WO2024062202A1 - In-flight testing of a gridded-ion thruster - Google Patents

Abstract

A method of testing a gridded-ion thruster (2) is performed on board a spacecraft (100) while the spacecraft is in thrust in extraterrestrial space. The test method is a perveance-type test which makes it possible to update a thrust operating point of the thruster and to monitor an ageing of said thruster, in particular an aging by erosion of an acceleration grid (25) disposed at the plasma chamber outlet (20).

Description

Description

Title: FLIGHT TEST OF A GRID ION PROPELLER

Technical area

[0001] The present description concerns a method of flight testing a grid ion thruster, as well as a power supply assembly for a grid ion thruster which is suitable for such a test.

Prior art

[0002] Ion thrusters with grids are used for space vehicles such as satellites or space probes although their use is still less common to date than that of Hall effect ion thrusters. These grid ion thrusters can be of the continuous discharge type, designated by the acronym GIT for “Gridded-lon Thruster” in English and also called Kaufman type, or of the RIT type for “Radiofrequency Ion Thruster”. These grid ion thrusters, RIT or GIT, each include at least:

- a plasma enclosure;

- a screen grille, which is located in front of or at the level of an outlet opening of the plasma enclosure; And

- an acceleration grid, which is parallel to the screen grid and located on one side of this screen grid opposite the plasma enclosure.

During operation of such a thruster, positive ions which are produced in the plasma enclosure pass through the screen grid then the acceleration grid. For this, an electrical power supply assembly, which is commonly referred to as PPU for “Power Processing Unit” in English, is also on board the space vehicle and arranged to, during operation of the thruster, simultaneously provide at least one positive electrical voltage to the screen grid and a negative electrical voltage to the acceleration grid, these positive and negative electrical voltages being determined with respect to a common reference node of the electrical power supply assembly, which is commonly designated by CRP for “Cathode Return Potential” in English, or sometimes NRP for “Neutralizer Return Potential” in English. [0003] The understanding and progressive improvement of grid ion thrusters requires knowledge of the wear phenomena and aging which progressively affect them during their use on board space vehicles. For this, measurements and tests are necessary, to be carried out on board space vehicles, in order to characterize the state of wear and aging of each grid ion thruster throughout its use on mission. Indeed, the tests carried out 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 use on mission. Such tests and measurements have the following purposes in particular:

- complete the general knowledge of grid ion thrusters, particularly regarding their aging in space environment conditions;

- predict the future wear and/or aging of a grid ion thruster which is on a mission aboard a space vehicle, in particular to assess the lifespan of this thruster; And

- characterize the state of wear and/or aging of a grid ion thruster which is on a mission on board a space vehicle, in order to optimize the operating point at which this thruster is used, or to achieve a compromise between speed of wear and intensity of the thrust force which is to be produced by the propeller.

[0004] The erosion of the acceleration grid of an ion thruster with grids is one of the main causes of its aging, which limits its lifespan. This erosion produces a progressive widening of the holes in the acceleration grid, and is caused by the bombardment of ions which come from the plasma enclosure and part of which hits the acceleration grid. For a fixed value of the negative electrical voltage which is applied to the acceleration grid, the state of erosion of this grid is revealed in particular by a modification of the limit of retro-circulation of the electrons generated by a neutralizer of the thruster.

[0005] Document US 6,964,396 describes carrying out an EBS type test, for “Electron Back-Streaming” in English or measuring the retro-circulation of electrons, on board a space vehicle which is in extraterrestrial space. During such an EBS test, the negative electrical voltage which is applied to the acceleration gate is gradually decreased in value absolute, that is to say that this electrical voltage is varied from very negative values to less negative values. Simultaneously, a beam current control loop, designated by BCC for “Beam Current Control” in English, maintains constant a difference between an electric current which is supplied to the screen grid and an electric current which comes from the screen grid. acceleration, by controlling an electrical power to generate plasma which is delivered to the plasma enclosure of the thruster. This plasma generation electrical power is then measured for each value of the negative electrical voltage which is applied to the acceleration grid. Such an EBS test makes it possible to determine a value to be used for the negative electrical voltage which is applied to the acceleration grid, the absolute value of which is minimum to maximize the thrust produced at a given value of electrical power which is supplied to the acceleration grid. screen, while reducing heating of the plasma enclosure and securing the operation of the thruster.

Technical problem

[0006] From this situation, an aim of the present invention is to propose another test to be executed while the space vehicle is in extraterrestrial space, to characterize the state of a grid ion thruster of this vehicle. spatial, in particular the state of erosion of the acceleration grid of this thruster.

[0007] Another aim of the invention is to provide new information for managing the operation of a grid ion thruster which is on board a space vehicle, in particular such information which relates to the operation and to the life of the propellant.

Summary of the invention

[0008] To achieve at least one of these goals or another, a first aspect of the invention proposes a new method for testing an ion thruster with grids on board a space vehicle, such as a satellite , a space probe, etc., this test method being carried out while the space vehicle is in extraterrestrial space. The thruster comprises at least the components which have been recalled previously: the plasma enclosure, the screen grid and the acceleration grid, the latter being on one side of the screen grid which is opposite the plasma enclosure so that, during operation of the thruster, positive ions which are produced in the plasma enclosure pass through the screen grid then the acceleration grid. In particular, the propellant to which the test method of the invention is applied can be of one of the following types: ion propellant with grids and continuous discharge, such as known under the designation GIT or also called Kaufman type, or ion propellant with grids and radiofrequency discharge, as known under the designation RIT. Possibly, the propellant to which the test method of the invention is applied may comprise at least one additional grid, in addition to the screen grid and the acceleration grid. Such a third grid is commonly called a deceleration grid.

