EP3574719B1 - System for generating a plasma jet of metal ions - Google Patents

System for generating a plasma jet of metal ions Download PDF

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Publication number
EP3574719B1
EP3574719B1 EP18705434.1A EP18705434A EP3574719B1 EP 3574719 B1 EP3574719 B1 EP 3574719B1 EP 18705434 A EP18705434 A EP 18705434A EP 3574719 B1 EP3574719 B1 EP 3574719B1
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Prior art keywords
metal
tube
generating
plasma
anode
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EP18705434.1A
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German (de)
French (fr)
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EP3574719A1 (en
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Tiberiu Minea
Thomas Petty
Daniel LUNDIN
Charles BALLAGE
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Centre National de la Recherche Scientifique CNRS
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Centre National de la Recherche Scientifique CNRS
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    • 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
    • H05H1/00—Generating plasma; Handling plasma
    • H05H1/24—Generating plasma
    • H05H1/26—Plasma torches
    • H05H1/32—Plasma torches using an arc
    • 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
    • H05H2007/022—Pulsed systems

Definitions

  • the present invention relates to a system for generating a plasma jet.
  • Systems for generating plasma of a metal from a solid block of this metal are known. Such systems are used to deposit a metallic coating on a substrate, in particular a thin film coating. These systems essentially produce neutral metal vapours, ie metal atoms of which only a part is ionized.
  • such a system comprises a vacuum chamber in which is placed a block of metal which is brought to a positive potential to become an anode, a cathode which generates electrons, and a substrate intended to receive a coating of this metal.
  • the system further comprises a series of magnets which are intended to guide the metal ions formed by vaporization of the metal.
  • the electrons emitted in a beam by the cathode are attracted by the block of metal forming the anode. Under the effect of the bombardment by the electrons of this beam and the local and intense rise in temperature which results from it, part of the block melts and is transformed into metallic gas. The atoms of this gas are then partially ionized by the flow of electrons emitted by the cathode and form a plasma of positive metal ions and electrons. These positive metal ions are accelerated towards the cathode and towards the substrate which is also placed at a negative potential.
  • the cathode is generally annular in shape, so that the ions, guided by the series of magnets arranged around the path between the block of metal and the substrate, pass through the cathode and impact the substrate in order to form a metallic coating.
  • the electron emitter is placed in the path of the flow of metal ions, and is therefore progressively damaged by this flow, in particular because of an undesirable deposit of metal ions which forms on the emitter.
  • the lifetime of the emitter, and consequently of the plasma generation system, is therefore reduced.
  • the present invention aims to remedy these drawbacks.
  • the invention aims to propose a system for generating a plasma jet comprising metal ions which is able to generate a directional flow, whose service life is improved, whose manufacture is simplified, and which operates without magnets.
  • the system for generating a plasma jet comprises a tube of electrically insulating material containing a metal in solid form at room temperature and an anode in contact with this metal, a generator connected to the anode suitable to create a positive electrical potential at this anode, a heating element capable of heating part of the metal to a heating temperature Tc sufficient to vaporize this part of the metal, a source of electrons located outside the tube and outside the longitudinal axis of the tube, and being able to generate a flow of electrons capable of ionizing the vapor of the metal to form metal ions, so that the metal ions thus produced are able to be repelled and thus accelerated by this potential and ejected out of the tube by the downstream end of the tube, and being partially neutralized by electrons in order to form a plasma flow, the system operating without magnets, without access grid leration.
  • the system for generating a plasma jet is simplified because no magnets are used to direct the plasma flow. Indeed, it is the specific distribution of the electric field inside and near the tube which directs the plasma.
  • the electron source being located outside the tube and outside its longitudinal axis, is not damaged by the plasma beam.
  • the lifetime of the plasma generation system is therefore increased.
  • the metal used has an atomic mass greater than or equal to that of gold or has a melting point less than or equal to that of gold.
  • the system according to the invention can operate with a metal whose melting point is lower than other metals, since the system does not use a concentrated electron beam, the characteristic of which is to heat the metal very strongly and therefore evaporate it too quickly, unlike existing systems.
  • the heating element surrounds the downstream part of the tube.
  • the tube is made of ceramic, providing electrical and thermal insulation.
  • the anode is separate from the metal contained in the tube.
  • the electron source comprises the heating element.
  • the electron source comprises an external electron emitter separate from the heating element.
  • the generator provides a direct electric current.
  • the generator provides pulses generating an electric current.
  • inside and outside indicate the region inside and outside the tube, respectively.
  • upstream and downstream designate the parts of the tube and of the metal cylinder with respect to the direction of circulation of the ions in the tube.
  • the system according to the invention comprises a tube 10, which contains a metal cylinder 20 which provides the metal atoms immediately ionized by the high current density of electrons, the expulsion of which from the tube constitutes the plasma jet.
  • this metal is referred to as "plasma metal” to distinguish it from other metals used in the system.
  • the tube 10 is made of a material whose melting temperature is higher than the melting temperature Tm of the plasma metal 20.
  • the tube 10 is made of ceramic. This ceramic is for example an aluminum oxide, or a boron nitride.
  • Tube 10 is electrically insulating.
  • a heating element 40 surrounds at least the downstream part 12 of the tube 10. This heating element 40 is powered by a heating source 42. For example, the heating element 40 surrounds the entire tube 10. The heating element is for example a filament wound around the tube 10 helically to form a turn.
  • the system according to the invention also comprises an electron source 60.
  • This source of electrons is necessary to balance the positive charge of the ions emitted by the plasma metal 20, so that the particles emitted by the system and used for propulsion are globally electrically neutral, downstream of the cylinder.
  • the heating element 40 emits electrons, and is therefore the whole of the electron source 60. This is the case when the heating element 40 is a filament. This filament is for example made of tungsten.
  • the heating element 40 being the only source of electrons 60, the manufacture of the system is simplified, since the system does not include a separate source of electrons.
  • heating element 40 is a cathode (negatively charged).
  • the heating element 40 does not emit electrons.
  • a source of electrons 60 distinct from the heating element 40, and external to the tube 10, is necessary. This situation is represented on the figure 2 .
  • the heating element 40 is a ring which surrounds the downstream part 12 of the tube 10.
  • the electron source 60 is an external emitter 62, which is a cathode located close to the downstream end 15 of the downstream part 12 of the tube 10, or an arc generator.
  • the external emitter 62 is the only cathode in the system.
  • the heating element 40 is for example made of a material such as an Ni-Cr alloy (for example Nichrome ® ), an Fe-Cr-Al alloy (such as Kanthal ® ), or a cupronickel.
  • both the heating element 40 and the external emitter 62 are a cathode.
  • the electron source 60 then consists of the heating element 40 and the external emitter 62.
  • the electron source is located outside the tube 10 and outside the longitudinal axis of the tube 10.
  • the heating element 40 is for example made of a material such as lanthanum hexaboride, cerium hexaboride, or mixtures of oxides of barium, strontium, and calcium.
  • the heating element 40 is surrounded by an electrical insulator.
  • the system comprises an anode 30 (positively charged) which is in contact with the plasma metal 20 when this metal is in solid form.
  • the anode 30 is therefore in contact with the plasma metal 20 located in the tube 10.
  • the anode 30 is separate from the plasma metal 20 and is located inside the tube 10.
  • the anode 30 is made of a conductive material which remains solid during operation of the system for generating a plasma jet.
  • the anode 30 is a metal with a melting temperature much higher than that of the plasma metal 20.
  • the anode is made of tungsten, tantalum, molybdenum, rhenium, or an alloy of these metals.
  • Anode 30 is a wire that extends through the center of plasma metal cylinder 20, from its upstream end to its downstream end.
  • An electrical generator 50 is connected to anode 30 and maintains positive electrical potential at anode 30.
  • the anode 30 can have any geometry, for example one or more wires embedded in the plasma metal 20, or a grid embedded in the plasma metal 20, or a grid that lines the internal face of the tube 10. Whatever its geometry , the anode 30 is still in contact with the plasma metal 20, which makes it possible to maintain the arrival of the flow of electrons in the plasma metal 20.
  • This embodiment has the advantage that the electrical potential is maintained on the plasma metal 20 even when part of the plasma metal 20 passes into the liquid phase.
  • Another advantage is that in the event of formation of metal droplets downstream of the plasma metal cylinder 20 during its partial vaporization, the electrical connection to the anode 30 is still made. In fact, these droplets are liable to disturb this electrical connection.
  • the anode 30 is formed by the plasma metal 20 itself.
  • anode in contact with the metal covers the embodiment where the anode is an element distinct from the metal and in contact with the metal, and the embodiment where the anode is formed by the metal .
  • the plasma metal cylinder 20 is fed continuously, that is to say that the cylinder 20 slides in the tube 10 from upstream to downstream in such a way that its solid downstream end is always substantially at the same position in the tube 10 as the plasma metal 20 located at the level of the downstream end 15 of the tube 10 is vaporized.
  • the plasma metal cylinder 20 is supplied from a coil.
  • Plasma metal 20 is solid at ambient temperature and pressure (about 20° C., 1 atmosphere).
  • the plasma generation system according to the invention preferably uses a plasma metal 20 whose atomic mass is greater than or equal to that of gold (whose atomic mass is 197), or whose melting temperature is lower or lower. equal to that of gold (1064°C).
  • the plasma metals are chosen from lead (atomic mass of 207, melting temperature of 327°C), bismuth (atomic mass of 208, melting temperature of 271°C), tin (melting temperature of 232°C), zinc (melting temperature 420°C), tellurium (melting temperature 450°C), indium (melting temperature 156°C), thallium (atomic mass 204 , melting temperature of 303°C).
