EP3344873B1 - Gridded ion thruster with integrated solid propellant - Google Patents

Description

The invention relates to a plasma thruster comprising an integrated solid propellant.

The invention relates more precisely to an ionic thruster, grid, comprising an integrated solid propellant.

The invention may find application for a satellite or a space probe.

More particularly, the invention may find application for small satellites. Typically, the invention will find an application for satellites having a mass of between 6 kg and 100 kg, possibly up to 500 kg. A particularly interesting application case concerns the “CubeSat”, of which a basic module (U) weighs less than 1 kg and has dimensions of 10cm ^∗ 10cm ^∗ 10cm. The plasma thruster according to the invention can in particular be integrated into a 1U module or a half-module (1 / 2U) and used in stacks of several modules by 2 (2U), 3 (3U), 6 (6U), 12 (12U) or more.

The term “propellant” is used here to denote a propellant in an ionic propellant, and not a product consisting of one or more propellants capable of supplying the propulsion energy of a rocket motor by chemical reaction.

A solid propellant plasma thruster has already been proposed. They can be classified into two categories, depending on whether they use a plasma chamber or not.

In the article by Keidar & al., “Electric propulsion for small satellites”, Plasma Phys. Control. Merger, 57 (2015 ) ( D1 ), different techniques are described for generating a plasma from a solid propellant, all based on ablation of a solid propellant. The solid propellant gives directly onto the external space, namely space for satellites or space probes, without a plasma chamber.

According to a first technique, Teflon (solid propellant) is placed between an anode and a cathode between which an electric discharge is carried out. This electric discharge causes the ablation of the Teflon, its ionization and its acceleration mainly by electromagnetic means to generate an ion beam directly in the external space.

According to a second technique, a laser beam is used to ablate and ionize a solid propellant, for example PVC. or Kapton®. The acceleration of the ions is generally carried out electromagnetically.

According to a third technique, an insulator is placed between an anode and a cathode, the whole being under vacuum. The cathode, metallic, serves as ablation material to generate ions. The acceleration takes place electromagnetically.

The techniques described in this document make it possible to obtain a relatively compact propellant. Indeed, the solid propellant is ablated, ionized and the ions are accelerated to provide propulsion with an all-in-one device.

However, the consequence is that there is no separate control of the sublimation of the solid propellant, the plasma and the ion beam.

In particular, the ion beam is more or less controlled because there is no separate means for controlling the density of the plasma induced by the ablation of the solid propellant and the speed of the ions. Accordingly, the thrust and specific impulse of the thruster cannot be controlled separately.

We generally do not have this type of disadvantage when a plasma chamber is implemented.

The article of Polzin & al., “Iodine Hall Thruster Propellant Feed System for a CubeSat”, American Institute of Aeronautics and Astronautics (D2) proposes a solid propellant supply system for a thruster operating by Hall effect.

This supply system can be used for any thruster using a plasma chamber.

In fact, in article D2, the solid propellant (iodine I ₂ , in this case) is stored in a tank. A heating means is associated with the tank. This heating means may be an element capable of receiving external radiation, placed on the outside of the tank. Thus, when the tank is heated, the diode is sublimated. The diode in the gas state leaves the reservoir and is directed to a chamber, located remote from the reservoir, where it is ionized to form a plasma. The ionization is carried out, in this case, by the Hall effect. The flow of gas entering the plasma chamber is controlled by a valve arranged between the reservoir and this chamber. We can thus achieve better control of the sublimation of the diode and of the characteristics of the plasma, compared with the techniques described in document D1.

Moreover, the characteristics of the ion beam exiting the chamber can then be controlled by means for extracting and accelerating the ions separated from the means used to sublimate the solid propellant and generate the plasma.

This system therefore has many advantages over those described in document D1.

However, in document D2, the presence of such a power supply system makes the plasma thruster not very compact and consequently not very conceivable for small satellites, in particular for a “CubeSat” type module.

In US 8,610,356 (D3), a system is also proposed using a propellant such as iodine (I ₂ ) stored in a reservoir located at a distance from a plasma chamber. The flow of diode gas leaving the tank is controlled by temperature and pressure sensors installed at the tank outlet and connected to a tank temperature control loop.

Here too, the system is not very compact.

In the same type of system as those proposed in documents D2 or D3, we can also cite document US 6,609,363 (D4), which describes an ion propellant according to the preamble of claim 1.

