission potential, or application of a potential of opposite sign to that of the emission potential.

[0027] In the case of potential extraction V ext of the same sign as the emission potential V In this case, it is possible to increase, adjust and maintain the beam potential at a value where the propulsive efficiency of the thruster is maximized, while remaining within the limits of the value of available potentials, for example on board a satellite.

[0028] In the case of potential extraction V ext of opposite sign to that of the emission potential VIt is possible to reduce, adjust, and maintain the beam potential at a value available to the thruster. For example, if the threshold potential is 7 kV, while the maximum available potential is only 5 kV, it is possible, by applying -2 kV to the extraction electrode, to obtain an ion emission beam potential of 5 kV and thus remain within the range of available voltages.

[0029] In the case of an extraction potential of 0 V, ion emission is obtained for the threshold potential applied to the emission electrode.

[0030] For example, the transmission current adjustment step may include a step for controlling the setpoint value. I c of the emission current.

[0031] To achieve this, the emitted current can be measured at the extraction electrode, or by a current intensity measurement device located in the current generator and controlled by comparison to the desired setpoint value. This control step allows the setpoint current to be adjusted automatically.

[0032] According to one embodiment of the invention, the emission current adjustment step may further include an emission current adjustment step.

[0033] As an example, adjusting the emission current can be achieved by adjusting the potential applied to the emission electrode. V em.

[0034] To achieve this, the setpoint value of the emission current can be slightly varied around the setpoint value. Ic of origin. This adjustment step allows the flow of ions, and therefore the mass ejected by the emission electrode, to be regulated very finely.

[0035] According to a non-limiting embodiment of the invention, the emission speed adjustment step may further include an emission speed adjustment step.

[0036] For example, the step of adjusting the emission rate can be achieved by adjusting the potential applied to the extraction electrode. V ext.

[0037] This step allows, in particular, a return to a desired emission speed after a flow adjustment that could have altered the emission speed.

[0038] The steps of adjusting the emission current and / or emission speed allow for very fine control or regulation of the thrust of the propulsion system.

[0039] According to a non-limiting example, the method of the invention may further include a step for stopping the ion propellant. The stopping step may include the following steps: progressive decrease in the value of the extraction potential V ext, so as to obtain V ext = 0 and V em = V threshold, progressive decrease in the emission potential value V em, so as to obtain V em = 0.

[0040] Advantageously, the method according to the invention may include an iteration of the steps of setting and adjusting the emission current, setting and adjusting the emission speed, and the stopping step, wherein at each new cycle of the iteration, the polarities of the emission potential V em and extraction potential are reversed compared to the previous cycle.

[0041] By regularly changing the polarity of ions, the process according to the invention makes it possible, in particular, to extend the lifespan of the propellant's conductive fluid by slowing down the depletion of chemical species and simultaneously the accumulation of counter-ions. This limits the mass and volume of conductive fluid required.

[0042] According to another aspect of the same invention, an ion propulsion system is proposed, comprising an ion propulsion unit including an emission electrode, an extraction electrode and a conductive liquid deposited on the emission electrode, the system further comprising a current generator linked to the emission electrode and a voltage generator linked to the extraction electrode, the system including a controller linked to the current generator and the voltage generator, the controller being adapted to implement the process according to the invention.

[0043] The conductive liquid may include an ionic liquid or a metallic liquid, that is, a liquid made conductive or a liquid or molten metal.

[0044] Sources of liquid metal ions (“ Liquid metal ion sources » (in English, LMIS) may, for example, include gallium, indium, gold, and alkali metals or alloys.

[0045] Ionic liquids can include, for example, salts of bulky organic anions and cations. Cations can include, for example, salts of phosphate, ammonium, or sulfate. Anions can include, for example, chloride, bromide, tetrafluoroborate, hexafluorophosphate, etc.

[0046] Advantageously, the system according to the invention can be implemented in a satellite, in particular of the CubeSat type.

[0047] The system and method according to the invention are particularly suitable for controlling the orientation and position of small satellites. Description of the figures and methods of implementation

[0048] Other advantages and features will become apparent upon examination of the detailed description of examples, which are by no means exhaustive, and the accompanying drawings on which: there Figure 1 is a schematic representation of a non-limiting example of a propulsion system implemented in the present invention; the Figure 2 is a schematic representation of a non-limiting example of a method for controlling a propulsion system according to the present invention; the Figure 3 illustrates a characteristic curve of a propellant for a step of the process according to the present invention; the Figure 4 illustrates a characteristic curve of a propellant for another step of the process according to the present invention; the Figure 5illustrates a characteristic curve of a propellant for another step of the process according to the present invention; and the Figure 6 illustrates a characteristic curve of a propellant for another step of the process according to the present invention.

