EP1957792B1 - Electronegative plasma thruster - Google Patents

Description

The invention lies in the field of plasma thrusters. These thrusters may for example be used in satellites or in spacecraft whose propulsion requires low thrusts over long periods, such as probes.

The propulsion of machines in space (where terrestrial gravitation becomes negligible) requires low thrusts (low ejected material flow), but high ejection speeds of the "fuel" to minimize the onboard weight. Indeed, the speed increase Δu of a spacecraft is related to the ejection velocity of the gases u _e and the initial masses m ₀ and final m _f of fuel by the following equation called "rocket equation": $$\mathrm{\Delta u}={\mathrm{u}}_{\mathrm{e}}\ln \left(\frac{{\mathrm{m}}_{\mathrm{0}}}{{\mathrm{m}}_{\mathrm{f}}}\right)$$

exprimée en secondes, où g₀ est la constante de gravité à la surface de la terre, et la poussée : $$\mathrm{T}=\dot{\mathrm{m}}{\mathrm{u}}_{\mathrm{e}}$$

où m est le débit massique.A high gas ejection speed is therefore imperative if we want to save fuel. Plasma thrusters achieve these high ejection speeds. Two quantities are used to characterize a thruster, the specific impulse: $${\mathrm{I}}_{\mathrm{s}}=\frac{{\mathrm{u}}_{\mathrm{e}}}{{\mathrm{boy\; Wut}}_{\mathrm{o}}}$$

expressed in seconds, where g ₀ is the gravitational constant at the Earth's surface, and the thrust: $$\mathrm{T}=\dot{\mathrm{m}}{\mathrm{u}}_{\mathrm{e}}$$

where m is the mass flow.

The principle of plasma thrusters described in the diagram illustrated in figure 1 , is the following: the "fuel" (gas) X is first ionized in a plasma to form positive ions X ⁺ and e ^- electrons, then ejected by acceleration in an electric field E (often created by accelerating gates ), before being neutralized by a Fe ^- annex electron beam positioned downstream of the accelerating zone. Neutralization is essential to prevent the spacecraft from charging electrically.

The various prototypes of plasma propellants existing to date, generally use an ionization stage to generate a source of positively charged material (positive ions), an acceleration stage and a neutralization structure. Sources of ionization, accelerating and neutralizing structures can be varied. But, all the propellers existing today use only the positively charged material (positive ions) for propulsion, the negative charge (the electrons) serving only for ionization and neutralization.

The article of J. SHIAO et al "The Dual-Plasma Jet Thrusters (with Electric Starters) Using Dual-Plasma Fusion Fuel Cells as Their Power Source", AIAA 2005-5385, XP008069911 , describes a plasma propellant comprising two ionization stages, electronegative ionizable gas supply means of one of the two ionization stages, means for creating an electric field so as to produce the ionization of the gas in the two ionization stages, and means for extracting a flow of negative and positive ions, respectively connected to the two ionization stages and allowing the extraction of a flow of positive ions and the extracting a flow of negative ions ensuring the electrical neutrality of the propellant.

In this context, the main idea proposed in the present invention is to use a positive ion flux and a negative ion flux for the thrust. For this, an electronegative gas (gas with high electron affinity) is used as fuel.

The thrust is ensured by the two types of ions, one of the types being positively charged and the other negatively. These ion beams are neutralized (for example by recombination) downstream to form a beam of fast neutral molecules which makes it possible to dispense with a neutralization structure downstream of the acceleration.

• un unique étage d'ionisation,
• des moyens d'alimentation en gaz électronégatif ionisable dudit étage d'ionisation,
• des moyens de création d'un champ électrique de manière à produire l'ionisation du gaz dans l'étage d'ionisation,
• des premiers moyens d'extraction d'un flux d'ions négatifs, des seconds moyens d'extraction d'un flux d'ions positifs, reliés à l'étage d'ionisation ;
• l'extraction d'un flux d'ions positifs et l'extraction d'un flux d'ions négatifs de même amplitude assurant la neutralité électrique du propulseur.