  • the melting temperature of plasma metal 20 is less than 500°C.
  • the plasma metal 20 has an atomic mass greater than or equal to that of gold, and a melting temperature is less than or equal to that of gold.
  • these metals have a lower melting temperature than other metals.
  • the heating temperature necessary to melt these metals which is at most of the order of the melting temperature Tm of the metal, is then lower, which makes it possible to dispense with a device for cooling the tube 10.
  • the power required to heat the plasma metal 20 and produce the ions is lower, which means a lower energy expenditure.
  • the only ions are metal ions.
  • the system according to the invention can be used in a space vehicle propulsion system.
  • the ejection of the plasma generates a moment which can be used for propulsion (see below the description of the propulsion systems).
  • the higher the plasma metal 20 has a high atomic mass in particular if it is greater to that of xenon (of atomic mass 131), the more the impulse generated during the expulsion of this metal is higher than that generated when xenon is used, for the same state of ionization.
  • a metal with a high atomic mass has a lower first ionization potential than other materials. For example, it is 6.1 eV for thallium, 7.4 eV for lead, and 9.2 eV for gold, which is lower than the ionization potential of xenon (12.1 eV). Thus, the probability of ionizing these metals is higher than that of ionizing xenon.
  • a metal with a high atomic mass has a higher probability of being doubly ionized, i.e., it loses two electrons to form metal ions.
  • an ion of this metal is accelerated more than ions losing only one electron, as is generally the case for Xenon.
  • the double ionization potentials of lead (15 eV), thallium (20.4 eV) and gold (20.2 eV) are lower than the double ionization potential of xenon (21 eV).
  • the invention also relates to a method for generating plasma, the operation of which is described below.
  • the plasma metal cylinder 20, in solid form, is placed in the tube 10.
  • the plasma metal 20 is then heated by the heating element 40, supplied by the heating source 42, to a heating temperature Tc sufficient to vaporize the downstream end of the plasma metal cylinder 20.
  • the heating temperature Tc is therefore much higher than the ambient temperature.
  • plasma metal 20 is placed at a non-zero positive potential by generator 50 (either directly or via anode 30 in contact with plasma metal 20).
  • the metallic gas, product of this vaporization is ionized by the electrons emitted by the electron source 60 (which is either the heating element 40, or the external emitter 62, or both together). These metal ions are repelled by the metal cylinder 20 because they have the same positive charge, and are accelerated towards the downstream end 15 of the tube 10. These metal ions, which form a plasma, also collide with the electrons emitted by the electron source 60 such that the plasma flux 70 emitted by the tube 10 at its downstream end 15 is partly a flux of electrically neutral metallic particles, partly a flow of metal ions, partly a flow of electrons. The direction of propagation of the flux 70 is indicated by an arrow on the figures 1 and 2 .
  • the metal ions are accelerated and then ejected from the tube 10, and during their ejection a part of these metal ions is neutralized by collision with the electrons emitted by the electron source 60. These metal ions which are neutralized are transformed into metal particles electrically neutral.
  • the system according to the invention does not include an ion acceleration grid, unlike the HC thrusters (see below). Indeed, these grids are useless because the ions are repelled by the anode and accelerated under a sufficiently high positive voltage (see explanation below). Thus, the manufacture of the system is simplified.
  • the system according to the invention does not include magnets, unlike HE thrusters (see below).
  • the system therefore does not use a magnetic field generated by magnets to act on the electrons, or on the ions ejected from the metal.
  • the system is therefore simpler and less expensive to manufacture.
  • the system according to the invention is therefore more compact than other systems according to the prior art.
  • the length of the system is of the order of 10 cm, and its diameter is less than 1 cm, for example equal to 0.5 cm.
  • the tube 10 Since the tube 10 is heated when the system is operating, the metal vapor particles which could have deposited on the internal surface of the downstream part of the tube 10 will be easily vaporized and will come off the surface during future operation. Thus, the tube 10 is not clogged with deposits.
  • the system according to the invention operates with direct current generated by the generator 50, which avoids interference with any electronic components located close to the system which could occur if radio frequency or high frequencies are used. .
  • the potential supplied to the anode 30 by the generator 50 is of the order of several hundred volts.
  • the intensity of the current is of the order of 1 Amp and more, being able to reach for example 5A or more in pulsed mode.
  • the system operates with a series of electrical pulses (pulsed current), using a pulse generator.
  • This mode operating mode has the advantage of providing a higher thrust in the case where the system according to the invention is used in a space vehicle propulsion system (see below).
  • the pulse generator is powered by generator 50.
  • the tests carried out by the inventors show that a stable current of 2 A (Amps) can be reached with an average voltage jump of 2 kV (kiloVolts), which gives a power at each pulse of 4 kW (kiloWatts) per pulse.
  • the duration of the pulse is variable between 10 and a few hundred ⁇ s. In the operating example given in picture 3 , the duration of the pulse is approximately 40 ⁇ s (microseconds).
  • the curve referenced S represents the pulse signal (in Volts)
  • the curve referenced V represents the discharge potential at the anode (in kiloVolts)
  • the curve referenced I represents the discharge current at the anode (in Amperes ).
  • the duration of the pulse is 40 ⁇ s (microseconds), the unit on the abscissa axis of the picture 3 being in ⁇ s.
  • the system allows the efficient transfer of momentum to the heavy ions, all the greater as the voltage applied to the anode is great.
  • the system according to the invention does not operate in arc mode standard.
  • the voltage supplied initially is of the order of several thousand volts, and remains at a few hundred volts after formation of the arc (breakage or breakdown phenomenon).
  • the high value of this voltage (voltage) even after breakdown is due to the formation at the downstream output of the tube 10 of a plasma sphere whose surface is the boundary of the shock wave generated by the expansion of the ion flux in vacuum.
  • this boundary is strongly electrically charged, which contributes to accelerating the metal ions ejected by the plasma metal cylinder 20.
  • this operating mode from the standard arc, it will be called “anomalous arc”.
  • the metal ions are naturally repelled by the anode, and in steady state the plasma is self-sustaining with a heating maintained by the discharge current (i.e., the electrons of the plasma which join the anode), especially for regimes high current.
  • the discharge current i.e., the electrons of the plasma which join the anode
  • an external electron emitter 62 will be used as an electron source only to emit electrons serving to neutralize the ion plasma towards the downstream end 15 of the tube 10.
  • the cathode when the anomalous arc is maintained, can be operated without additional heating.
  • This mode of operation of the plasma generation system has the advantage that in steady state the source of electrons 60, in this case the external emitter 62, can operate at a lower electrical power consumption.
  • the system (and the method) for generating plasma according to the invention is used in a propulsion system of a space vehicle, the ejection of the plasma serving for the propulsion of this vehicle.
  • Hall effect thrusters or HE (“Hall Effect Thruster”) thrusters
  • This thruster has an annular space with a bottom at one end, and open at the other end, in which a magnetic field is established.
  • a cathode which emits electrons, is located at the open end of the annular space often operating with a gas supply (hollow cathode).
  • the bottom of the annular space constitutes an anode, through which are injected atoms of xenon or another propellant gas, often stored in liquefied form.
  • the electrons emitted by the cathode are trapped at the entrance to the annular space by the magnetic field, where they accumulate, part of the electrons continuing their journeys towards the anode.
  • the propellant gas atoms are ionized by collision with the electrons in the annular space, and accelerated by the electric field towards the open end of this space.
  • the ions are neutralized by crossing the cloud of electrons and ejected out of space in the form of a plasma with zero charge. The ejection of this plasma provides propulsion to the space vehicle.
  • a gas is injected through a tube (hollow cylinder) forming the anode, the internal surface of which is covered with a material which emits electrons when heated (thermionic emission).
  • heating the tube causes ionization of the gas as it passes through the tube.
  • the ions thus formed are then accelerated by the potential difference between the anode and the cathode which is located at the end of the tube which is opposite to that through which the gas is injected.
  • the HC propellant has drawbacks.
  • the HC thruster operates with a low potential difference (approximately 30 V) and therefore a low intrinsic thrust. Accelerating the ions further to achieve greater thrust requires voltages (voltage) of several hundred volts, which involves the use of biased grids. These grids are placed downstream of the tube. This complicates the propulsion system. Moreover, these grids, being subjected to the flow of accelerated ions, wear out, which reduces their long-term effectiveness.
  • the propulsion system is simplified because it is not necessary to deposit a coating of an additional material, a source of electrons, on the internal face of the tube. Indeed, the source of electrons is located outside the tube.
  • the initial source (precursor material) of material for the ions is, at ambient temperature, not a gas, nor a liquid, but a solid.
  • the precursor material which is used by the system according to the invention before the start of its operation, therefore before the heating of this precursor material is a solid metal.
  • the acceleration potential of the ions of the propulsion system is higher than that of HC thrusters and the ions are accelerated under a sufficiently high voltage (see explanation above), which makes it possible to overcome the use of polarized grids and therefore to reduce the weight of the system, and which increases its efficiency.
  • the system therefore operates without acceleration grids.
  • the system works without magnets and therefore without a magnetic field, unlike HE thrusters.
  • the system is therefore simpler and less expensive to manufacture.
  • the system according to the invention is therefore more compact than other systems according to the prior art.
  • the length of the system is of the order of 10 cm, and its diameter is less than 1 cm, for example equal to 0.5 cm.
  • the system according to the invention can also be used for other applications, such as the production of multicharged heavy ions for particle accelerators, or for thermonuclear fusion by heavy ions.
  • the system according to the invention thus advantageously replaces the existing systems for the production of heavy ions, which use magnetic fields.
  • the pulses supplied by the generator are of high power, of the order of several hundred kV.