It should be noted that a propellant plasma thruster integrated in a plasma chamber has already been proposed in US 7,059,111 (D5). This plasma thruster, based on the Hall effect, is therefore likely to be more compact than that proposed in documents D2, D3 or D4. It is also capable of better controlling the evaporation of the propellant, the plasma and the extraction of the ions, compared to document D1. However, the propellant is stored in the liquid state and uses an additional system of electrodes to control the flow of gas leaving the tank.

An objective of the invention is to overcome at least one of the aforementioned drawbacks.

• une chambre,
• un réservoir comprenant un propergol solide, ledit réservoir étant logé dans la chambre et comportant une enveloppe conductrice munie d'au moins un orifice ;
• un ensemble de moyens pour former un plasma ions-électrons dans la chambre, ledit ensemble étant apte à sublimer le propergol solide dans le réservoir pour former un propergol à l'état de gaz, puis à générer ledit plasma dans la chambre à partir du propergol à l'état de gaz provenant du réservoir à travers ledit au moins orifice ;
• un moyen d'extraction et d'accélération d'au moins les ions du plasma hors de la chambre, ledit moyen d'extraction et d'accélération comprenant :
  • soit une électrode logée dans la chambre à laquelle est associée une grille située à une extrémité de la chambre, ladite électrode présentant une surface plus importante que la surface de la grille,
  • soit un ensemble d'au moins deux grilles situées à une extrémité de la chambre ;
• une source de tension continue ou une source de tension alternative radiofréquence disposée en série avec un condensateur et adaptée pour générer un signal dont la radiofréquence est comprise entre la fréquence plasma des ions et la fréquence plasma des électrons, ladite source de tension continue ou alternative radiofréquence étant connectée, par l'une de ses sorties, au moyen d'extraction et d'accélération d'au moins les ions du plasma hors de la chambre, et plus précisément:
  • soit à l'électrode,
  • soit à l'une des grilles dudit ensemble d'au moins deux grilles,

la grille associée à l'électrode ou, selon le cas, l'autre grille dudit ensemble d'au moins deux grilles étant soit mise à un potentiel de référence, soit connectée à l'autre des sorties de ladite source de tension alternative radiofréquence ;
ledit moyen d'extraction et d'accélération et ladite source de tension continue ou alternative radiofréquence permettant de former, en sortie de la chambre, un faisceau comportant au moins des ions.To achieve this objective, the invention provides an ion propellant, comprising:

• room,
• a tank comprising a solid propellant, said tank being housed in the chamber and comprising a conductive envelope provided with at least one orifice;
• a set of means for forming an ion-electron plasma in the chamber, said set being able to sublimate the solid propellant in the tank to form a propellant in the gas state, then to generate said plasma in the chamber from the propellant in the form of gas coming from the reservoir through said at least orifice;
• means for extracting and accelerating at least the ions from the plasma out of the chamber, said means for extracting and accelerating comprising:
  • either an electrode housed in the chamber with which is associated a grid located at one end of the chamber, said electrode having a larger area than the area of the grid,
  • either a set of at least two grids located at one end of the chamber;
• a direct voltage source or a radiofrequency alternating voltage source arranged in series with a capacitor and adapted to generate a signal the radiofrequency of which is between the plasma frequency of the ions and the plasma frequency of the electrons, said direct or alternating radiofrequency voltage source being connected, by one of its outputs, by means of extraction and acceleration of at least the ions of the plasma outside the chamber, and more precisely:
  • either to the electrode,
  • either to one of the grids of said set of at least two grids,

the gate associated with the electrode or, as the case may be, the other gate of said set of at least two gates being either set to a reference potential, or connected to the other of the outputs of said radiofrequency alternating voltage source;
said extraction and acceleration means and said radiofrequency direct or alternating voltage source making it possible to form, at the outlet of the chamber, a beam comprising at least ions.

