EP1101938B1 - Closed electron drift plasma thrustor with orientable thrust vector - Google Patents

Description

Field of the invention

The invention relates to a steerable thrust vector electron drift plasma thruster comprising at least one main annular ionization and acceleration channel equipped with an anode and means for supplying ionizable gas, a circuit magnetic field for creating a magnetic field in said main annular channel, and a hollow cathode associated with means for supplying ionizable gas.

Prior art

The orientation of the thrust vector of ionic thrusters or closed-electron drift thrusters allows attitude control operations to be performed by offsetting the thrust vector from the center of gravity of the satellite or, on the contrary, to eliminate parasitic couples by aligning the thrust vector so as to follow the displacements of the center of gravity of the satellite induced by the thermal deformations and the exhaustion of the propellants.

This need has been recognized since the 1970s. Since the control mechanisms of the thrust vector are inherently quite complex, numerous attempts have been made to replace this mechanical thrust control with electrostatic or electromagnetic control.

In the case of ionic thrusters to bombardment, electrostatic deflection may have seemed the most suitable. The most commonly used technique has been to divide each accelerator gate hole into four sectors whose potential can be controlled independently, the deflection angle being up to 3 °. However, no industrial realization has been carried out with this type of technique.

The ionic bomb thrusters thus generally use a mechanical thrust steering device.

By way of example, mention may be made of the Hughes XIPS 13 thrusters on the HS 601 HP satellite and the RIT 10 and UK 10 thrusters on the ARTEMIS experimental satellite.

With regard to closed-electron drift thrusters, electromagnetic deflection seemed the most suitable. Indeed, the electric field in a plasma thruster is determined by the radial magnetic field in the gap. If the radial magnetic field is varied in azimuth, the electric field is also varied. The deformation of the equipotentials then causes an angular deflection of the thrust vector.

This solution is presented for example in the document US-A-5,359,258.

In such a case, the outer pole piece is divided into four sectors, each sector being mounted on a magnetic core with a coaxial coil. The differential supply of the coils makes it possible to modify the azimuthal distribution of the magnetic field.

This provision has however never been used on an operational thruster.

EP 0 800 196 A1 also discloses a thrust orientation system according to which four coils mounted on four magnetic cores in the shape of an arc allow the radial magnetic field to be varied in azimuth.

If the various electromagnetic control techniques of the thrust vector of a closed-drift electron propellant make it possible to obtain deflection angles of up to 3 °, they present a series of disadvantages due to the very physics of these thrusters. In particular, locally increasing the electric field changes the position of the erosion zone. The wear profile, instead of being axisymmetric will thus be more pronounced on one side (the displacement of the center of gravity of a satellite is deterministic). Since it is necessary to change the aiming point of the beam, the interface between the plasma and the used wall of the channel will not be symmetrical. This will result in a more pronounced wear on the side previously subjected to moderate wear but especially a displacement of the wear threshold, which can greatly disrupt the operation.

It should also be noted that a life test is difficult to specify with an electromagnetic control device. Indeed, since the service life may be a function of the thrust vector control law, it becomes almost impossible to demonstrate that the law of thrust vector control used in lifetime testing is more severe than a random law encountered in actual operation.

Another drawback is related to the significant yield drop when the ion beam (the thrust vector) is deflected.

Indeed, in an axisymmetric thruster, nothing is opposed to the electron drift movement in the annular channel under the effect of electric and magnetic cross fields (hence the name of drift propellants closed electrons).

If we shift the walls of the channel vis-à-vis the pole pieces, there is a decrease in efficiency that is due to the increase in electron-wall collisions.

The same effect occurs if the magnetic field is increased locally. It will be aggravated by asymmetrical wear.

A simple way of controlling the thrust vector may be to use several thrusters whose thrust is individually controlled.

It is then very easy to fix the direction and amplitude of the resulting thrust vector and the service life becomes independent of the thrust orientation law. Such a method however has the disadvantage of being very expensive when it takes at least three thrusters and three power supplies.

Object and brief description of the invention

The invention aims to remedy the aforementioned drawbacks and in particular to allow control of the thrust vector using a system that does not increase excessively the mass of the onboard assembly and its cost, and therefore does not include a complete set of multiple thrusters, while ensuring easy and effective control of the orientation of the thrust vector, with sufficiently large deviation angles, and without the creation of uncontrollable asymmetries.