More specifically, the subject of the present invention is a plasma thruster comprising the extraction of a flow of positive ions, characterized in that it comprises:

• a single ionization stage,
• ionisable electronegative gas supply means of said ionization stage,
• means for creating an electric field so as to produce the ionization of the gas in the ionization stage,
• first means for extracting a flow of negative ions, second means for extracting a flow of positive ions, connected to the ionization stage;
• extracting a flow of positive ions and extracting a flow of negative ions of the same amplitude ensuring the electrical neutrality of the propellant.

The advantage of the invention lies in particular in the use of a single ionization stage and a single ionizable gas for delivering a flow of negative ions and a flow of positive ions of the same amplitude.

Advantageously, the plasma thruster according to the invention may further comprise means for filtering the electrons released in the ionization stage, during the ionization of the gas.

Advantageously, the plasma thruster may comprise ion flux extraction means comprising at least one polarized gate.

Advantageously, the plasma thruster may comprise means for creating an electric field comprising two conductive elements placed at the ends of the ionization stage for placing said stage under voltage, or comprising a coil supplied with a radiofrequency current.

The means for creating an electric field may also be of the helicon antenna type powered by a radio frequency current

According to a variant of the invention, the electronegative gas may be diiodine.

According to a variant of the invention, the electronegative gas can be oxygen.

According to a variant of the invention, the plasma thruster can comprise means for creating an alternating field generating a pulsed plasma (alternation of ON and OFF periods) allowing the extraction of the ion fluxes in the OFF period, during which the electrons have disappeared (temporal filter of electrons).

Advantageously, the plasma thruster may comprise means for generating a static magnetic field within the ionization stage so as to filter the electrons in stationary regime (spatial filter).

These means may be permanent magnets placed at the periphery of the ionization stage to create the magnetic field within said ionization stage.

According to a variant of the invention, the plasma thruster may comprise negative and positive ion flux extraction means in a direction perpendicular to the direction of the magnetic field applied at the level of the ionization stage. Advantageously, in this case, the Plasma propellant may comprise a constituent cylinder of the ionization stage and at least one peripheral extraction stage mounted on said cylinder and equipped on the surface with polarized grids.

• la figure 1 schématise un propulseur plasma selon l'art antérieur comprenant la propulsion d'un gaz positif accompagné d'un neutraliseur.
• la figure 2 schématise un exemple de propulseur selon l'invention comportant un gaz électronégatif pour générer simultanément un flux d'ions positifs et un flux d'ions négatifs
• la figure 3 illustre un exemple de propulseur selon l'invention, présentant deux grilles d'extraction polarisées positivement et négativement
• la figure 4 illustre une vue en perspective de variante d'étage d'extraction comprenant des paires de grilles polarisées positivement et négativement, selon un exemple de propulseur similaire à celui illustré en figure 3.

The invention will be better understood and other details will appear on reading the description which follows, given by way of non-limiting example, and thanks to the appended figures in which:

• the figure 1 schematizes a plasma propellant according to the prior art comprising the propulsion of a positive gas accompanied by a neutralizer.
• the figure 2 schematizes an example of a propellant according to the invention comprising an electronegative gas for simultaneously generating a flow of positive ions and a flow of negative ions
• the figure 3 illustrates an example of a propellant according to the invention, having two extraction grids polarized positively and negatively
• the figure 4 illustrates a perspective view of extraction stage variant comprising pairs of positively and negatively polarized grids, according to an example of a propeller similar to that illustrated in FIG. figure 3 .

• un étage d'ionisation 1
• un étage de filtrage 2
• un étage d'extraction 3.

In the example described below, the propellant according to the invention comprises a structure fed with electronegative gas as schematized in FIG. figure 2 and comprising:

• an ionization stage 1
• a filtering stage 2
• an extraction stage 3.

An electronegative gas flow A ₂ is introduced into the ionization stage 1. Under the action of an electric field schematized by the arrow representative of the electric power Pe, the electronegative gas generates positive ions A ⁺ , ions Negatives A ^- and electrons e ^- . The ionization stage is coupled to a filter stage 2 of the electrons so as to have in the extraction stage 3 a positive ion plasma and negative ions without electrons by means of filtering, which can be for example a static magnetic field. Plasma extraction is ensured in the case here schematized by two grids polarized negatively 4 and positively 5.

