FLIGHT TESTING OF A GRID ION ENGINE

Description

Domaine technique

[0001] This description relates to a method for flight testing a gridded ion thruster, and to a gridded ion thruster power supply assembly that is suitable for such a test. Technique antérieure

[0002] Gridded ion thrusters are used for spacecraft such as satellites or space probes, although their use is still less common than that of Hall-effect ion thrusters. These gridded ion thrusters can be of the continuous discharge type, designated by the acronym GIT for "Gridded-Ion Thruster" and also called the Kaufman type, or of the RIT type for "Radiofrequency Ion Thruster". These gridded ion thrusters, whether RIT or GIT, each include at least: a plasma enclosure; a screen grille, which is located in front of or at an outlet opening of the plasma enclosure; and an acceleration grille, which is parallel to the screen grille and located on one side of that screen grille opposite the plasma enclosure. During the operation of such a thruster, positive ions produced in the plasma chamber pass through the screen grid and then the acceleration grid. For this purpose, a power supply unit, commonly referred to as a PPU for "Power Processing Unit", is also on board the spacecraft and arranged to, during thruster operation, simultaneously provide at least one positive electrical voltage to the screen grid and one negative electrical voltage to the acceleration grid, these positive and negative electrical voltages being determined with respect to a common reference node of the power supply unit, commonly referred to as CRP for "Cathode Return Potential", or sometimes NRP for "Neutralizer Return Potential".

[0003] Understanding and progressively improving gridded ion thrusters requires knowledge of the wear and aging phenomena that gradually affect them during their use aboard spacecraft. To this end, measurements and tests must be conducted aboard spacecraft to characterize the wear and aging state of each gridded ion thruster throughout its mission. Indeed, tests performed on Earth are not sufficiently representative of the environmental conditions to which each thruster is subjected in space, and it is necessary to collect measurement results throughout its mission. Such tests and measurements have the following objectives: to complete the general knowledge of gridded ion thrusters, particularly concerning their aging in space environment conditions; to predict the future wear and / or aging of a gridded ion thruster that is on a mission aboard a space vehicle, in particular to assess the lifespan of this thruster; and to characterize the state of wear and / or aging of a gridded ion thruster that is on a mission aboard a space vehicle, in order to optimize the operating point at which this thruster is used, or to achieve a compromise between the rate of wear and the intensity of the thrust force that is to be produced by the thruster.

[0004] Erosion of the acceleration grid in a gridded ion thruster is one of the main causes of its aging, which limits its lifespan. This erosion produces a progressive enlargement of the holes in the acceleration grid and is caused by bombardment from ions originating in the plasma chamber, some of which strike the acceleration grid. For a fixed value of the negative electrical voltage applied to the acceleration grid, the erosion state of this grid is revealed, in particular, by a change in the backflow limit of electrons generated by a neutralizer in the thruster.

[0005] US patent 6,964,396 describes conducting an Electron Back-Streaming (EBS) test aboard a spacecraft in outer space. During such an EBS test, the negative voltage applied to the accelerator grid is gradually decreased in absolute value, meaning it is varied from very negative to less negative values. Simultaneously, a Beam Current Control (BCC) loop maintains a constant difference between the current supplied to the shield grid and the current from the accelerator grid by controlling the plasma-generating power delivered to the propellant's plasma chamber.This plasma generation power is then measured for each value of the negative electrical voltage applied to the accelerating grid. Such an EBS test makes it possible to determine a value to use for the negative electrical voltage applied to the accelerating grid, the absolute value of which is minimal to maximize the thrust produced at a given value of electrical power supplied to the screen grid, while reducing heating of the plasma chamber and ensuring the safe operation of the thruster.

[0006] The article entitled "In-orbit demonstration of an iodine electric propulsion system" by Rafalskyi Dmytro et al., Nature, Nature Publishing Group UK, London, vol. 599, no. 7885, 17 November 2021, pp. 411-415, XP037627779, ISSN: 0028-0836, DOI: 10.1038 / S41586-021-04015-Y, reports results obtained in flight with a propulsion system that uses iodine. Problème technique

[0007] From this situation, one aim of the present invention is to propose another test to be carried out while the space vehicle is in outer space, to characterize the state of a gridded ion thruster of this space vehicle, in particular the erosion state of the acceleration grid of this thruster.

[0008] Another objective of the invention is to provide new information for managing the operation of a gridded ion thruster that is carried on board a space vehicle, in particular such information which is related to the operation and lifetime of the thruster. Résumé de l'invention

[0009] To achieve at least one of these goals, or another, a first aspect of the invention proposes a new method for testing a gridded ion thruster on board a spacecraft, such as a satellite, a space probe, etc., this test method being carried out while the spacecraft is in outer space. The thruster comprises at least the components mentioned previously: the plasma chamber, the screen grid, and the acceleration grid, the latter being on one side of the screen grid opposite the plasma chamber so that, during thruster operation, positive ions produced in the plasma chamber pass through the screen grid and then the acceleration grid.In particular, the thruster to which the test method of the invention is applied may be of one of the following types: a continuous discharge gridded ion thruster, known as a GIT or Kaufman type, or a radio frequency discharge gridded ion thruster, known as a RIT. Possibly, the thruster to which the test method of the invention is applied may include at least one additional grid, in addition to the screening grid and the acceleration grid. Such a third grid is commonly called a deceleration grid.

[0010] An electrical power supply is also on board the spacecraft and arranged to, during thruster operation and particularly during an execution of the test method of the invention, simultaneously provide at least one positive electrical voltage to the screen grid and one negative electrical voltage to the acceleration grid, these electrical voltages being determined with respect to the common reference node of the electrical power supply. The method includes a thruster thrust stage, itself comprising adjusting the electrical power supply to a predetermined thrust operating point with predetermined values ​​of the positive electrical voltage (VPHV) and the negative electrical voltage (VNHV) and with a predetermined electrical supply power.