These objects are achieved by means of a steerable thrust vector electron drift plasma propellant comprising at least one main ionization and acceleration annular channel equipped with an anode and means for supplying ionizable gas, a magnetic circuit for creating a magnetic field in said main annular channel, and a hollow cathode associated with gas supply means ionizable, characterized in that it comprises a plurality of main annular ionization and acceleration channels having non-parallel axes which converge on the downstream output side of said main annular channels, in that the magnetic circuit for creating a magnetic field comprises a first external downstream pole piece common to all the annular channels, a second external pole piece common to all the annular channels and arranged upstream of the first external downstream pole piece, a plurality of internal pole pieces in number equal to number of main annular channels and mounted on first cores arranged around the axes of the main annular channels, a plurality of first coils disposed respectively around the plurality of first cores, a plurality of second coils mounted on second cores arranged in free spaces arranged between the main ring channels blades, said second cores of the second coils being interconnected in their upstream portion by ferromagnetic bars and being connected in their downstream portion to said first outer downstream pole piece, and in that it comprises means for regulating the flow rate of the supply of ionizable gas from each main annular channel and means for controlling the discharge current and acceleration of the ions in the main annular channels.

The axes of the main annular channels of ionization and acceleration converge on the geometric axis of the thruster and can form with the geometric axis of the propeller angles between 5 ° and 20 °.

Each main annular ionization and acceleration channel comprises an anode associated with a distributor supplied with ionizable gas by means of a pipe connected by an insulator to a flow regulator.

The hollow cathode is fed by a pipe connected by an insulator to a pressure drop member.

The flow regulators and the pressure drop member are fed by a common pipe controlled by a solenoid valve.

The thruster comprises a power supply circuit for establishing the discharge between the hollow cathode and the anodes and the Discharge oscillators of the main annular channels are decoupled by filters placed between the cathode and the anodes.

To control the discharge currents of the anodes, the thruster comprises servocontrol loops comprising current sensors and a current regulator acting on the flow regulators and receiving a total discharge current setpoint and at least one deflection setpoint of thrust vector for control over at least one axis, the ion discharge and acceleration current being controlled by a magnetic field distribution determined by said magnetic circuit in which the plurality of first coils and the plurality of second coils are mounted series between the cathode and the negative terminal of the power supply circuit.

Flow controllers can be constituted by thermocapillaries controlled by servocontrol loops of the discharge currents or by microelectrovalves dosing thermal actuator, piezoelectric or magnetostrictive.

The current sensors can be galvanically isolated to measure the current of each of the anodes at a potential of several hundred volts.

Advantageously, the flow range in each main annular channel is between 50% and 120% of the nominal flow rate.

The number of second coils can be between 4 and 10.

According to various possible embodiments, the thruster may comprise two main annular channels, or three main annular channels distributed in a triangle around the axis of the thruster or four main annular channels distributed in a square around the axis of the thruster.

According to a particular embodiment, the number of second coils is a multiple of the number of main annular channels, the coils of each subset of second coils assigned to each channel are connected in series and the different subsets of second coils are mounted. in parallel, the impedances of the coils connected in series being equal.

According to another particular embodiment, the number of second coils is a multiple of the number of main annular ionization and acceleration channels and the coils of each of subassemblies of second coils assigned to the different channels are fed by a current vernier.

According to a particular embodiment, the thruster comprises a digital loop for controlling the orientation of the thrust vector, the total thrust and the thrust vector deflection instructions being given in digital form, and the thrust vector deviation instruction having priority on the total thrust instruction in case of incompatibility between the two instructions.

Advantageously, the thruster comprises a common base acting as a radiator and housing for the electrical and fluidic connections.

According to one embodiment, the means for regulating the flow rate of the ionizable gas supply receive two guidelines for a thrust vector deviation for a control along two axes.

According to a particular embodiment, the thruster comprises two main annular ionization and acceleration channels making it possible to perform a control along a first axis using means for regulating the flow rate of the ionizable gas supply, and it further comprises mechanical means of articulation of the base of the thruster around another axis.

In this case, the base of the thruster is articulated around the second axis with a maximum angle of 50 °.

According to a particular aspect, the base of the thruster is articulated around said second axis on two bearings prestressed by at least one flexible membrane mounted on a fixed platform and directly fixed to the base, the center of gravity of the moving assembly. being located in the vicinity of the axis of rotation and the angle of rotation being controlled by an electric motor and a gearbox ensuring the angular locking.