Description

La présente invention concerne un système de génération d'un jet plasma. On connait des systèmes de génération de plasma d'un métal à partir d'un bloc solide de ce métal. De tels systèmes sont utilisés pour déposer un revêtement métallique sur un substrat, en particulier un revêtement en couche mince. Ces systèmes produisent essentiellement des vapeurs métalliques neutres, c'est à dire des atomes de métal dont seulement une partie est ionisée.The present invention relates to a system for generating a plasma jet. Systems for generating plasma of a metal from a solid block of this metal are known. Such systems are used to deposit a metallic coating on a substrate, in particular a thin film coating. These systems essentially produce neutral metal vapours, ie metal atoms of which only a part is ionized.

Par exemple, un tel système comprend une chambre à vide dans laquelle est placé un bloc de métal qui est porté à un potentiel positif pour devenir une anode, une cathode qui génère des électrons, et un substrat destiné à recevoir un revêtement de ce métal. Le système comprend en outre une série d'aimants qui sont destinés à guider les ions métalliques formés par vaporisation du métal.For example, such a system comprises a vacuum chamber in which is placed a block of metal which is brought to a positive potential to become an anode, a cathode which generates electrons, and a substrate intended to receive a coating of this metal. The system further comprises a series of magnets which are intended to guide the metal ions formed by vaporization of the metal.

Dans un tel système, les électrons émis en faisceau par la cathode sont attirés par le bloc de métal formant anode. Sous l'effet du bombardement par les électrons de ce faisceau et de l'élévation locale et intense de température qui en résulte, une partie du bloc fond et se transforme en gaz métallique. Les atomes de ce gaz sont alors partiellement ionisés par le flux d'électrons émis par la cathode et forment un plasma d'ions positifs de métal et d'électrons. Ces ions métalliques positifs sont accélérés vers la cathode et vers le substrat qui est également placé à un potentiel négatif. La cathode est généralement de forme annulaire, de telle sorte que les ions, guidés par la série d'aimants disposés autour du trajet entre le bloc de métal et lè substrat, passent au travers de la cathode et impactent le substrat afin d'y former un revêtement métallique.In such a system, the electrons emitted in a beam by the cathode are attracted by the block of metal forming the anode. Under the effect of the bombardment by the electrons of this beam and the local and intense rise in temperature which results from it, part of the block melts and is transformed into metallic gas. The atoms of this gas are then partially ionized by the flow of electrons emitted by the cathode and form a plasma of positive metal ions and electrons. These positive metal ions are accelerated towards the cathode and towards the substrate which is also placed at a negative potential. The cathode is generally annular in shape, so that the ions, guided by the series of magnets arranged around the path between the block of metal and the substrate, pass through the cathode and impact the substrate in order to form a metallic coating.

Un tel système présente cependant des inconvénients.However, such a system has drawbacks.

En effet, l'émetteur d'électrons est placé sur le trajet du flux d'ions métalliques, et est donc progressivement endommagé par ce flux, notamment à cause d'un dépôt indésirable d'ions métalliques qui se forme sur l'émetteur. La durée de vie de l'émetteur, et par voie de conséquence du système de génération de plasma, est donc réduite.Indeed, the electron emitter is placed in the path of the flow of metal ions, and is therefore progressively damaged by this flow, in particular because of an undesirable deposit of metal ions which forms on the emitter. The lifetime of the emitter, and consequently of the plasma generation system, is therefore reduced.

De plus, l'utilisation d'aimants, nécessaire pour former un flux concentré et directionnel d'ions métalliques (plasma), complexifie le système de génération du plasma. En outre, un dispositif de refroidissement des aimants doit être intégré au système pour éviter que les aimants soient chauffés au-dessus de leur température de Curie sous l'influence du plasma. Des exemples de systèmes et procédés pour la génération d'un jet plasma sont divulgués en:

  • US 2003033797 ;
  • US 4328667 et
  • " POWER SOURCE", product engineering, vol.35, no.16, p.75; 6 août 1964 .
In addition, the use of magnets, necessary to form a concentrated and directional flow of metal ions (plasma), complicates the plasma generation system. In addition, a magnet cooling device must be integrated into the system to prevent the magnets from being heated above their Curie temperature under the influence of the plasma. Examples of systems and methods for generating a plasma jet are disclosed in:
  • US 2003033797 ;
  • US 4328667 and
  • " POWER SOURCE", product engineering, vol.35, no.16, p.75; August 6, 1964 .

La présente invention vise à remédier à ces inconvénients.The present invention aims to remedy these drawbacks.

L'invention vise à proposer un système de génération d'un jet plasma comprenant des ions métalliques qui soit apte à générer un flux directionnel, dont la durée de vie soit améliorée, dont la fabrication soit simplifiée, et qui fonctionne sans aimants.The invention aims to propose a system for generating a plasma jet comprising metal ions which is able to generate a directional flow, whose service life is improved, whose manufacture is simplified, and which operates without magnets.

Ce but est atteint grâce au fait que le système de génération d'un jet plasma comprend un tube en matériau isolant électriquement contenant un métal sous forme solide à température ambiante et une anode en contact avec ce métal, un générateur connecté à l'anode apte à créer un potentiel électrique positif au niveau de cette anode, un élément de chauffage apte à chauffer une partie du métal à une température de chauffage Tc suffisante pour vaporiser cette partie du métal, une source d'électrons située à l'extérieur du tube et hors de l'axe longitudinal du tube, et étant apte à générer un flux d'électrons capable d'ioniser la vapeur du métal pour former des ions métalliques, de telle sorte que les ions métalliques ainsi produits sont aptes à être repoussés et ainsi accélérés par ce potentiel et éjectés hors du tube par l'extrémité aval du tube, et étant neutralisés pour une partie par des électrons afin de former un flux de plasma, le système fonctionnant sans aimants, sans grille d'accélération.This object is achieved thanks to the fact that the system for generating a plasma jet comprises a tube of electrically insulating material containing a metal in solid form at room temperature and an anode in contact with this metal, a generator connected to the anode suitable to create a positive electrical potential at this anode, a heating element capable of heating part of the metal to a heating temperature Tc sufficient to vaporize this part of the metal, a source of electrons located outside the tube and outside the longitudinal axis of the tube, and being able to generate a flow of electrons capable of ionizing the vapor of the metal to form metal ions, so that the metal ions thus produced are able to be repelled and thus accelerated by this potential and ejected out of the tube by the downstream end of the tube, and being partially neutralized by electrons in order to form a plasma flow, the system operating without magnets, without access grid leration.

Grâce à ces dispositions, on simplifie le système de génération d'un jet plasma car on n'utilise pas d'aimants pour diriger le flux de plasma. En effet, c'est la distribution spécifique du champ électrique à l'intérieur et à proximité du tube qui dirige le plasma.Thanks to these arrangements, the system for generating a plasma jet is simplified because no magnets are used to direct the plasma flow. Indeed, it is the specific distribution of the electric field inside and near the tube which directs the plasma.

De plus, la source d'électrons étant située à l'extérieur du tube et hors de son axe longitudinal, n'est pas endommagé par le faisceau de plasma. La durée de vie du système de génération de plasma est donc augmentée.In addition, the electron source being located outside the tube and outside its longitudinal axis, is not damaged by the plasma beam. The lifetime of the plasma generation system is therefore increased.

Avantageusement, le métal utilisé possède une masse atomique supérieure ou égale à celle de l'or ou possède une température de fusion inférieure ou égale à celle de l'or.Advantageously, the metal used has an atomic mass greater than or equal to that of gold or has a melting point less than or equal to that of gold.

Le système selon l'invention peut fonctionner avec un métal dont la température de fusion est plus faible que d'autres métaux, car le système n'utilise pas de faisceau d'électrons concentré dont la caractéristique est de chauffer très fortement le métal et donc de l'évaporer trop rapidement, contrairement aux systèmes existants.The system according to the invention can operate with a metal whose melting point is lower than other metals, since the system does not use a concentrated electron beam, the characteristic of which is to heat the metal very strongly and therefore evaporate it too quickly, unlike existing systems.

Avantageusement, l'élément de chauffage entoure la partie aval du tube.Advantageously, the heating element surrounds the downstream part of the tube.

Avantageusement, le tube est en céramique, assurant l'isolation électrique et thermique.Advantageously, the tube is made of ceramic, providing electrical and thermal insulation.

