Classifications

• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05H—PLASMA TECHNIQUE; PRODUCTION OF ACCELERATED ELECTRICALLY-CHARGED PARTICLES OR OF NEUTRONS; PRODUCTION OR ACCELERATION OF NEUTRAL MOLECULAR OR ATOMIC BEAMS
  • H05H7/00—Details of devices of the types covered by groups H05H9/00, H05H11/00, H05H13/00
  • H05H7/08—Arrangements for injecting particles into orbits
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J27/00—Ion beam tubes
  • H01J27/02—Ion sources; Ion guns
  • H01J27/16—Ion sources; Ion guns using high-frequency excitation, e.g. microwave excitation
  • H01J27/18—Ion sources; Ion guns using high-frequency excitation, e.g. microwave excitation with an applied axial magnetic field
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F03—MACHINES OR ENGINES FOR LIQUIDS; WIND, SPRING, OR WEIGHT MOTORS; PRODUCING MECHANICAL POWER OR A REACTIVE PROPULSIVE THRUST, NOT OTHERWISE PROVIDED FOR
  • F03H—PRODUCING A REACTIVE PROPULSIVE THRUST, NOT OTHERWISE PROVIDED FOR
  • F03H1/00—Using plasma to produce a reactive propulsive thrust
  • F03H1/0081—Electromagnetic plasma thrusters
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05H—PLASMA TECHNIQUE; PRODUCTION OF ACCELERATED ELECTRICALLY-CHARGED PARTICLES OR OF NEUTRONS; PRODUCTION OR ACCELERATION OF NEUTRAL MOLECULAR OR ATOMIC BEAMS
  • H05H1/00—Generating plasma; Handling plasma
  • H05H1/24—Generating plasma
  • H05H1/46—Generating plasma using applied electromagnetic fields, e.g. high frequency or microwave energy
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05H—PLASMA TECHNIQUE; PRODUCTION OF ACCELERATED ELECTRICALLY-CHARGED PARTICLES OR OF NEUTRONS; PRODUCTION OR ACCELERATION OF NEUTRAL MOLECULAR OR ATOMIC BEAMS
  • H05H1/00—Generating plasma; Handling plasma
  • H05H1/54—Plasma accelerators
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05H—PLASMA TECHNIQUE; PRODUCTION OF ACCELERATED ELECTRICALLY-CHARGED PARTICLES OR OF NEUTRONS; PRODUCTION OR ACCELERATION OF NEUTRAL MOLECULAR OR ATOMIC BEAMS
  • H05H13/00—Magnetic resonance accelerators; Cyclotrons
  • H05H13/005—Cyclotrons
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05H—PLASMA TECHNIQUE; PRODUCTION OF ACCELERATED ELECTRICALLY-CHARGED PARTICLES OR OF NEUTRONS; PRODUCTION OR ACCELERATION OF NEUTRAL MOLECULAR OR ATOMIC BEAMS
  • H05H7/00—Details of devices of the types covered by groups H05H9/00, H05H11/00, H05H13/00
  • H05H7/02—Circuits or systems for supplying or feeding radio-frequency energy
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F03—MACHINES OR ENGINES FOR LIQUIDS; WIND, SPRING, OR WEIGHT MOTORS; PRODUCING MECHANICAL POWER OR A REACTIVE PROPULSIVE THRUST, NOT OTHERWISE PROVIDED FOR
  • F03H—PRODUCING A REACTIVE PROPULSIVE THRUST, NOT OTHERWISE PROVIDED FOR
  • F03H1/00—Using plasma to produce a reactive propulsive thrust
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05H—PLASMA TECHNIQUE; PRODUCTION OF ACCELERATED ELECTRICALLY-CHARGED PARTICLES OR OF NEUTRONS; PRODUCTION OR ACCELERATION OF NEUTRAL MOLECULAR OR ATOMIC BEAMS
  • H05H1/00—Generating plasma; Handling plasma
  • H05H1/24—Generating plasma
  • H05H1/46—Generating plasma using applied electromagnetic fields, e.g. high frequency or microwave energy
  • H05H1/461—Microwave discharges
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05H—PLASMA TECHNIQUE; PRODUCTION OF ACCELERATED ELECTRICALLY-CHARGED PARTICLES OR OF NEUTRONS; PRODUCTION OR ACCELERATION OF NEUTRAL MOLECULAR OR ATOMIC BEAMS
  • H05H7/00—Details of devices of the types covered by groups H05H9/00, H05H11/00, H05H13/00
  • H05H7/02—Circuits or systems for supplying or feeding radio-frequency energy
  • H05H2007/027—Microwave systems
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05H—PLASMA TECHNIQUE; PRODUCTION OF ACCELERATED ELECTRICALLY-CHARGED PARTICLES OR OF NEUTRONS; PRODUCTION OR ACCELERATION OF NEUTRAL MOLECULAR OR ATOMIC BEAMS
  • H05H7/00—Details of devices of the types covered by groups H05H9/00, H05H11/00, H05H13/00
  • H05H7/08—Arrangements for injecting particles into orbits
  • H05H2007/081—Sources
  • H05H2007/082—Ion sources, e.g. ECR, duoplasmatron, PIG, laser sources
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05H—PLASMA TECHNIQUE; PRODUCTION OF ACCELERATED ELECTRICALLY-CHARGED PARTICLES OR OF NEUTRONS; PRODUCTION OR ACCELERATION OF NEUTRAL MOLECULAR OR ATOMIC BEAMS