Brief description of the drawings

• la figure 1 est une vue schématique de côté montrant un premier exemple de propulseur à plasma selon l'invention, à deux canaux annulaires principaux,
• la figure 2 est une vue de face prise de l'aval montrant le propulseur à plasma de la figure 1,
• la figure 3 est une vue en perspective, avec coupe partielle, d'un mode particulier de réalisation du propulseur à plasma selon les figures 1 et 2,
• la figure 4 est un schéma électrique et fluidique d'un deuxième exemple de propulseur à plasma selon l'invention, à trois canaux annulaires principaux,
• la figure 5 est une vue schématique de côté montrant un exemple de propulseur à plasma selon l'invention, à trois canaux annulaires principaux répartis en triangle à sept bobines externes,
• la figure 6 est une vue de face prise de l'aval montrant le propulseur à plasma de la figure 5,
• la figure 6 A est un schéma montrant l'inclinaison des canaux du propulseur des figures 5 et 6,
• la figure 7 est une vue schématique de côté montrant un autre exemple de propulseur à plasma selon l'invention, à trois canaux annulaires principaux répartis en triangle et à dix bobines externes,
• la figure 8 est une vue de face prise de l'aval montrant le propulseur à plasma de la figure 7,
• la figure 9 est une vue schématique de côté montrant un exemple de propulseur à plasma selon l'invention, à quatre canaux annulaires principaux répartis en carré et à neuf bobines externes,
• la figure 10 est une vue de face prise de l'aval montrant le propulseur à plasma de la figure 9,
• la figure 10A est un schéma montrant l'inclinaison des canaux du propulseur des figures 9 et 10,
• la figure 11 est une vue schématique de côté montrant encore un autre exemple de propulseur à plasma selon l'invention, à deux canaux annulaires principaux et six bobines externes, équipé en outre d'un axe de pointage mécanique,
• la figure 12 est une vue de face prise de l'aval montrant le propulseur à plasma de la figure 11,
• la figure 13 est une vue de côté prise selon la flèche F de la figure 12 et montrant des détails de réalisation de l'axe de pointage mécanique,
• la figure 14 est une vue en perpective avec coupe axiale, d'une anode pouvant être incorporée dans chacun des canaux annulaires principaux du propulseur selon l'invention,
• la figure 15 est une vue en demi-coupe axiale, montrant un mode de réalisation possible d'un canal annulaire principal d'un propulseur selon l'invention, et
• la figure 16 est une vue de côté montrant un propulseur à plasma de l'art antérieur, comprenant un seul canal annulaire principal et des moyens de pointage mécanique.

Other features and advantages of the invention will emerge from the following description of particular embodiments, given by way of example, with reference to the appended drawings, in which:

• FIG. 1 is a schematic side view showing a first example of a plasma thruster according to the invention, with two main annular channels,
• FIG. 2 is a front view taken downstream showing the plasma thruster of FIG. 1,
• FIG. 3 is a perspective view, partly in section, of a particular embodiment of the plasma thruster according to FIGS. 1 and 2,
• FIG. 4 is an electric and fluidic diagram of a second example of a plasma thruster according to the invention, with three main annular channels,
• FIG. 5 is a schematic side view showing an example of a plasma thruster according to the invention, with three main annular channels distributed in a triangle with seven outer coils,
• FIG. 6 is a front view taken downstream showing the plasma thruster of FIG. 5,
• FIG. 6A is a diagram showing the inclination of the thruster channels of FIGS. 5 and 6,
• FIG. 7 is a schematic side view showing another example of a plasma thruster according to the invention, with three main annular channels distributed in a triangle and with ten external coils,
• FIG. 8 is a front view taken downstream showing the plasma thruster of FIG. 7,
• FIG. 9 is a schematic side view showing an example of a plasma thruster according to the invention, with four main annular channels divided into squares and nine external coils,
• FIG. 10 is a front view taken downstream showing the plasma thruster of FIG. 9,
• FIG. 10A is a diagram showing the inclination of the thruster channels of FIGS. 9 and 10,
• FIG. 11 is a schematic side view showing yet another example of a plasma thruster according to the invention, with two main annular channels and six external coils, furthermore equipped with a mechanical pointing axis,
• FIG. 12 is a front view taken downstream showing the plasma thruster of FIG. 11,
• FIG. 13 is a side view taken along the arrow F of FIG. 12 and showing details of the realization of the mechanical pointing axis,
• FIG. 14 is a perspective view in axial section of an anode that can be incorporated into each of the main annular channels of the thruster according to the invention,
• FIG. 15 is an axial half-sectional view, showing a possible embodiment of a main annular channel of a thruster according to the invention, and
• Fig. 16 is a side view showing a plasma thruster of the prior art, comprising a single main annular channel and mechanical pointing means.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS OF THE INVENTION

In the following description of various examples of electron drift plasma thrusters equipped with several main annular ionization and acceleration channels, the similar elements of the different main annular channels, or associated with the different channels, will carry the same references, but followed by the letter A, B, C or D depending on whether it is a first, a second, a third or a fourth annular channel of the same thruster.

Figures 1 to 3 show a plasma thruster with two main annular channels 124A, 124B arranged side by side and defining a substantially rectangular configuration. The axes 241A, 241B of the two channels 124A, 124B are inclined at an angle 242 with respect to the geometric axis 752 of the thruster. A single hollow cathode 140 is associated with the two main channels 124A, 124B.