Avantageusement, l'anode est distincte du métal contenu dans le tube.Advantageously, the anode is separate from the metal contained in the tube.

Avantageusement, la source d'électrons comprend l'élément de chauffage.Advantageously, the electron source comprises the heating element.

Avantageusement, la source d'électrons comprend un émetteur externe d'électrons distinct de l'élément de chauffage.Advantageously, the electron source comprises an external electron emitter separate from the heating element.

L'invention concerne également un procédé de génération de plasma, qui comprend les étapes suivantes :

  1. (a) On fournit un tube en matériau isolant électriquement, contenant un métal sous forme solide à température ambiante et une anode
    en contact avec ce métal, un générateur électrique connecté à cette anode, et une source d'électrons située à l'extérieur du tube,
  2. (b) On applique un potentiel électrique positif au niveau de l'anode à l'aide du générateur,
  3. (c) On chauffe une partie du métal à une température de chauffage Tc suffisante pour vaporiser cette partie du métal,
  4. (d) On ionise la vapeur de métal ainsi produite par les électrons émis par la source d'électrons, pour former des ions métalliques qui sont accélérés par ce potentiel et éjectés hors du tube par l'extrémité aval du tube après avoir été pour une partie neutralisés par des électrons émis par ladite source d'électrons, afin de former un flux de plasma,
    le procédé n'utilisant pas d'aimants, pas de grille d'accélération, et pas de gaz en tant que source initiale de matière à ioniser.
The invention also relates to a method for generating plasma, which comprises the following steps:
  1. (a) A tube of electrically insulating material is provided, containing a metal in solid form at room temperature and an anode
    in contact with this metal, an electric generator connected to this anode, and a source of electrons located outside the tube,
  2. (b) A positive electric potential is applied to the anode using the generator,
  3. (c) a part of the metal is heated to a heating temperature Tc sufficient to vaporize this part of the metal,
  4. (d) The metal vapor thus produced is ionized by the electrons emitted by the electron source, to form metal ions which are accelerated by this potential and ejected out of the tube by the downstream end of the tube after having been for a partly neutralized by electrons emitted by said electron source, in order to form a plasma flow,
    the method using no magnets, no accelerating grid, and no gas as the initial source of material to be ionized.

Par exemple, le générateur fournit un courant électrique continu.For example, the generator provides a direct electric current.

Par exemple, le générateur fournit des impulsions générant un courant électrique.For example, the generator provides pulses generating an electric current.

L'invention sera bien comprise et ses avantages apparaîtront mieux, à la lecture de la description détaillée qui suit, d'un mode de réalisation représenté à titre d'exemple non limitatif. La description se réfère aux dessins annexés sur lesquels :

  • la figure 1 est une vue en coupe longitudinale du système selon l'invention.
  • la figure 2 est une vue en coupe longitudinale d'un autre mode de réalisation du système selon l'invention,
  • la figure 3 est un graphe montrant l'évolution en fonction du temps de certaines quantités lorsque le système selon l'invention fonctionne avec une série d'impulsions électriques.
The invention will be well understood and its advantages will appear better, on reading the detailed description which follows, of an embodiment represented by way of non-limiting example. The description refers to the accompanying drawings in which:
  • the figure 1 is a longitudinal sectional view of the system according to the invention.
  • the picture 2 is a view in longitudinal section of another embodiment of the system according to the invention,
  • the picture 3 is a graph showing the evolution as a function of time of certain quantities when the system according to the invention operates with a series of electrical pulses.

Dans la description qui suit les termes "intérieur" et "extérieur" indiquent la région à l'intérieur et à l'extérieur du tube, respectivement. Les termes « amont » et « aval » désignent les parties du tube et du cylindre de métal par rapport au sens de circulation des ions dans le tube.In the following description the terms "inside" and "outside" indicate the region inside and outside the tube, respectively. The terms “upstream” and “downstream” designate the parts of the tube and of the metal cylinder with respect to the direction of circulation of the ions in the tube.

Comme représenté en figure 1, le système selon l'invention comporte un tube 10, qui contient un cylindre en métal 20 qui fournit les atomes métalliques immédiatement ionisés par la forte densité de courant d'électrons dont l'expulsion hors du tube constitue le jet plasma. Dans la description qui suit, ce métal est appelé « métal plasma » afin de le distinguer d'autres métaux utilisés dans le système.As depicted in figure 1 , the system according to the invention comprises a tube 10, which contains a metal cylinder 20 which provides the metal atoms immediately ionized by the high current density of electrons, the expulsion of which from the tube constitutes the plasma jet. In the following description, this metal is referred to as "plasma metal" to distinguish it from other metals used in the system.

Le tube 10 est réalisé en un matériau dont la température de fusion est supérieure à la température de fusion Tf du métal plasma 20. Par exemple le tube 10 est en céramique. Cette céramique est par exemple un oxyde d'aluminium, ou un nitrure de bore.The tube 10 is made of a material whose melting temperature is higher than the melting temperature Tm of the plasma metal 20. For example, the tube 10 is made of ceramic. This ceramic is for example an aluminum oxide, or a boron nitride.

Le tube 10 est isolant électriquement.Tube 10 is electrically insulating.

Un élément de chauffage 40 entoure au moins la partie aval 12 du tube 10. Cet élément de chauffage 40 est alimenté par une source de chauffage 42. Par exemple l'élément de chauffage 40 entoure tout le tube 10. L'élément de chauffage est par exemple un filament enroulé autour du tube 10 de façon hélicoïdale pour former une spire.A heating element 40 surrounds at least the downstream part 12 of the tube 10. This heating element 40 is powered by a heating source 42. For example, the heating element 40 surrounds the entire tube 10. The heating element is for example a filament wound around the tube 10 helically to form a turn.

Le système selon l'invention comporte également une source d'électrons 60.The system according to the invention also comprises an electron source 60.

Cette source d'électrons est nécessaire pour équilibrer la charge positive des ions émis par le métal plasma 20, de telle sorte que les particules émises par le système et servant à la propulsion soient globalement électriquement neutres, en aval du cylindre.This source of electrons is necessary to balance the positive charge of the ions emitted by the plasma metal 20, so that the particles emitted by the system and used for propulsion are globally electrically neutral, downstream of the cylinder.

Par « globalement » électriquement neutre, on entend que le flux qui sort du tube est un mélange d'ions positifs et d'électrons et d'atomes, formant un plasma. La neutralité du jet plasma permet ainsi de préserver son fort caractère directionnel.By “globally” electrically neutral, it is meant that the flux leaving the tube is a mixture of positive ions and electrons and atoms, forming a plasma. The neutrality of the plasma jet thus makes it possible to preserve its strong directional character.

Dans un premier mode de réalisation, l'élément de chauffage 40 émet des électrons, et est donc la totalité de la source d'électrons 60. Cela est le cas lorsque l'élément de chauffage 40 est un filament. Ce filament est par exemple en tungstène.In a first embodiment, the heating element 40 emits electrons, and is therefore the whole of the electron source 60. This is the case when the heating element 40 is a filament. This filament is for example made of tungsten.

L'élément de chauffage 40 étant la seule source d'électrons 60, la fabrication du système est simplifiée, puisque le système ne comprend pas de source d'électrons distincte.The heating element 40 being the only source of electrons 60, the manufacture of the system is simplified, since the system does not include a separate source of electrons.

Dans ce mode de réalisation, l'élément de chauffage 40 est une cathode (chargée négativement).In this embodiment, heating element 40 is a cathode (negatively charged).

Selon un second mode de réalisation, l'élément de chauffage 40 n'émet pas d'électrons. Dans ce cas, une source d'électrons 60 distincte de l'élément de chauffage 40, et externe au tube 10, est nécessaire. Cette situation est représentée sur la figure 2. L'élément de chauffage 40 est un anneau qui entoure la partie aval 12 du tube 10.According to a second embodiment, the heating element 40 does not emit electrons. In this case, a source of electrons 60 distinct from the heating element 40, and external to the tube 10, is necessary. This situation is represented on the figure 2 . The heating element 40 is a ring which surrounds the downstream part 12 of the tube 10.

La source d'électrons 60 est un émetteur externe 62, qui est une cathode située à proximité de l'extrémité aval 15 de la partie aval 12 du tube 10, ou un générateur d'arc.The electron source 60 is an external emitter 62, which is a cathode located close to the downstream end 15 of the downstream part 12 of the tube 10, or an arc generator.

L'émetteur externe 62 est la seule cathode du système. Dans ce cas, l'élément de chauffage 40 est par exemple réalisé en un matériau tel qu'un alliage Ni-Cr (par exemple le Nichrome ®), un alliage Fe-Cr-Al (tel que le Kanthal ®), ou un cupronickel.The external emitter 62 is the only cathode in the system. In this case, the heating element 40 is for example made of a material such as an Ni-Cr alloy (for example Nichrome ® ), an Fe-Cr-Al alloy (such as Kanthal ® ), or a cupronickel.

Selon un troisième mode de réalisation, à la fois l'élément de chauffage 40 et l'émetteur externe 62 sont une cathode. La source d'électrons 60 est alors constituée de l'élément de chauffage 40 et de l'émetteur externe 62.According to a third embodiment, both the heating element 40 and the external emitter 62 are a cathode. The electron source 60 then consists of the heating element 40 and the external emitter 62.