[0011] The process of the invention then comprises the following steps: / 1 / fix the value of the negative electrical voltage corresponding to the predetermined thrust operating point of the thruster; / 2 / provide two sweep limit values ​​for the positive electrical voltage, which are respectively higher and lower than the value of this positive electrical voltage at the predetermined thrust operating point, or provide a sweep start limit value for the positive electrical voltage which is higher than the value of this positive electrical voltage at the predetermined thrust operating point, and provide a maximum limit for an electrical current which flows from the acceleration grid to the power supply assembly, called the acceleration grid current;and / 3 / vary the positive electrical voltage in a direction of variation that is constant between the sweep limit values ​​provided in step / 2 / , verifying that the accelerating grid current remains below the maximum limit provided, or decrease the positive electrical voltage from the start-of-sweep limit value until the accelerating grid current reaches the maximum limit provided, and for each produced value of the positive electrical voltage, measure the accelerating grid current and record at least one measurement result of said accelerating grid current with the corresponding value of the positive electrical voltage. ;

[0012] The maximum limit provided in step / 2 / for the accelerating grid current can depend on the beam current exiting the thruster. In particular, it can be equal to the intensity of this beam current multiplied by a constant coefficient. Generally, in this description, the beam current exiting the thruster is equal to the difference between an electric current flowing from the power supply assembly to the screen grid and the accelerating grid current, from which can be further subtracted a backflow current of electrons emitted by a neutralizer of the thruster and then collected by the screen grid. For most implementations of the invention, the maximum limit provided in step / 2 / for the accelerating grid current can be between 0.5% and 5% of the beam current value.For example, it can be approximately equal to 3.125% of the beam current value.

[0013] The test method proposed by the invention is a perveance test. It allows the identification of the positive voltage applied to the screen grid at which the accelerating grid current is minimal, that is, at which a minimal quantity of ions from the plasma chamber strikes the accelerating grid. This positive voltage value, which minimizes the accelerating grid current, corresponds to a thruster operating point that reduces erosion of its accelerating grid. The thruster's lifetime can then be increased by adopting this thrust operating point, or its lifetime can be managed if other mission constraints require the use of a thruster operating point that does not correspond to the minimum accelerating grid current.

[0014] The range of variation of the positive electrical voltage which is implemented in step / 3 / , being limited on the side of the lowest values ​​of this positive electrical voltage either by one of the sweep limit values ​​provided in step / 2 / , or because the maximum limit for the accelerating grid current is reached, depends on the amount of information which is desired on the thruster.

[0015] One advantage of the method of the invention is that it allows for regular measurements to be taken during thruster operation, to detect the erosion status of its grids, and to determine whether its predetermined thrust operating point needs to be corrected. The thrust force produced by the thruster is not interrupted by the execution of the test method of the invention.

[0016] Another advantage of the method of the invention is that it allows the operation of the thruster to be adjusted according to its state of aging, without damaging the thruster, in a way that is particularly safe and robust so that it can be carried out automatically and in a space environment.

[0017] Yet another advantage of the invention's process is that it allows the space vehicle to be tested in flight, and multiple operating parameters of the propulsion system to be transmitted remotely in order to understand its behavior under operational conditions.

[0018] The present invention thus improves the understanding of the evolution of the characteristics, particularly electrical, thermal, and material-related, of a gridded ion thruster used on board a spacecraft throughout the spacecraft's lifetime. It provides measurements of certain thruster characteristics during the spacecraft mission.

[0019] Finally, another advantage of the invention lies in the superior accuracy of the tests and measurements performed while the spacecraft is in outer space, compared to tests and measurements performed on Earth. Indeed, many characteristics of the thruster have very different values ​​on Earth compared to their actual values ​​in outer space, notably due to the lack of representativeness of environmental parameters, e.g., pressure, narrowness of vacuum chambers, etc., which are used on Earth to simulate the vacuum of space. Thus, the testing method of the invention provides a more accurate understanding of the behavior of gridded ion thrusters in outer space throughout their operational life. This knowledge allows, in particular, for better adjustment of the electrical parameters of a gridded ion thruster, i.e., electrical currents and voltages, and also fluidic parameters, i.e.flow rates and pressures of the gases used in this propulsion system. Such an improved adjustment in turn allows for optimized use of the propulsion system's grids, notably by minimizing their erosion.

[0020] Advantageously, the positive electrical voltage can be varied at stage / 3 / automatically by the power supply assembly, according to a programming of this power supply assembly.

[0021] Possibly, and in order for the results of the test method of the invention to be studied and / or used on Earth, the method may further include transmitting from the spacecraft to a station located on Earth at least some of the measurement results and values ​​recorded in step / 3 / . Such transmission may be included in a telemetry procedure used for the spacecraft.

[0022] In certain implementations of the invention, particularly those for the purpose of reducing acceleration grid erosion, the method may further include the following step: / 4 / determining a minimum value of the acceleration grid current from among the measurement results recorded in step / 3 / for this acceleration grid current, as well as the respective corresponding values ​​of the positive electrical voltage and the negative electrical voltage.

[0023] When step / 4 / is executed, the process may also include, in addition: update the thruster operating point according to the respective values ​​of the positive electrical voltage and the negative electrical voltage that correspond to the minimum value of the accelerating grid current, as determined in step / 4 / , possibly with a predetermined margin of error between the value of the positive electrical voltage that was determined for the minimum value of the accelerating grid current and the value of the positive electrical voltage of the updated thrust operating point; then activate a new thruster operation that conforms to the updated thrust operating point. Such an update and activation of the thrust operating point of the thruster can be remotely commanded from the station which is located on Earth, or executed automatically on board the spacecraft.

