EP2798209B1 - Plasma thruster and method for generating propulsive plasma thrust - Google Patents

Description

The invention relates to a plasma thruster and a method for generating a propulsive thrust using said plasma thruster.

Artificial satellites typically use booster motors or thrusters to perform trajectory or attitude correction maneuvers. In the same way, space probes intended for the exploration of the solar system have thrusters allowing them to position themselves very precisely around a planet, or even to land on an asteroid to collect samples of matter.

In general, these thrusters, known as chemical or propellant propellants, provide thrusts of a few newtons or less by using liquid propellants such as hydrazine (N ₂ H ₂ ) or hydrogen peroxide (hydrogen peroxide). During the decomposition of these propellants, the chemical energy is converted into heat and then thrust during the relaxation of hot gases in a nozzle. The main drawbacks of these chemical thrusters are that their specific impulse is limited, that the propellants necessary for their operation represent half of the total mass of the satellite and that their high consumption of ergol limits the lifetime of the latter.

To allow for longer and more distant space missions, recent years have seen the emergence of plasma thrusters that have the advantage, compared to chemical thrusters, of offering higher specific impulses, of significantly increasing the mass the payload relative to the mass of the propulsion system and the life of the satellite. Their main drawbacks, as we will see, are the unreliability of the priming, especially when the propellant pressure is low, their lifetime limited by the ionic bombardment of certain elements, and their miniaturization constraints for their use for example on miniature satellites. It should be noted that while their fuel efficiency, although better than that of chemical thrusters, was improved, even longer or longer missions could be considered.

Plasma thrusters can be classified in different ways depending on whether one considers their plasma initiation mode or the mode of acceleration of the plasma towards the exit of the nozzle. It should be noted that these two criteria are relatively independent of one another and just as important as the other. In fact, the priming mode conditions the completeness of the ionization of the propellant gas and the reliability of this priming, thus that of the propellant, and can determine the size of the plasma discharge chamber, the bulk, the weight and the efficiency. thruster energy. As for the acceleration mode of the plasma, it determines the thrust, the specific impulse, the energy efficiency and can determine the size, weight, and life of the thruster.

If we consider as a classification criterion their plasma initiation mode, a first category of plasma thruster is the so-called "arc-jet" propellant, as described by the patent application. US 5,640,843 whose principle is the priming of the plasma by an electric arc in the jet of propellant gas. The advantage of this category of thruster is to provide, all things being equal, higher thrusts than those of other types of plasma thrusters, but it has the following major drawbacks: these thrusters have a low specific impulse compared to that of other plasma thrusters; consume a lot of electrical power; have a limited lifetime by the bombardment of electrodes and internal walls of the discharge chamber by ions and electrons that reach temperatures of the order of a few thousand to a few tens of thousands of degrees; need to evacuate excess heat in space which leads to poor energy efficiency. In addition, the priming of the plasma when the partial pressure of propellant gas is low, unreliability.

According to this same criterion, a second category of plasma thruster is that of plasma thrusters initiating their plasma by the single resonance of an electromagnetic wave (EM), often microwave, in a discharge chamber containing a propellant gas to be ionized. The major disadvantage of propellers in this category is the relatively low energy efficiency since only a small fraction of the EM energy is absorbed by the plasma. Furthermore, the ionization of the propellant gas is rarely complete, especially when the flow of propellant gas is high, and the plasma ignition is unreliable when the partial pressure of propellant gas is low.

According to this same criterion, a third category of plasma thruster is that of the plasma thrusters with "gyromagnetic resonance" of the magnetized free electrons of the plasma or ECR ("Electron Cyclotron Resonance" according to the Anglo-Saxon name). The application of a magnetic field to the plasma leading its free electrons to rotate in the same direction and at a given frequency, the plasma can theoretically be initiated and then maintained with an energy efficiency equal to 1 by the total absorption of an electromagnetic wave whose electric field rotates at the same speed and in the same direction as these magnetized electrons. To maximize this energy efficiency in practice, the length of the discharge chamber is substantially equal to an integer number of half-length of the electromagnetic wave in the vacuum, which raises the problem of the miniaturization of the discharge chamber and therefore thruster. Indeed, to be able to increase the resonance frequency of the EM wave while having the conditions of the ECR, it is necessary to increase correspondingly the intensity of the magnetic field, which supposes quickly the use of powerful magnetic coils or the congestion and the weight of these coils goes against the goal of miniaturization of the thruster. This problem of miniaturization is also complicated by the multiplicity of sources to emit in the discharge chamber: source of propellant gas, EM wave source and magnetic field source. The patent EP 0 505 327 describes such a propellant. Other technical fields also use ECR plasma sources, such as for example the production of integrated circuits. The patent application US 2005 0 287 discloses an ECR resonance ion source, provided with magnetic coils, for ion implantation in microelectronics. The use of magnetic coils leads to a weight and a large bulk for a relatively low energy efficiency due to losses by Joule effect, which is poorly suited for use as a space thruster. Furthermore, the ionization of the propellant gas is rarely complete, especially when the flow of propellant gas is high, and the plasma ignition is unreliable when the partial pressure of propellant gas is low. Finally, these thrusters often deplore the existence of upstream plasma spurious jets known as the ion pump effect.

Whatever the way in which their plasma is primed, plasma thrusters can also be classified according to the second criterion which is the mode of acceleration of the plasma in the nozzle.

According to this second criterion, a first family is that of so-called "electrostatic" plasma thrusters, which is characterized by the electrostatic nature of the force accelerating the plasma towards the outlet of the nozzle. This family can in turn be divided into three categories: accelerator thrusters, Hall effect thrusters, and field effect thrusters.