[0009] An electrical power supply assembly is also on board the space vehicle and arranged to, during operation of the thruster and in particular during execution of the test method of the invention, simultaneously supply at least one positive electrical voltage to the screen grid and a negative electrical voltage to the acceleration grid, these electrical voltages being determined relative to the common reference node of the electrical power supply assembly. The method comprises a step of pushing the thruster, itself comprising an adjustment of the electrical power supply assembly 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 power supply.

[0010] The process of the invention then comprises the following steps:

/1/ freeze the value of the negative electrical voltage corresponding to the predetermined thrust operating point of the thruster;

121 provide two scan limit values for the positive electrical voltage, which are respectively greater and lower than the value of this positive electrical voltage at the predetermined thrust operating point, or else provide a scan start limit value for the voltage positive electric voltage which is greater than the value of this positive electric voltage at the predetermined thrust operating point, and provide a maximum limit for an electric current which flows from the acceleration gate to the electric power supply assembly, called current of acceleration grid; And

/3/ vary the positive electrical voltage according to a direction of variation which is constant between the scan limit values provided in step 121, checking that the acceleration gate current remains lower than the maximum limit provided, or else reduce the positive electrical voltage from the start limit value of scanning until the acceleration gate current reaches the maximum limit provided, and for each generated value of the positive electrical voltage, measuring the acceleration gate current and recording at least one measurement result of said gate current acceleration with the corresponding value of the positive electrical voltage.

[0011] The maximum limit which is provided in step 121 for the acceleration gate current may depend on the beam current which exits the thruster. In particular, it can be equal to the intensity of this beam current multiplied by a constant coefficient. Generally in the present description, the beam current exiting the thruster is equal to a difference between an electrical current flowing from the power supply assembly to the screen grid and the acceleration grid current, at which can be subtracted, in addition, a current of back-circulation of electrons which are emitted by a neutralizer of the propellant then collected by the screen grid. For most implementations of the invention, the maximum limit provided in step 121 for the acceleration gate current can be between 0.5% and 5% of the beam current value. For example, it can be substantially equal to 3.125% of the value of the beam current.

[0012] The test method proposed by the invention is of the perveance test type. It makes it possible to identify the value of the positive electrical voltage which is applied to the screen grid, for which the acceleration grid current is minimal, that is to say for which a minimum quantity of ions which come from the plasma enclosure hits the acceleration grid. This value of the positive electrical voltage which minimizes the acceleration grid current corresponds to operation of the thruster which reduces the erosion of its acceleration grid. The life of the thruster can then be increased by adopting this operation for thrust, or its life can be managed if other mission constraints require the use of an operating point for the thrust of the thruster which does not correspond not at the minimum of the acceleration gate current. [0013] The variation interval 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 or by one of the scanning limit values provided in step 121, whether because the maximum limit for the acceleration gate current is reached, depends on the amount of information that is desired on the thruster.

[0014] An advantage of the method of the invention is that it makes it possible to regularly carry out measurements during the thrust of the thruster, to detect the state of erosion of its grids, and to determine whether its predetermined thrust operating point needs to be corrected. The thrust force that is produced by the thruster is not interrupted by the execution of the test method of the invention.

[0015] Another advantage of the method of the invention is that it makes it possible to adjust the operation of the thruster according to its state of aging, without damaging the thruster, in a way which is particularly safe and robust so that it can be executed automatically. and in a space environment.

[0016] Yet another advantage of the method of the invention is that it makes it possible to test the space vehicle in flight, and to remotely transmit multiple operating parameters of the thruster to understand its behavior in operational conditions.

[0017] The present invention therefore makes it possible to improve the understanding of the evolution of the characteristics, in particular electrical, thermal or related to materials, of an ion thruster with grids which is used on board a space vehicle throughout the duration life of this space vehicle. It provides measurements of certain characteristics of the propellant during the space vehicle mission.

[0018] Finally, yet another advantage of the invention lies in the greater precision of the tests and measurements which are carried out while the space vehicle is in extraterrestrial space, compared to tests and measurements carried out on Earth. Indeed, many characteristics of the propellant have very different values on Earth compared to their effective values in extraterrestrial space, in particular because of the lack of representativeness of the environmental parameters, eg pressure, narrowness of the vacuum chambers, etc. ., which are used on Earth to simulate the vacuum of space. Thus, the test method of the invention provides more accurate knowledge of the behavior of the grid ion thrusters in extraterrestrial space throughout their operation. This knowledge makes it possible in particular to better adjust the electrical parameters of an ion thruster with grids, ie electrical currents and voltages, and also fluidic parameters, ie flow rates and pressures of the gases which are used in this thruster. Such an improved adjustment in turn allows optimized exploitation of the propeller grids, in particular by minimizing their erosion.

[0019] Advantageously, the positive electrical voltage can be varied at step /3Z automatically by the power supply assembly, in accordance with programming of this power supply assembly.

[0020] Possibly, and so that the results of execution of the test method of the invention can be studied and/or exploited on Earth, the method may further comprise transmitting from the space vehicle to a station which is 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 that is used for the space vehicle.

[0021] In certain implementations of the invention, in particular those with the aim of reducing the erosion of the acceleration grid, the method may also comprise the following step:

Z4Z determine a minimum value of the acceleration gate current among the measurement results recorded in step /3Z for this acceleration gate current, as well as the respective corresponding values of the positive electrical voltage and the negative electrical voltage .

[0022] When step Z4Z is executed, the method can also include:

- update the thrust operating point of the thruster in accordance with the respective values of the positive electrical voltage and the negative electrical voltage which correspond to the minimum value of the acceleration grid current, as determined in step Z4Z, possibly with a predetermined margin of difference between the value of the positive electrical voltage which has been determined for the minimum value of the acceleration gate current and the value of the positive electrical voltage of the updated thrust operating point; Then - activate a new thruster operation that is consistent with the updated thrust operating point.