  • H05H7/00—Details of devices of the types covered by groups H05H9/00, H05H11/00, H05H13/00
  • H05H7/08—Arrangements for injecting particles into orbits
  • H05H2007/087—Arrangements for injecting particles into orbits by magnetic means

Definitions

• the invention relates to a plasma thruster and a method for generating a propulsive thrust using said plasma thruster.
• Artificial satellites generally use booster motors or thrusters to perform trajectory or attitude correction maneuvers.
• 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.
• these thrusters provide thrusts of a few newtons or less by using liquid propellants such as hydrazine (N2H2) or hydrogen peroxide (hydrogen peroxide).
• liquid propellants such as hydrazine (N2H2) or hydrogen peroxide (hydrogen peroxide).
• N2H2 hydrazine
• hydrogen peroxide hydrogen peroxide
• 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.
• 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.
• 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.
• a first category of plasma thruster is the so-called "arc-jet" propellant, as described by the patent application US 5,640,843, the principle of which is priming of the plasma by an electric arc in the jet of propellant gas.
• 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 the 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 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.
• 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.
• EM electromagnetic wave
• the major disadvantage of the thrusters of this class is the relatively low energy efficiency since only a small fraction of the EM energy is absorbed by the plasma.
• 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.
• 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).
• ECR Electro Cyclotron Resonance
• 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.
• 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 quickly supposes the use of powerful magnetic coils or 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.
• Patent EP 0 505 327 describes such a propellant.
• ECR plasma sources such as for example the production of integrated circuits.
• US patent application 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 footprint for a relatively low energy efficiency due to Joule losses, which is poorly suited for use as a space thruster.
• 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.
• these thrusters often deplore the existence of upstream plasma spurious jets known as the ion pump effect.
• plasma thrusters can also be classified according to the second criterion which is the mode of acceleration of the plasma in the nozzle.
• 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.
• electrostatic plasma thrusters
• the family can 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.
• accelerator gate thrusters are given in patent applications JP 01 310 179 and US 2004/161579 A1, in US Pat. No. 7,400,096 B1, and in the article by MORRISON NA et al "High rate deposition of ta -C: H using an electron cyclotron wave resonance plasma source, "published in THIN SOLID FILMS, ELSEVIER-SEQUOIA SA LAUSANNE, CH, vol. 337, No.
• 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). US 2006/290287 discloses such a propellant.
• the category of field effect thrusters is characterized by the ionization of a metallic liquid, its acceleration then its electrical neutralization.
• a second family is that of so-called "electromagnetic" plasma thrusters.
• This family can be divided into six categories: pulsed induction boosters, magnetoplasmadynamic boosters, non-electrode boosters, electrothermal boosters, helical double-layer boosters and mugradB boosters.
• the category of pulsed induction thrusters 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 propellant 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.
• US Pat. No. 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.
• 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 plasma ions and electrons) of the VASIMR type (Variable Specifies 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 a high power EM wave.
• RF Radio Frequency
• the category of helical double layer propellants 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.