The category of accelerator gate thrusters is characterized in that ions from a discharge chamber are accelerated by an electrically biased grid system. It should be noted that the ejected plasma is not electrically neutral. The accelerator gate thrusters have the following drawbacks which limit their effectiveness and their lifetime: the positive ion beams crossing the accelerating grid erode it, which limits the life of these thrusters; the ejected ions recombine with the ejected electrons and generate obscuration deposits of matter on the solar panels of the satellites on which they are mounted; the discharge chamber must be of large volume; the energy efficiency is relatively low due to plasma leakage at the walls of the discharge chamber and the acceleration grid; and the thrust is limited by the limitation of the density of the ions inside the grids due to the secondary electrons. Examples of accelerator gate thrusters are given in patent applications JP 01 310179 and US 2004/161579 A1 in the patent US 7,400,096 B1 and in the article MORRISON NA et al. High rate deposition of ta-C: H using an electron cyclotron wave resonance source plasma, "published in THIN SOLID FILMS, ELSEVIER-SEQUOIA SA LAUSANNE, CH, vol. 337, No. 1-2, January 11, 1999, pages 71-73, XP004197099, ISSN: 0040-6090, DOI: 10.1016 / S0040-6090 (98) 01187-0 and the article of NISHIYAMA K AND AL: "Microwave power absorption coefficient of an ECR Xenon ion thruster", SURFACE AND COATINGS TECHNOLOGY, ELSEVIER, AMSTERDAM, NL, vol.202, no. 22-23, August 30, 2088 (2008-08-30), pages 5262-5265, XP025875510, ISSN: 0257-8972, DOI: 10.1016 / J SURFCOAT.2008.06.069 .

The class of Hall effect thrusters is characterized by a cylindrical anode and a negatively charged plasma. Hall effect thrusters use the drift of charged particles in crossed magnetic and electric fields. Their disadvantages are on the one hand the presence of a continuous electric field which involves polarized electrodes and on the other hand the limitation in plasma density which is related to the formation of sheaths around these electrodes which oppose the penetration of the continuous electric field within the plasma, unlike the microwave field which easily penetrates inside the ionized medium, hence the interest of high frequency discharges (HF). The document US 2006/290287 describes such a propellant.

The category of field effect thrusters is characterized by the ionization of a metallic liquid, its acceleration then its electrical neutralization.

According to this second criterion, a second family is that of so-called "electromagnetic" plasma thrusters. This family can be divided into six categories: pulsed induction boosters, magnetoplasmadynamic boosters, electrodeless boosters, electrothermal boosters, helical double layer boosters and mugradB boosters.

The category of pulsed induction boosters is characterized by acceleration during discontinuous time intervals.

The category of magnetoplasmadynamic thrusters is characterized by electrodes that ionize the propellant gas and create a current that in turn creates a magnetic field that accelerates the plasma via the Lorentz force.

The category of thrusters without electrodes is characterized by the absence of electrodes which removes a weak point for the lifetime of the plasma thrusters. The propulsive gas is ionized in a first chamber by an EM wave and then transferred to a second chamber where the plasma is accelerated by inhomogeneous and oscillating electric and magnetic fields generating a so-called ponderomotive force. The patent US 7,461,502 describes such a propellant. A disadvantage of this class of thrusters is their use of magnetic coils to generate the oscillating magnetic field, because their size, their weight and energy loss Joule effect, relatively high, are poorly suited to space applications.

The category of electrothermal thrusters is characterized by heating the plasma at temperatures of the order of one million degrees and then the partial conversion of this temperature into axial speed. These thrusters require high power magnetic coils to generate very intense magnetic fields in order to be able to confine a plasma whose electrons have very high speeds because of their temperature. In addition to the size and weight of these coils, their joule heat dissipation significantly degrades the energy efficiency of these thrusters. The patent US 6,293,090 describes such a thruster, more specifically it is a radio frequency (RF) booster in low hybrid resonance (energy absorption by coupling a very low frequency RF wave via a combined oscillation of the ions and electrons of the plasma) of the VASIMR type (Variable Specific Impulse Magnetoplasma Rocket), where the plasma is not heated by resonance of its electrons as is generally the case for thrusters of this category but by excitation of its ions by an EM wave of strong power.

The category of helical double-layer thrusters is characterized by the injection of the propellant gas into a tubular chamber around which is wound an antenna emitting an electromagnetic wave of sufficiently high power to ionize the gas and then generate, in the plasma thus created, a helicon wave which further increases the temperature of the plasma.
The category of "mugradB" thrusters, also called "space charge field" is characterized by the diamagnetic nature of its force. Chapter 5.1 of J.-M. Rax's book "Physics of Plasmas, Course and Application" rigorously exposes the theory of the motion of an electron animated by an electromagnetic HF field in a static or slowly variable magnetic field. It is written in particular on page 152 the presence of a convergence or a divergence of the induced field lines and therefore of a force along the direction of this field, a force which is proportional to the magnetic moment mu and to the gradient of this magnetic field. This force is called "mugradB" or diamagnetic force. The thruster object of the present patent application is actually based on the quite "classical" physical principles described in this chapter, the adiabaticity hypotheses mentioned on page 153 for the invariance of the mu magnetic moment being largely satisfied in the case of the invention. However, this book does not disclose how to design an ECR plasma-maintenance plasma thruster that is smaller in size than the half-wavelength of the electromagnetic wave and whose priming reliability is improved even in conditions of very low partial pressure of propellant gas. The article of STALLARD BW ET AL: "Whistler-driver, electron-cyclotron-resonance-heated thruster: experimental status", JOURNAL OF PROPULSION AND POWER 1996 JUL-AUG AIAA, vol. 12, no. 4, July 1996 (1996-07), pp. 814-816, XP008133752 describes a propellant with a diamagnetic force, the plasma of which is initiated and maintained by electronic waves generated by an EM wave, of a frequency lower than the gyromagnetic frequency, emitted by two helically-wound antennas, and by a magnetic field, generated by magnetic coils, of an intensity greater than the resonance intensity ECR. The propellant gas is injected into an area where the magnetic field has decreased below the resonance intensity RCC. It raises the problem of the incomplete ionization of the propellant gas of this propellant. To limit this incompleteness of this ionization, the gas chamber is segmented. Despite this precaution, although it is explained that the ionization becomes more complete when the flow of gas decreases, it remains incomplete even for low flow rates. It also does not disclose improving the reliability of the priming for very low propulsive gas flow rates or means of reducing the size of this propellant.