[0049] It is understood that the embodiments described below are by no means exhaustive. In particular, variants of the invention may be conceived comprising only a selection of the features described below, isolated from the other features described, if this selection of features is sufficient to confer a technical advantage or to differentiate the invention from the prior art. This selection includes at least one preferably functional feature without structural details, or with only a portion of the structural details if this portion alone is sufficient to confer a technical advantage or to differentiate the invention from the prior art.

[0050] In particular, all the variants and embodiments described can be combined with each other if there are no technical obstacles to this combination.

[0051] In the figures, elements common to several figures retain the same reference.

[0052] There Figure 1 This is a schematic representation of a non-limiting example of an ion propulsion system that can be implemented within the scope of the present invention. The system can, in particular, be used to implement the method of the invention.

[0053] System 1, represented on the Figure 1 , is arranged to produce an ion beam, adapted to provide ion propulsion to system 1.

[0054] System 1 includes an ion thruster 10, comprising an emission electrode 11 and an extraction electrode 12.

[0055] The emitting electrode 11 comprises a plurality of emitters 14, for example, in the form of points. The emitters 14 are coated with a conductive liquid. This conductive liquid can be, for example, an ionic liquid, a conductive liquid, or a liquid or molten metal. When an electric field is generated between the two electrodes 11 and 12, a very strong local electric field (on the order of 10⁹ V / m) is generated at the points, causing the conductive liquid to form a Taylor cone located at a plurality of points 14 from the emitting electrode 10. Ions are then emitted from the apex of each cone. The charged particles are subsequently accelerated to high speeds on the order of tens of kilometers per second by the applied electric field.

[0056] The extraction electrode 12 can consist, for example, of a metal plate arranged opposite the emission electrode 11 and having openings 16 to allow the flow of ions to pass through.

[0057] The ion beam generated by the thruster 10 provides it with thrust. The thrust depends on an emission current. I em and the potential applied to the emitter corresponding to an ion emission velocity.

[0058] System 1 is represented on the Figure 1 also includes a first high voltage power supply 20 to power the emission electrode 11 and a second high voltage power supply 22 to power the extraction electrode 12.

[0059] The first power supply 20 is a high-voltage generator operating in current source mode. This means that the generator 20 is designed to deliver a constant current corresponding to a setpoint value and to minimize variations in that current.

[0060] The second power supply 22 is a high-voltage generator operating in voltage source mode. This means that generator 22 is designed to deliver a stable output voltage and minimize variations in that voltage.

[0061] As illustrated on the Figure 1 , the first 20 and the second power supply 22 are independent of each other, both being directly connected to ground.

[0062] Finally, system 1 includes a controller 24 linked to the high voltage power supplies 20, 22 allowing control of the power supplies 20, 22, according to a method of controlling the thruster within the framework of the present invention.

[0063] There Figure 2 is a schematic representation of a non-limiting example embodiment of a method for controlling an ionic propellant according to the invention.

[0064] Process 100, shown on the Figure 2, includes a step 102 of adjusting the emission current to a setpoint value I c. The emission current corresponds to the ion fluxes emitted by the emitters 14. The setpoint value can be, for example, 50 µA.

[0065] The transmission current is adjusted by applying a threshold transmission voltage. V The threshold at the emitting electrode 11 is set by means of the current generator 20. Thanks to the current generator 20, the emitted ion current can be precisely regulated. The setpoint current value I c is kept constant. To achieve this, system 1 according to the invention may include a device for measuring the emission current in order to control this current value. The measuring device may include, for example, a microammeter placed in the current generator 20, or an ammeter placed at the extraction electrode 12.

[0066] The initial conditions for performing step 102 of adjusting the emission current are as follows: voltage applied to the emission electrode V em = 0 and voltage applied to the extraction electrode V ext = 0. The voltage V em applied to the emission electrode 11 is then gradually increased, for example in steps of 500 V. V em is increased until the current emitted by the current generator 20 reaches its setpoint value I c. To maintain the setpoint value I c constant, the value of V em is adjusted automatically thanks to the control of the emission current.

[0067] The value of V em corresponds to the threshold value V threshold of the potential for which an ion emission is obtained, this value being characteristic of the ion propellant 10. This value can be, for example, 5000 V.

[0068] At the end of step 102 of emission current adjustment, the thruster 10 emits an ion flux corresponding to a certain mass of ejected material, but whose emission velocity is not yet nominal, or optimal, with respect to a thrust value T requested.