Dans tous les cas, la source d'électrons est située à l'extérieur du tube 10 et hors de l'axe longitudinal du tube 10.In all cases, the electron source is located outside the tube 10 and outside the longitudinal axis of the tube 10.

Dans le cas d'une cathode chauffée indirectement, l'élément de chauffage 40 est par exemple réalisé en un matériau tel que l'hexaborure de lanthane, l'hexaborure de cérium, ou des mélanges d'oxides de barium, de strontium, et de calcium.In the case of an indirectly heated cathode, the heating element 40 is for example made of a material such as lanthanum hexaboride, cerium hexaboride, or mixtures of oxides of barium, strontium, and calcium.

Alternativement, dans le cas d'une cathode chauffée directement, l'élément de chauffage 40 est entouré par un isolant électrique.Alternatively, in the case of a directly heated cathode, the heating element 40 is surrounded by an electrical insulator.

Le système comporte une anode 30 (chargée positivement) qui est en contact avec le métal plasma 20 lorsque ce métal est sous forme solide. L'anode 30 est donc en contact avec le métal plasma 20 situé dans le tube 10.The system comprises an anode 30 (positively charged) which is in contact with the plasma metal 20 when this metal is in solid form. The anode 30 is therefore in contact with the plasma metal 20 located in the tube 10.

Selon un mode de réalisation, illustré en figure 1, l'anode 30 est distincte du métal plasma 20 et est située à l'intérieur du tube 10. L'anode 30 est réalisée en un matériau conducteur qui reste solide pendant le fonctionnement du système de génération d'un jet plasma. Ainsi, l'anode 30 est un métal avec une température de fusion largement supérieure à celle du métal plasma 20. Par exemple, l'anode est en tungstène, tantale, molybdène, rhénium, ou un alliage de ces métaux.According to one embodiment, illustrated in figure 1 , the anode 30 is separate from the plasma metal 20 and is located inside the tube 10. The anode 30 is made of a conductive material which remains solid during operation of the system for generating a plasma jet. Thus, the anode 30 is a metal with a melting temperature much higher than that of the plasma metal 20. For example, the anode is made of tungsten, tantalum, molybdenum, rhenium, or an alloy of these metals.

L'anode 30 est un fil qui s'étend au centre du cylindre de métal plasma 20, de son extrémité amont à son extrémité aval.Anode 30 is a wire that extends through the center of plasma metal cylinder 20, from its upstream end to its downstream end.

Un générateur électrique 50 est connecté à l'anode 30 et maintient le potentiel électrique positif à l'anode 30.An electrical generator 50 is connected to anode 30 and maintains positive electrical potential at anode 30.

L'anode 30 peut avoir une géométrie quelconque, par exemple un ou plusieurs fils noyés dans le métal plasma 20, ou une grille noyée dans le métal plasma 20, ou une grille qui tapisse la face interne du tube 10. Quelle que soit sa géométrie, l'anode 30 est toujours en contact avec le métal plasma 20, ce qui permet de maintenir l'arrivée du flux d'électrons dans le métal plasma 20.The anode 30 can have any geometry, for example one or more wires embedded in the plasma metal 20, or a grid embedded in the plasma metal 20, or a grid that lines the internal face of the tube 10. Whatever its geometry , the anode 30 is still in contact with the plasma metal 20, which makes it possible to maintain the arrival of the flow of electrons in the plasma metal 20.

Ce mode de réalisation présente l'avantage que le potentiel électrique est maintenu sur le métal plasma 20 même lorsqu'une partie du métal plasma 20 passe en phase liquide.This embodiment has the advantage that the electrical potential is maintained on the plasma metal 20 even when part of the plasma metal 20 passes into the liquid phase.

Un autre avantage est qu'en cas de formation de gouttelettes de métal en aval du cylindre de métal plasma 20 pendant sa vaporisation partielle, la connexion électrique à l'anode 30 est toujours réalisée. En effet ces gouttelettes sont susceptibles de perturber cette connexion électrique.Another advantage is that in the event of formation of metal droplets downstream of the plasma metal cylinder 20 during its partial vaporization, the electrical connection to the anode 30 is still made. In fact, these droplets are liable to disturb this electrical connection.

Alternativement, l'anode 30 est formée par le métal plasma 20 luimême.Alternatively, the anode 30 is formed by the plasma metal 20 itself.

Par l'expression « anode en contact avec le métal », on couvre le mode de réalisation où l'anode est un élément distinct du métal et en contact avec le métal, et le mode de réalisation où l'anode est formée par le métal.The expression "anode in contact with the metal" covers the embodiment where the anode is an element distinct from the metal and in contact with the metal, and the embodiment where the anode is formed by the metal .

Avantageusement, l'alimentation du cylindre en métal plasma 20 s'effectue en continu, c'est-à-dire que le cylindre 20 coulisse dans le tube 10 de l'amont vers l'aval de telle sorte que son extrémité aval solide se situe toujours sensiblement à la même position dans le tube 10 au fur et à mesure où le métal plasma 20 situé au niveau de l'extrémité aval 15 du tube 10 est vaporisée. Par exemple le cylindre en métal plasma 20 est fourni à partir d'une bobine.Advantageously, the plasma metal cylinder 20 is fed continuously, that is to say that the cylinder 20 slides in the tube 10 from upstream to downstream in such a way that its solid downstream end is always substantially at the same position in the tube 10 as the plasma metal 20 located at the level of the downstream end 15 of the tube 10 is vaporized. For example the plasma metal cylinder 20 is supplied from a coil.

Le métal plasma 20 est solide à température et pression ambiantes (environ 20°C, 1 atmosphère). Le système de génération de plasma selon l'invention utilise de préférence un métal plasma 20 dont la masse atomique est supérieure ou égale à celle de l'or (dont la masse atomique est de 197), ou dont la température de fusion est inférieure ou égale à celle de l'or (1064°C).Plasma metal 20 is solid at ambient temperature and pressure (about 20° C., 1 atmosphere). The plasma generation system according to the invention preferably uses a plasma metal 20 whose atomic mass is greater than or equal to that of gold (whose atomic mass is 197), or whose melting temperature is lower or lower. equal to that of gold (1064°C).

Par exemple les métaux plasmas sont choisis parmi le plomb (masse atomique de 207, température de fusion de 327°C), le bismuth (masse atomique de 208, température de fusion de 271°C), l'étain (température de fusion de 232°C), le zinc (température de fusion de 420°C), le tellure (température de fusion de 450°C), , l'indium (température de fusion de 156°C), le thallium (masse atomique de 204, température de fusion de 303°C).For example, the plasma metals are chosen from lead (atomic mass of 207, melting temperature of 327°C), bismuth (atomic mass of 208, melting temperature of 271°C), tin (melting temperature of 232°C), zinc (melting temperature 420°C), tellurium (melting temperature 450°C), indium (melting temperature 156°C), thallium (atomic mass 204 , melting temperature of 303°C).

Avantageusement, la température de fusion du métal plasma 20 est inférieure à 500°C.Advantageously, the melting temperature of plasma metal 20 is less than 500°C.

Avantageusement, le métal plasma 20 a une masse atomique supérieure ou égale à celle de l'or, et une température de fusion est inférieure ou égale à celle de l'or.Advantageously, the plasma metal 20 has an atomic mass greater than or equal to that of gold, and a melting temperature is less than or equal to that of gold.

L'utilisation de métaux avec une masse atomique élevée présente plusieurs avantages.The use of metals with a high atomic mass has several advantages.

En effet, d'une part ces métaux présentent une température de fusion plus faible que d'autres métaux.Indeed, on the one hand these metals have a lower melting temperature than other metals.

La température de chauffage nécessaire pour fondre ces métaux, qui est au maximum de l'ordre de la température de fusion Tf du métal, est alors moins élevée, ce qui permet de s'affranchir d'un dispositif de refroidissement du tube 10.The heating temperature necessary to melt these metals, which is at most of the order of the melting temperature Tm of the metal, is then lower, which makes it possible to dispense with a device for cooling the tube 10.

De plus, la puissance nécessaire pour chauffer le métal plasma 20 et produire les ions est plus faible, ce qui signifie une dépense d'énergie plus faible. Dans le jet plasma généré par le système selon l'invention, les seuls ions sont des ions métalliques.In addition, the power required to heat the plasma metal 20 and produce the ions is lower, which means a lower energy expenditure. In the plasma jet generated by the system according to the invention, the only ions are metal ions.

D'autre part, le système selon l'invention peut être utilisé dans un système de propulsion de véhicule spatial. En effet, l'éjection du plasma génère un moment pouvant servir à la propulsion (voir ci-dessous la description des systèmes de propulsion). Ainsi, plus le métal plasma 20 présente une masse atomique élevée (en particulier si elle est supérieure à celle du xénon (de masse atomique 131), plus l'impulsion générée lors de l'expulsion de ce métal est supérieure à celle générée lorsque le xénon est utilisé, pour le même état d'ionisation.On the other hand, the system according to the invention can be used in a space vehicle propulsion system. Indeed, the ejection of the plasma generates a moment which can be used for propulsion (see below the description of the propulsion systems). Thus, the higher the plasma metal 20 has a high atomic mass (in particular if it is greater to that of xenon (of atomic mass 131), the more the impulse generated during the expulsion of this metal is higher than that generated when xenon is used, for the same state of ionization.