The web page http://www.reiszengrs.com/space.php4, "Resonance Electron-Cyclotron Throttle Rocket", XP055038206, describes an ECR thruster and a corresponding method for generating a propulsive thrust. The thruster comprises an interior cavity, a gas injection nozzle, an electromagnetic wave generator at the ECR frequency, and a magnetic field generator having a local maximum near the outlet end of the injection nozzle.

None of the state-of-the-art plasma thrusters combines the advantages of reliable priming (systematic and instantaneous ignition) and complete ionization under all power operating conditions of the electromagnetic wave and propellant gas flow, especially for very low flow and partial pressure of propellant gas; absence of parasitic plasma jet upstream; a discharge chamber of reduced size with respect to the half wavelength of the EM wave used for plasma maintenance; capable of operating with magnetic field intensities permitting the use of permanent magnets thus avoiding the bulk, weight and Joule losses of magnetic coils; allowing a controlled variation of the thrust and the specific impulse; can achieve an energy efficiency close to 1; accelerating a neutral plasma, thus not requiring a neutralizer; and whose service life is not limited by the wear of parts by the plasma or by the deposition of propellant gas on the solar panels.

The object of the present invention is to provide a propellant that can have an energy efficiency close to 1, such as ECR-initiated thrusters, and be smaller than the state-of-the-art ECR-ignition thrusters. As will be seen in the following description, the inventors will find that this propellant has all the advantages mentioned above, in particular by virtue of the implementation of a new type of plasma priming resulting from the conjunction of particular geometrical configurations. magnetic field lines, propellant gas injection and EM wave emission.

The principle of the invention is to reduce the size of a plasma thruster ECR on the one hand by reducing the length of its discharge chamber and on the other hand by injecting the propellant gas by means of the antenna emitting the EM wave, the reduction of the length of the discharge chamber being obtained by the use of an electron resonance plasma zone, confined by a magnetic field, as a cavity resonant of the EM wave, since the refractive index of the ECR resonance plasma is 5 to 10 times greater than that of the discharge chamber that the state of the art of plasma thrusters uses as a resonant cavity of the wave EM.

More specifically, the subject of the invention is a plasma thruster according to claim 1 and a method for generating a propulsive thrust by means of a plasma thruster according to claim 9.

Note that said local minimum intensity of the magnetic field functions as an electron trap that will allow the initiation of plasma by hollow cathode micro-discharge even at very low pressure.

Note also the importance of the shape of the magnetic field lines that lead to a position of the surface ECRjust output (at a distance of about one millimeter) from the injection nozzle of the propellant gas ionized by the micro-discharge with hollow cathode. This position contributes to the fact that all the neutral gas leaving the injection nozzle is ionized while crossing the ECR surface.

Note also that the injection of the propellant gas and the electromagnetic wave (EM) by the same means makes it possible, on the one hand, to have a more compact discharge chamber and, on the other hand, to guarantee that the EM wave radiates a zone where the gas density is maximum, which maximizes the ionization rate of the neutral gas leaving the injection nozzle, which was one of the problems of the "mu.gradB" thruster described by STALLARD BW ET AL.

Note finally that the conjunction of the EM wave antenna and the ECR surface positions allows the irradiation to be concentrated in the volume delimited by the ECR surface where the EM wave resonates, which maximizes the absorption of EM energy by the plasma and thus maximizes the energy efficiency of the propellant.

According to particular embodiments, the plasma thruster comprises one or more of the features of the dependent claims.

The interest of the sleeve is explained below. The "mu.gradB" plasma thruster comprises an open cavity of dimensions much smaller than the incident wavelength, a significant loss of power related to the diffraction of the EM wave in the orifice and radiation outside the engine could, in the absence of a sleeve, occur in the ignition phase of the engine.

Moreover, in the absence of a sleeve, only the fraction of the EM wave corresponding to the right circular polarization would be used for the ECR resonance with the plasma inside the engine, the rest of the EM wave returning to the EM generator or radiating outwardly by diffraction in the outlet. The presence of a sleeve characterized as above allows all of the power EM arriving on the sleeve to be reflected towards the inside of the engine, the portion which rises towards the generator can then be returned to the thruster cavity again. by means of said circulator disposed at the output of said generator EM. Upon entering the cavity, a fraction of the power reflected by the circulator is in turn circularly polarized and absorbed by the ECR resonance plasma, the unabsorbed EM wave fraction at this stage being again subjected to same circulation cycle until all EM energy is absorbed by the ECR resonance plasma. The combination of such a sleeve coupled with such a circulator provides an energy efficiency close to unity in all operating configurations of the thruster. Note that a sleeve can be made of fine wire mesh and therefore be lightweight.

Note that the priming of the plasma is not performed by ECR as is commonly the case in the state of the art of the diamagnetic force thrusters, but by hollow cathode micro-discharge. Once the plasma is primed and positioned in the so-called priming volume at the outlet of the injection nozzle, this plasma is put into ECR resonance via the electromagnetic wave, which multiplies by a factor 5 to 10 its refractive index and then makes it possible to use this volume as a resonant cavity of the electromagnetic wave then increasing the energy efficiency. This index of refraction of the EM wave resonance medium, higher than in the state of the art, on the one hand, makes it possible to reduce the length of the discharge chamber, since the priming of the plasma and its maintenance no longer require that the length of the discharge chamber be equal to a whole number of half a wave EM wave in the vacuum, and secondly to use a magnetic field of lower intensity, achievable with a simple permanent magnet, since a lower frequency of the EM wave can be used.

Plasma priming by hollow cathode micro-discharge provides a systematic and almost instantaneous initiation whatever the operating conditions, in particular of gas flow and EM power, and therefore greatly increases the reliability of the thruster. The propellant according to the invention therefore belongs to a new class of plasma thruster.