[0069] There Figure 3 illustrates a characteristic curve of the emission current I em as a function of the beam potential V applied between the electrodes 11, 12 of the thruster 10. The setpoint value I c current is reached for the voltage V threshold applied to the emission electrode, and no voltage applied to the extraction electrode, which defines an operating point of 200 for the propellant.

[0070] During step 104 of adjusting the emission speed, of process 100 according to the embodiment of the Figure 2 , an extraction tension Vext is applied to the extraction electrode 12 by means of the voltage generator 22. Thus, the emission potential between the electrodes 11, 12 is brought to a programmed value V borrow, so that V loan = V threshold + V ext.

[0071] Programmed potential V empr corresponds to a predetermined beam potential for which the thrust reaches a required value. The empr value can be predetermined, for example, by calculation by the onboard controller 24 or by a remote control center and sent to the controller 24.

[0072] The initial conditions for performing step 104 of adjusting the emission speed are as follows: voltage applied to the emission electrode V em = V threshold and voltage applied to the extraction electrode V ext = 0. According to an example implementation, the voltage VThe voltage applied to the extraction electrode 12 is then gradually increased, for example, in 50 V increments. The sign of the voltage V ext is the same as that of the voltage V threshold. The increase in the voltage value V ext to the extraction electrode is governed by a time-varying, asymptotic type law, which helps to avoid discharge phenomena or electrical breakdowns. V ext is increased until the emission potential reaches the programmed value V borrowed

[0073] There Figure 4 illustrates the shift in the characteristic curve I em (V) for step 104 of setting the emission speed. In this example, the value of V is increased by V threshold at V borrowed by the application of the extraction voltage V ext to the extraction electrode, and the emission current value remains the setpoint valueI c.

[0074] According to other examples of implementation, the tension V ext can be 0, or even have the opposite sign to that of the emission voltage V em.

[0075] At the end of step 104 of the transmission speed adjustment, the transmission current value still corresponds to the setpoint value. I c.

[0076] The process 100 according to the embodiment shown on the Figure 2 It also includes a step 106 for adjusting the emission current. This step 106 allows the thrust of the propellant 10 to be regulated and controlled by acting on the ion flow and therefore the ejected mass.

[0077] During this step 106, the emission current can be reduced or increased by changing the setpoint value of this current sent to the current generator 20. The effect of this step is illustrated on the Figure 5 A slight variation in voltage Vapplied em to the emission electrode can significantly modify the emission current, allowing for very fine adjustment of the ion flux.

[0078] There Figure 5 illustrates the displacement of the operating point 200 on the characteristic curve I em (V) for step 106 of adjusting the transmission current when the transmission voltage V em is varied.

[0079] The process 100 according to the embodiment shown on the Figure 2 It also includes a step 108 for adjusting the emission velocity. This step 108 allows for fine-tuning and maintaining the emission velocity to obtain the required thrust of the propellant. Indeed, during step 106 for adjusting the emission current, the emission potential may no longer correspond to the emission potential. V predetermined loan.

[0080] The adjustment during step 108 can be achieved by varying the extraction voltage.V ext, as illustrated on the Figure 6 The emission potential is given by the sum of the voltages applied to the electrodes. V em = V threshold + V ext. The threshold emission potential V The threshold is a physical characteristic determined by the geometry of the emitter, which can be altered by various aging mechanisms affecting, for example, the morphology of the emitter or the physicochemical characteristics of the conductive liquid used as a propellant. It is then possible to bring the emission potential back to the desired value Vempr by varying the extraction voltage. V ext.

[0081] This step 108 of adjusting the emission rate also makes it possible to compensate for a depletion of the species of ions of interest present in the conductive liquid, requiring more energy to be supplied to extract this rarefied species and leading to an increase in the threshold value of the emission potential.

[0082] There Figure 6 illustrates the shift in the characteristic curve I em (V) for step 108 of adjusting the emission speed, carried out by varying the value of the extraction voltage V ext.

[0083] Thanks to the adjustable amplitudes of the emission and extraction voltages, it is possible to keep the propulsion system in operation for long periods, thanks to the compensation of the depletion of the emitted ionic species.

[0084] According to an advantageous embodiment of the invention, the process 100 includes a propellant shutdown phase 110. During the shutdown phase 110, the supply to the electrodes and therefore the emission of the ion beam are stopped.