En outre, un métal avec une masse atomique élevée présente un premier potentiel d'ionisation plus faible que d'autres matériaux. Par exemple, il est de 6.1 eV pour le thallium, de 7.4 eV pour le plomb, et de 9.2 eV pour l'or, ce qui est plus faible que le potentiel d'ionisation du xénon (12.1 eV). Ainsi, la probabilité d'ioniser ces métaux est plus élevée que celle d'ioniser le xénon.Also, a metal with a high atomic mass has a lower first ionization potential than other materials. For example, it is 6.1 eV for thallium, 7.4 eV for lead, and 9.2 eV for gold, which is lower than the ionization potential of xenon (12.1 eV). Thus, the probability of ionizing these metals is higher than that of ionizing xenon.

De plus, un métal avec une masse atomique élevée présente une probabilité plus élevée d'être doublement ionisé, c'est-à-dire qu'il perd deux électrons pour former des ions métalliques. Ainsi, à puissance d'alimentation électrique égale, un ion de ce métal est davantage accéléré que les ions ne perdant qu'un seul électron, tel que généralement le cas pour le Xénon. Par exemple, les potentiels de double ionisation du plomb (15 eV), du thallium (20,4 eV) et de l'or (20,2 eV) sont inférieurs au potentiel de double ionisation du Xénon (21 eV).Also, a metal with a high atomic mass has a higher probability of being doubly ionized, i.e., it loses two electrons to form metal ions. Thus, at equal power supply, an ion of this metal is accelerated more than ions losing only one electron, as is generally the case for Xenon. For example, the double ionization potentials of lead (15 eV), thallium (20.4 eV) and gold (20.2 eV) are lower than the double ionization potential of xenon (21 eV).

L'invention concerne également un procédé de génération de plasma, dont le fonctionnement est décrit ci-après.The invention also relates to a method for generating plasma, the operation of which is described below.

Le cylindre de métal plasma 20, sous forme solide, est placé dans le tube 10. Le métal plasma 20 est ensuite chauffé par l'élément de chauffage 40, alimenté par la source de chauffage 42, à une température de chauffage Tc suffisante pour vaporiser l'extrémité aval du cylindre de métal plasma 20. La température de chauffage Tc est donc bien supérieure à la température ambiante. Simultanément, le métal plasma 20 est placé à un potentiel positif non nul par le générateur 50 (soit directement, soit par l'intermédiaire de l'anode 30 en contact avec le métal plasma 20).The plasma metal cylinder 20, in solid form, is placed in the tube 10. The plasma metal 20 is then heated by the heating element 40, supplied by the heating source 42, to a heating temperature Tc sufficient to vaporize the downstream end of the plasma metal cylinder 20. The heating temperature Tc is therefore much higher than the ambient temperature. Simultaneously, plasma metal 20 is placed at a non-zero positive potential by generator 50 (either directly or via anode 30 in contact with plasma metal 20).

Le gaz métallique, produit de cette vaporisation, est ionisé par les électrons émis par la source d'électrons 60 (qui est soit l'élément de chauffage 40, soit l'émetteur externe 62, soit l'ensemble des deux). Ces ions métalliques sont repoussés par le cylindre de métal 20 car ils sont de même charge positive, et sont accélérés vers l'extrémité aval 15 du tube 10. Ces ions métalliques, qui forment un plasma, entrent en outre en collision avec les électrons émis par la source d'électrons 60 de telle sorte que le flux plasma 70 émis par le tube 10 à son extrémité aval 15 est pour partie un flux de particules métalliques électriquement neutres, pour partie un flux d'ions métalliques, pour partie un flux d'électrons. La direction de propagation du flux 70 est indiquée par une flèche sur les figures 1 et 2.The metallic gas, product of this vaporization, is ionized by the electrons emitted by the electron source 60 (which is either the heating element 40, or the external emitter 62, or both together). These metal ions are repelled by the metal cylinder 20 because they have the same positive charge, and are accelerated towards the downstream end 15 of the tube 10. These metal ions, which form a plasma, also collide with the electrons emitted by the electron source 60 such that the plasma flux 70 emitted by the tube 10 at its downstream end 15 is partly a flux of electrically neutral metallic particles, partly a flow of metal ions, partly a flow of electrons. The direction of propagation of the flux 70 is indicated by an arrow on the figures 1 and 2 .

Ainsi, les ions métalliques sont accélérés puis éjectés du tube 10, et pendant leur éjection une partie de ces ions métalliques est neutralisée par collision avec les électrons émis par la source d'électrons 60. Ces ions métalliques qui sont neutralisés sont transformés en particules métalliques électriquement neutres.Thus, the metal ions are accelerated and then ejected from the tube 10, and during their ejection a part of these metal ions is neutralized by collision with the electrons emitted by the electron source 60. These metal ions which are neutralized are transformed into metal particles electrically neutral.

Le système selon l'invention ne comporte pas de grille d'accélération des ions, contrairement aux propulseurs HC (voir ci-dessous). En effet, ces grilles sont inutiles car les ions sont repoussés par l'anode et accélérés sous une tension positive suffisamment élevé (voir explication ci-dessous). Ainsi, la fabrication du système est simplifiée.The system according to the invention does not include an ion acceleration grid, unlike the HC thrusters (see below). Indeed, these grids are useless because the ions are repelled by the anode and accelerated under a sufficiently high positive voltage (see explanation below). Thus, the manufacture of the system is simplified.

Le système selon l'invention ne comporte pas d'aimants, contrairement aux propulseurs HE (voir ci-dessous). Le système n'utilise donc pas de champ magnétique généré par des aimants pour agir sur les électrons, ou sur les ions éjectés du métal. Le système est donc plus simple et moins coûteux à fabriquer.The system according to the invention does not include magnets, unlike HE thrusters (see below). The system therefore does not use a magnetic field generated by magnets to act on the electrons, or on the ions ejected from the metal. The system is therefore simpler and less expensive to manufacture.

Le système selon l'invention est par conséquent plus compact que d'autres systèmes selon l'art antérieur. Par exemple, la longueur du système est de l'ordre de 10 cm, et son diamètre est inférieur à 1 cm, par exemple égal à 0,5 cm.The system according to the invention is therefore more compact than other systems according to the prior art. For example, the length of the system is of the order of 10 cm, and its diameter is less than 1 cm, for example equal to 0.5 cm.

Le tube 10 étant chauffé lorsque le système fonctionne, les particules de vapeur métallique qui auraient pu se déposer sur la surface interne de la partie aval du tube 10 seront aisément vaporisées et se décolleront de la surface lors d'un futur fonctionnement. Ainsi, le tube 10 n'est pas encrassé par des dépôts.Since the tube 10 is heated when the system is operating, the metal vapor particles which could have deposited on the internal surface of the downstream part of the tube 10 will be easily vaporized and will come off the surface during future operation. Thus, the tube 10 is not clogged with deposits.

Avantageusement, le système selon l'invention fonctionne en courant continu généré par le générateur 50, ce qui évite l'interférence avec les composants électroniques éventuellement situés à proximité du système qui pourrait se produire si l'on utilise de la radiofréquence ou des hautes fréquences.Advantageously, the system according to the invention operates with direct current generated by the generator 50, which avoids interference with any electronic components located close to the system which could occur if radio frequency or high frequencies are used. .

Le potentiel fournit à l'anode 30 par le générateur 50 est de l'ordre de plusieurs centaines de Volts. L'intensité du courant est de l'ordre de 1 Ampère et plus, pouvant atteindre par exemple 5A ou plus en mode impulsionnel.The potential supplied to the anode 30 by the generator 50 is of the order of several hundred volts. The intensity of the current is of the order of 1 Amp and more, being able to reach for example 5A or more in pulsed mode.

Alternativement, le système fonctionne avec une série d'impulsions électriques (courant pulsé), à l'aide d'un générateur d'impulsions. Ce mode de fonctionnement présente l'avantage de fournir une poussée plus élevée dans le cas où le système selon l'invention est utilisé dans un système de propulsion de véhicule spatial (voir ci-dessous). Le générateur d'impulsions est alimenté par le générateur 50. Les essais réalisés par les inventeurs montrent que l'on peut atteindre un courant stable de 2 A (Ampères) avec un saut de tension moyen de 2 kV (kiloVolts), ce qui donne une puissance à chaque impulsion de 4 kW (kiloWatts) par impulsion. La durée de l'impulsion est variable entre 10 et quelques centaines de µs. Dans l'exemple de fonctionnement donné en figure 3, la durée de l'impulsion est de 40 µs (microsecondes) environ. La courbe référencée S représente le signal de l'impulsion (en Volts), la courbe référencée V représente le potentiel de décharge à l'anode (en kiloVolts), la courbe référencée I représente le courant de décharge à l'anode (en Ampères). La durée de l'impulsion est de 40 µs (microsecondes), l'unité sur l'axe des abscisses de la figure 3 étant en µs.Alternatively, the system operates with a series of electrical pulses (pulsed current), using a pulse generator. This mode operating mode has the advantage of providing a higher thrust in the case where the system according to the invention is used in a space vehicle propulsion system (see below). The pulse generator is powered by generator 50. The tests carried out by the inventors show that a stable current of 2 A (Amps) can be reached with an average voltage jump of 2 kV (kiloVolts), which gives a power at each pulse of 4 kW (kiloWatts) per pulse. The duration of the pulse is variable between 10 and a few hundred µs. In the operating example given in picture 3 , the duration of the pulse is approximately 40 µs (microseconds). The curve referenced S represents the pulse signal (in Volts), the curve referenced V represents the discharge potential at the anode (in kiloVolts), the curve referenced I represents the discharge current at the anode (in Amperes ). The duration of the pulse is 40 µs (microseconds), the unit on the abscissa axis of the picture 3 being in µs.