Such updating and activation of the thruster operating point can be remotely controlled from the station which is located on Earth, or performed automatically on board the space vehicle.

[0023] Also when step /4/ is executed, and when the positive electrical voltage has been reduced in step /3/ from the scan start limit value until the gate current acceleration reaches the maximum limit provided, the method may further comprise the following steps:

/5/ determine a threshold value for the positive electrical voltage which corresponds to a bend in a curve of the measured values of the acceleration gate current as a function of the values of the positive electrical voltage, on the side of the lowest values of this positive electrical voltage; And

/6/ calculate a difference between the value of the positive electrical voltage which corresponds to the minimum value of the acceleration gate current, as it was determined in step /4/, and the threshold value determined in step /5/ for the positive electrical voltage.

The difference result calculated in step /6/ is a measure of the state of erosion of the acceleration grid, and therefore constitutes a measure of the aging of the propellant. The sequence of steps /1/ to /6/ can be repeated several times, each time at a different time while the space vehicle is in extraterrestrial space, and then a new value is determined for the difference between the voltage value positive electrical voltage which corresponds to the minimum value of the acceleration gate current, and the threshold value of this positive electrical voltage, by extrapolation of 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 erosion profile of the acceleration grid, as a function of the operating time of the thruster, and this temporal profile can be extended by extrapolation to provide a forecast of the difference value that is relative to a future date.

[0024] The electrical power supply assembly may comprise a device for controlling electrical power for generating plasma which is delivered to the plasma enclosure by this power supply assembly, this control device being designed to maintain constant a value of difference between the electric current which flows from the power supply assembly to the screen grid and the grid current acceleration. Such a servo device corresponds to the beam current control loop, or BCC for “Beam Current Control” in English. Then, in first possible implementations of the test method of the invention, step /3/ can be executed while the value of the difference between the electric current which flows from the power supply assembly to the grid d The screen and the acceleration grid current is kept constant by the servo device. This difference value is equal to the beam current in the absence of additional so-called retro-circulation current.

[0025] Alternatively, in other possible implementations of the test method of the invention, step /3/ can be executed while the electrical plasma generation power which is delivered to the plasma enclosure by the entire power supply is kept constant. For this, and if it is present, the plasma generation electrical power control device is kept deactivated while step /3/ is executed. The value of the difference between the electric current which flows from the power supply assembly to the screen grid and the acceleration grid current is then no longer constant, because of the profile adopted by the screen grid current. acceleration under the effect of the variation of the positive electrical voltage which is supplied to the screen grid.

[0026] A second aspect of the invention proposes a power supply assembly for an ion thruster with grids, the thruster comprising: a plasma enclosure, a screen grid which is located in front of or at the level of an outlet opening of the plasma enclosure, and an acceleration grid which is parallel to the screen grid and located on a side thereof which is opposite to the plasma enclosure, the power supply assembly comprising :

- the common reference node;

- a positive electrical power supply unit, which is intended to be connected to the screen grid of the thruster to provide this screen grid, during operation of the thruster, with a positive electrical voltage relative to the common reference node;

- a negative power supply unit, which is intended to be connected to the acceleration grid of the thruster to provide this acceleration grid, during operation of the thruster, 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 electrical 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 gate current is measured and recorded for at least one supplied value of the positive electrical voltage, when the electrical power supply assembly is connected to the thruster to allow operation of this thruster.

Such a power supply assembly is adapted to carry out a test method which conforms to the first aspect of the invention.

[0027] Each of the variation, measurement and recording modules of the power supply assembly can be of the software or recorded program type, designated by “software” in English, or of the hardware type, designated by “hardware ".

[0028] Possibly, the power supply assembly may also include:

- a device for controlling the plasma generation electrical power which is delivered to the plasma enclosure, designed to maintain the beam current constant, and as designated by the beam current control loop, or BCC.

[0029] A third aspect of the invention proposes a plasma propulsion system which comprises:

- the grid ion thruster; And

- an electrical power supply assembly which conforms to the second aspect of the invention, and which is connected to the thruster to produce operation of the latter. Finally, a fourth aspect of the invention proposes a space vehicle which comprises a plasma propulsion system conforming to the third aspect.

Brief description of the figures

[0031] The characteristics and advantages of the present invention will appear more clearly in the detailed description below of non-limiting exemplary embodiments, with reference to the appended figures among which:

[0032] [Fig. 1 a] shows, in a schematic and simplified manner, a space vehicle which is equipped with a GIT type grid ion thruster, and on board which the test method of the invention can be used;

[0033] [Fig. 1 b] corresponds to [Fig. 1 a] for an RIT type grid ion thruster;

[0034] [Fig. 2a] is an example of a diagram presenting results which are obtained by the test method of the invention, according to a first possible sequence of the method;

[0035] [Fig. 2b] corresponds to [Fig. 2a] for a second possible sequence of the process;

[0036] [Fig. 3a] illustrates a situation of over-perversion;

[0037] [Fig. 3b] illustrates an optimal situation of perveance; And

[0038] [Fig. 3c] illustrates a situation of under-performance.

Detailed description of the invention

[0039] In these figures, all the elements are only represented symbolically, and identical references which are indicated in different figures designate identical elements or which have identical functions.