• la source de tension connectée au moyen d'extraction et d'accélération est une source de tension alternative radiofréquence, et l'ensemble de moyens pour former le plasma ions-électrons comprend au moins une bobine alimentée par cette même source de tension alternative radiofréquence par l'intermédiaire d'un moyen pour gérer le signal fourni par ladite source de tension radiofréquence en direction d'une part, de ladite au moins une bobine et d'autre part, du moyen d'extraction et d'accélération, pour former un faisceau d'ions et d'électrons en sortie de la chambre ;
• l'ensemble de moyens pour former le plasma ions -électrons comprend au moins une bobine alimentée par une source de tension alternative radiofréquence différente de la source de tension continue ou alternative radiofréquence connectée au moyen d'extraction et d'accélération ou au moins une antenne micro-ondes alimentée par une source de tension alternative micro-ondes ;
• la source de tension connectée au moyen d'extraction et d'accélération est une source de tension alternative radiofréquence, pour former, en sortie de la chambre, un faisceau d'ions et d'électrons ;
• le moyen d'extraction et d'accélération est un ensemble d'au moins deux grilles situées à une extrémité de la chambre, l'électroneutralité du faisceau d'ions et d'électrons est obtenue au moins en partie par réglage de la durée d'application des potentiels positifs et/ou négatifs issus de la source de tension alternative radiofréquence connectée au moyen d'extraction et d'accélération ;
• le moyen d'extraction et d'accélération est un ensemble d'au moins deux grilles situées à une extrémité de la chambre, l'électroneutralité du faisceau d'ions et d'électrons est obtenue au moins en partie par réglage de l'amplitude des potentiels positifs et/ou négatifs issus de la source de tension alternative radiofréquence connectée au moyen d'extraction et d'accélération ;
• la source de tension connectée au moyen d'extraction et d'accélération est une source de tension continue, pour former, en sortie de la chambre, un faisceau d'ions, le propulseur comprenant en outre des moyens pour injecter des électrons dans ledit faisceau d'ions afin d'assurer une électroneutralité ;
• le réservoir comporte une membrane située entre le propergol solide et l'enveloppe munie d'au moins un orifice, ladite membrane comportant au moins un orifice, la surface de la ou chaque orifice de la membrane étant plus grande que la surface de la ou chaque orifice de l'enveloppe du réservoir ;
• la ou chaque grille présente des orifices dont la forme est choisie parmi les formes suivantes : circulaires, carrés, rectangles ou en formes de fentes, notamment de fentes parallèles ;
• la ou chaque grille présente des orifices circulaires, dont le diamètre est compris entre 0,2mm et 10mm, par exemple entre 0,5mm et 2mm ;
• lorsque le moyen d'extraction et d'accélération hors de la chambre comprend un ensemble d'au moins deux grilles situées à l'extrémité de la chambre, la distance entre les deux grilles est comprise entre 0,2mm et 10mm, par exemple entre 0,5mm et 2mm ;
• le propergol solide est choisi parmi : le diiode, le diiode mélangé à d'autres composants chimiques, le ferrocène, l'adamantane ou l'arsenic.

The propellant may also include at least one of the following characteristics, taken alone or in combination:

• the voltage source connected to the extraction and acceleration means is a radiofrequency alternating voltage source, and the assembly of means for forming the ion-electron plasma comprises at least one coil supplied by this same radiofrequency alternating voltage source by means of a means for managing the signal supplied by said radiofrequency voltage source in the direction of, on the one hand, said at least one coil and on the other hand, the extraction and acceleration means, to form a beam of ions and electrons at the outlet of the chamber;
• the set of means for forming the ion-electron plasma comprises at least one coil supplied by a radiofrequency alternating voltage source different from the radiofrequency direct or alternating voltage source connected to the extraction and acceleration means or at least one antenna microwave powered by an alternating microwave voltage source;
• the voltage source connected to the extraction and acceleration means is a radiofrequency alternating voltage source, to form, at the output of the chamber, a beam of ions and electrons;
• the extraction and acceleration means is a set of at least two grids located at one end of the chamber, the electroneutrality of the ion and electron beam is obtained at least in part by adjusting the duration d application of the positive and / or negative potentials coming from the radiofrequency alternating voltage source connected to the extraction and acceleration means;
• the extraction and acceleration means is a set of at least two grids located at one end of the chamber, the electroneutrality of the ion and electron beam is obtained at least in part by adjusting the amplitude positive and / or negative potentials coming from the radiofrequency alternating voltage source connected to the extraction and acceleration means;
• the voltage source connected to the extraction and acceleration means is a direct voltage source, to form, at the output of the chamber, an ion beam, the propellant further comprising means for injecting electrons into said beam ions in order to ensure electroneutrality;
• the reservoir comprises a membrane located between the solid propellant and the envelope provided with at least one orifice, said membrane comprising at least one orifice, the area of the or each orifice of the membrane being greater than the area of the or each orifice of the tank shell;
• the or each grid has orifices the shape of which is chosen from the following shapes: circular, square, rectangular or in the form of slots, in particular of parallel slots;
• the or each grid has circular orifices, the diameter of which is between 0.2mm and 10mm, for example between 0.5mm and 2mm;
• when the means of extraction and acceleration out of the chamber comprises a set of at least two grids located at the end of the chamber, the distance between the two grids is between 0.2mm and 10mm, for example between 0.5mm and 2mm;
• the solid propellant is selected from: diodine, diodine mixed with other chemical components, ferrocene, adamantane or arsenic.