A conventional single-channel main plasma thruster, such as that shown in FIG. 16, comprises in principle four outer coils 31 associated with an outer pole piece 34.

In the case of a plasma thruster according to the invention, with two main channels 124A, 124B, it is possible to merge the two adjacent outer coils 131 located in the vicinity of the median portion between the two channels 124A, 124B. In this way, it is possible to use only six external coils 131 connected to a common external pole piece 134 having a very open V shape (FIGS. 1 and 2)

Internal pole pieces 135A, 135B are mounted on first cores 138A, 138B arranged around the axes 241A, 241B of the main annular channels 124A, 124B, and are therefore equal in number to the number of annular channels 124A, 124B. Internal coils or first coils 133A, 133B disposed around the first cores 138A, 138B are also equal in number to the number of annular channels 124A, 124B (Figure 3).

The outer coils 131, or second coils, are mounted on second cores 137 disposed in free spaces formed between the main annular channels 124A, 124B. The cores 137 of the coils 131 are connected in their downstream part to the outer downstream pole piece 134. Another upstream outer pole piece 311 comprising portions 311A, 311B arranged around the annular channels 124A, 124B is arranged upstream of the first pole piece external downstream 134 (Figures 3 and 15).

The channels 124A, 124B and the magnetic circuit elements are integral with a base 175, preferably light alloy, which acts as a radiator. The electrical and fluid connections are housed in cavities formed in this base.

The magnetic circuit can be produced for example in a manner similar to that described in US Pat. No. 5,359,258 or in a manner similar to that described in French patent application 98,10674 filed on August 25, 1998, and illustrated on Figures 3 and 15.

If we consider more particularly Figures 3, 14 and 15, we see that each annular channel such as 124A is delimited by insulating walls 122A, is open at its downstream end and has a frustoconical section at its upstream portion and cylindrical at its downstream part. An annular anode 125A has a conical section shaped frustum open downstream. The anode 125A may have slits 117A made in the solid portion 116A of the anode 125A to increase the area of contact with the plasma. Injection holes 120A of an ionizable gas from an ionizable gas distributor 127A are formed in the wall of the anode 125A. The distributor 127 A is supplied with ionizable gas via a pipe 126A. The anode 125A may be supported with respect to the 122A ceramic material delimiting the channel 124A, for example by a solid column 114A circular section and by at least two columns 115A thinned flexible blades. An isolator 300A is interposed between the pipe 126A and the anode 125A which is connected by an electrical connection 145A to the positive pole of the power supply of the anode-cathode discharge.

The inner pole piece 135A is extended by a central axial magnetic core 138A which is itself extended to the upstream portion of the thruster by a plurality of radial arms 352A connected to a second conical upstream inner pole piece 351A. A second internal magnetic coil 132A can be placed in the upstream portion of the second inner pole piece 351 A, outside thereof. The magnetic field of the internal coil 132A is channeled by radial arms 136 placed in the extension of the radial arms 352A, as well as by the outer pole piece 311A and the inner pole piece 351A. A small gap 361 may be provided between the radial arms 352A and the radial arms 136.

Sheets of screen-insulating material 130A are disposed upstream of the annular channel 124A and superimposing material sheets 301A forming a screen are also interposed between the channel 124A and the inner coil 133A. The screens 130A, 301A eliminate most of the flux radiated by the channel 124A to the coils 133A, 132A and the base 175.

In the context of the multichannel plasma thruster 124A, 124B according to the invention it is possible to use a single cathode 140 for supplying the two channels 124A, 124B. Indeed, the cathode 140 creates a plasma cloud which makes its positioning relatively insensitive with respect to one of the beams and moreover the axes 241A, 241B of the channels 124A, 124B being convergent, this leads to a crossing of the plasma beams which significantly decreases the impedance between the beams. However, it is not excluded to add a redundant cathode if this is necessary, especially if the number of channels is greater than or equal to four.

The two-channel thruster 124A, 124B of FIGS. 1 to 3 allows control of the thrust vector along an axis.

Three-channel thruster configurations 124A-124C, such as those shown in FIGS. 5-8, provide two-axis thrust vector control.

In the embodiment of FIGS. 5 and 6, the axes 241A, 241B, 241C of the three main annular channels 124A, 124B, 124C arranged in a triangle converging towards the axis 752 of the thruster. Each channel 124A-124C is surrounded by four outer coils 131 in a "diamond" configuration. Some coils 131 cooperate with two neighboring channels, so that the total number of external coils 131 is reduced to 7 instead of 12.