[0040] In [Fig. 1 a] and [Fig. 1 b], the reference 100 designates a space vehicle, whatever the type of this vehicle, for example a satellite or a space probe. Inside this space vehicle 100, the reference 10 designates a plasma propulsion system, commonly referred to by the acronym PPS for “Plasma Propulsion Subsystem” in English. This plasma propulsion system 10, which is sometimes referred to as a subsystem with respect to the space vehicle 100, itself comprises at least one electrical power supply assembly 1, commonly designated by the acronym PPU for “Power Processing Unit” in English, and at least one grid ion thruster 2. The assembly electrical power supply 1 and the grid ion thruster 2 which are represented in each of these two figures are associated with one another so that the thruster 2 is supplied with electrical energy appropriately by the power supply assembly electric

1, in order to produce the thrust force which is desired at each moment of operation of the plasma thruster, while the space vehicle 100 is in extraterrestrial space. In a usual manner, the electrical power supply assembly 1 is connected between an electrical power bus and an electrical mass (not shown) of the space vehicle 100, while the various components of the thruster 2 are supplied with currents and voltages. electrical via the electrical power supply assembly 1. During operation of each of the grid ion thrusters of [Fig. 1a] and [Fig. 1 b], the thrust force which is produced by this thruster and applied to the space vehicle 100 results from the production, by the ion grid thruster, of a beam of ions which is denoted ions in the figures.

[0041] For reasons of clarity of the present description, the components of the plasma propulsion system 10 which are shown in [Fig. 1 a] and [Fig. 1 b] are limited to those concerned by the invention. In particular, each of the grid ion thrusters 2 of [Fig. 1 a] and [Fig. 1 b] includes one or more neutralizer power supply units, as well as a gas management system, which are not shown, and the power supply assembly 1 includes additional power units and interfaces corresponding ones, which are also not represented.

[0042] For the space vehicle 100 of [Fig. 1 a], the grid ion thruster 2 is of the continuous discharge type designated by GIT for “Gridded Ion Thruster”, or called the Kaufman type. It comprises a plasma enclosure 20, which is provided with an anode 21 and a cathode 22. The anode 21 can be located around an outlet opening of the plasma enclosure 20, through which the ions are intended to come out during operation of the thruster

2. The cathode 22 can be located at the bottom of the plasma enclosure 20, opposite its outlet opening. In known manner, the plasma enclosure 20 is provided with an electromagnet 23, commonly called a “magnet”, which helps to confine the plasma inside the the enclosure 20. To produce the plasma in the enclosure 20, and thus constitute a source of the ions which are ejected towards the outside of 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 there.

The grid ion thruster 2 further comprises at least two electrically conductive grids, which are arranged parallel and at a distance from one another in front of the outlet opening of the plasma enclosure 20. The first grid , called screen grid and designated by the reference 24, has the main functions of controlling the quantity of ions which leave the plasma enclosure 20 and to participate in accelerating the ions which leave the plasma enclosure 20. For this, the screen grid 24 is brought to a positive electrical voltage which is denoted VPHV. The second grid, called acceleration grid and designated by the reference 25, contributes to accelerating the ions which leave the plasma enclosure 20, in cooperation with the screen grid 24, and creates an electrical potential barrier between the screen grid and the neutralizer 27 described below, for electrons emitted by the latter. For this, the acceleration grid 25 is brought to a negative electrical voltage which is denoted VNHV. The screen grid 24 is intermediate between the outlet opening of the plasma enclosure 20 and the acceleration grid 25. Possibly, the thruster 2 may include at least a third additional grid. Such a third grid, called the deceleration grid, can be electrically connected to the electrical ground of the space vehicle 100.

The electrical power supply assembly 1 comprises a first power supply unit which is dedicated to powering the plasma enclosure 20, designated by the reference 11 and denoted DC-plasma, a second power supply unit which is dedicated to the polarization of the screen grid 24, designated by the reference 12 and denoted PHV for "positive high voltage" in English, and a third power supply unit which is dedicated to the polarization of the acceleration grid 25, designated by the reference 13 and denoted NHV for “negative high voltage”. In the general part of the present description, the power supply unit 12 has been referred to as the positive power supply unit, and the power supply unit 13 has been referred to as the negative power supply unit. The power supply assembly 1 includes a common reference node which is commonly designated by CRP for “Cathode Return Potential” in English, or sometimes NRP for “Neutralizer Return Potential” in English. This common CRP reference node is itself even electrically connected to the electrical mass of the space vehicle 100 by a charge-conducting system 19, sometimes called a “bleed resistor” in English. The positive electrical voltages VPHV and negative VNHV are defined in relation to this common reference node CRP. For this, the power supply unit 12, which is dedicated to the polarization of the screen grid 24, has a positive output terminal which is electrically connected to this screen grid 24, and a negative output terminal which is electrically connected to the common CRP reference node. Thus, the electrical voltage VPHV which is applied to the screen grid 24 is positive. The electric current which flows through the positive output terminal of the power supply unit 12 towards the screen grid 24 is denoted IPHV, and called screen grid current. In a similar manner but by reversing the polarity, the power supply unit 13, which is dedicated to the polarization of the acceleration grid 25, has a negative output terminal which is electrically connected to this acceleration grid 25, and a positive output terminal which is electrically connected to the common reference node CRP. The electric current which flows through the negative output terminal of the power supply unit 13 coming from the acceleration grid 25 is denoted INHV, and called acceleration grid current. The electrical voltage VNHV which is thus applied to the acceleration grid 25 is negative.

[0045] The power supply unit 11, which is dedicated to powering the plasma enclosure 20, has a positive output terminal which is electrically connected to the anode 21, and a negative output terminal which is connected to the common CRP reference node. The electric current which flows through the positive output terminal of the power supply unit 11 towards the anode 21 is denoted Id and called plasma discharge current. The cathode 22 is electrically connected to the common reference node CRP. The electrical power for generating the plasma, which is supplied by the power supply unit 11 to the plasma enclosure 20, through the anode 21 and the cathode 22, is equal to the product of the value of the discharge current Id by the value of an electrical voltage Vd of the anode 21, determined relative to the common reference node CRP. This plasma generation power can be determined internally to the power supply unit 11.