Cette puissance génère une poussée qui est bien supérieure à celle obtenue avec les propulseurs HE, pour une masse embarquée équivalente (voir ci-dessous).This power generates a thrust which is much greater than that obtained with HE thrusters, for an equivalent on-board mass (see below).

De plus, l'utilisation des générateurs d'impulsions de plus grande puissance permet en plus d'augmenter le courant, donc le jet plasma, de réaliser d'ionisations multiples, très utiles dans le cas où le système serait utilisé pour un tel objectif.In addition, the use of higher power pulse generators also makes it possible to increase the current, and therefore the plasma jet, to carry out multiple ionizations, very useful in the event that the system is used for such an objective. .

De plus, le système permet le transfert efficace de moment aux ions lourds, d'autant plus grand que la tension appliquée à l'anode est grande.Moreover, the system allows the efficient transfer of momentum to the heavy ions, all the greater as the voltage applied to the anode is great.

Contrairement aux systèmes de l'art antérieur qui utilisent exclusivement une source d'électrons externe (cathode) et pour lesquels il se forme un arc entre la cathode et l'anode, le système selon l'invention ne fonctionne pas en régime d'arc standard. Au contraire, la tension fournie initialement est de l'ordre de plusieurs milliers de Volts, et se maintient à quelques centaines de Volts après formation de l'arc (phénomène de rupture ou claquage). La valeur élevée de ce voltage (tension) même après claquage (comparé au régime d'arc standard où le voltage est inférieur à 100 Volts), est due à la formation en sortie aval du tube 10 d'une sphère de plasma dont la surface est la frontière de l'onde de choc générée par l'expansion du flux d'ions dans le vide. Ainsi, cette frontière est fortement chargée électriquement, ce qui contribue à accélérer les ions métalliques éjectés par le cylindre de métal plasma 20. Pour distinguer ce mode de fonctionnement de l'arc standard, il sera nommé « arc anomal ».Unlike the systems of the prior art which exclusively use an external electron source (cathode) and for which an arc is formed between the cathode and the anode, the system according to the invention does not operate in arc mode standard. On the contrary, the voltage supplied initially is of the order of several thousand volts, and remains at a few hundred volts after formation of the arc (breakage or breakdown phenomenon). The high value of this voltage (voltage) even after breakdown (compared to the standard arc regime where the voltage is less than 100 Volts), is due to the formation at the downstream output of the tube 10 of a plasma sphere whose surface is the boundary of the shock wave generated by the expansion of the ion flux in vacuum. Thus, this boundary is strongly electrically charged, which contributes to accelerating the metal ions ejected by the plasma metal cylinder 20. To distinguish this operating mode from the standard arc, it will be called “anomalous arc”.

C'est ce fonctionnement particulier du système de d'accélération selon l'invention qui permet de s'affranchir de l'utilisation de grilles d'accélération des ions dans le cas où le système selon l'invention est utilisé dans un système de propulsion de véhicule spatial (voir ci-dessous).It is this particular operation of the acceleration system according to the invention which makes it possible to dispense with the use of ion acceleration grids in the case where the system according to the invention is used in a propulsion system. space vehicle (see below).

Avantageusement, une fois le plasma généré depuis le cylindre de métal 20 comme expliqué ci-dessus, il est possible sous certaines conditions d'éteindre la source de chauffage 42 alors que l'arc anomal continue à fonctionner. En effet, les ions métalliques sont naturellement repoussés par l'anode, et en régime stationnaire le plasma s'autoentretient avec un chauffage entretenu par le courant de décharge (i.e., les électrons du plasma qui rejoignent l'anode), surtout pour des régimes de courant élevé. Ainsi la formation d'un arc anomal pérenne dans le vide est entretenue entre la cathode et l'anode. Dans ce cas on utilisera un émetteur d'électrons externe 62 comme source d'électrons uniquement pour émettre des électrons servant à neutraliser le plasma d'ions vers l'extrémité aval 15 du tube 10.Advantageously, once the plasma has been generated from the metal cylinder 20 as explained above, it is possible under certain conditions to turn off the heating source 42 while the anomalous arc continues to operate. Indeed, the metal ions are naturally repelled by the anode, and in steady state the plasma is self-sustaining with a heating maintained by the discharge current (i.e., the electrons of the plasma which join the anode), especially for regimes high current. Thus the formation of a perennial anomalous arc in vacuum is maintained between the cathode and the anode. In this case, an external electron emitter 62 will be used as an electron source only to emit electrons serving to neutralize the ion plasma towards the downstream end 15 of the tube 10.

Avantageusement, lorsque l'arc anomal est entretenu, on peut faire fonctionner la cathode sans chauffage additionnel. Ce mode de fonctionnement du système de génération de plasma présente l'avantage qu'en régime stationnaire la source d'électrons 60, en l'espèce l'émetteur externe 62, peut fonctionner à une puissance électrique consommée plus faible.Advantageously, when the anomalous arc is maintained, the cathode can be operated without additional heating. This mode of operation of the plasma generation system has the advantage that in steady state the source of electrons 60, in this case the external emitter 62, can operate at a lower electrical power consumption.

Avantageusement, le système (et le procédé) de génération de plasma selon l'invention est utilisé dans un système de propulsion d'un véhicule spatial, l'éjection du plasma servant à la propulsion de ce véhicule.Advantageously, the system (and the method) for generating plasma according to the invention is used in a propulsion system of a space vehicle, the ejection of the plasma serving for the propulsion of this vehicle.

Pour la propulsion dans l'espace d'un véhicule spatial, tel un satellite, on connait les propulseurs à effet Hall (ou propulseur HE (« Hall Effect Thruster »)). Ce propulseur comporte un espace annulaire avec un fond à une extrémité, et ouvert à l'autre extrémité, dans lequel un champ magnétique est établi. Une cathode, qui émet des électrons, est située à l'extrémité ouverte de l'espace annulaire souvent fonctionnant avec une alimentation en gaz (cathode creuse). Le fond de l'espace annulaire constitue une anode, au travers de laquelle sont injectés des atomes de xénon ou un autre gaz propulsif, stocké souvent sous forme liquéfiée. Les électrons émis par la cathode sont piégés à l'entrée de l'espace annulaire par le champ magnétique, où ils s'accumulent, une partie des électrons poursuivant leurs trajets vers l'anode. Les atomes de gaz propulsif sont ionisés par collision avec les électrons dans l'espace annulaire, et accélérés par le champ électrique en direction de l'extrémité ouverte de cet espace. En sortie de cet espace, les ions sont neutralisés en traversant le nuage d'électrons et éjectés hors de l'espace sous forme d'un plasma à charge nulle. L'éjection de ce plasma fournit la propulsion au véhicule spatial.For the propulsion in space of a space vehicle, such as a satellite, Hall effect thrusters (or HE (“Hall Effect Thruster”) thrusters) are known. This thruster has an annular space with a bottom at one end, and open at the other end, in which a magnetic field is established. A cathode, which emits electrons, is located at the open end of the annular space often operating with a gas supply (hollow cathode). The bottom of the annular space constitutes an anode, through which are injected atoms of xenon or another propellant gas, often stored in liquefied form. The electrons emitted by the cathode are trapped at the entrance to the annular space by the magnetic field, where they accumulate, part of the electrons continuing their journeys towards the anode. The propellant gas atoms are ionized by collision with the electrons in the annular space, and accelerated by the electric field towards the open end of this space. On leaving this space, the ions are neutralized by crossing the cloud of electrons and ejected out of space in the form of a plasma with zero charge. The ejection of this plasma provides propulsion to the space vehicle.

Pour diminuer le poids du système de propulsion, on cherche à en diminuer la taille. Or cette diminution implique une augmentation du champ magnétique pour conserver le même rendement, ce qui implique une consommation en énergie supplémentaire, et souvent le besoin d'un système de refroidissement des aimants afin de ne pas dépasser la température de Curie ou l'emploi d'électroaimants très consommateurs d'énergie.To reduce the weight of the propulsion system, it is sought to reduce its size. However, this reduction implies an increase in the magnetic field to maintain the same efficiency, which implies additional energy consumption, and often the need for a magnet cooling system so as not to exceed the Curie temperature or the use of energy-intensive electromagnets.

Il a alors été développé des systèmes de propulsion fonctionnant sans champ magnétique, en particulier le propulseur à cathode creuse (ou propulseur HC (« Hollow Cathode Thruster »)).Propulsion systems operating without a magnetic field were then developed, in particular the hollow cathode thruster (or HC thruster (“Hollow Cathode Thruster”)).