[0024] Also, when step / 4 / is executed, and when the positive electrical voltage has been decreased in step / 3 / from the start-scan limit value until the accelerating grid current reaches the supplied maximum limit, the method may further include the following steps: / 5 / determine a threshold value for the positive electrical voltage that corresponds to a bend in a curve of measured values ​​of the accelerating grid current as a function of the values ​​of the positive electrical voltage, on the side of the lower values ​​of this positive electrical voltage; and / 6 / calculate a difference between the value of the positive electrical voltage that corresponds to the minimum value of the accelerating grid current, as determined in step / 4 / , and the threshold value determined in step / 5 / for the positive electrical voltage. The difference calculated in step / 6 / is a measure of the acceleration grid erosion state and thus constitutes a measure of the thruster's aging. The sequence of steps / 1 / to / 6 / can be repeated several times, each time at a different time while the spacecraft is in outer space. A new value is then determined for the difference between the positive voltage corresponding to the minimum acceleration grid current and the threshold value of this positive voltage, by extrapolating the difference values ​​calculated at each execution of the sequence of steps / 1 / to / 6 / . Indeed, these difference values ​​calculated at each execution of the sequence of steps / 1 / to / 6 / characterize an acceleration grid erosion profile as a function of the thruster's operating time. The time profile can be extended by extrapolation to provide a forecast of the difference value that is relative to a future date.

[0025] The power supply assembly may include a control device for the plasma-generating electrical power delivered to the plasma chamber by this power supply assembly. This control device is designed to maintain a constant difference between the electrical current flowing from the power supply assembly to the screen grid and the accelerating grid current. Such a control device corresponds to the beam current control loop, or BCC. Therefore, in early possible implementations of the test method of the invention, step / 3 / can be performed while the difference between the electrical current flowing from the power supply assembly to the screen grid and the accelerating grid current is maintained constant by the control device.This difference value is equal to the beam current in the absence of additional backflow current.

[0026] Alternatively, in other possible implementations of the test method of the invention, step / 3 / can be performed while the plasma generation power supplied to the plasma chamber by the power supply assembly is kept constant. For this purpose, and if present, the plasma generation power control device is kept deactivated during step / 3 / . The value of the difference between the electrical current flowing from the power supply assembly to the screen grid and the accelerating grid current is then no longer constant, due to the profile adopted by the accelerating grid current under the effect of the variation in the positive electrical voltage supplied to the screen grid.

[0027] A second aspect of the invention proposes a power supply assembly for a gridded ion thruster, the thruster comprising: a plasma enclosure, a screen grid located in front of or at an outlet opening of the plasma enclosure, and an acceleration grid parallel to the screen grid and located on a side thereof opposite the plasma enclosure, the power supply assembly comprising: the common reference node; a positive power supply unit, which is intended to be connected to the thruster's screen grid to provide this screen grid, during thruster operation, with a positive electrical voltage relative to the common reference node; a negative power supply unit, which is intended to be connected to the thruster's acceleration grid to provide this acceleration grid, during thruster operation, with a negative electrical voltage relative to the common reference node; a variation module, which is arranged to vary, preferably automatically, the positive electrical voltage supplied by the positive power supply unit; measurement and recording modules, arranged to measure and record values ​​of the acceleration grid current;and a controller, which is configured to activate the variation module and the measurement and recording modules, so that variable values ​​are produced successively for the positive electrical voltage while the negative electrical voltage is kept constant, and that a value of the acceleration grid current is measured and recorded for at least one supplied value of the positive electrical voltage, when the power supply assembly is connected to the thruster to enable operation of that thruster. Such a power supply assembly is suitable for carrying out a test procedure which conforms to the first aspect of the invention.

[0028] Each of the variation, measurement and recording modules of the power supply assembly can be of the software or recorded program type, designated as "software" in English, or of the hardware type, designated as "hardware".

[0029] The power supply assembly may also include: a device for controlling the electrical power of plasma generation which is delivered to the plasma chamber, designed to maintain the beam current constant, and as designated by beam current control loop, or BCC.

[0030] A third aspect of the invention proposes a plasma propulsion system which includes: the gridded ion thruster; and a power supply assembly which conforms to the second aspect of the invention, and which is connected to the thruster to produce an operation of the latter.

[0031] Finally, a fourth aspect of the invention proposes a space vehicle which includes a plasma propulsion system conforming to the third aspect. Brève description des figures

[0032] The features and advantages of the present invention will become clearer in the following detailed description of non-limiting embodiments, with reference to the accompanying figures, among which: [ Fig. 1a ] shows, schematically and in a simplified manner, a space vehicle equipped with a GIT-type gridded ion thruster, and on board which the test method of the invention can be used; [ Fig. 1b ] corresponds to [ Fig. 1a ] for a RIT-type gridded ion thruster; [ Fig. 2a ] is an example of a diagram presenting the results obtained by the testing method of the invention, according to a first possible sequence of the method; [ Fig. 2b ] corresponds to [ Fig. 2a ] for a second possible sequence of events in the process; [ Fig. 3a ] illustrates a situation of over-perseverance; [ Fig. 3b ] illustrates an optimal situation of perveance; and [ Fig. 3c ] illustrates a situation of under-perveance. Description détaillée de l'invention

[0033] In these figures, all elements are represented only symbolically, and identical references that are indicated in different figures denote identical elements or elements that have identical functions.