The grid ion thruster 2 further comprises a neutralizer 27, denoted NEUTR., whose function is to emit electrons towards the outside of the thruster 2 in order to neutralize the ions which are emitted to produce the thrust. In known manner, this neutralizer 27 is electrically powered by several dedicated power units which are integrated into the power supply assembly 1, but not shown in the figures. A dedicated electrical connection connects the neutralizer 27 to the common reference node CRP to conduct to this node a neutralizer return current which is denoted INEUTR. The neutralizer 27 thus determines the electrical potential of the common reference node CRP. Generally, the beam current, denoted beam, is given by the formula: beam = IPHV - INHV - IPHV_EBS, WHERE IPHV_EBS is an additional current which is supplied by the power supply unit 12 to the screen grid 14 , without participating in the thrust which is produced because it is consumed by the back-circulation of electrons from the neutralizer 27 to the screen grid 14. The beam beam current is the electric current which corresponds to the beam of ions which are emitted by the thruster 2 towards the outside of the space vehicle 100. During nominal operation of the thruster 2, that is to say during operation for which the retro-circulation of electrons is very small and negligible, the beam beam current is substantially equal to the difference between the screen grid current and the acceleration grid current ■ beam * IPHV - INHV.

[0047] The other references which are indicated in [Fig. 1 a], and which will eventually be included in [Fig. 1 b], will be described later.

[0048] For the space vehicle 100 of [Fig. 1 b], the grid ion thruster 2 is of the radiofrequency discharge type designated by RIT for “Radiofrequency Ion Thruster”. It also includes a plasma enclosure 20, but this is provided with a coil 26 which surrounds a side wall of this enclosure to generate a radio frequency electromagnetic field inside the plasma enclosure 20. The radio frequency electromagnetic field produces the ionization of the gas which is introduced into the plasma enclosure 20, and which constitutes the ion source. For such a plasma generation mode, the coil 26 is supplied with radio frequency current, denoted IRF, by a generator 3, denoted RFG for “radiofrequency generator”, which can also be integrated into the electrical power supply assembly 1. This RFG generator 3 is itself powered by a dedicated power supply unit of the electrical power supply assembly 1, designated by the reference 14 and called RF ionization stage driver, or “RF ionization stage driver” in English . This power supply unit 14 is denoted RF-driver in [Fig. 1 b] and replaces the power supply unit 11 of [Fig. 1a] in the power supply assembly 1. Electric power generation of plasma in such a case of RIT thruster is calculated from the value of the radio frequency current IRF and the value of the electrical output voltage VRF of the generator RFG 3.

[0049] 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. 1 a], apply identically to the RIT type grid ion thruster of [Fig. 1 b].

[0050] The electrical power supply assembly 1, for the two types of thrusters GIT and RIT, further comprises a variation module 15, denoted VAR., to control the power supply unit 12, in order to vary the voltage positive electrical VPHV which is produced by it. It also includes measurement and recording modules, which are generally designated by the reference 16, in order to measure and record the values of the acceleration gate current INHV. Finally, a controller 17, denoted CTRL and internal to the electrical power supply assembly 1, controls the executions of a perveance test process, as now described.

The perveance test method of the present invention is executed when the space vehicle 100 is in flight in extraterrestrial space, from ongoing operation of the propeller 2, GIT or RIT, electrically powered by the assembly 1. This operation is controlled by the controller 17 by imposing several output values of the electrical supply assembly 1, including those of the positive electrical voltages VPHV and negative VNHV. These output values of the electrical power supply assembly 1 which are thus produced participate in determining the thrust operating point of the thruster 2. Each thrust operating point which is available for the thruster 2 is identified, in order to distinguish it other thrust operating points in the case where several are possible.

[0052] The thrust operating point also includes the setting by the controller 17 of an output value of the power supply unit 11 or 14, depending on the GIT or RIT type of the propeller 2. For the GIT case of [Fig. 1 a], this output value is that of the plasma generation power which is supplied by the power supply unit 11 to the plasma enclosure 20. Two operating modes are possible alternatively: beam current which is constant, or at plasma generation power which is constant. For this, a servo device is associated with the power supply unit 11, which is adapted to vary in real time the plasma generation power so as to maintain the constant beam current during the current operation of the thruster. 2. This servo device, called BCC for “beam current control”, can include a module 18a which is arranged to measure the IPHV current which is transmitted by the power supply unit 12 to the screen grid 24, and a module 18b which is adapted to control the electrical power which is supplied at output by the power supply unit 11. The module 18a transmits to the module 18b the value of the current IPHV, at the same time as the value of the current INHV which returns from the acceleration grid 25 to the power supply unit 13 is also transmitted to the module 18b by the measurement and recording module 16. The operating mode of the thruster 2 for which the beam current is constant is obtained by activating the BCC servo device, and the operating mode for which the plasma generation power is constant, is obtained by deactivating the BCC servo device. For the RIT case of [Fig. 1 b], the BCC servo device still includes the module 18b, but arranged to control the electrical power which is supplied by the power supply unit 14 to the RFG generator 3. The current measurement modules 16 and 18a are arranged for the RIT case as for the GIT case, respectively at the outputs of the power supply units 12 and 13. When the BCC servo device is activated, for the RIT case, the plasma generation power which is supplied by the RFG generator 3 at coil 26 is adjusted in real time to keep the beam current constant. Still in the RIT case, when the BCC servo device is deactivated, the coil 26 is powered from the power supply unit 14, via the RFG generator 3, with a plasma generation power which is kept constant.