• la figure 1 est une vue en coupe axiale d'un propulseur plasmique selon l'invention ;
• la figure 2 est une vue agrandie d'une partie de la figure 1 représentant les lignes de champ du champ magnétique généré par un générateur du propulseur plasmique selon l'invention ;
• la figure 3 est un diagramme des étapes du procédé selon l'invention ;
• La figure 4 est une vue en coupe axiale d'un propulseur selon une variante de réalisation de l'invention ; et
• La figure 5 est un graphe représentant le champ magnétique le long de l'axe A-A du propulseur.

The invention will be better understood on reading the description which follows, given solely by way of example and with reference to the drawings, in which:

• the figure 1 is an axial sectional view of a plasma thruster according to the invention;
• the figure 2 is an enlarged view of some of the figure 1 representing the field lines of the magnetic field generated by a plasma thruster generator according to the invention;
• the figure 3 is a diagram of the steps of the process according to the invention;
• The figure 4 is an axial sectional view of a propellant according to an alternative embodiment of the invention; and
• The figure 5 is a graph representing the magnetic field along the AA axis of the thruster.

With reference to the figure 1 , the plasma thruster 2 according to the invention comprises a support body 4 supporting a discharge chamber 6 opening on an outlet opening 48.

The support body 4 is a non-magnetic hollow body open at each of its ends 9, 11. It has a cylindrical inner cavity 14 of axis of revolution AA, hereinafter called predefined axis AA.

This cavity 14 comprises a central injection channel 10 coaxial with the predefined axis A-A. This central injection channel 10 is for example constituted by a magnetic metal conduit. It has an outer diameter less than the diameter of the cavity 14 so that it forms with the support body 4, a peripheral injection channel 12 arranged between the inner wall of the support body 4 and the outer wall of the channel. central injection 10.

In particular, the central injection channel 10 has an internal diameter of between 0.5 and 2 mm, and preferably between 1 mm and 1.5 mm. The peripheral injection channel 12 has an outside diameter of between 3 and 20 mm, and preferably between 6 mm and 12 mm, the inside diameter of the peripheral injection channel 12 being the outside diameter of the central injection channel 10 .

In other words, the central injection channel 10 has an inner section of between 0.7 square millimeters and 3 square millimeters. In a variant, the central injection channel 10 and the peripheral injection channel 12 have a square section.

The central injection channel 10 is fixed to the support body 4 by means of an insulating block 16 and a clamping ring 20. In particular, a portion of the central injection channel 10 is fitted into a hole through the insulating block 16. The insulating block 16 is arranged and fixed in the cavity 14 between a shoulder 18 of the support body 4 and a bearing face 21 of the clamping ring 20. The clamping ring 20 is screwed on the outside rim of the end 9 of the support body 4.

A first O-ring 22 is interposed between the insulating block 16 and the shoulder 18. A second O-ring 24 is interposed between the insulating block 16 and the bearing face 21 of the clamping ring 20.

The central injection channel 10 and the peripheral injection channel 12 form two propellant gas injection means in the chamber 6, within the meaning of the invention.

For this purpose, one end of the central injection channel 10 is connected, by a pipe 28, to a source of propellant gas 30. An opening 31 is arranged in the support body 4. This opening 31 opens into the channel of peripheral injection 12. This opening 31 is connected by a pipe 44 to the source of propellant gas 30 for supplying the peripheral injection channel 12 with propellant gas, during the operation of the plasma thruster in a second "arc-jet" operating mode, as described herein. -after.

This source 30 is provided with a device 32 for controlling the gas flow rate.

In a first so-called "conventional" operating mode, the flow rate of the propellant gas is between 0.1 gram per hour and 40 gram per hour.

In a second mode of operation known as "arc jet", the flow rate of the propellant gas is between 1 gram per hour and 400 gram per hour, and preferably between 10 gram per hour and 400 gram per hour.

The other end of the central injection channel 10 comprises a tip 36, for example, formed by a beveling of the annular stop of the channel.

The tip 36 extends outside the support body 4, in the discharge chamber 6. It contributes to the ionization of the propellant gas by an effect called "peak effect". The peak effect makes it possible to concentrate the magnetic field in a volume of the discharge chamber, called the priming volume. It is not a Corona ionization discharge, which concentrates the lines of the electric field, but a hollow cathode micro-discharge between the two mentioned maxima of magnetic field strength in the immediate vicinity of an output an injection nozzle.

It should be noted that the presence of a local maximum of the intensity of the magnetic field in the priming volume and therefore inside the injection tube is possible for two reasons. First, because the present diamagnetic force propellant constitutes an open cavity for the magnetic field, or more precisely a coaxial system open at one end. Secondly, because the complex magnetic circuit of the thruster comprises parts whose role is precisely to channel a large part of the magnetic field in this volume via including the injection channel 10 of magnetic material and especially via its tip 36.

In the present example, the priming volume is between 0.5 mm ³ and 5 mm ³ . It is disposed 12 mm to 15 mm downstream of the tip 36 of the central injection channel 10.

The central injection channel 10 is further adapted to emit electromagnetic waves, in particular microwaves. For this, the central injection channel 10 is made of an electrically conductive material and is electrically connected to an electromagnetic wave generator 38 via a connector 40 fixed, for example by screwing, to the support body 4 The connector 40 is, for example, a connector of the SMA (registered trademark) type.

The electromagnetic wave generator 38 is able to irradiate the propulsive gas present in the discharge chamber 6 with at least one electromagnetic wave whose electric field rotates in the same direction and at the same frequency as the magnetized electrons of the propellant gas. in order to obtain a total absorption of the electromagnetic energy by the electrons ECR. More precisely, the electric field has a right circular polarization and a frequency equal to the gyromagnetic resonance frequency of the electrons of the propellant gas magnetized by the magnetic field generator.

The electromagnetic wave generator 38 is provided with a device 42 for electromagnetic power modulation. It is capable of generating electromagnetic waves with a power of between 0.5 and 300 Watts, and preferably between 0.5 and 30 Watts in a first so-called "classical" operating mode, and electromagnetic waves with a power of between 50 and and 500 Watts, and preferably between 200 and 500 Watts in the second mode of operation called "arc jet".