[0034] In [ Fig. 1a ] And [ Fig. 1b [ ], the reference 100 designates a space vehicle, regardless of its type, for example, a satellite or a space probe. Within this space vehicle 100, the reference 10 designates a plasma propulsion system, commonly referred to by the acronym PPS for "Plasma Propulsion Subsystem". This plasma propulsion system 10, which is sometimes referred to as a subsystem in relation to the space vehicle 100, itself comprises at least one power processing unit 1, commonly referred to by the acronym PPU for "Power Processing Unit", and at least one gridded ion thruster 2.The power supply assembly 1 and the gridded ion thruster 2, shown in each of these two figures, are linked together so that the thruster 2 is appropriately supplied with electrical energy by the power supply assembly 1, in order to produce the desired thrust force at each instant of the plasma thruster's operation, while the spacecraft 100 is in outer space. Typically, the power supply assembly 1 is connected between an electrical power bus and an electrical ground (not shown) of the spacecraft 100, while the various components of the thruster 2 are supplied with electrical currents and voltages via the power supply assembly 1. During the operation of each of the gridded ion thrusters of [. Fig. 1a ] And [ Fig. 1b ], the thrust force which is produced by this thruster and applied to the space vehicle 100 results from the production, by the gridded ion thruster, of an ion beam which is denoted ions in the figures.

[0035] For the sake of clarity in this description, the components of the plasma propulsion system 10 that are shown in [ Fig. 1a ] And [ Fig. 1b ] are limited to those relevant to the invention. In particular, each of the gridded ion thrusters 2 of [ Fig. 1a ] And [ Fig. 1b ] includes one or more neutralizer power supply units, as well as a gas management system, which are not shown, and power supply assembly 1 includes additional power supply units and corresponding interfaces, which are also not shown.

[0036] For the space vehicle 100 of [ Fig. 1a The gridded ion thruster 2 is of the continuous discharge type designated by GIT for "Gridded Ion Thruster," or Kaufman type. It comprises a plasma chamber 20, which is equipped with an anode 21 and a cathode 22. The anode 21 can be located around an outlet opening of the plasma chamber 20, through which the ions are intended to exit during the operation of the thruster 2. The cathode 22 can be located at the bottom of the plasma chamber 20, opposite its outlet opening. As is known, the plasma enclosure 20 is equipped with an electromagnet 23, commonly called a "magnet", which helps to confine the plasma inside the enclosure 20. To produce the plasma in the enclosure 20, and thus constitute a source of the ions which are ejected outside the vehicle 100, a continuous electrical discharge is created in the enclosure 20 between the anode 21 and the cathode 22, at the same time as an ionizable gas is introduced.

[0037] The gridded ion thruster 2 further comprises at least two electrically conductive grids arranged parallel to each other and spaced apart in front of the outlet opening of the plasma chamber 20. The first grid, called the screening grid and designated by reference numeral 24, has the primary functions of controlling the quantity of ions exiting the plasma chamber 20 and contributing to accelerating the ions exiting the plasma chamber 20. For this purpose, the screening grid 24 is held to a positive electrical voltage, denoted VPHV. The second grid, called the accelerating grid and designated by reference numeral 25, contributes to accelerating the ions exiting the plasma chamber 20, in cooperation with the screening grid 24, and creates an electrical potential barrier between the screening grid and the neutralizer 27 described later, for electrons emitted by the latter.For this purpose, the acceleration grid 25 is brought to a negative electrical voltage, denoted V NHV. The screen grid 24 is intermediate between the outlet aperture of the plasma chamber 20 and the acceleration grid 25. Possibly, the thruster 2 may include at least one additional third grid. Such a third grid, called the deceleration grid, may be electrically connected to the electrical ground of the spacecraft 100.

[0038] The power supply assembly 1 comprises a first power supply unit dedicated to powering the plasma enclosure 20, designated by reference number 11 and noted as DC-plasma; a second power supply unit dedicated to biasing the screen grid 24, designated by reference number 12 and noted as PHV for "positive high voltage"; and a third power supply unit dedicated to biasing the accelerator grid 25, designated by reference number 13 and noted as NHV for "negative high voltage." In the general section of this description, power supply unit 12 has been referred to as the positive power supply unit, and power supply unit 13 as the negative power supply unit. The power supply assembly 1 includes a common reference node commonly designated as CRP for "Cathode Return Potential," or sometimes NRP for "Neutralizer Return Potential."This common reference node (CRP) is itself electrically connected to the electrical ground of the spacecraft 100 via a charge-conducting system 19, sometimes called a "bleed resistor." The positive voltage VPHV and negative voltage VNHV are defined with respect to this common reference node CRP. For this purpose, the power supply unit 12, which is dedicated to biasing the screen grid 24, has a positive output terminal that is electrically connected to this screen grid 24, and a negative output terminal that is electrically connected to the common reference node CRP. Thus, the voltage VPHV applied to the screen grid 24 is positive. The current flowing through the positive output terminal of the power supply unit 12 toward the screen grid 24 is denoted IPHV and is called the screen grid current.Similarly, but with the polarity reversed, the power supply unit 13, which is dedicated to biasing the acceleration grid 25, has a negative output terminal that is electrically connected to this acceleration grid 25, and a positive output terminal that is electrically connected to the common reference node CRP. The electric current flowing through the negative output terminal of the power supply unit 13 from the acceleration grid 25 is denoted INHV and is called the acceleration grid current. The resulting voltage VNHV applied to the acceleration grid 25 is negative.

[0039] The power supply unit 11, which is dedicated to powering the plasma enclosure 20, has a positive output terminal electrically connected to the anode 21, and a negative output terminal connected to the common reference node CRP. The electric current flowing through the positive output terminal of the power supply unit 11 towards the anode 21 is denoted Id and is called the plasma discharge current. The cathode 22 is electrically connected to the common reference node CRP. The plasma-generating power supplied by the power supply unit 11 to the plasma enclosure 20, via the anode 21 and cathode 22, is equal to the product of the discharge current Id and the voltage Vd of the anode 21, determined with respect to the common reference node CRP. This plasma-generating power can be determined internally within the power supply unit 11.