The number of ampere-turns of the outer coils 131 is adjusted according to the perimeter of the pole pieces to feed. This number of ampere-turns is identical for the four most central coils while the three outer coils 131 located near the vertices of the triangle defined by the channels 124A to 124C comprise two-thirds of the number of revolutions of the outer coils 131 .

The other main elements of the three-channel thruster 124A, 124B, 124C are similar to those of the two-channel thruster 124A, 124B, in particular with regard to the light alloy common base 175, the common cathode 140, the magnetic cores 138A at 138C of the internal coils 133A to 133C and the magnetic cores 137 of the external coils 131 interconnected by a network of ferromagnetic bars 136.

Figures 7 and 8 show a thruster with three main annular channels 124A, 124B, 124C which differs from the embodiment of Figures 5 and 6 only by the number and arrangement of the outer coils 131.

In the case of the embodiment of Figures 7 and 8, there are ten outer coils 131. These are distributed so that each main annular channel 124A, 124B, 124C is surrounded by five coils forming an irregular pentagon. This irregular character is due to the angle of convergence of the channels, which is of the order of 10 °. A regular pentagon could be obtained if the angle of convergence of the channels was greater, of the order of 37 °. Some of the outer coils 131 play a role simultaneously for two or three channels 124A-124C, so that the total number of outer coils 131 is reduced to 10 instead of 15. The common pole piece 134 averages the field.

The arrangement of FIGS. 7 and 8 is of interest for large thrusters, for which it is preferable to split the outer coils 131 in order to lighten the outer pole piece 134. The outer pole piece 134 and the base 175 have the shape of an irregular hexagon with six outer coils 131 placed in the vicinity of the vertices of the hexagon and four outer coils 131 distributed in a star between the three channels 124A to 124C.

FIGS. 9 and 10 show a thruster with four main annular channels 124A, 124B, 124C, 124D disposed essentially in a square and associated with nine external coils 131. Each channel 124A to 124D is surrounded by four external coils 131. External coils 131 play a role vis-à-vis several channels. Only the coils 131 located near the corners of the pole piece 134 and the base 175 of substantially square shape, play a role vis-à-vis only one channel 124A to 124D. In this way, the number of external coils 131 can be reduced from 16 to 9.

To obtain a determined deflection, it is necessary to increase the angle 242 of the axes 241A to 241D with respect to the axis 752, this angle 242 becoming twice that envisaged in the case of a two-channel thruster.

Referring to FIGS. 11 to 13, a propellant according to the invention with two channels 124A, 124B is shown essentially similar to the thruster of FIGS. 1 to 3. However, in the case of FIGS. 11 to 13, the thruster is in addition equipped with means of uniaxial mechanical orientation.

The two main annular channels 124A, 124B and their associated six outer coils 131 provide smooth and easy control of the orientation of the vector pushed along a first axis, with an angle that can be between 5 ° and 20 °. The uniaxial mechanical orientation means make it possible to control the orientation of the thrust vector along a second axis, with an important angle 783, for example of the order of 50 °.

It should be noted that a uniaxial mechanical orientation system is much simpler, lighter and more robust than a two-axis mechanical orientation system. In particular, in the case of a uniaxial system, the center of gravity 751 of the thruster may be located on the axis of rotation 782 of the orientation device, which then dispenses to implement a locking device. The angular locking can indeed be obtained directly by means of an irreversible rotation control mechanism comprising for example an electric motor 177 and a reducer 179. The axis of rotation 782 of the cradle 175 of the mechanically-oriented thruster may be embodied by two angular contact bearings 178 capable of withstanding the dynamic forces during the launching of the thruster. At least one of the angular contact bearings 178 may be mounted on an elastic membrane 781 to ensure a constant and independent preload of thermal gradients preventing jamming, as described for example in European Patent 0 325 073. The elastic membrane 781 is itself mounted on a fixed base 176. The electrical connections are provided by flexible cables and the supply of ionizable gas by elastic conduits.

The two-channel thruster 124A, 124B with uniaxial mechanical orientation is particularly useful when it comes to pointing the thrust vector at a large angle on one axis and at a lower angle on the other.

This is particularly the case for telecommunications satellites using plasma propulsion for the end of a transfer between a geostationary transfer orbit (GTO) and a final geostationary orbit (GEO), then for a North-South control, as well as only for missions requesting a push vector law in the orbital plane, then outside the orbital plane (tilt correction for GTO-GEO transfer or for some planetary missions).

In general, according to the invention, the control of the thrust vector is obtained by separately supplying to the propulsion fluid several main annular ionization and acceleration channels 124A to 124D included in a common magnetic circuit 134, connected to a cathode single hollow 140 and a single power supply 190 (Figure 4).