The power of the electromagnetic waves is large enough to obtain the ECR and eject the electrons before they have time to radiate, but not too high so as to avoid any radiation of these electrons before ejection, which makes it possible to avoid radiant heating and maintain efficiency optimal energy. The electromagnetic power that the propellant can absorb without degrading the energy efficiency is related to the size of the Larmor Rb radius of the electrons in the plasma. This must remain substantially less than the radius of the cavity so that the electrons do not strike at any time the inner wall of the thruster (plasma called "magnetic levitation"). However, for an electron, of electric charge qe and mass me, in a magnetic field B0 of the order of 0.1 Tesla (1000 Gauss), a radius of gyration Rb of 1 millimeter would correspond to a velocity of the electrons ve = Rb.qe.B0 / me = 1.76.10 ⁷ m / s in a direction perpendicular to the magnetic field. Expressed in electronvolts, the kinetic energy corresponding to the rotation of the electrons would then be of the order of 0.92.10 ⁵ eV. Compared to the ionization energy of the gas of the order of 10 to 20 eV for example, such a limit seems difficult to achieve with the electromagnetic power of a few tens to a few hundred watts where one is located.

It will also be noted that in an adiabatic process, the acceleration of the electrons in the nozzle retains the magnetic moment mu = qe ² . Rb ² . B0 / 2 me. A reduction of B0 by a factor of 10, for example, would therefore only lead to an increase by a factor of approximately 3 of the electronic radius of gyration Rb.

Finally, if a much greater electromagnetic power was to be used, it is possible without increasing its size, to increase the upper operating limit of the motor, by correlatively increasing the magnetic field B0 and the frequency of the exciter EM wave. Magnets about ten times more powerful than those used in our experiments are already available on the market.

The discharge chamber 6 comprises a generator of the magnetic field 46 fixed, for example by screwing, to the end 11 of the support body 4. This generator 46 comprises a source 50 of magnetic field having two poles, a washer 52 secured to an end surface constituting a pole of said source 50, a clamping nut 54 in contact with the washer 52, and a washer 58 integral with an end surface constituting the other pole of said source 50.

The discharge chamber 6 further comprises an outlet opening 48 of the plasma.

The magnetic field source 50 is constituted, for example, by a permanent magnet of toroidal shape coaxial with the predefined axis A-A. To simplify the description, it is hereinafter referred to as magnet 50.

The magnetic field emitted by the magnet 50 has an intensity of between 0.05 Tesla and 1 Tesla, and preferably between 0.085 Tesla and 0.2 Tesla.

The washer 52 and the clamping nut 54 form a first magnetic element and the washer 58 forms a second magnetic element within the meaning of the invention.

The washers 52, 58 are each secured to an annular face of the magnet 50. The washer 52 is further fixed, for example by screwing, on the outer periphery of the end 11 of the support body.

The clamping nut 54 has a protrusion 62 substantially frustoconical axis of revolution, the predefined axis A-A. The protrusion 62 extends towards the central injection channel 10.

The washer 52, the clamping nut 54 and the washer 58 consist of paramagnetic steel, and preferably of ferromagnetic steel.

With reference to the figure 2 since the washer 52 and the clamping nut 54 are adapted to conduct the magnetic field emitted by the permanent magnet 50, the end surface of the protuberance 62 closest to the central injection channel 10 forms a first pole. magnetic 64 disposed upstream of the injection nozzle 65 by considering the flow direction F1 of the propellant gas, and at a first distance D1 of the predefined axis AA.

Since the washer 58 is also adapted to conduct the magnetic field, the end surface of the washer 58 closest to the central injection channel 10 forms a second magnetic pole 66 disposed downstream of the injection nozzle 65 of the channel central injection, considering the direction F1, and a second distance D2 of the predefined axis AA; said second distance D2 being longer than the first distance D1.

The field lines 68 of the field emitted by the magnetic field generator 46 have a nozzle shape. They cut the injection nozzle 65 of the central injection channel 10 and form an angle between 10 ° and 70 ° with the predefined axis A-A. In other words, the magnetic field emitted by the magnetic field generator 46 diverges. At the predefined axis A-A, the magnetic field gradient is parallel to the predefined axis A-A. In addition, this magnetic field gradient is negative from upstream to downstream by considering the direction of ejection of the propellant gas.

The magnetic field also has a first local maximum intensity of the magnetic field at the injection nozzle 65 of the central injection channel. This intensity is sufficient to completely ionize, by ECR resonance, the propellant gas leaving said injection nozzle 65. This intensity is for example between 0.087 Tesla (ECR for a microwave frequency of 2.45 GHz), and about 0 , 5 Tesla (upper limit achievable with permanent magnets). The particular shape of the field lines 68 causes the ECR surface to be very close to said first local intensity maximum and for this ECR surface to envelop the output end 165 of the injection nozzle 65. For a frequency 2.45 GHz EM wave, the ECR surface is located at a distance of millimeter downstream of the output end 165.

In this patent application, the term "ECR surface" is a region of the space where the free electron gyration rate in the local magnetic field is substantially equal to the frequency of the exciting electromagnetic wave.

The magnetic field generator 46 is further able to accelerate towards the outlet opening 48, by a diamagnetic force, the plasma initiated at the injection nozzle 65, said plasma ejected from said thruster being electrically neutral. It should be noted that one of the main interests of ECR plasma sources lies in the possibility of acting only on the free electrons of the plasma and not on the ions, which requires only relatively small magnetic fields, approximately 0, 1 Teslas (1000 Gauss) in our example. Electrical neutrality of the plasma is provided very efficiently by the ambipolar electric field, or space charge field, which appears immediately within the plasma and against any imbalance between the populations of positive ions and electrons. It is therefore not necessary to use a neutralizer. In the absence of an electric field applied by a possible accelerating grid, the ambipolar electric field is not disturbed and the electrons subjected to the only diamagnetic force will then carry with them in their movement the non-magnetized positive ions (hence the so-called "diamagnetic" character of the plasma). Conversely, at the output of the thruster, the electrons connected to the ions by the space charge will be able to escape the residual magnetic field due to the inertia of these previously accelerated ions within the propellant. Unlike other thrusters of the state of the art, the acceleration of the plasma in the magnetic nozzle does not require additional power expenditure in the case where, as in this example, the magnetic nozzle is generated by simple permanent magnets. This saving of electrical power is an important asset for a spatial application.