[0085] According to an implementation example, to perform this shutdown phase 110, the extraction voltage value V The voltage applied to the extraction electrode 12 is gradually decreased to 0, so as to return to an emission potential V em = V threshold. When V ext = 0, the value of the transmission voltage V em applied to the emission electrode 11 is gradually decreased to 0.

[0086] Other types of shutdown phase procedures can of course be applied.

[0087] The implementation of the shutdown phase 110 allows, in particular, the propulsion unit to be restarted with reversed polarity, enabling the use of ions with the opposite polarity to those used in the previous propulsion unit operating cycle. This significantly slows the depletion of a species in the conductive fluid. Restarting the propulsion unit with reverse polarity after a shutdown is indicated by reference 120 on the... Figure 2 . Re-marketing can be achieved by reversing the polarities of the emission and extraction voltages compared to those used during the previous cycle.

[0088] For prolonged operation of the thruster, it is then possible to perform an iteration of the steps of setting and adjusting the emission current, setting and adjusting the emission speed, and the shutdown step, where at each new cycle of the iteration, the polarities of the emission potential V em and extraction potentialV ext are reversed compared to the previous cycle.

[0089] Of course, the invention is not limited to the examples just described and many modifications can be made to these examples without departing from the scope of the invention.

Claims

1. Method (100) for controlling an ion thruster (10), the ion thruster comprising an emission electrode (11), an extraction electrode (12) and a conductive liquid deposited on the emission electrode (11), the ion thruster (10) being suitable for emitting an ion beam when an electric field is applied to the conductive liquid, the ion beam being suitable for providing the thruster (10) with thrust, the thrust depending on an emission current Iem and an ion emission speed, characterized in that the method (100) comprises the following steps: - adjusting (102) the emission current to a setpoint Ic by applying a threshold emission potential Vthresh to the emission electrode by means of a current generator; and - when the setpoint Ic of the emission current is reached, adjusting (104) the emission speed by applying an extraction potential Vext to the extraction electrode by means of a voltage generator, so as to bring the emission potential Vem to a predetermined value Vempr = Vthresh + Vext.

2. Method (100) according to claim 1, characterized in that the step (102) of adjusting the emission current is implemented by gradually increasing the value of the potential applied to the emission electrode from 0 to the threshold value Vthresh for which the emission current Iem reaches the setpoint Ic.

3. Method (100) according to claim 1 or 2, characterized in that the step (104) of adjusting the emission speed is implemented by one of the following steps: - applying a potential of 0 V, - applying a potential having the same sign as the emission potential or - applying a potential having the opposite sign relative to that of the emission potential.

4. Method (100) according to any one of the preceding claims, characterized in that the step (102) of adjusting the emission current may comprise a step of automatically controlling the setpoint Ic of the emission current.

5. Method (100) according to any one of the preceding claims, characterized in that the step (102) of adjusting the emission current further comprises a step (106) of setting the emission current.

6. Method (100) according to claim 5, characterized in that setting the emission current is implemented by setting the potential applied to the emission electrode Vem.

7. Method (100) according to any one of the preceding claims, characterized in that the step (104) of adjusting the emission speed further comprises a step (108) of setting the emission speed.

8. Method (100) according to claim 7, characterized in that the emission speed is set by adjusting the extraction potential Vext.

9. Method (100) according to any one of the preceding claims, characterized in that it further comprises a step (110) of stopping the ion thruster (10), the stopping step comprising the following steps. - gradually reducing the value of the extraction potential Vext so as to obtain Vext = 0 and Vem = Vthresh, - gradually reducing the value of the emission potential Vem so as to obtain Vem = 0.

10. Method (100) according to the preceding claim in combination with claims 5 and 7, characterized in that it comprises an iteration of the steps of adjusting (102) and setting (106) the emission current, adjusting (104) and setting (108) the emission speed and the stopping step (110), the polarities of the emission potential Vem and of the extraction potential being inverted in each new repetition cycle compared with the previous cycle.

11. Ion thruster system (1) comprising an ion thruster (10) which comprises an emission electrode (11), an extraction electrode (12) and a conductive liquid deposited on the emission electrode (11), the system (1) further comprising a current generator (20) connected to the emission electrode (11), and a voltage generator (22) connected to the extraction electrode (12), the system (1) comprising a controller (24) connected to the current generator (20) and to the voltage generator (22), the controller being suitable for carrying out the method (100) according to any one of the preceding claims.

12. System (1) according to claim 11, characterized in that the conductive liquid comprises one from an ionic liquid, a liquid made to be conductive, and a liquid or molten metal.

13. Satellite, particularly of the CubeSat type, comprising an ion thruster system (1) according to one of claims 11 or 12.