METHOD FOR CONTROLLING AN ION THRUSTER, AND ION THRUSTER SYSTEM

Abstract

A method for controlling an ion thruster including an emission electrode, an extraction electrode and a conductive liquid which is deposited on the emission electrode, the ion thruster configured for emitting an ion beam when an electric field is applied to the conductive liquid, the ion beam providing thrust to the thruster, the thrust depending on an emission current I.sub.em and an ion emission speed, the method including the following steps: adjusting the emission current to a setpoint value I.sub.c by applying a threshold emission potential V.sub.thresh to the emission electrode by means of a current generator; and when the setpoint value I.sub.c of the emission current is reached, adjusting the emission speed by applying an extraction potential V.sub.ext to the extraction electrode by means of a voltage generator in order to bring the emission potential V.sub.em to a predetermined value V.sub.empr=V.sub.thresh+V.sub.ext.

Claims

1. A method for controlling an ion thruster, the ion thruster comprising an emission electrode, an extraction electrode and a conductive liquid deposited on the emission electrode, the ion thruster 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 with thrust, the thrust depending on an emission current I.sub.em and an ion emission speed, the method includes the following steps: adjusting the emission current to a setpoint I.sub.c by applying a threshold emission potential V.sub.thresh to the emission electrode by means of a current generator; and when the setpoint I.sub.c of the emission current is reached, adjusting the emission speed by applying an extraction potential V.sub.ext to the extraction electrode by means of a voltage generator, so as to bring the emission potential V.sub.em to a predetermined value V.sub.empr=V.sub.thresh+V.sub.ext.

2. The method according to claim 1, characterized in that the step 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 V.sub.thresh for which the emission current I.sub.em reaches the setpoint I.sub.c.

3. The method according to claim characterized in that the step 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. The method according to claim 1, characterized in that the step of adjusting the emission current may comprise a step of automatically controlling the setpoint I.sub.c of the emission current.

5. The method according to claim 1, characterized in that the step of adjusting the emission current further comprises a step of setting the emission current.

6. The method according to claim 5, characterized in that setting the emission current is implemented by setting the potential applied to the emission electrode V.sub.em.

7. The method according to claim 1, characterized in that the step of adjusting the emission speed further comprises a step of setting the emission speed.

8. The method according to claim 7, characterized in that the emission speed is set by adjusting the extraction potential V.sub.ext.

9. The method according to claim 1, characterized in that it further comprises a step of stopping the ion thruster, the stopping step comprising the following steps: gradually reducing the value of the extraction potential V.sub.ext so as to obtain V.sub.ext=0 and V.sub.em=V.sub.thresh; and gradually reducing the value of the emission potential V.sub.em so as to obtain V.sub.em=0.

10. The method according to claim 9, characterized in that it comprises an iteration of the steps of adjusting and setting the emission current including setting the emission current, adjusting and setting the emission speed including setting the emission speed, and the stopping step, the polarities of the emission potential V.sub.em and of the extraction potential being inverted in each new repetition cycle compared with the previous cycle.

11. An ion thruster system comprising: an ion thruster which includes an emission electrode; an extraction electrode and a conductive liquid deposited on the emission electrode, the system further comprising a current generator connected to the emission electrode; and a voltage generator connected to the extraction electrode, the system being suitable for carrying out the method according to claim 1.

12. The system 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. A satellite, particularly of the CubeSat type, comprising the ion thruster system according to claim 11.

Description

DESCRIPTION OF THE FIGURES AND EMBODIMENTS

[0056] Other advantages and characteristics will become apparent on examination of the detailed description of examples that are in no way limitative, and from the attached drawings, in which:

[0057] FIG. 1 is a diagrammatic representation of a non-limitative embodiment of a propulsion system implemented in the present invention;

[0058] FIG. 2 is a diagrammatic representation of a non-limitative example of a method for controlling a thruster according to the present invention;

[0059] FIG. 3 shows a characteristic curve of a thruster for a step of the method according to the present invention;

[0060] FIG. 4 shows a characteristic curve of a thruster for another step of the method according to the present invention;

[0061] FIG. 5 shows a characteristic curve of a thruster for another step of the method according to the present invention; and

[0062] FIG. 6 shows a characteristic curve of a thruster for another step of the method according to the present invention.

[0063] It is well understood that the embodiments which will be described hereinafter are in no way limitative. It is possible in particular to envisage variants of the invention comprising only a selection of the characteristics described hereinafter, in isolation from the other characteristics described, if this selection of characteristics is sufficient to confer a technical advantage or to differentiate the invention with respect to the state of the prior art. This selection comprises at least one, preferably functional, characteristic without structural details, or with only a part of the structural details if this part alone is sufficient to confer a technical advantage or to differentiate the invention with respect to the state of the prior art.

[0064] In particular, all the variants and all the embodiments described can be combined together if there is no objection to this combination from a technical point of view.

[0065] In the figures, the elements common to several figures retain the same reference.