For a fixed radial magnetic field (determined by the current flowing in the common hollow cathode 140), there is a certain margin of mass flow, and therefore of discharge current, for a closed-electron drift motor operating in an unfocused mode ( still says "fashion rod" or "spike mode" in English). Since the thrust is substantially proportional to the discharge current and to the mass flow rate in a small area around the nominal operating point, it becomes easy to control the individual thrust of each channel 124A to 124D. modifying the mass flow. This is easily achieved with the aid of individual flow controllers 185A through 185D including, for example, a thermocapillary controlled by a discharge current control loop. It is also possible to use a metering micro-solenoid valve (with thermal actuator, or piezoelectric or magnetostrictive).

In conventional stationary plasma thrusters, a current sensor is located on the current return line (at a potential near the ground, because equal to the cathode potential minus the voltage drop in the coils)

In this case, it is necessary to measure the current of each anode. Since the anode potential is 300 V, it is preferable to carry out this measurement by a galvanically isolated current sensor 193A to 193D. For example, the current differential between two wires can be measured by placing a Hall sensor on the axis of two coiled-in solenoids, each solenoid being traversed by the current of an anode.

Figure 4 shows the wiring diagram of a three-channel thruster 124A-124C (thus three anodes 125A-125C). Each anode 125A to 125C is connected to the common supply via a filter consisting of an L-C circuit (911A to 911C). This makes it possible to decouple the oscillation frequencies between each channel which may be slightly different because of the different flow rates.

With respect to a power supply unit supplying a single thruster, the only complication is the addition of additional flow regulator controls and galvanically isolated differential current sensors (92, 921, 922).

The diagram of Figure 4 is naturally applicable to a four-channel embodiment 124A to 124D such as that of Figures 9 and 10. In this case, it is only added an additional branch whose elements are assigned the letter D.

In each branch corresponding to a channel 124A to 124D, a chamber comprises an anode 125A to 125D and a distributor 127A to 127D, supplied with ionizable gas by means of a line 118A to 118D, from an isolator (300A to 300D) and a flow regulator (185A to 185D), connected by a common supply pipe section 126 controlled by a solenoid valve 187. The common channel 126 also feeds the hollow cathode 140 by means of a pressure drop member 186 and an isolator 300. The discharge is established between the hollow cathode 140 and the anodes 125A to 125D by means of a power supply circuit 191 The discharge oscillations of the different channels are decoupled by filters 911 A to 911 D placed between the different anodes 125A to 125D and the cathode 140. The discharge current of each anode is controlled by a servo control loop comprising a sensor. current 193A to 193D, preferably electrically isolated, a regulator 192 receiving a setpoint 922 of thrust vector deflection for one-axis control, or two setpoints 922 of thrust vector deflection for a two-axis control, and a setpoint 921 current total discharge. The discharge current and the acceleration of the ions are controlled by the magnetic field distribution determined by the external downstream pole piece 134 common to all the channels, the upstream external pole piece 311 common to all the channels, the external coils 131 mounted on cores 137 and inner pole pieces 135A-135D mounted on cores 138A-138D provided with coils 133A-133D. The ends of all the pole pieces have coaxial core profiles at axes 241A through 241D of channels 124A through 124D. The internal coils 133A to 133D and external coils 131 are connected in series between the cathode and the negative terminal of the power supply circuit 191 while the various cores are connected upstream by the ferromagnetic bars 136. The control circuits make it possible to define in each channel 124A to 124D a flow range typically between 50% and 120% of the nominal flow rate.

Various embodiments of the control circuits are possible.

Thus, according to a particular variant, the number of external coils 131 is a multiple of the number of main annular channels 124A to 124D, the coils of each subset of coils 131 assigned to each channel 124A to 124D are connected in series and the different Subsets of coils 131 are connected in parallel, the impedances of the coils connected in series being equal.

According to another variant, the number of external coils 131 is a multiple of the number of annular channels 124A to 124D and the coils each of the subsets of coils 131 allocated to the different channels are fed by a current vernier.

According to yet another variant, a digital loop for controlling the orientation of the thrust vector is provided, the commands for total thrust and deflection of the thrust vector being given in digital form, and the thrust reference of the thrust vector takes precedence. on the total thrust instruction in case of incompatibility between the two instructions.

It will be noted that the multi-channel thruster according to the invention is capable of providing the same thrust control capability as a single thruster mounted on a turntable allowing a travel of 3 °.