The invention also relates to a satellite comprising a thruster according to the invention and an energy source, for example a battery or a solar panel, connected to the or each source of direct or alternating voltage of the thruster.

The invention also relates to a space probe comprising a thruster according to the invention and an energy source, for example a battery or a solar panel, connected to the or each source of direct or alternating voltage of the thruster.

• la figure 1 est une vue schématique d'un propulseur plasma selon un premier mode de réalisation de l'invention ;
• la figure 2 est une vue schématique d'une variante au premier mode de réalisation représenté sur la figure 1 ;
• la figure 3 est une vue schématique d'une autre variante au premier mode de réalisation représenté sur la figure 1 ;
• la figure 4 est une vue schématique d'une autre variante au premier mode de réalisation représenté sur la figure 1 ;
• la figure 5 est une vue schématique d'un propulseur plasma selon un deuxième mode de réalisation de l'invention ;
• la figure 6 est une vue schématique d'une variante au deuxième mode de réalisation représenté sur la figure 5 ;
• la figure 7 est une vue schématique d'une autre variante au deuxième mode de réalisation représenté sur la figure 5 ;
• la figure 8 est une vue schématique d'une autre variante au deuxième mode de réalisation représenté sur la figure 5 ;
• la figure 9 est une vue schématique d'une variante de réalisation du propulseur plasma représenté sur la figure 8
• la figure 10 est une vue schématique d'un troisième mode de réalisation de l'invention ;
• la figure 11 est une vue en coupe d'un réservoir à propergol solide susceptible d'être employé dans un propulseur plasma selon l'invention, quel que soit le mode de réalisation envisagé, avec son environnement permettant son montage à l'intérieur de la chambre plasma ;
• la figure 12 est une vue éclatée du réservoir représenté sur la figure 9 ;
• la figure 13 est une courbe fournissant, dans le cas du diiode (I₂) utilisé comme propergol solide, l'évolution de la pression de vapeurs de diode en fonction de la température ;
• la figure 14 représente, de façon schématique, un satellite comportant un propulseur plasma selon l'invention ;
• la figure 15 représente, de façon schématique, une sonde spatiale comportant un propulseur plasma selon l'invention.

The invention will be better understood and other aims, advantages and characteristics thereof will emerge more clearly on reading the description which follows and which is given with regard to the appended figures, in which:

• the figure 1 is a schematic view of a plasma thruster according to a first embodiment of the invention;
• the figure 2 is a schematic view of a variant of the first embodiment shown in figure 1 ;
• the figure 3 is a schematic view of another variant of the first embodiment shown in figure 1 ;
• the figure 4 is a schematic view of another variant of the first embodiment shown in figure 1 ;
• the figure 5 is a schematic view of a plasma thruster according to a second embodiment of the invention;
• the figure 6 is a schematic view of a variant of the second embodiment shown in figure 5 ;
• the figure 7 is a schematic view of another variant of the second embodiment shown in figure 5 ;
• the figure 8 is a schematic view of another variant of the second embodiment shown in figure 5 ;
• the figure 9 is a schematic view of an alternative embodiment of the plasma thruster shown in figure 8
• the figure 10 is a schematic view of a third embodiment of the invention;
• the figure 11 is a sectional view of a solid propellant tank capable of being used in a plasma thruster according to the invention, regardless of the embodiment envisaged, with its environment allowing its mounting inside the plasma chamber;
• the figure 12 is an exploded view of the tank shown in figure 9 ;
• the figure 13 is a curve providing, in the case of the diode (I ₂ ) used as solid propellant, the evolution of the pressure of diode vapors as a function of the temperature;
• the figure 14 represents, schematically, a satellite comprising a plasma thruster according to the invention;
• the figure 15 represents, schematically, a space probe comprising a plasma thruster according to the invention.

A first embodiment of an ion propellant 100 according to the invention is shown in figure 1 .

The propellant 100 comprises a plasma chamber 10 and a reservoir 20 of solid propellant PS housed in the chamber 10. More precisely, the reservoir 20 comprises a conductive envelope 21 comprising the solid propellant PS, this envelope 21 being provided with one or more orifices 22. The fact of accommodating the solid propellant reservoir 20 in the chamber 10 gives the propellant greater compactness.