The thrust is therefore ensured by the two types of ions (the negative charge and the positive charge). Downstream neutralization is no longer necessary because the ion beams neutralize downstream (recombination) to form a beam of fast neutral molecules.

The ionization stage 1 may use any type of coupling of electrical energy to the plasma (for example: two plates that are continuously polarized, at low frequency or radiofrequency, a coil supplied with radiofrequency for coupling inductive, or a microwave source.

• (i) en modulant la création du plasma (plasmas pulsés : alternance ON-OFF de la puissance électrique) et en utilisant la période OFF pour l'extraction, période durant laquelle les électrons ont disparus par attachement sur les molécules. Selon cette configuration, les étages d'ionisation et de filtrage sont communs.
• (ii) en utilisant un champ magnétique statique pour piéger les électrons qui ont un rayon de Larmor beaucoup plus faible en raison du rapport de leurs masses respectives. Le rayon de Larmor est proportionnel à la masse des particules, il s'écrit : $${\mathrm{R}}_{\mathrm{L}}=\frac{{\mathrm{m}}_{\mathrm{e},\mathrm{i}}{\mathrm{u}}_{\mathrm{e},\mathrm{i}}}{\mathrm{eB}}$$
  

The filter stage 2 can be realized in at least two ways:

• (i) modulating the creation of the plasma (pulsed plasmas: ON-OFF alternation of the electrical power) and using the OFF period for the extraction, during which the electrons disappeared by attachment on the molecules. According to this configuration, the ionization and filtering stages are common.
• (ii) using a static magnetic field to trap electrons that have a much lower Larmor radius due to the ratio of their respective masses. The radius of Larmor is proportional to the mass of the particles, it is written: $${\mathrm{R}}_{\mathrm{The}}=\frac{{\mathrm{m}}_{\mathrm{e},\mathrm{i}}{\mathrm{u}}_{\mathrm{e},\mathrm{i}}}{\mathrm{eB}}$$
  

Where m _{e, i} and u _{e, i} are respectively the mass and velocity of the electrons or ions, e is the elementary charge, and B is the amplitude of the magnetic field.

The extraction stage 3 may consist of accelerating grids whose dimensions are not necessarily similar to those of conventional grid thrusters, because the properties of the space charge sheaths are different in the absence of electrons.

The figure 3 illustrates an example of a possible prototype which is only an example among the possible prototypes.

The system comprises a horizontal cylinder: the ionization stage 1, where the dense plasma is generated by applying a radio frequency voltage at 13.56 MHz on a "helicon" type antenna, represented by the RF symbol. Helicon sources are known to produce very efficient ionization. This cylinder further comprises means 6 for introducing the ionizable gas into the ionization stage. Diode 1 ₂ is used as fuel. It is a very electronegative gas that makes it possible to form a large amount of heavy negative ions (the higher the mass the higher the thrust, the mass of l ₂ is 254 uma (atomic mass unit). the threshold of ionization of the diode is low (10.5 eV to form l ⁺ ) which favors the formation of the positive ions with low energetic cost.However, any electronegative gas can a priori be used (for example the oxygen). static magnetic field B of an intensity of the order of 0.01-0.1 Tesla is applied in the source cylinder, to confine the electrons in the cylinder, as shown in the figure 3 . It can be generated by direct current flow in coils or by permanent magnets (positioned at the periphery of the cylinder and not shown).

• (i) augmenter l'efficacité d'ionisation grâce à un meilleur confinement des électrons et un meilleur chauffage du plasma par l'onde hélicon,
• (ii) créer le filtre magnétique pour les électrons, i.e. « magnétiser » les électrons, pour les empêcher de diffuser dans les étages d'extraction ionique 3.

This magnetic field has two functions:

• (i) increase the ionization efficiency thanks to a better confinement of the electrons and a better heating of the plasma by the helicon wave,
• (ii) create the magnetic filter for the electrons, ie "magnetize" the electrons, to prevent them from diffusing into the ion extraction stages 3.