In the case of a single thruster applied for example to a constellation satellite, the distance between the thruster and the center of gravity of the satellite is of the order of 1m. The torque induced by a thrust F with a deflection angle of θ degrees is equal to C = F.sinθ. either for θ = 3 ° C = 0.0523 F

$$\mathrm{C}\mathrm{=}0,21136\left(\mathrm{\Delta }{\mathrm{F}}_{\mathrm{1}}\mathrm{-}\mathrm{\Delta }{\mathrm{F}}_{\mathrm{2}}\right)$$

In the case of a thruster according to the invention with two channels 140 mm apart, with a unitary beam diameter of 100 mm and a nominal unit thrust F1 = F / 2, if the axes of the individual channels have a divergence angle with a half-angle α of 10 °, the variation of torque allowed by the individual thrust variation of each channel will be: $$\mathrm{VS}\mathrm{=}\left(\mathrm{0}\mathrm{,}\mathrm{07}\mathrm{+}\sin \mathrm{10}\mathrm{°}\right)\left(\mathrm{\Delta }{\mathrm{F}}_{\mathrm{1}}\mathrm{-}\mathrm{\Delta }{\mathrm{F}}_{\mathrm{two}}\right)$$

$$\mathrm{VS}\mathrm{=}0,21136\left(\mathrm{\Delta }{\mathrm{F}}_{\mathrm{1}}\mathrm{-}\mathrm{\Delta }{\mathrm{F}}_{\mathrm{two}}\right)$$

If the absolute values of the variations are equal, by execution of a control law, we obtain: $$\mathrm{<figure-callout\; id="1"\; label="\Delta \; F"\; filenames="imgf0009.png"\; state="\{\{state\}\}">\Delta \; </figure-callout>}{\mathrm{F}}_{}{\mathrm{}}_{\mathrm{1}}\mathrm{=}\mathrm{0}\mathrm{,}\mathrm{215}\mathrm{}{\stackrel{\mathrm{~}}{\mathrm{F}}}_{\mathrm{1}}$$

The variation of thrust which is thus of the order of 20%, is easily controllable.

In terms of additional mass of ionizable gas embedded on a satellite, such as a telecommunications satellite of 150 kg, it may be noted that in the case of embodiments of the prior art comprising two orientation plates, the additional onboard mass is greater at 12kg. In the case of a thruster according to the invention to a single plate but multiple channels, it is necessary to embark a additional mass of ionizable gas such as xenon of the order of 2kg which is very significantly lower than the additional mass induced by the devices of the prior art with two orientation plates.

Claims (24)