[0053] From the thrust operating point which is in use for the thruster 2, whether GIT or RIT, and to execute the perveance test, the controller 17 commands to keep the negative electrical voltage constant. VNHV which is applied to the acceleration grid 25, and to vary, via the module 15, the positive electrical voltage VPHV which is applied to the screen grid 24. For each value of the positive electrical voltage VPHV which is produced thus, the module 16 measures and records the value of the acceleration gate current INHV. Such a perveance test can be carried out alternatively while the control device BCC is activated, or deactivated, in both cases GIT and RIT. When it is carried out while the BCC servo device is deactivated, it may be necessary to turn off the thruster 2 after the end of the perveance test, then restart it once the BCC servo device is reactivated to begin a new thrust production duration. The perveance test is preferably carried out automatically by the power supply assembly 1, in accordance with programming of its controller 17.

[0054] [Fig. 2a] and [Fig. 2b] illustrate two possible procedures for such a perveance test. The horizontal axis of the diagrams in these figures marks the time, denoted t, the left vertical axis marks the values of the positive electrical voltage VPHV which are successively controlled by the controller 17, and the right vertical axis marks the values of the acceleration gate current INHV which are measured by the module 16 for the commanded values of the voltage VPHV, as test results. The test can start from a scan limit value VPHV_ _SUP which is greater than the value of the positive electrical voltage VPHV which corresponds to the thrust operating point in use for the thruster 2, then the electrical voltage positive VPHV is gradually reduced during the test from this VPHV_ _SUP value. This reduction in the voltage VPHV is carried out by successive descending steps, with a decrement AVPHV and an individual step duration At which are selected to establish a compromise between the duration of the test and its precision. The end of the sweep of the values of the positive electrical voltage VPHV can be determined in two alternative ways: either by another scanning limit value VpHvjnf which is lower than the value of the positive electrical voltage VPHV which corresponds to the thrust operating point in use for the thruster 2, or by a maximum limit relating to the acceleration grid current INHV which is measured. Indeed, in order not to degrade the thruster 2, the acceleration grid current INHV must remain less than a fraction of the beam beam current, for example less than 0.03125-Ibeam. During such a downward variation of the positive electrical voltage VPHV, the acceleration gate current INHV begins by gradually decreasing, reaching a minimum value lNHv_min, then increases, first slowly and then more rapidly while the positive electrical voltage HPVV continues to be reduced with a reduction speed which is kept constant. The variation curve of the acceleration gate current INHV thus has the following two characteristics: the minimum value lNHv_min of the acceleration gate current INHV which is obtained for the value VpHv_min of the positive electrical voltage VPHV, and the rapid change of slope in the curve of the values of the acceleration gate current INHV, occurring at the value VpHv_change of the positive electrical voltage VPHV. This last value VpHv_change, which is associated with the rapid change in slope in the curve of the values of the acceleration gate current INHV, has been called threshold value of the positive electrical voltage in the general part of the present description. The value VpHv_change, or preferably the value of the difference VpHv_min - VpHv_change, constitutes a characterization of the state of erosion of the grids 24 and 25, mainly of the acceleration grid 25. [Fig. 2a] corresponds to a progress of the perveance test for which the progressive reduction of the positive electrical voltage VPHV is stopped when the acceleration gate current INHV reaches the maximum limit set for this current, that is to say 0, 03125- beam in the example considered. Such a sequence provides the value VpHv_change. [Fig. 2b] corresponds to an alternative course of the perveance test for which the progressive reduction of the positive electrical voltage VPHV is stopped when this voltage VPHV reaches the scanning limit VpHvjnf. In the latter case, the scanning limit VpHvjnf is greater than the value of the positive electrical voltage VPHV for which the acceleration gate current INHV would have reached its fixed maximum limit.

[0055] The results of the perveance test, that is to say, the value of the negative electrical voltage VNHV for which it was performed, and the successive associated values of the positive electrical voltage VPHV and the gate current d INHV acceleration, can be used on board the space vehicle 100 or transmitted to a terrestrial station under the control of an on-board computer of the space vehicle 100, designated by the reference 110 and denoted OBC for “on-board computer” in English in [Fig. 1 a] and [Fig. 1b].

[0056] [Fig. 3a]-[Fig. 3c] schematically illustrate different perveance situations for the exit of ions from the plasma enclosure 20. Each of these figures is a section of the outlet opening of the plasma enclosure 20, with a hole in the grid screen 24 and, opposite it, a hole in the acceleration grid 25. For the case illustrated, the hole in the acceleration grid 25 has an opening surface which is more smaller than that of the hole in the screen grid 24. The positive electrical voltage VPHV which is applied to the screen grid 24 determines the shape of a plasma sheath which is contained in the enclosure 20, at the level of the hole in this grid screen 24. This plasma sheath is designated by the reference PI_Sh, for “plasma shealth” in English. The situation which is shown by [Fig. 3a] corresponds to an over-perveance, for which the sheath of the PI_Sh plasma is substantially flat in the opening of the hole in the screen grid 24. The ions which pass through this opening in the screen grid 24 have trajectories substantially parallel, so that a peripheral part of these ions hits the acceleration grid 25 on the edges of the hole of the latter, producing a significant value for the acceleration grid current INHV. Because of this ion bombardment, such a situation of overperversion causes a progressive erosion of the acceleration grid 25. Conversely, the situation which is shown by [Fig. 3c] corresponds to underperveance, for which the plasma sheath PI_Sh is too concave in the opening of the hole in the screen grid 24. The ions which cross this opening in the screen grid 24 then have trajectories inclined towards the central axis of the opening, and all the more inclined as the ions pass through the opening of the hole near its peripheral edge. Then a part of the ions which comes from the periphery of the hole in the screen grid 24 still hits the edges of the hole in the acceleration grid 25, now after having crossed the central axis. A significant value again results for the acceleration gate current INHV. Such a situation of under-performance therefore also causes an erosion of the acceleration grid 25. Finally, [Fig. 3b] shows an intermediate situation, for which the sheath of the plasma PI_Sh is slightly concave in the opening of the hole of the screen grid 24. The ions which cross this opening then have trajectories which tighten gently and pass through the opening the hole in the acceleration grid 25 without hitting the latter, or by intercepting it at a minimum. The perveance situation of [Fig. 3b] is optimal by producing an erosion of the acceleration grid 25 which is minimal, and corresponds to the minimum value lNHv_min of the acceleration grid current INHV.