[0040] The gridded ion thruster 2 further includes a neutralizer 27, denoted NEUTR., whose function is to emit electrons outward from the thruster 2 in order to neutralize the ions emitted to produce thrust. As is known, this neutralizer 27 is electrically powered by several dedicated power supply units integrated into the power supply assembly 1, but not shown in the figures. A dedicated electrical connection links the neutralizer 27 to the common reference node CRP to conduct a neutralizer return current, denoted I NEUTR. The neutralizer 27 thus determines the electrical potential of the common reference node CRP.In general, the beam current, denoted I beam, is given by the formula: I beam = I PHV - I NHV - I PHV_EBS, where I PHV_EBS is an additional current supplied by the power unit 12 to the screen grid 14, without participating in the thrust produced because it is consumed by the back-circulation of electrons from the neutralizer 27 to the screen grid 14. The beam current I beam is the electric current corresponding to the beam of ions emitted by the thruster 2 outwards from the spacecraft 100. During nominal operation of the thruster 2, i.e., during operation in which the back-circulation of electrons is very low and negligible, the beam current I beam is approximately equal to the difference between the screen grid current and the acceleration grid current: I beam ≈ I PHV - I NHV.

[0041] The other references that are indicated in [ Fig. 1a ], and which will possibly be included in [ Fig. 1b ], will be described later.

[0042] For the space vehicle 100 of [ Fig. 1b The gridded ion thruster 2 is of the radiofrequency discharge type designated by RIT for "Radiofrequency Ion Thruster". It still includes a plasma chamber 20, but this chamber is equipped with a coil 26 which surrounds a lateral wall of this chamber to generate a radiofrequency electromagnetic field inside the plasma chamber 20. The radiofrequency electromagnetic field produces the ionization of the gas which is introduced into the plasma chamber 20, and which constitutes the ion source. For such a mode of plasma generation, the coil 26 is supplied with radio frequency current, noted I RF, by a generator 3, noted RFG for "radiofrequency generator", which can also be integrated into the power supply assembly 1. This RFG 3 generator is itself supplied by a dedicated power supply unit of the power supply assembly 1, designated by the reference 14 and called the RF ionization stage driver, or "RF ionization stage driver" in English.This power supply unit 14 is labeled RF-driver in [. Fig. 1b ] and replaces the power supply unit 11 of [ Fig. 1a ] in the power supply assembly 1. The electrical power of plasma generation in such a case of RIT thruster is calculated from the value of the radio frequency current I RF and the value of the electrical output voltage V RF of the RFG generator 3.

[0043] The arrangement of the screen grid 24 and the acceleration grid 25, and their respective functions with their dedicated power supply units 12 and 13, as described above with reference to [ Fig. 1a ], apply identically to the RIT-type gridded ion thruster of [ Fig. 1b ].

[0044] The power supply assembly 1, for both the GIT and RIT thruster types, further includes a variation module 15, denoted VAR, for controlling the power supply unit 12, in order to vary the positive electrical voltage VPHV produced by it. It also includes measurement and recording modules, collectively designated by reference numeral 16, for measuring and recording the values ​​of the acceleration grid current INHV. Finally, a controller 17, denoted CTRL and internal to the power supply assembly 1, controls the execution of a perveance test procedure, as described below.

[0045] The pervection test method of the present invention is performed when the spacecraft 100 is in flight in outer space, from an ongoing operation of the thruster 2, GIT or RIT, electrically powered by the power supply assembly 1. This operation is controlled by the controller 17 by imposing several output values ​​of the power supply assembly 1, including the positive voltage VPHV and the negative voltage VNHV. These output values ​​of the power supply assembly 1 thus produced contribute to determining the thrust operating point of the thruster 2. Each thrust operating point available for the thruster 2 is identified, in order to distinguish it from other thrust operating points in the case where several are possible.

[0046] The thrust operating point also includes the setting, by the controller 17, of an output value from the power supply unit 11 or 14, depending on the type GIT ou RIT of the propeller 2. For the case GIT of [ Fig. 1a This output value represents the plasma generation power supplied by the power supply unit 11 to the plasma chamber 20. Two operating modes are possible: with a constant beam current Ibeam, or with a constant plasma generation power. To achieve this, a control device is associated with the power supply unit 11, which is adapted to vary the plasma generation power in real time to maintain a constant beam current Ibeam during the operation of the thruster 2. This control device, called BCC for "beam current control," may include a module 18a arranged to measure the current IPHV transmitted by the power supply unit 12 to the screen grid 24, and a module 18b adapted to control the electrical power supplied at the output by the power supply unit 11.Module 18a transmits the value of the current IPHV to module 18b, while the value of the current INHV, which returns from the acceleration grid 25 to the power supply unit 13, is also transmitted to module 18b by the measurement and recording module 16. The operating mode of thruster 2, for which the beam current Ibeam is constant, is obtained by activating the BCC control device, and the operating mode for which the plasma generation power is constant is obtained by deactivating the BCC control device. For the RIT case of [. Fig. 1b The BCC control device still includes module 18b, but arranged to control the electrical power supplied by power supply unit 14 to generator RFG 3. Current measurement modules 16 and 18a are arranged for both the RIT and GIT cases, respectively, at the outputs of power supplies 12 and 13. When the BCC control device is activated, for the RIT case, the plasma generation power supplied by generator RFG 3 to coil 26 is adjusted in real time to maintain a constant beam current (Ibeam). Also in the RIT case, when the BCC control device is deactivated, coil 26 is powered from power supply unit 14, via generator RFG 3, with a plasma generation power that is maintained constant.