The central injection channel 10 opens at the beginning of the diverging portion of the magnetic field, upstream of the resonance zone ECR.

Advantageously, the central injection channel 10 serves both as a microwave emission antenna 39 inside the discharge chamber 6 and as an injection nozzle 65 for the injection of the gas to be ionized. Injection nozzle 65 includes an outlet end 165.

The magnet 50, the washer 52, the clamping nut 54 and the washer 58 form the discharge chamber 6. This has a diameter of between 6 mm and 60 mm, and preferably between 12 mm and 30 mm. . The discharge chamber 6 thus has an inner section of between 0.7 square centimeters and 30 square centimeters.

The length, defined along the predefined axis AA, of the internal cavity 14 of the discharge chamber 6 is 5 to 10 times smaller than the half-wavelength in the vacuum of the electromagnetic wave emitted by the generator. electromagnetic wave 38.

Advantageously, the discharge chamber has a very small dimension.

The plasma thruster 2 further comprises a fastening flange 70 and a lock nut 72 screwed onto the outer periphery of the support body 4. An O-ring 74 is furthermore disposed between the fastening flange 70 and the lock nut. 72.

Advantageously, the plasma thruster according to the invention can be used by means of permanent magnets that do not consume energy.

Advantageously, the discharge chamber forms a high frequency resonant cavity having dimensions of the order of one centimeter with a relatively low frequency of the order of 2.3 to 2.8 GHz. This is possible because the optical index of the plasma at the ECR is very high, which makes it possible to have a relatively short wavelength even with a relatively low frequency. As the ECR frequency is proportional to the magnetic field, a cavity of this size is therefore possible even with a magnetic field of the order of 0.08 to 0.1 T, easily achievable by annular permanent magnets of small dimensions.

• génération 90 d'un champ magnétique 63 ;
• émission 100 des microondes par le générateur d'onde électromagnétique 38 ;
• injection 104 du gaz propulsif dans la chambre de décharge 6 par l'intermédiaire du canal d'injection central 10 ;
• amorçage 101 du plasma ;
• entretien 103 du plasma par ECR
• modulation 102 de la puissance de l'onde électromagnétique émis par le générateur d'onde électromagnétique 38, par le dispositif de modulation 42 ;
• réglage 106 du débit de gaz propulsif dans le canal d'injection central 10 par le dispositif de commande 32.

The method of generating a propulsive thrust according to the invention is achieved by means of a plasma thruster described above. In the first so-called "classic" mode of operation, it includes, with reference to the figure 3 , the following steps:

• generation 90 of a magnetic field 63;
• emission 100 of the microwaves by the electromagnetic wave generator 38;
• injection 104 of the propellant gas into the discharge chamber 6 via the central injection channel 10;
• priming 101 of the plasma;
• Plasma maintenance 103 by ECR
• modulation 102 of the power of the electromagnetic wave emitted by the electromagnetic wave generator 38, by the modulation device 42;
• setting 106 of the propellant gas flow in the central injection channel 10 by the control device 32.

Advantageously, the emission step 100 is implemented before the injection step 104 when the user wishes to save the propellant gas, and the injection step 104 is implemented before the transmission step 100 when the user wants to save electricity.

• injection 108 de gaz propulsif supplémentaire par l'intermédiaire du canal d'injection périphérique 12 ;
• réglage 110 du débit de gaz propulsif dans le canal d'injection périphérique 12 par le dispositif de commande 32 ; et
• modulation, avec le dispositif de modulation 42 de la puissance des microondes émises par le générateur d'onde électromagnétique 38, pour fonctionner dans le second mode de fonctionnement dit « arc jet ».

In the second operating mode called "arc-jet", it further comprises the following steps:

• injection 108 of additional propellant gas through the peripheral injection channel 12;
• setting 110 of the propellant gas flow in the peripheral injection channel 12 by the control device 32; and
• modulation, with the modulation device 42 of the power of the microwaves emitted by the electromagnetic wave generator 38, to operate in the second operating mode called "arc jet".

Advantageously, the axial injection of the propellant gas is completed in this operating mode by an injection of gas around the central injection duct. This is generally used during a temporary operation with high thrust of the thruster here called second mode of operation called "arc-jet". In this case, the rise in pressure of the discharge chamber 6 makes it possible to ignite a plasma arc-type - very dense and very hot under the effect of the injection of microwaves of high power (greater than a hundred watts). This makes it possible to operate the plasma thruster with much larger surges - of the order of several hundred milli-newtons, but with a much greater heat dissipation and a lower energy efficiency.

Advantageously, it is possible to optimize, for example, over the entire mission, both the consumption of gas and that of energy, by acting on a range of adjustment of the gas flow in the injection channel. and a range of electromagnetic wave power control, both of which vary the specific impulse and thrust of the thruster differently, and, where appropriate, by playing over a range of regulating the gas flow in the peripheral channel and a range of adjustment of the power of the electromagnetic waves.

Advantageously, it is possible to use each mode of propulsion independently or in combination, a combination making it possible, for example, to make fine adjustments to the total thrust, even for large amplitudes of this thrust.

According to the embodiment variant illustrated on the figure 4 the plasma thruster 120 further comprises, on the one hand, a circulator 80 connected to the electromagnetic wave generator 38 and to the connector 40 screwed onto the support body 4 and, on the other hand, an electrically conductive cylindrical sleeve 85 placed downstream of the plane of exit of the plasma thruster 120.

The circulator 80 is a device, generally made of ferrite, which is placed in a microwave circuit to protect the electromagnetic generator 38 or a possible amplifier against a return of EM waves, for example reflected by the plasma (which is for the generator). EM wave, the charge to be irradiated). The EM wave flow through the circulator 80 towards the plasma is not absorbed by the circulator. The flux reflected towards the EM wave generator rotates in the circulator 80 and returns towards the plasma so that the electromagnetic generator 38 is protected and there is no loss of EM wave flux by reflection. upstream.