[0057] Again with reference to the procedures of the perveance test illustrated by [Fig. 2a] and [Fig. 2b], the first part of the reduction in the positive electrical voltage, corresponding to VPHV greater than VpHv_min, corresponds to situations of under-perveance as shown by [Fig. 3c], and the last part of the tension reduction positive electrical, corresponding to VPHV smaller than VpHv_min, corresponds to situations of over-perveance as shown by [Fig. 3a]. The VpHv_min value produces the optimal perveance situation, as shown by [Fig. 3b].

[0058] A first possible use of the results of the perveance test consists of updating the thrust operating point of the thruster 2. The updated thrust operating point has the value of the negative electrical voltage VNHV for which the test was carried out, and a value close to VpHv_min is attributed to the positive electrical voltage VPHV. The updated value for the positive electrical voltage VPHV can take into account a predetermined safety margin, so that this value of VPHV for the updated thrust operating point is sufficiently far from the threshold value VpHv_change. This results in a possible margin of difference between the VPHV value used for updating 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 the thruster 2 according to this thrust operating point. Such an update can be carried out either automatically on board the space vehicle 100, by the controller 17, or controlled remotely by an operator after the test results have been transmitted to him. The operation of the thruster 2 can then be continued after the perveance test in accordance with the updated thrust operating point.

[0059] A second possible use of the results of the perveance test consists of monitoring the evolution of the aging of the propellant 2. In fact, the value VpHv_change corresponds to the effectiveness limit of the screen grid 2 in controlling the quantity of ions which leave the plasma enclosure 20. Due to the aging of the propellant 2, the VpHv_change values which are obtained by perveance tests carried out on different dates increase with the duration of use of the propellant 2, reducing the value of the difference VpHv_min - VpHv_change. The curve of these difference values Vp _H v_min - Vp _H v_change as a function of the time of use of the thruster makes it possible to predict its aging, and possibly to preferentially adopt operating points which cause slower aging of the thruster.

[0060] Furthermore, it is also possible to implement a perveance test which is still in accordance with the invention, but which is limited to a restricted interval of variation of the positive electrical voltage VPHV around its value for the thrust operating point in use. Such a test is shorter, but still allows the VpHv_min value to be determined and the thrust operating point to be updated accordingly. For such implementations, the positive electrical voltage VPHV is varied between two predetermined limit values VPHV_SU _P and VpHvjnf of the sweep, in the increasing direction or in the decreasing direction, while checking that the acceleration gate current INHV remains constantly lower than the maximum limit set, for example lower than 0.03125-beam.

[0061] It is understood that the invention can be reproduced by modifying secondary aspects of the modes of implementation which have been described in detail above, while retaining at least some of the advantages cited. In particular, all the numerical values which have been cited have been for illustration purposes only, and can be changed depending on the application considered.