• XP008133752 describes a propellant with 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 antennae wound helically, 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 incomplete ionization propellant gas of this propellant. To limit this incompleteness of this ionization, the gas chamber is segmented.
• 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.
• ECR-initiated thrusters such as ECR-initiated thrusters
• this thruster combines all the advantages mentioned above, in particular through the implementation of implementing a new type of plasma priming resulting from the conjunction of the particular geometrical configurations of the magnetic field lines, the propellant injection and the 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 resonant cavity of the EM wave, since the refractive index ECR resonance plasma is 5 to 10 times higher than that of the discharge chamber that the state of the art of plasma thrusters uses as the resonant cavity of the EM wave.
• the subject of the invention is a plasma thruster comprising a discharge chamber comprising an interior cavity and an outlet opening; at least one injection means comprising an outlet end called injection nozzle, capable of injecting into the discharge chamber (6) a propellant gas along a predefined axis; a magnetic field generator capable of rotating gyromagnetic electrons of the propellant gas present in the discharge chamber; and an electromagnetic wave generator adapted to irradiate the propellant gas present in the discharge chamber by generating at least one electromagnetic wave whose electric field has a right circular polarization and a frequency equal to the frequency, E E CR, of gyromagnetic resonance of the electrons of the propellant gas magnetized by said magnetic field generator, characterized in that:
• a magnetic field having: ⁇ a first local maximum (A) of the intensity, located within the injection nozzle (65) and the outlet end (165) of the injection nozzle (65), said first maximum of intensity being sufficient to ionize, by cyclotron resonance, under the effect of said electromagnetic wave, the propellant gas leaving said injection nozzle;
• ECR surface of equal intensity to that for a cyclotron resonance of electrons under the effect of said electromagnetic wave, said ECR surface being between 0.5 mm and 2 mm from said first local maximum intensity of the magnetic field and enveloping the outlet end of said injection nozzle, the volume defined by this ECR surface being the resonance cavity of the electromagnetic wave, which allows a total ionization of the propellant gas exiting said nozzle;
• ⁇ is made of an electrically conductive material and is electrically connected to the electromagnetic wave generator so as to also function as an electromagnetic antenna emitting said electromagnetic wave in the propellant gas at the exit of said nozzle;
• ⁇ is made of a magnetic conductive material, to obtain inside the latter, a second local maximum of the intensity of the magnetic field;
• ⁇ comprises, at the downstream end of said nozzle, a channel of outside diameter less than a few millimeters called point allowing, by concentrating the magnetic field lines, to obtain from a magnetic field generator of achievable intensity by permanent magnets, on the one hand the first local maximum of the intensity of the magnetic field, and on the other hand a hollow cathode micro-discharge in said local minimum of the intensity of the magnetic field, sufficient to ionize at least a portion of the propellant gas present in said nozzle regardless of its flow rate.
• 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.
• 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.
• 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.
• the plasma thruster comprises one or more of the following characteristics: plasma thruster according to the preceding embodiment, in which the magnetic field generator comprises as magnetic field source at least one permanent magnet of toric form arranged coaxially to the predefined axis and having two poles, a first magnetic element integral with a magnetic field pole and a second magnetic element integral with the other pole of said magnetic field source (50), said magnetic poles and being arranged at a first distance and a second distance respectively from the predefined axis; the second distance being longer than the first distance, the first magnetic pole and the second magnetic pole being arranged upstream and respectively downstream of the injection nozzle, considering the direction of flow of the propellant gas, the cutting field lines the injection nozzle and forming an angle between 10 ° and 70 ° with said predefined axis.
• the magnetic field generator comprises as magnetic field source at least one permanent magnet of toric form arranged coaxially to the predefined axis and having two poles, a first magnetic element integral with a magnetic field pole and a second magnetic element integral with the other pole of
• Plasma thruster according to one of the preceding embodiments, wherein the length, defined along the predefined axis, of the internal cavity of the discharge chamber is 5 to 10 times smaller than the half-wavelength of said electromagnetic wave in a vacuum, the discharge chamber having an inner section of between 0.7 square centimeters and 30 square centimeters; wherein the injection means comprises a central injection channel having an inner section of between 0.7 square millimeters and 3 square millimeters.
• Plasma thruster according to one of the preceding embodiments, wherein the magnetic field intensities of said first local maximum, local minimum and second local maximum are respectively about 0.18 Tesla, 0.01 Tesla and 0.05 Tesla. .