[0066] FIG. 1 is a diagrammatic representation of a non-limitative embodiment of an ion thruster system that may be implemented within the framework of the present invention. The system may in particular be used to carry out the method of the invention.

[0067] The system 1 shown in FIG. 1 is arranged so as to produce an ion beam suitable for providing the system 1 with ion thrust.

[0068] The system 1 comprises an ion thruster 10, comprising an emission electrode 11 and an extraction electrode 12.

[0069] The emission electrode 11 comprises a plurality of emitters 14, for example in the form of tips. The emitters 14 are covered by a conductive liquid. This conductive liquid may be, for example, an ionic liquid, a liquid made to be conductive, or a liquid or molten metal. When an electric field is generated between the two electrodes 11, 12, a very strong local electric field (of the order of 10.sup.9 V/m) is generated at the tips, which causes the conductive liquid to form a Taylor cone located at a plurality of tips 14 of the emission electrode 10. Ions are then emitted at the apex of each cone. The charged particles are then accelerated at high speeds of the order of several tens of kilometres per second by the applied electric field.

[0070] By way of example, the extraction electrode 12 may consist of a metal plate that is arranged opposite the emission electrode 11 and has openings 16 for allowing the flux of ions to pass through.

[0071] The ion beam generated by the thruster 10 provides a thrust thereto. The thrust depends on an emission current I.sub.em and on the potential applied to the emitter, corresponding to an ion emission speed.

[0072] The system 1 shown in FIG. 1 likewise comprises a first high-voltage power supply 20 for powering the emission electrode 11, and a second high-voltage power supply 22 for powering the extraction electrode 12.

[0073] The first power supply 20 is a high-voltage generator operating in current-source mode. This means that the generator 20 is suitable for delivering a constant current corresponding to a setpoint, and of minimizing the variations thereof.

[0074] The second power supply 22 is a high-voltage generator operating in voltage-source mode. This means that the generator 22 is suitable for delivering a stable output voltage and of minimizing the variations thereof.

[0075] As shown in FIG. 1, the first power supply 20 and the second power supply 22 are independent of one another, each of the two being directly earthed.

[0076] Lastly, the system 1 comprises a controller 24 connected to the high-voltage power supplies 20, 22, for controlling the power supplies 20, 22, for example in accordance with a method for controlling the thruster within the framework of the present invention.

[0077] FIG. 2 is a diagrammatic representation of a non-limitative embodiment of a method for controlling an ion thruster according to the invention.

[0078] The method 100 shown in FIG. 2 comprises a step 102 of adjusting the emission current to a setpoint I. The emission current corresponds to the fluxes of ions emitted by the emitters 14. The setpoint may be, for example, 50 μA.

[0079] The emission current is adjusted by applying a threshold emission voltage V.sub.thresh to the emission electrode 11 by means of the current generator 20. Owing to the current generator 20, the current of ions emitted can be precisely controlled. The value of the setpoint current I.sub.c is kept constant. To do so, the system 1 according to the invention may comprise a device for measuring the emission current in order to automatically control said current value. The measurement device may comprise, for example, a microammeter placed in the current generator 20, or an ammeter placed at the extraction electrode 12.

[0080] The initial conditions for carrying out the step 102 of adjusting the emission current are the following: voltage applied to the emission electrode V.sub.em=0 and voltage applied to the extraction electrode V.sub.ext=0. The voltage V.sub.em applied to the emission electrode 11 is then increased gradually, for example in 500 V steps. V.sub.em is increased until the current emitted by the current generator 20 reaches its setpoint I.sub.c. To keep the setpoint I.sub.c constant, the value of V.sub.em is set automatically owing to the automatic control of the emission current.

[0081] The value of V.sub.em corresponds to the threshold value V.sub.thresh of the potential for which emission of ions is achieved, this value being characteristic of the ion thruster 10. This value may be, for example, 5,000 V.

[0082] At the end of the step 102 of adjusting the emission current, the thruster 10 emits a flux of ions that corresponds to a certain mass of ejected matter but the emission speed of which is not yet nominal, or optimal, in relation to a requested thrust T value.

[0083] FIG. 3 shows a characteristic curve of the emission current I.sub.em plotted against the beam potential V applied between the electrodes 11, 12 of the thruster 10. The setpoint I.sub.c of the current is reached for the voltage V.sub.thresh applied to the emission electrode, with no voltage applied to the extraction electrode, and this defines a working point 200 of the thruster.

[0084] During a step 104 of controlling the emission speed in the method 100 according to the embodiment of FIG. 2, an extraction voltage V.sub.ext is applied to the extraction electrode 12 by means of the voltage generator 22. The emission potential between the electrodes 11, 12 is thus brought to a programmed value V.sub.empr such that V.sub.empr=V.sub.thresh+V.sub.ext.

[0085] The programmed potential V.sub.empr corresponds to a predetermined beam potential for which the thrust reaches a requested value. The value V.sub.empr may be predetermined, for example, by calculations by the on-board controller 24, or by a remote control centre, and sent to the controller 24.