Claims

Claims [Claim 1] Method for testing a grid ion thruster (2) on board a space vehicle (100), the method being executed while the space vehicle is in extraterrestrial space, the thruster comprising at least: - a plasma enclosure (20); - a screen grid (24), which is located in front of or at an outlet opening of the plasma enclosure (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 enclosure (20), so that, when operation of the thruster (2), positive ions which are produced in the plasma enclosure (20) pass through the screen grid (24) then the acceleration grid (25), a power supply assembly (1) also being on board the space vehicle (100) and arranged to, during operation of the thruster (2), simultaneously supply at least one positive electrical voltage (VPHV) to the screen grid (24) and an electrical voltage negative (VNHV) to the acceleration gate (25), the positive and negative electrical voltages being determined with respect to a common reference node (CRP) of the electrical power supply assembly, and the method comprising a pushing step of the thruster (2), said thrust step comprising an adjustment of the electrical power supply assembly (1) 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 power supply, the method being characterized in that it then comprises the following steps: /1/ freeze the value of the negative electrical voltage (VNHV) corresponding to the predetermined thrust operating point of the thruster (2); 121 provide two scan limit values for the positive electrical voltage (VPHV), which are respectively greater and lower than the value of said positive electrical voltage at the predetermined thrust operating point, or else provide a scan start limit value (VPHV_ _SUP ) for the positive electrical voltage which is greater to the value of said positive electrical voltage at the predetermined operating point of thrust, and provide a maximum limit for an electrical current which flows from the acceleration gate (25) to the electrical power supply assembly (1), called current acceleration grid (INHV); And /3/ vary the positive electrical voltage (VPHV) in a direction of variation which is constant between the scan limit values provided in step 121, checking that the acceleration gate current (INHV) remains less than the maximum limit provided, or decrease the positive electrical voltage from the scan start limit value (VpHv_su _P ) until the acceleration gate current reaches the maximum limit provided, and for each value produced of the positive electric voltage, measure the acceleration gate current and record at least one measurement result of said acceleration gate current with the corresponding value of the positive electric voltage. [Claim 2] Method according to claim 1, according to which the positive electrical voltage (VPHV) is varied in step /3/ automatically by the electrical power supply assembly (1), in accordance with programming of said power supply assembly electric [Claim 3] Method according to claim 1 or 2, further comprising transmitting from the space vehicle (100) to a station located on Earth, at least some of the measurement results and values recorded in step /3/. [Claim 4] Method according to any one of the preceding claims, further comprising the following step: /4/ determine a minimum value (lNHv_min) of the acceleration gate current (INHV) among the measurement results recorded in step /3/ for said acceleration gate current, as well as the respective corresponding values (VpHv_min , VNHV) of the positive electrical voltage (VPHV) and the negative electrical voltage (VNHV). [Claim 5] Method according to claim 4, further comprising: - update 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 (lNHv_min) of the acceleration gate current (INHV), as determined in step /4/, possibly with a predetermined margin of difference between the value of the positive electrical voltage determined for said minimum value of the acceleration gate current and the value of the positive electrical voltage of the updated thrust operating point; Then - activate a new operation of the thruster (2) which conforms to the updated thrust operating point. [Claim 6] Method according to claim 4 or 5, according to which the positive electrical voltage (VPHV) is reduced in step 73/ from the scan start limit value (VPHV_ _SUP ) until the Acceleration gate current (INHV) reaches the maximum limit provided, and the method further comprises the following steps: /5/ determine a threshold value (VpHv_change) for the positive electrical voltage (VPHV) which corresponds to a bend in a curve of the measured values of the acceleration gate current (INHV) as a function of the values of the positive electrical voltage, on the side of the lowest values of said positive electrical voltage; And 76/ calculate a difference between the value (VpHv_min) of the positive electrical voltage (VPHV) which corresponds to the minimum value (lNHv_min) of the acceleration gate current (INHV), as determined in step 747, the value threshold (VpHv_change) determined in step 75/ for said positive electrical voltage. [Claim 7] Method according to claim 6, according to which the sequence of steps /1 / to 76/ is repeated several times, each time at a different time while the space vehicle (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 (lNHv_min) of the acceleration gate current (INHV) and the threshold value (VpHv_change) of said positive electrical voltage, by extrapolation of the difference values calculated at each execution of the sequence of steps /1 / to 767. [Claim 8] Method according to any one of claims 1 to 7, according to which the electrical power supply assembly (1) comprises a device for controlling (18a, 18b) an electrical power for generating plasma which is delivered to the plasma enclosure (20) by said electrical power supply assembly, said servo device being designed to maintain constant a value of difference between an electric current (IPHV) which flows from the power supply assembly to the screen grid (24) and the acceleration grid current (INHV), and according to which step /3/ is executed while the value of the difference between the current electrical (IPHV) which circulates from the power supply assembly (1) to the screen grid (24) and the acceleration grid current (INHV) is kept constant by the servo device (18a, 18b). [Claim 9] Method according to any one of claims 1 to 7, according to which step 73/ is executed while an electrical plasma generation power which is delivered to the plasma enclosure (20) by the power supply assembly is kept constant. [Claim 10] Method according to any one of the preceding claims, wherein the maximum limit provided in step 121 for the acceleration gate current (INHV) is between 0.5% and 5% of a value of a beam current (beam) which exits the thruster (2). [Claim 11] Method according to claim 10, according to which the maximum limit provided in step 121 for the acceleration gate current (INHV) is substantially equal to 3.125% of the value of the beam current (beam). [Claim 12] Method according to any one of the preceding claims, according to which the thruster (2) is of one of the following types: ion thruster with grids and continuous discharge, or ion thruster with grids and radio frequency discharge. [Claim 13] Electrical power supply assembly (1) for ion thruster with grids (2), the thruster comprising: a plasma enclosure (20), a screen grid (24) which is located in front of or at the level of an outlet opening of the plasma enclosure, and an acceleration grid (25) which is parallel to the screen grid and located on one side of said screen grid opposite the plasma enclosure, power supply assembly (1) comprising: - a common reference node (CRP); - a positive electrical power supply unit (12), intended to be connected to the screen grid (24) of the thruster (2) to provide said screen grid, during operation of said thruster, with a positive electrical voltage (VPHV) relative to the common reference node (CRP); - a negative electrical power supply unit (13), intended to be connected to the acceleration grid (25) of the thruster (2) to supply said acceleration grid, during operation of said thruster, with a negative electrical voltage ( VNHV) relative to the common reference node (CRP); - a variation module (15), arranged to vary the positive electrical voltage (VPHV) supplied by the positive electrical power supply unit (12); - measurement and recording modules (16), arranged to measure and record values of an electric current which circulates from the acceleration grid (25) to the electrical power supply assembly (1), called current acceleration grid (INHV); And - a controller (17), configured to activate the variation module (15) and the measurement and recording modules (16), so that variable values are produced successively for the positive electrical voltage (VPHV) while the negative electrical voltage (VNHV) is kept constant, and a value of the acceleration gate current (INHV) is measured and recorded for at least one supplied value of the positive electrical voltage, when the power supply assembly ( 1) is connected to the thruster (2) to enable operation of said thruster, the electrical power supply assembly (1) being adapted to carry out a method which is in accordance with any one of claims 1 to 12. [Claim 14] Plasma propulsion system (10) comprising: - an ion thruster with grids (2), the thruster comprising: a plasma enclosure (20), a screen grid (24) which is located in front of or at the level of an outlet opening of the plasma enclosure, and an acceleration grid (25) which is parallel to the screen grid and located on one side of said screen grid opposite the plasma enclosure, so that during operation of said thruster, positive ions which are produced in the plasma enclosure pass through the screen grid then the acceleration grid; And - an electrical power supply assembly (1) which conforms to claim 13, and which is connected to the thruster (2) to produce operation of said thruster. [Claim 15] Space vehicle (100), comprising a plasma propulsion system (10) which conforms to claim 14.