• Plasma thruster wherein said electromagnetic wave is able to propagate along an axis parallel to the predefined axis and in which, at the level of the predefined axis, the magnetic field gradient is parallel to the predefined axis; said field gradient magnetic being negative from upstream to downstream of the direction of ejection of the propellant gas said nozzle.
• Plasma thruster according to one of the preceding embodiments, which comprises a device for modulating the power of the electromagnetic wave and a device for controlling the flow of the propellant gas, said power of the electromagnetic wave being between 0.5 watts. and 300 watts, and preferably between 0.5 watts and 30 watts in a first mode of operation.
• Plasma thruster according to one of the preceding embodiments, which comprises firstly a circulator disposed at the output of said electromagnetic wave generator and secondly an electrically conductive cylindrical sleeve disposed downstream of the exit plane. the thruster, whose diameter is substantially equal to a quarter of the wavelength of the electromagnetic radiation and whose length is substantially equal to three quarters of the wavelength of the electromagnetic radiation.
• 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.
• Plasma thruster comprising two coaxial injection means to the axis, one supplying gas to ionize the ECR surface, and the other significantly increasing the thrust by a gas flow rate. significantly larger and an arc-jet operation.
• the invention also relates to a method for generating a propulsive thrust by means of a plasma thruster comprising the following steps:
• ⁇ injecting into a discharge chamber comprising an interior cavity and an outlet opening, with at least one injection means comprising an outlet end known injection nozzle, a propellant gas along an axis predefined; ⁇ generation, using a magnetic field generator, a magnetic field capable of rotating gyromagnetic electrons propellant gas present in the discharge chamber;
• ⁇ emission in the propellant gas present in the discharge chamber with an electromagnetic wave generator at least one electromagnetic wave whose electric field has a right circular polarization and a frequency equal to the frequency, ⁇ E CR, of gyromagnetic resonance of electrons of the propulsive gas magnetized by said magnetic field generator;
• plasma priming is carried out by hollow-cathode micro-discharge by means of injection which is made of magnetic material and comprises, at the downstream end of its nozzle, a channel of outside diameter less than a few millimeters, called tip allowing by concentrating the magnetic field lines, obtaining from a magnetic field generator of intensity achievable by permanent magnets, on the one hand a first local maximum of the intensity of the magnetic field inside.
• the injection of the propellant gas and the emission of the electromagnetic wave are carried out by the same means and therefore at the same place in the chamber of discharge, said injection means being made of an electrically conductive material and electrically connected to the electromagnetic wave generator for emitting the wave in the propellant gas at the outlet of said nozzle, so as to maximize the ionization rate propellant gas when leaving;
• the magnetic field has:
• ⁇ a first local maximum of the intensity at the output end and the interior of the injection nozzle (65), sufficient to ionize, by electron cyclotron resonance under the effect of said electromagnetic wave, gas propellant leaving said injection nozzle;
• ECR surface of equal intensity to that for a cyclotron resonance of electrons under the effect of said electromagnetic wave, said ECR surface being very close to said first local maximum of intensity magnetic field and enveloping the outlet end of said injection nozzle so as to maximize the ionization rate of the propellant gas leaving said nozzle;
• the magnetic field accelerates towards the exit opening by the diamagnetic force, along a nozzle magnetic, the free electrons of the plasma initiated at the injection nozzle, the positive ions, not magnetized, following these electrons because of the ambipolar electric field, or space charge field, which appears immediately within the plasma and opposes any imbalance between the populations of positive ions and electrons, this electric field, which is not disturbed by any applied electric field, ensuring very effectively the electrical neutrality of the plasma ejected from said thruster;
• the maintenance of the electron cyclotron resonance plasma is carried out by resonance of the electromagnetic wave in the volume delimited by the ECR surface, so as to be able to take advantage of the very high refractive index in this volume to reduce the length of the discharge chamber and consequently the plasma thruster.
• 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.
• This index of refraction of the EM wave resonance medium which is higher than in the state of the art, makes it possible on the one hand to reduce the length of the discharge chamber, since the plasma initiation and maintenance thereof no longer require that the length of the discharge chamber be equal to a whole number of half wavelength of the EM wave in the vacuum, and secondly to use a magnetic field of lower intensity, reachable 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.