[0047] Starting from the thrust operating point currently in use for the thruster 2, whether GIT or RIT, and to perform the perveance test, the controller 17 commands the negative electrical voltage VNHV applied to the acceleration grid 25 to remain constant, and the positive electrical voltage VPHV applied to the screen grid 24 to vary, via module 15. For each value of the positive electrical voltage VPHV thus produced, module 16 measures and records the value of the acceleration grid current INHV. Such a perveance test can be performed alternatively while the BCC control device is activated, or deactivated, in both GIT and RIT modes.When performed with the BCC control device deactivated, it may be necessary to shut down thruster 2 after the completion of the perveance test, and then restart it once the BCC control device is reactivated to begin a new thrust production period. The perveance test is preferably performed automatically by the power supply unit 1, according to the programming of its controller 17.

[0048] [ Fig. 2a ] And [ Fig. 2b [ ] illustrate two possible procedures for such a perveance test. The horizontal axis of the diagrams in these figures represents time, denoted t; the left vertical axis represents the values ​​of the positive electrical voltage VPHV that are successively controlled by the controller 17; and the right vertical axis represents the values ​​of the acceleration grid current INHV that are measured by the module 16 for the controlled values ​​of the voltage VPHV, as test results. The test can begin from a sweep limit value VPHV_sup that is greater than the value of the positive electrical voltage VPHV that corresponds to the thrust operating point in use for the thruster 2; then the positive electrical voltage VPHV is progressively reduced during the test from this value VPHV_sup.This reduction of the voltage V PHV is achieved through successive downward steps, with a decrement ΔV PHV and an individual step duration Δt selected to establish a compromise between test duration and accuracy. The end of the sweep of the positive electrical voltage values ​​V PHV can be determined in two alternative ways: either by another sweep limit value V PHV_inf that is lower than the positive electrical voltage value V PHV corresponding to the thrust operating point in use for thruster 2, or by a maximum limit relative to the acceleration grid current I NHV that is measured. Indeed, to avoid degrading thruster 2, the acceleration grid current I NHV must remain below a fraction of the beam current I beam, for example, less than 0.03125·I beam.During such a downward variation of the positive electrical voltage VPHV, the accelerating grid current INHV initially decreases gradually, reaching a minimum value INHV_min, and then increases, first slowly and subsequently more rapidly, while the positive electrical voltage VPHV continues to decrease at a constant rate. The curve of the accelerating grid current INHV thus exhibits the following two characteristics: the minimum value INHV_min of the accelerating grid current INHV, which is obtained at the value VPHV_min of the positive electrical voltage VPHV, and the rapid change in slope of the curve of the accelerating grid current values ​​INHV, occurring at the value VPHV_change of the positive electrical voltage VPHV.This latter value V PHV_change, which is associated with the rapid change in slope in the curve of the accelerating grid current values ​​I NHV, has been called the threshold value of the positive electrical voltage in the general part of this description. The value V PHV_change, or preferably the value of the difference V PHV_min - V PHV_change, constitutes a characterization of the erosion state of grids 24 and 25, primarily of the accelerating grid 25. [ Fig. 2a ] corresponds to a permeability test procedure in which the gradual reduction of the positive electrical voltage VPHV is stopped when the accelerating grid current INHV reaches the maximum limit set for this current, i.e., 0.03125·Ibeam in the example considered. Such a procedure yields the value VPHV_change. Fig. 2b ] corresponds to an alternative execution of the perveance test in which the gradual reduction of the positive electrical voltage V PHV is stopped when this voltage V PHV reaches the slew limit V PHV_inf. In this latter case, the slew limit V PHV_inf is greater than the value of the positive electrical voltage V PHV for which the accelerating grid current I NHV would have reached its fixed maximum limit.

[0049] The results of the perveance test, that is, the value of the negative electrical voltage VNHV at which it was performed, and the successive associated values ​​of the positive electrical voltage VPHV and the accelerating grid current INHV, can be used on board Space Vehicle 100 or transmitted to a ground station under the control of an onboard computer of Space Vehicle 100, designated by reference 110 and noted as OBC for "on-board computer" in English in [ Fig. 1a ] And [ Fig. 1b ].

[0050] [ Fig. 3a ]-[ Fig. 3c ] schematically illustrate different permeability situations for the exit of ions from the plasma chamber 20. Each of these figures is a cross-section of the exit opening of the plasma chamber 20, with a hole in the screen grid 24 and, opposite it, a hole in the acceleration grid 25. In the illustrated case, the hole in the acceleration grid 25 has an opening area that is smaller than that of the hole in the screen grid 24. The positive electrical voltage VPHV applied to the screen grid 24 determines the shape of a plasma sheath contained within the chamber 20, at the level of the hole in this screen grid 24. This plasma sheath is designated by the reference PI_Sh, for "plasma sheath". The situation shown by [ Fig. 3a ] corresponds to an over-perveance, for which the plasma sheath PI_Sh is substantially planar in the opening of the screen grid hole 24. The ions that pass through this opening in the screen grid 24 have substantially parallel trajectories, so that a peripheral part of these ions strikes the accelerating grid 25 at the edges of its hole, producing a significant value for the accelerating grid current I NHV. Because of this ion bombardment, such an over-perveance situation causes progressive erosion of the accelerating grid 25. Conversely, the situation shown by [ Fig. 3c ] corresponds to an under-perveance, for which the plasma sheath PI_Sh is too concave in the opening of the screen grid hole 24. The ions that pass through this opening in the screen grid 24 then have trajectories inclined towards the median axis of the opening, and all the more inclined the closer the ions pass through the opening of the hole to its peripheral edge. Then some of the ions that come from the periphery of the hole in the screen grid 24 still hit the edges of the hole in the accelerating grid 25, now after having crossed the median axis. A significant value again results for the accelerating grid current I NHV. Such an under-perveance situation therefore also causes erosion of the accelerating grid 25. Finally, [ Fig. 3b ] shows an intermediate situation, in which the plasma sheath PI_Sh is slightly concave in the opening of the screen grid hole 24. The ions that pass through this opening then have trajectories that gradually narrow and pass through the opening of the acceleration grid hole 25 without colliding with it, or with minimal interception. The perveance situation of [ Fig. 3b ] is optimal by producing minimal acceleration grid erosion 25, and corresponds to the minimum value I NHV_min of the acceleration grid current I NHV.