The sleeve 85 has a diameter greater than the diameter of the permanent magnet 50 and a flange 86 fixed against the washer 58 of the magnetic field generator 46. In particular, the sleeve 85 is, for example, a circular waveguide section of diameter equal to 1/2 wavelength and length equal to 1/4 or 3/4 wavelength of EM wave in vacuum. The sleeve 85 blocks the propagation of the EM wave which would otherwise radiate in the free space by diffraction from the propeller outlet. Instead of being emitted into the free space, the microwave EM wave flux is thus reflected towards the plasma inside the propellant and its non-absorbed part by the plasma is directed towards the circulator 80. The circulator 80 returns then in turn this flow retrograde to the plasma thruster 120, and so on ... until complete absorption of EM wave flux by the plasma.

The figure 5 represents the variation of the magnetic field generated by the generator 46 with respect to the distance to the output plane DD of the plasma thruster along the predefined axis AA. The zero of the abscissa axis defines in this figure the output plane DD. As visible on the figure 2 , the exit plane is the plane parallel to the median plane of the fastening flange 70 located at the outlet opening 48.

As visible in this figure, the magnetic field has a first local maximum, A, and a second local maximum, C, located inside the injection nozzle 65, and a local minimum located between the first local maximum A and the second local maximum C.

The first local maximum A is located at the outlet end 165 of the injection nozzle 165. The first local maximum A is sufficient to ionize, by cyclotron resonance, electrons of the propellant gas under the effect of said electromagnetic wave, the propellant gas leaving said injection nozzle 65.

The first local maximum A has an intensity greater than the threshold value B _ECR necessary to obtain a cyclotron resonance defined by the following formula: $${\mathrm{B}}_{\mathrm{ECR}}=\mathrm{2}*\pi *{\mathrm{f}}_{\mathrm{ECR}}*\mathrm{me}/\mathrm{qe},$$

• me est la masse d'un électron,
• qe est la charge électrique d'un électron,
• F_ECR est la fréquence de résonance gyromagnétique.

In which

• me is the mass of an electron,
• qe is the electric charge of an electron,
• F _ECR is the gyromagnetic resonance frequency.

The magnetic field generator 50 is able to accelerate towards the outlet opening 48 by the diamagnetic force, the free electrons of the plasma initiated at the injection nozzle (65), the positive ions, not magnetized, following these electrons. free because of the ambipolar electric field, or charge field of space, which appears almost immediately within the plasma and opposes any imbalance between the populations of positive ions and electrons, this electric field, which does not is disturbed by no field applied electrically, ensuring very effectively the electrical neutrality of the plasma ejected said thruster.

The tip 36 of the injection means 10 makes it possible, by concentrating the magnetic field lines, to obtain from the magnetic field generator 50, on the one hand, the first local maximum of the intensity A, and on the other hand a hollow cathode micro-discharge between the first local maximum A and the local minimum B of the intensity of the magnetic field. This micro-discharge is sufficient to ionize at least a portion of the propellant gas present in said injection nozzle 65 regardless of its flow rate. The magnetic field generator 50 comprises for example permanent magnets.

Claims (10)

1. Plasma thruster (2, 120) comprising a discharge chamber (6) comprising an internal cavity (14) and an outlet opening (48); at least one injection means (10, 12) comprising an injection nozzle (65) capable of injecting into the discharge chamber (6) a propellant gas along a predefined axis (A-A); said injection nozzle (65) having an outlet end (165); a magnetic field generator (50, 52, 54, 58) capable of setting electrons of the propellant gas present in the discharge chamber (6) in gyromagnetic rotation; and an electromagnetic wave generator (38) capable of irradiating the propellant gas present in the discharge chamber (6) by generating at least one electromagnetic wave the electric field of which has a right-hand circular polarization and a frequency equal to the frequency, F_ECR, of gyromagnetic resonance of the electrons of the propellant gas magnetized by said magnetic field generator (50, 52, 54, 58),

   - said magnetic field generator (50, 52, 54, 58) is capable:

   ∘ on the one hand, of generating a magnetic field having field lines (68) which determine an iso-field surface, known as the "ECR surface", with an intensity equal to that allowing a cyclotron resonance of the electrons under the effect of said electromagnetic wave;

   ∘ on the other hand, of giving said field lines (68) the shape of a nozzle, so as to generate a diamagnetic propulsion force;

   - said injection means (10) being produced from an electrically conductive material and being electrically connected to the electromagnetic wave generator (38) so as to also operate as an electromagnetic antenna (39) emitting said electromagnetic wave into the propellant gas at the outlet of said injection nozzle (65);
   wherein said magnetic field generator (50, 52, 54, 58) is capable of generating a magnetic field

   ■ producing an ECR surface enveloping the outlet end (165) of said injection nozzle (65), the volume delimited by this ECR surface being the resonant cavity of the electromagnetic wave ;

   ■ having a first local maximum (A) of intensity inside the injection nozzle (65) and at the outlet end (165) of the injection nozzle (65);

   ■ having a second local maximum (C) of the intensity of the magnetic field inside the injection nozzle (65), separated from the first local maximum (A) by a local minimum (B) of the intensity of the magnetic field inside said injection nozzle (65);

   - said injection means (10):

   ■ is produced from a magnetically conductive material, making it possible to achieve, inside the latter, said second local maximum (C) of the intensity of the magnetic field;