Dans le propulseur HC, un gaz est injecté au travers d'un tube (cylindre creux) formant l'anode, dont la surface interne est recouverte d'un matériau qui émet des électrons lorsqu'il est chauffé (émission thermoionique). Ainsi, le chauffage du tube entraîne une ionisation du gaz lors de son passage au travers du tube. Les ions ainsi formés sont ensuite accélérés par la différence de potentiel entre l'anode et la cathode qui est située à l'extrémité du tube qui est opposée à celle par laquelle le gaz est injecté.In the HC thruster, a gas is injected through a tube (hollow cylinder) forming the anode, the internal surface of which is covered with a material which emits electrons when heated (thermionic emission). Thus, heating the tube causes ionization of the gas as it passes through the tube. The ions thus formed are then accelerated by the potential difference between the anode and the cathode which is located at the end of the tube which is opposite to that through which the gas is injected.

Le propulseur HC présente des inconvénients.The HC propellant has drawbacks.

En effet, le propulseur HC fonctionne avec une différence de potentiel faible (30 V environ) et donc une poussée intrinsèque faible. Accélérer davantage les ions afin d'obtenir une poussée supérieure requiert des voltages (tension) de plusieurs centaines de volts, ce qui implique l'utilisation de grilles polarisées. Ces grilles sont placées en aval du tube. Ceci complexifie le système de propulsion. De plus ces grilles, étant soumises au flux des ions accélérés, s'usent, ce qui diminue leur efficacité à long terme.Indeed, the HC thruster operates with a low potential difference (approximately 30 V) and therefore a low intrinsic thrust. Accelerating the ions further to achieve greater thrust requires voltages (voltage) of several hundred volts, which involves the use of biased grids. These grids are placed downstream of the tube. This complicates the propulsion system. Moreover, these grids, being subjected to the flow of accelerated ions, wear out, which reduces their long-term effectiveness.

Ainsi, en utilisant dans le système de propulsion un système de génération d'un jet plasma tel que décrit ci-dessus et dans lequel c'est le flux 70 de plasma qui propulse le véhicule spatial, on simplifie le système de propulsion car il n'est pas nécessaire de déposer un revêtement d'un matériau supplémentaire, source d'électrons, sur la face interne du tube. En effet, la source d'électrons est située à l'extérieur du tube.Thus, by using in the propulsion system a system for generating a plasma jet as described above and in which it is the flow 70 of plasma which propels the space vehicle, the propulsion system is simplified because it is not necessary to deposit a coating of an additional material, a source of electrons, on the internal face of the tube. Indeed, the source of electrons is located outside the tube.

Selon l'invention, la source initiale (matériau précurseur) de matière pour les ions (matière à ioniser) n'est, à température ambiante, pas un gaz, ni un liquide, mais un solide. En d'autres termes, le matériau précurseur qui est utilisé par le système selon l'invention avant le début de son fonctionnement, donc avant le chauffage de ce matériau précurseur, est un métal solide.According to the invention, the initial source (precursor material) of material for the ions (material to be ionized) is, at ambient temperature, not a gas, nor a liquid, but a solid. In other words, the precursor material which is used by the system according to the invention before the start of its operation, therefore before the heating of this precursor material, is a solid metal.

L'utilisation d'un métal solide comme source initiale de matière pour les ions et non d'un gaz tel que le xénon ou d'un liquide permet de simplifier la fabrication et de diminuer la masse (embarquée) du système de propulsion puisqu'il n'est plus nécessaire d'utiliser des conteneurs pressurisés de gaz avec un contrôle de température, et les équipements associés (tuyaux de circulation du gaz, valves).The use of a solid metal as the initial source of material for the ions and not of a gas such as xenon or of a liquid makes it possible to simplify manufacture and to reduce the (on-board) mass of the propulsion system since it is no longer necessary to use pressurized gas containers with temperature control, and the associated equipment (gas circulation pipes, valves).

Le potentiel d'accélération des ions du système de propulsion est supérieur à celui des propulseurs HC et les ions sont accélérés sous un voltage suffisamment élevé (voir explication ci-dessus), ce qui permet de s'affranchir de l'utilisation de grilles polarisées et donc de diminuer le poids du système, et ce qui en augmente l'efficacité.The acceleration potential of the ions of the propulsion system is higher than that of HC thrusters and the ions are accelerated under a sufficiently high voltage (see explanation above), which makes it possible to overcome the use of polarized grids and therefore to reduce the weight of the system, and which increases its efficiency.

Le système fonctionne donc sans grilles d'accélération.The system therefore operates without acceleration grids.

Le système fonctionne sans aimants donc sans champ magnétique, contrairement aux propulseurs HE. Le système est donc plus simple et moins coûteux à fabriquer.The system works without magnets and therefore without a magnetic field, unlike HE thrusters. The system is therefore simpler and less expensive to manufacture.

Le système selon l'invention est par conséquent plus compact que d'autres systèmes selon l'art antérieur. Par exemple, la longueur du système est de l'ordre de 10 cm, et son diamètre est inférieur à 1 cm, par exemple égal à 0,5 cm.The system according to the invention is therefore more compact than other systems according to the prior art. For example, the length of the system is of the order of 10 cm, and its diameter is less than 1 cm, for example equal to 0.5 cm.

Le système selon l'invention peut également être utilisé pour d'autres applications, telles que la production d'ions lourds multichargés pour des accélérateurs de particules, ou pour la fusion thermonucléaire par ions lourds. Le système selon l'invention remplace ainsi avantageusement les systèmes existants de production d'ions lourds, qui utilisent des champs magnétiques.The system according to the invention can also be used for other applications, such as the production of multicharged heavy ions for particle accelerators, or for thermonuclear fusion by heavy ions. The system according to the invention thus advantageously replaces the existing systems for the production of heavy ions, which use magnetic fields.

Dans les accélérateurs, les impulsions fournies par le générateur sont d'une haute puissance, de l'ordre de plusieurs centaines de kV.In accelerators, the pulses supplied by the generator are of high power, of the order of several hundred kV.

Claims (11)

  1. A system for generating a plasma jet, comprising a tube (10) made of electrically insulating material containing a metal (20) that is in a solid form at room temperature and an anode (30) in contact with said metal (20), an electrical generator (50) connected to said anode (30) that is capable of producing a positive electrical potential at said anode (30), a heating element (40) that is capable of heating a portion of said metal (20) to a heating temperature Tc that is sufficient to vaporize said portion of the metal (20), an electron source (60) located on the outside of the tube (10) and out of the longitudinal axis of the tube (10), and being capable of generating an electron stream that is able to ionise the vapour of said metal so as to form metal ions, such that the metal ions thus produced are capable of being repelled and thus accelerated by said potential and ejected out of said tube (10) via the downstream end (15) of said tube (10), and a portion of which being neutralised by electrons emitted by said electron source (60) so as to form a plasma stream (70).
  2. The system for generating a plasma jet according to claim 1, wherein said metal (20) has an atomic mass higher than or equal to that of gold, or a melting point lower than or equal to that of gold.
  3. The system for generating a plasma jet according to claim 1 or 2, wherein said heating element (40) surrounds the downstream portion (12) of said tube (10).
  4. The system for generating a plasma jet according to any one of claims 1 to 3, wherein said tube (10) is made of ceramic.
  5. The system for generating a plasma jet according to any one of claims 1 to 4, wherein the anode (30) is distinct from the metal (20) contained in said tube (10).
  6. The system for generating a plasma jet according to any one of claims 1 to 5, wherein said electron source (60) comprises said heating element (40).
  7. The system for generating a plasma jet according to any one of claims 1 to 6, wherein said electron source (60) comprises an external electron emitter (62) that is distinct from said heating element (40).
  8. A propulsion system for a space vehicle comprising a system for generating a plasma jet according to any one of the preceding claims, the ejection of said plasma generating the thrust.
  9. A method for generating a plasma jet, characterised in that it comprises the following steps:
    (a) a tube (10) made of electrically insulating material containing a metal (20) that is in the solid form at room temperature and an anode (30) in contact with said metal (20), a generator (50) connected to said anode (30) and an electron source (60) located on the outside of the tube (10) and out of the longitudinal axis of the tube (10) are provided,
    (b) a positive electrical potential is applied to said anode (30) using said generator (50),
    (c) a portion of said metal (20) is heated to a heating temperature Tc that is sufficient to vaporise said portion of the metal (20),
    (d) the vapour of said metal thus produced is ionised by the electrons emitted by said electron source (60) so as to form metal ions that are accelerated by said potential and ejected out of said tube (10) via the downstream end (15) of said tube (10), and a portion of which being neutralized by electrons emitted by said electron source (60), so as to form a plasma stream (70).
  10. The method for generating a plasma jet according to claim 9, wherein said generator (50) delivers a DC electric current.
  11. The method for generating a plasma jet according to claim 9, wherein said generator (50) provides pulses generating an electric current.
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US6769241B2 (en) * 2001-07-09 2004-08-03 W. E. Research Llc Description of methods to increase propellant throughput in a micro pulsed plasma thruster
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US7701145B2 (en) * 2007-09-07 2010-04-20 Nexolve Corporation Solid expellant plasma generator
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