[0051] Again, with reference to the perviousness test procedures illustrated by [ Fig. 2a ] And [ Fig. 2b ], the first part of the reduction of the positive electrical voltage, corresponding to V PHV greater than V PHV_min, corresponds to situations of sub-perveance such as shown by [ Fig. 3c ], and the last part of the reduction of the positive electrical voltage, corresponding to V PHV less than V PHV_min, corresponds to over-perveance situations such as those shown by [ Fig. 3a ]. The value V PHV_min produces the optimal perveance situation, as shown by [ Fig. 3b ].

[0052] One possible initial application of the perveance test results is to update the thrust operating point of thruster 2. The updated thrust operating point retains the negative voltage value VNHV for which the test was performed, and a value close to VPHV_min is assigned to the positive voltage VPHV. The updated value for the positive voltage VPHV may incorporate a predetermined safety margin, ensuring that this VPHV value for the updated thrust operating point is sufficiently far from the threshold value VPHV_change. This results in a possible margin of error between the VPHV value used to update the thrust operating point and the VPHV_min value. In this way, the erosion of the acceleration grid 25 is reduced for the future operation of thruster 2 at this thrust operating point.Such an update can be performed either automatically on board spacecraft 100 by controller 17, or remotely by an operator after the test results have been transmitted to them. The operation of thruster 2 can then continue after the perveance test according to the updated thrust operating point.

[0053] A second possible use of the perveance test results is to monitor the aging of thruster 2. The VPHV_change value corresponds to the limit of effectiveness of the screen grid 2 in controlling the quantity of ions exiting the plasma chamber 20. Due to the aging of thruster 2, the VPHV_change values ​​obtained from perveance tests performed at different times increase with the duration of thruster 2's use, reducing the value of the difference between VPHV_min and VPHV_change. The curve of these difference values, VPHV_min and VPHV_change, as a function of thruster usage time allows for predicting its aging and potentially for preferentially adopting operating points that result in slower thruster aging.

[0054] Furthermore, it is also possible to implement a perveance test that is still consistent with the invention, but which is limited to a restricted range of variation of the positive electrical voltage V PHV around its value for the thrust operating point during use. Such a test is shorter, but still allows the value V PHV_min to be determined and the thrust operating point to be updated accordingly. For such implementations, the positive electrical voltage V PHV is varied between two predetermined limit values ​​V PHV_sup and V PHV_inf of the sweep, in the increasing or decreasing direction, while verifying that the accelerating grid current I NHV remains constantly below the fixed maximum limit, for example, less than 0.03125·I beam.

[0055] It is understood that the invention can be reproduced by modifying minor aspects of the embodiments described in detail above, while retaining at least some of the aforementioned advantages. In particular, all numerical values ​​cited are for illustrative purposes only and may be changed depending on the application.

Claims

1. A method for testing a gridded ion thruster (2) on board a spacecraft (100), the method being performed while the spacecraft is in extraterrestrial space, the thruster comprising at least: - a plasma chamber (20); - a screen grid (24), which is located in front of or at an outlet opening of the plasma chamber (20); and - an acceleration grid (25), which is parallel to the screen grid (24) and located on one side of said screen grid opposite the plasma chamber (20), so that, during an operation of the thruster (2), positive ions produced in the plasma chamber (20) pass through the screen grid (24) and then the acceleration grid (25), a power supply unit (1) also being on board the spacecraft (100) and aranged to simultaneously provide, during the operation of the thruster (2), at least one positive electrical voltage (VPHV) to the screen grid (24) and one negative electrical voltage (VNHV) to the acceleration grid (25), with the positive and negative electrical voltages being determined with respect to a common reference node (CRP) of the power supply unit, and the method comprising a thrusting step of the thruster (2), said thrusting step comprising adjusting the power supply unit (1) to a predetermined thrust operating point with predetermined values of positive electrical voltage (VPHV) and negative electrical voltage (VNHV), along with a predetermined electrical supply power, the method being characterised in that it then comprises the following steps: / 1 / fixing the value of the negative electrical voltage (VNHV) corresponding to the predetermined thrust operating point of the thruster (2); / 2 / providing two scan limit values for the positive electrical voltage (VPHV), which are respectively above and below the value of said positive electrical voltage at the predetermined thrust operating point, or providing a scan start limit value (VPHV_sup) for the positive electrical voltage that is above the value of said positive electrical voltage at the predetermined thrust operating point, and providing a maximum limit for an electrical current that flows from the acceleration grid (25) to the power supply unit (1), referred to as the acceleration grid current (INHV); and / 3 / varying the positive electrical voltage (VPHV) in a constant direction between the scan limit values provided in step / 2 / , ensuring that the acceleration grid current (INHV) remains below the provided maximum limit, or reducing the positive electrical voltage from the scan start limit value (VPHV_sup) until the acceleration grid current reaches the provided maximum limit, and for each value of the positive electrical voltage produced, measuring the acceleration grid current and recording at least one measurement result of said acceleration grid current with the corresponding value of the positive electrical voltage.

2. The method according to Claim 1, wherein the positive electrical voltage (V PHV) is automatically varied in step / 3 / by the power supply unit (1), in accordance with a programming of said power supply unit.

3. The method according to Claim 1 or 2, further comprising transmission from the spacecraft (100) to a station located on Earth of at least some of the measurement results and values recorded in step / 3 / .