   ■ comprises, at the downstream end of said injection nozzle (65), an injection channel (10) with an external diameter strictly comprised between 0.5 and 3 mm.
2. Plasma thruster (2, 120) according to the previous claim, in which the magnetic field generator (50, 52, 54, 58) comprises as magnetic field source at least one permanent magnet (50) with a toric shape arranged coaxially to the predefined axis (A-A) and having a first magnetic pole (64) and a second magnetic pole (66), a first magnetic element (52, 54) integral with the first magnetic pole (64) and a second magnetic element (58) integral with the second magnetic pole (66), said first (64) and second (66) magnetic poles being arranged at a first distance (D1) and, respectively, a second distance (D2) from the predefined axis (A-A); the second distance (D2) being longer than the first distance (D1), the first magnetic pole (64) and the second magnetic pole (66) being arranged upstream and, respectively, downstream of the injection nozzle (65) with respect to the direction (F1) of flow of the propellant gas, the field lines (68) intersecting with the injection nozzle (65) and forming an angle comprised between 10° and 70° with said predefined axis (A-A).
3. Plasma thruster (2, 120) according to one of the previous claims, in which the length, defined along the predefined axis (A-A), of the internal cavity (14) of the discharge chamber (6) is 5 to 10 times smaller than the half-wavelength of said electromagnetic wave in a vacuum, the discharge chamber (6) having an internal cross-sectional area comprised between 0.7 square centimeters and 30 square centimeters; in which the central injection channel (10) has an internal cross-sectional area comprised between 0.7 square millimeters and 3 square millimeters.
4. Plasma thruster (2, 120) according to one of the previous claims, in which the magnetic field intensities of said first local maximum (A), local minimum (B) and second local maximum (C) are, respectively, approximately 0.18 tesla, 0.01 tesla and 0.05 tesla.
5. Plasma thruster (2, 120) according to one of the previous claims, in which said electromagnetic wave is capable of propagating along an axis parallel to the predefined axis (A-A) and in which, at the predefined axis (A-A), the magnetic field gradient is parallel to the predefined axis (A-A); said magnetic field gradient being negative from upstream to downstream in a direction defined by the direction in which the propellant gas is ejected.
6. Plasma thruster (2, 120) according to one of the previous claims, which comprises a device (42) for modulating the power of the electromagnetic wave and a device (32) for controlling the flow rate of the propellant gas, said power of the electromagnetic wave being comprised between 0.5 watts and 300 watts, and preferably between 0.5 watts and 30 watts in a first operating mode.
7. Plasma thruster (120) according to one of the previous claims, which comprises, on the one hand, a circulator (80), arranged at the outlet of said electromagnetic wave generator (38) and, on the other hand, an electrically conductive cylindrical sleeve (85), arranged downstream of the plane defined by the outlet opening (48) known as the outlet plane (D-D) of the plasma thruster (120), the diameter of which is substantially equal to one quarter of the wavelength of the electromagnetic wave and the length of which is substantially equal to three quarters of the wavelength of the electromagnetic wave.
8. Plasma thruster (2, 120) according to one of the previous claims, comprising two injection means (10, 12) coaxial to the axis (A-A), one supplying gas to be ionized to the ECR surface and the other increasing the thrust via a gas flow rate and an arcjet operation.
9. Method for generating a propulsion thrust by means of a plasma thruster (2, 120) comprising the following steps:

   ■ injection (104), into a discharge chamber (6) comprising an internal cavity (14) and an outlet opening (48), using at least one injection means (10, 12) comprising an outlet end called the injection nozzle (65), of a propellant gas along a predefined axis (A-A);

   - generation (90), using a magnetic field generator (50, 52, 54, 58), of a magnetic field (63) capable of setting electrons of the propellant gas present in the discharge chamber (6) in gyromagnetic rotation; said magnetic field generation (90) being such that:

   ∘ on the one hand, the magnetic field has field lines (68) which determine an iso-field surface, known as the ECR surface, with an intensity equal to that allowing a cyclotron resonance of the electrons under the effect of said electromagnetic wave,

   ∘ on the other hand, the magnetic field gives said field lines the shape of a nozzle, so as to generate a diamagnetic force;

   - emission (100) into the propellant gas present in the discharge chamber (6), using an electromagnetic wave generator (38), of at least one electromagnetic wave the electric field of which has a right-hand circular polarization and a frequency equal to the gyromagnetic resonance frequency, F_ECR, of the electrons of the propellant gas magnetized by said magnetic field generator (50, 52, 54, 58), the injection (104) of the propellant gas and the emission (100) of the electromagnetic wave being carried out by one and the same injection means (10, 12) and at the same location in the discharge chamber, said injection means (10, 12) being produced from an electrically conductive material and electrically connected to the electromagnetic wave generator (50, 52, 54, 58) in order to emit the electromagnetic wave into the propellant gas at the outlet of the gas from said injection nozzle (65), so as to maximize the level of ionization of the propellant gas on exiting;

   ■ ignition (101) of the plasma by ionization of the propellant gas;

   ■ sustaining (103) of the plasma by cyclotron resonance of the electrons; wherein the ignition (101) of the plasma is realized by microhollow cathode discharge using the injection means (10) which is made of magnetic material and comprises, at the downstream end of its injection nozzle (65), an injection channel (10) with an external diameter strictly comprised between 0.5 mm and 3 mm;

   - said magnetic field generation (90) being such that the magnetic field has:

   ■ said ECR surface which envelops the outlet end (165) of said injection nozzle (65), the volume delimited by the ECR surface being the resonance cavity of the electromagnetic wave;

   ■ a first local maximum (A) of intensity situated inside the injection nozzle (65) and at the outlet end (165) of the injection nozzle (65);

   ■ a second local maximum (C) of the intensity of the magnetic field inside the injection nozzle (65), separated from the first local maximum (A) by a local minimum (B) of the intensity of the magnetic field inside said injection nozzle (65);

   - the sustaining (103) of the plasma by cyclotron resonance of the electrons being realized by resonance of the electromagnetic wave in the volume delimited by the ECR surface.
10. Method according to the previous claim, in which the plasma thruster (2, 120) moreover comprises a device (42) for modulating the power of the electromagnetic wave, a device (32) for controlling the gas flow rate, a peripheral injection channel (12) capable of injecting the propellant gas into the discharge chamber (6); and in which the method comprises the following steps:

    - injection (108) of propellant gas into the discharge chamber (6) via the peripheral injection channel (12);

    - regulation (110) of the flow rate of propellant gas injected into the discharge chamber (6) via the peripheral injection channel (12);

    - modulation (112) of the power of the electromagnetic wave.

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