• the method according to the preceding embodiment wherein the plasma thruster further comprises a device for modulating the power of the electromagnetic wave, a device for controlling the flow of gas and a peripheral injection channel capable of injecting. propellant gas in the discharge chamber, comprises the following steps:
• Figure 1 is an axial sectional view of a plasma propellant according to the invention
• FIG. 2 is an enlarged view of a portion of FIG. 1 representing the field lines of the magnetic field generated by a plasma thruster generator according to the invention
• Figure 3 is a diagram of the steps of the method according to the invention
• Figure 4 is an axial sectional view of a propellant according to an alternative embodiment of the invention.
• FIG. 5 is a graph representing the magnetic field along the axis A-A of the thruster.
• the plasma thruster 2 comprises a support body 4 supporting a discharge chamber 6 opening onto an outlet opening 48.
• the support body 4 is a non-magnetic hollow body open at each of its ends 9, 11. It comprises a cylindrical inner cavity 14 of axis of revolution A-A, hereinafter called predefined axis A-A.
• 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.
• 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 .
• the central injection channel 10 has an inner section of between 0.7 square millimeters and 3 square millimeters.
• the channel central injection 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.
• 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.
• 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 operating mode called "arc -jet" As described below.
• This source 30 is provided with a device 32 for controlling the gas flow rate.
• the flow rate of the propellant gas is between 0.1 gram per hour and 40 gram per hour.
• 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 a outlet of an injection nozzle.
• 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.
• the present diamagnetic force propellant constitutes an open cavity for the magnetic field, or more precisely a coaxial system open at one end.
• 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.
• 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.
• 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 any radiation heating and maintain optimum energy efficiency.
• 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").
• 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 AA. 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.
• the end surface of the protuberance 62 closest to the injection channel central 10 forms a first magnetic pole 64 disposed upstream of the injection nozzle 65 by considering the flow direction Fl of the propellant gas, and at a first distance D1 of the predefined axis AA.
• 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 Fl, 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 off the injection nozzle 65 of the central injection channel 10 and form an angle between 10 ° and 70 ° with the predefined axis AA. In other words, the magnetic field emitted by the magnetic field generator 46 diverges. At the predefined axis AA, the magnetic field gradient is parallel to the predefined axis AA. 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).
• ECR electromagnetic resonance
• 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.
• the ECR surface is located at a distance of millimeter downstream of the output end 165.
• 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.
• ECR plasma sources lie 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.
• the 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.
• 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).
• 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.
• 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.
• 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.
• 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 counter nut. 72.
• the plasma thruster according to the invention can be used by means of permanent magnets that do not consume energy.
• 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.
• 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.
• the method of generating a propulsive thrust according to the invention is achieved by means of a plasma thruster described above.
• a plasma thruster described above.
• it comprises, with reference to FIG. 3, the following steps:
• 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.
• the axial injection of the propellant gas is completed in this operating mode by an injection of gas around the central injection duct.
• This one is generally used during a temporary operation with strong thrust of the thruster here called second mode of operation said "arc -jet".
• the pressure rise of the discharge chamber 6 allows to ignite a plasma type arc - 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.
• 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.
• the plasma thruster 120 furthermore 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 a electrically conductive cylindrical sleeve 85 disposed downstream of the plasma thruster 120 exit plane.
• the circulator 80 is a device, generally made of ferrite, which is placed in a microwave circuit to protect the electromagnetic generator 38 or any amplifier against a return wave EM, for example reflected by the plasma (which is, for the generator of EM wave, the charge to be irradiated).
• the EM wave flow that passes 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.
• 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 retrograde flow to the plasma thruster 120, and so on ... until complete absorption of EM wave flux by the plasma.
• FIG. 5 represents the variation of the magnetic field generated by the generator 46 with respect to the distance to the output plane D-D of the plasma thruster along the predefined axis A-A.
• the zero of the abscissa axis defines in this figure the output plane D-D.
• the exit plane is the plane parallel to the median plane of the fastening flange 70 located at the outlet opening 48.
• 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, for example cyclotron resonance of the 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 E CR necessary to obtain a cyclotron resonance defined by the following formula:
• 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 space charge field, 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 any applied electric field, ensuring very effectively the electrical neutrality of the plasma ejected from 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.

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