4. The method according to any one of the preceding claims, further comprising the following step: / 4 / determining a minimum value (INHV_min) of the acceleration grid current (INHV) among the measurement results recorded in step / 3 / for said acceleration grid current, along with the respective corresponding values (VPHV_min, VNHV) of the positive electrical voltage (VPHV) and the negative electrical voltage (VNHV).

5. The method according to Claim 4, further comprising: - updating the thrust operating point of the thruster (2) in accordance with the respective values (VPHV_min, VNHV) of the positive electrical voltage (VPHV) and the negative electrical voltage (VNHV) which correspond to the minimum value (INHV_min) of the acceleration grid current (INHV), as determined in step / 4 / , possibly with a predetermined margin of deviation between the positive electrical voltage value determined for said minimum value of the acceleration grid current and the positive electrical voltage value of the updated thrust operating point; then - activating a new operation of the thruster (2) that is in accordance with the updated thrust operating point.

6. The method according to Claim 4 or 5, wherein the positive electrical voltage (VPHV) is reduced in step / 3 / from the scan start limit value (VPHV_sup) until the acceleration grid current (INHV) reaches the provided maximum limit, and the method further comprises the following steps: / 5 / determining a threshold value (VPHV_change) for the positive electrical voltage (VPHV) that corresponds to a bend in a curve of the measured values of the acceleration grid current (INHV) as a function of the positive electrical voltage values, on the lowest-value side of said positive electrical voltage; and / 6 / calculating a difference between the value (VPHV_min) of the positive electrical voltage (VPHV) which corresponds to the minimum value (INHVmin) of the acceleration grid current (INHV), as determined in step / 4 / , and the threshold value (VPHV_change) determined in step / 5 / for said positive electrical voltage.

7. The method according to Claim 6, wherein the sequence of steps / 1 / to / 6 / is repeated several times, each time at a different moment while the spacecraft (100) is in extraterrestrial space, then a new value is determined for the difference between the value (VPHV_min) of the positive electrical voltage (VPHV) which corresponds to the minimum value (INHV_min) of the acceleration grid current (INHV) and the threshold value (VPHV_change) of said positive electrical voltage, by extrapolating the difference values calculated at each execution of the sequence of steps / 1 / to / 6 / .

8. The method according to any one of Claims 1 to 7, wherein the power supply unit (1) comprises a control device (18a, 18b) for regulating an electrical power used for plasma generation, which is delivered to the plasma chamber (20) by said power supply unit, said control device being designed to keep constant a difference value between an electrical current (IPHV) flowing from the power supply unit to the screen grid (24) and the acceleration grid current (INHV), and wherein step / 3 / is executed while the difference between the electrical current (IPHV) flowing from the power supply unit (1) to the screen grid (24) and the acceleration grid current (INHV) is maintained at a constant value by the control device (18a, 18b).

9. The method according to any one of Claims 1 to 7, wherein step / 3 / is executed while an electrical power which is used for plasma generation and delivered to the plasma chamber (20) by the power supply unit is kept constant.

10. The method according to any one of the preceding claims, wherein the maximum limit provided in step / 2 / for the acceleration grid current (INHV) is between 0.5% and 5% of a value of a beam current (Ibeam) exiting the thruster (2).

11. The method according to claim 10, wherein the maximum limit provided in step / 2 / for the acceleration grid current (INHV) is substantially equal to 3.125% of the beam current value (Ibeam).

12. The method according to any one of the preceding claims, wherein the thruster (2) is of one of the following types: gridded ion thruster with continuous discharge, or gridded ion thruster with radio frequency discharge.

13. A power supply unit (1) for a gridded ion thruster (2), the thruster comprising: a plasma chamber (20), a screen grid (24) located in front of or at an outlet opening of the plasma chamber, and an acceleration grid (25), which is parallel to the screen grid and located on one side of said screen grid opposite the plasma chamber, the power supply unit (1) comprising: - a common reference node (CRP); - a positive power supply unit (12), designed to be connected to the screen grid (24) of the thruster (2) to provide said screen grid, during an operation of said thruster, with a positive electrical voltage (VPHV) relative to the common reference node (CRP); - a negative power supply unit (13), designed to be connected to the acceleration grid (25) of the thruster (2) to provide said acceleration grid, during the operation of said thruster, with a negative electrical voltage (VNHV) relative to the common reference node (CRP); - a variation module (15), designed to vary the positive electrical voltage (VPHV) supplied by the positive power supply unit (12); - measurement and recording modules (16), arranged to measure and record values of an electrical current flowing from the acceleration grid (25) to the power supply unit (1), referred to as the acceleration grid current (INHV); and - a controller (17), configured to activate the variation module (15) and the measurement and recording modules (16), so that variable values are successively produced for the positive electrical voltage (VPHV) while the negative electrical voltage (VNHV) remains constant, and a value of the acceleration grid current (INHV) is measured and recorded for at least one provided value of the positive electrical voltage, when the power supply unit (1) is connected to the thruster (2) to enable an operation of said thruster, the power supply unit (1) being adapted to execute a method that is in accordance with any one of Claims 1 to 12.

14. A plasma propulsion system (10) comprising: - an ion gridded thruster (2), the thruster comprising: a plasma chamber (20), a screen grid (24) located in front of or at an outlet opening of the plasma chamber, and an acceleration grid (25) which is parallel to the screen grid and located on one side of said screen grid opposite the plasma chamber, so that during operation of said thruster, positive ions produced in the plasma chamber pass through the screen grid and then the acceleration grid; and - a power supply unit (1) that is in accordance with Claim 13, and which is connected to the thruster (2) to enable an operation of said thruster.

15. A spacecraft (100), comprising a plasma propulsion system (10) in accordance with Claim 14.