1. A closed electron drift plasma thruster having a steerable thrust vector, the thruster comprising at least one main annular ionization and acceleration channel fitted with an anode and ionizable gas feed means, a magnetic circuit for creating a magnetic field in said main annular channel, and a hollow cathode (140) associated with the ionizable gas feed means, the thruster being characterised in that it comprises a plurality of main annular ionization and acceleration channels (124A to 124D) having axes that are not parallel (241A to 241D) and that converge downstream from the outlets of said main annular channels (124A to 124D), in that the magnetic circuit for creating a magnetic field comprises a first external polepiece (134) that is downstream and common to all of the annular channels (124A to 124D), a second external polepiece (311) common to all of the annular channels (124A to 124D) and that is disposed upstream from the downstream first external polepiece (134), a plurality of internal polepieces (135A to 135D) in number equal to the number of main annular channels (124A to 124D) and mounted on first cores (138A to 138D) disposed about the axes (241A to 241D) of the main annular channels (124A to 124D), a plurality of first coils (133A to 133D) disposed respectively around the plurality of first cores (138A to 138D), and a plurality of second coils (131) mounted on second cores (137) disposed in empty spaces left between the main annular channels (124A to 124D), said second cores (137) of the second coils (131) being interconnected via their upstream portions by ferromagnetic bars (136) and being connected via their downstream portions to said downstream first external polepiece (134), and in that it comprises means (192) for regulating the ionizable gas feed flow rate to each of the main annular channels (124A to 124D) and means (191) for controlling the ion discharge and acceleration current in the main annular channels (124A to 124D).
2. A plasma thruster according to claim 1, characterised in that the axes (241A to 241D) of the main annular ionization and acceleration channels (124A to 124D) converge on the geometrical axis (752) of the thruster.
3. A plasma thruster according to claim 1 or claim 2, characterised in that the axes (241A to 241A) of the main annular ionization and acceleration channels (124A to 124D) form angles lying in the range 5° to 20° with the geometrical axis (752) of the thruster.
4. A plasma thruster according to any one of claims 1 to 3, characterised in that each main annular ionization and acceleration channel (124A to 124D) comprises an anode (125A to 125D) associated with a manifold (127A to 127D) fed with ionizable gas by means of a pipe (118A to 118D) connected via an isolator (300A to 300D) to a flow rate regulator (185A to 185D).
5. A plasma thruster according to any one of claims 1 to 4, characterised in that the hollow cathode (140) is fed by a pipe connected via an isolator (300) to a head loss member (186).
6. A plasma thruster according to claim 4 and claim 5, characterised in that the flow rate regulators (185A to 185D) and the head loss member (186) are fed from a common pipe (126) controlled by an electrically controlled valve (187).
7. A plasma thruster according to claims 4 and 5, characterised in that it comprises an electrical power supply circuit (191) for setting up discharge between the hollow cathode (140) and the anodes (125A to 125D), and in that the discharge oscillations of the main annular channels (124A to 124D) are decoupled by filters (911A to 911D) placed between the cathode (140) and the anodes (125A to 125D).
8. A plasma thruster according to claim 7, characterised in that to control the discharge currents of the anodes (125A to 125D), it comprises servo-control loops comprising current pickups (193A to 193D) and a current regulator (192) acting on the flow rate regulators (185A to 185D) and receiving a total current discharge reference value (921) and at least one thrust vector deflection reference value (922) for steering about at least one axis, the ion discharge and acceleration current being controlled by a magnetic field distribution determined by said magnetic circuit in which the plurality of first coils (133A to 133D) and the plurality of second coils (131) are connected in series between the cathode (140) and the negative terminal of the electricity power supply circuit (191).
9. A plasma thruster according to claim 8, characterised in that the flow rate regulators (185A to 185D) are constituted by thermocapillary means controlled by discharge current servo-control loops.
10. A plasma thruster according to claim 8, characterised in that the flow rate regulators (185A to 185D) are constituted by electrically controlled micromeasuring valves that are actuated thermally, piezoelectrically, or magnetostrictively.
11. A plasma thruster according to claim 8, characterised in that the current pickups (193A to 193D) are electrically-isolated in order to measure the current in each of the anodes (125A to 125D) at a potential of several hundred volts.
12. A plasma thruster according to any one of claims 1 to 11, characterised in that the range of flow rates in each main annular channel (124A to 124D) extends from 50% to 120% of the nominal flow rate.
13. A plasma thruster according to any one of claims 1 to 12, characterised in that the number of second coils (131) lies in the range 4 to 10.
14. A plasma thruster according to any one of claims 1 to 13, characterised in that it comprises a common baseplate (175) acting as a radiator and as a housing for the electrical and fluid connections.
15. A plasma thruster according to any one of claims 1 to 14, characterised in that it comprises two main annular ionization and acceleration channels (124A, 124B).
16. A plasma thruster according to claims 14 and 15, characterised in that it comprises two main annular ionization and acceleration channels (124A, 124B) making it possible to provide control about a first axis using means (192) for adjusting the ionizable gas feed rate, and in that it further comprises mechanical hinge means to the baseplate (175) of the thruster about a different axis.
17. A plasma thruster according to claim 16, characterised in that the baseplate (175) of the thruster is hinged about said second axis (782) with a maximum angle (783) of 50°.
18. A plasma thruster according to claim 16 or claim 17, characterised in that the baseplate (175) of the thruster is hinged about said second axis (782) on two ball bearings (178) which are prestressed by at least one flexible membrane (781) mounted on a fixed platform (176) and which are fixed directly to the baseplate (175), the center of gravity (751) of the moving assembly being situated close to the vicinity of the axis of rotation (782) and the angle of rotation (783) being controlled by an electric motor (177) and a stepdown gear (179) that provide angular locking.
19. A plasma thruster according to any one of claims 1 to 14, characterised in that it comprises three main annular ionization and acceleration channels (124A to 124C) distributed in a triangle about the axis (752) of the thruster.
20. A plasma thruster according to any one of claims 1 to 14, characterised in that it comprises four main annular ionization and acceleration channels (124A to 124D) disposed in a square about the axis (752) of the thruster.
21. A plasma thruster according to any one of claims 1 to 20, characterised in that the number of second coils (131) is a multiple of the number of main annular ionization and acceleration channels (124A to 124D), in that the coils of each subset of second coils (131) allocated to each channel (124A to 124D) are connected in series, and in that the various subsets of second coils (131) are connected in parallel, with the impedances of the coils connected in series being equal.
22. A plasma thruster according to any one of claims 1 to 20, characterised in that the number of second coils (131) is a multiple of the number of main annular ionization and acceleration channels (124A to 124D), and in that the coils of each of the subsets of second coils (131) allocated to the various channels (124A to 124D) are powered via a current vernier.
23. A plasma thruster according to any one of claims 1 to 20, characterised in that it comprises a digital servo-control loop for steering the thrust vector, the total thrust reference value and the thrust vector deflection value being given in digital form, and the thrust vector deflection reference value having priority over the total thrust reference value in the event of the two reference values being incompatible.
24. A plasma thruster according to any one of claims 1 to 15, 19, and 20, characterised in that the means (192) for regulating the ionizable gas feed rate receive two reference values (922) for thrust vector deflection to provide control about two axes.

Images