[0086] The initial conditions for carrying out the step 104 of adjusting the emission speed are the following: voltage applied to the emission electrode V.sub.em=V.sub.thresh and voltage applied to the extraction electrode V.sub.ext=0. According to an embodiment, the voltage V.sub.ext applied to the extraction electrode 12 is then increased gradually, for example in 50 V steps. The sign of the voltage V.sub.ext is the same as that of the voltage V.sub.thresh. The increase of the value of the voltage V.sub.ext at the extraction electrode is governed by an asymptotic-type law that is variable over time, thus making it possible to avoid electrical discharge or breakdown phenomena. V.sub.ext is increased until the emission potential reaches the programmed value V.sub.empr.

[0087] FIG. 4 shows the shift of the characteristic curve I.sub.em(V) for the step 104 of controlling the emission speed. In this example, the value of V is increased from V.sub.thresh to V.sub.empr by applying the extraction voltage V.sub.ext to the extraction electrode, and the value of the emission current remains at the setpoint I.sub.c.

[0088] According to other embodiments, the voltage V.sub.ext may be 0, or even have the opposite sign to that of the emission voltage V.sub.em.

[0089] At the end of the step 104 of adjusting the emission speed, the value of the emission current still corresponds to the setpoint I.sub.c.

[0090] The method 100 according to the embodiment shown in FIG. 2 further comprises a step 106 of setting the emission current. This step 106 makes it possible to adjust and control the thrust of the thruster 10 by acting on the flux of ions and thus on the ejected mass.

[0091] During this step 106, the emission current may be reduced or increased by changing the setpoint of said current sent to the current generator 20. The effect of this step is shown in FIG. 5. A small variation in the voltage V.sub.em applied to the emission electrode may significantly change the emission current, thus allowing the flux of ions to be set very finely.

[0092] FIG. 5 shows the shift of the working point 200 on the characteristic curve I.sub.em(V) for the step 106 of setting the emission current while the emission voltage V.sub.em is varied.

[0093] The method 100 according to the embodiment shown in FIG. 2 further comprises a step 108 of setting the emission speed. By means of this step 108, the emission speed may be finely adjusted and maintained in order to obtain the thrust requested from the thruster. Indeed, during the step 106 of setting the emission current, the emission potential may no longer correspond to the predetermined emission potential V.sub.empr.

[0094] During the step 108, setting may be implemented by varying the extraction voltage V.sub.ext, as shown in FIG. 6. The emission potential is given by the sum of the voltages applied to the electrodes, V.sub.em=V.sub.thresh+V.sub.ext. The threshold emission potential V.sub.thresh is a physical characteristic given by the geometry of the emitter and may be distorted by various ageing mechanisms that affect, for example, the morphology of the emitter or even the physico-chemical characteristics of the conductive liquid used as the propellant. It is then possible to revert the emission potential to the desired value V.sub.empr by varying the extraction voltage V.sub.ext.

[0095] Due to this step 108 of setting the emission speed, therefore, it is likewise possible to compensate for depletion of the ionic species of interest present in the conductive liquid, making it necessary to provide more energy in order to extract this depleted species and leading to an increase in the threshold value of the emission potential.

[0096] FIG. 6 shows the shift of the characteristic curve I.sub.em(V) for the step 108 of setting the emission speed, which is implemented by varying the value of the extraction voltage V.sub.ext.

[0097] Owing to the setting amplitudes of the emission and extraction voltages, it is possible to maintain the thruster in operation for long periods because the depletion of the emitted ionic species is compensated for.

[0098] According to an advantageous embodiment of the invention, the method 100 comprises a phase 110 of stopping the thruster. During the stopping phase 110, the power supply to the electrodes, and thus the emission of the ion beam are stopped.

[0099] According to an embodiment example, to carry out this stopping phase 110, the value of the extraction voltage V.sub.ext applied to the extraction electrode 12 is reduced gradually to 0, so as to revert to an emission potential of V.sub.em=V.sub.thresh. When V.sub.ext=0, the value of the emission voltage V.sub.em applied to the emission electrode 11 is reduced gradually to 0.

[0100] Of course, other ways of carrying out the stopping phase may be applied.

[0101] By implementing the stopping phase 110, it is possible in particular to restart the thruster with the opposite polarity, as a result of which ions having the opposite polarity to those of the previous operating cycle of the thruster can be used. The depletion of a species in the conductive liquid is thus greatly slowed down. The restarting of the thruster with the opposite polarity after stopping is indicated by the reference 120 in FIG. 2. The thruster may be restarted by inverting the polarities of the emission and extraction voltages in relation to those used during the previous cycle.

[0102] To be able to use the thruster for a longer period, it is thus possible to perform an iteration of the steps of adjusting and setting the emission current, adjusting and setting the emission speed and the stopping step, the polarities of the emission potential V.sub.em and of the extraction potential V.sub.ext being inverted in each new repetition cycle compared with the previous cycle.

[0103] Of course, the invention is not limited to the examples which have just been described and numerous adjustments can be made to these examples without exceeding the scope of the invention.