These stages can typically be equipped with polarized grids, as shown in FIG. figure 3 , on the one hand to generate a flow of negative ions l _x ^- and a flow of positive ions l _y ⁺ . The positive and negative ions generated in the ionization stage (the horizontal cylinder) diffuse radially in the extraction stages because, unlike the electrons, they are not magnetized (the magnetic field is quite weak and their mass is very high). , so that their Larmor radius is much greater than the radius of the cylinder).

According to a variant of the invention, the extraction stages 3 illustrated in perspective on the figure 4 can also work with pairs of grids 41 and 51, (the system shown in the figures has four pairs, two on each side); one is polarized negatively, to accelerate the positive ions, the other is polarized positively, to accelerate the negative ions. Note that the extraction zones can have different geometric shapes; any geometry is conceivable and will seek to maximize the extraction area.

Finally, the two extracted ion beams, of opposite signs, neutralize each other downstream (in space). Neutralization is therefore automatic and does not require additional electron beam. The two beams can also recombine to form a beam of fast neutral molecules.

Typically with a thruster having an overall extraction area of about 500 cm ² , an acceleration voltage of 1000 V (obtained by polarizing the extraction grids so as to optimize the ion optics), it is possible to achieve a density of ion current of 10 mA / cm ² , and thus a total current extracted of the order of 5A. Taking the mass of iodine, this current corresponds to a mass flow of fuel ejected by 6.5 mg / s. Considering an acceleration voltage of 1000 V, the ion ejection speed will be 40 km / s. Referring to the equations presented in the introduction, this mass flow rate and this ejection speed lead to the following performances: a thrust of 250 mN for a specific pulse of 4000s.

Claims (13)

1. A plasma thruster comprising the extraction of a positive ion flow, characterized in that it comprises:

   • a single ionization stage (1),

   • supply means (6) for supplying ionizable electronegative gas for said ionization stage,

   • means for creating an electric field so as to produce the ionization of the gas in the ionization stage,

   • first means for extracting a negative ion flow, second means for extracting a positive ion flow, connected to the ionization stage;

   • extracting a positive ion flow and extracting a negative ion flow with the same amplitude ensuring the electrical neutrality of the thruster.
2. The plasma thruster according to claim 1, characterized in that it further includes means for filtering electrons (2) released in the ionization stage, during the ionization of the gas.
3. The plasma thruster according to one of claims 1 or 2, characterized in that the ion flow extraction means comprise at least one polarized gate (4, 5).
4. The plasma thruster according to one of claims 1 or 2, characterized in that the means for creating an electric field comprise two conductive elements placed at the end of the ionization stage to power said stage on.
5. The plasma thruster according to one of claims 1 to 4, characterized in that the means for creating an electric field comprise a coil powered by a radiofrequency current.
6. The plasma thruster according to one of claims 1 to 4, characterized in that the means for creating an electric field comprise a helicon antenna powered by a radiofrequency (RF) current.
7. The plasma thruster according to one of claims 1 to 6, characterized in that the electronegative gas is of the diiode type.
8. The plasma thruster according to one of claims 1 to 6, characterized in that the electronegative gas is oxygen.
9. The plasma thruster according to one of claims 2 to 8, characterized in that it comprises means for creating an alternating field generating a pulsed plasma allowing the simultaneous extraction of the ion flows without an electric field and filtration of the electrons.
10. The plasma thruster according to one of claims 2 to 8, characterized in that it comprises means for generating a static magnetic field within the ionization stage, so as to filter the electrons.
11. The plasma thruster according to claim 10, characterized in that it comprises permanent magnets placed on the periphery of the ionization stage to create a magnetic field within said ionization stage.
12. The plasma thruster according to one of claims 10 or 11, characterized in that it comprises means for extracting negative and positive ion flows (41, 51) in a direction perpendicular to the direction of the magnetic field applied at the ionization stage.
13. The plasma thruster according to claim 12, characterized in that it comprises a cylinder making up the ionization stage, at least one peripheral extraction stage mounted on said cylinder and equipped on the surface with positively and negatively polarized gates.

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