The thruster 100 also includes a radiofrequency alternating voltage source 30 and one or more coils 40 supplied by the radiofrequency alternating voltage source 30. The or each coil 40 may have one or more windings. On the figure 1 , a single coil 40 comprising several windings is provided.

The coil 40, supplied by the radiofrequency alternating voltage source 30, induces a current in the reservoir 20, which is conductive (eddy current). The current induced in the tank causes a Joule effect which heats the tank 20. The heat thus produced is transmitted to the solid propellant PS by thermal conduction and / or thermal radiation. Heating the solid propellant PS then makes it possible to sublimate the latter, the propellant thus being put into the state of gas. Then, the propellant in the gas state then passes through the orifice (s) 22 of the reservoir 20, in the direction of the chamber 10. This same assembly 30, 40 also makes it possible to generate a plasma in the chamber 10. by ionizing the propellant in the gas state which is in chamber 10. The plasma thus formed will generally be an ion-electron plasma (it should be noted that, the plasma chamber will also include neutral species - propellant in the state of gas - because generally not all gas is ionized to form plasma).

The same source 30 of radiofrequency alternating voltage is therefore used to sublimate the solid propellant PS and create the plasma in the chamber 10. In the present case, a single coil 40 is also used for this purpose. However, it is conceivable to provide several coils, for example a coil to sublimate the solid propellant PS and a coil to create the plasma. By using several coils 40, it is then possible to increase the length of the chamber 10.

More precisely, the chamber 10 and the reservoir 20 are initially at the same temperature.

When the source 30 is activated, the temperature of the reservoir 20, heated by the coil (s) 40, increases. The temperature of the solid propellant PS also increases, the propellant being in thermal contact with the shell 21 of the tank.

This causes sublimation of the solid propellant PS, within the tank 20, and consequently an increase in the pressure P1 of propellant in the gas state within the tank 20 accompanying the increase in temperature T1 in this tank.

Then, under the effect of the pressure difference between the reservoir 20 and the chamber 10, the propellant in the gas state passes through the or each orifice 22 in the direction of the chamber 10.

When the temperature and pressure conditions are sufficiently high in the chamber 10, the assembly formed by the source 30 and the coil (s) 40 makes it possible to generate the plasma in the chamber 10. At this stage, the solid propellant PS is then more fully heated by the charged particles of the plasma, the coil (s) being screened by the presence of the sheath in the plasma (skin effect) as well as by the presence of the charged particles themselves within the plasma.

In the presence of the plasma (propellant in operation), it should be noted that the temperature of the tank 20 can be better controlled by the presence of a heat exchanger (not shown) connected to the tank 20.

One or more orifice (s) 22 can be provided on the reservoir 20, this does not matter. Only the total area of the orifice or, if several orifices are provided, of all of these orifices is of importance. Their size will depend on the nature of the solid propellant used, and the desired operating parameters for the plasma (temperature, pressure).

This sizing will therefore be carried out on a case-by-case basis.

In general, the sizing of the propellant according to the invention will take up the following steps.

The volume of the chamber 10 is first of all defined, as well as the nominal operating pressure P2 desired in this chamber 10 and the mass flow rate m 'of positive ions desired at the outlet of the chamber 10. These data can be obtained by numerical modeling or by routine testing. It should be noted that this mass flow rate (m ′) corresponds substantially to that which is found between the reservoir 20 and the chamber 10.

Then, the temperature T1 desired for the tank 20 is chosen.

This temperature T1 being fixed, it is possible to know the propellant pressure in the corresponding gas state, namely the pressure P1 of this gas in the tank 20 (cf. figure 13 in the case of diode I ₂ ).

Knowing in this way P2, m ', P1 and T1, it is possible to deduce therefrom the area A of the orifice or, if several orifices are provided, of all of the orifices. Advantageously, however, several orifices will be provided to ensure a more homogeneous distribution of the propellant in the gas state within the chamber 10.

An example of sizing is however provided below.

It is then possible to estimate the leakage of propellant in the gas state between the reservoir 20 and the chamber 10 when the thruster 100 is stopped. In fact, in this case, the surface A of the orifices is known, just like P1, T1 and P2, which makes it possible to obtain m '(leakage rate). In practice, it turns out that when stopping, the leak is minimal compared to the flow rate of propellant in the gas state passing from the tank 20 to the chamber 10 during use. This is why, in the context of the invention, the presence of valves at the level of the orifices is not compulsory.

For the solid propellant, one can consider: diiodine (I ₂ ), a mixture of diodine (I ₂ ) with other chemical components, adamantane (gross chemical formula: C ₁₀ H ₁₆ ) or ferrocene (formula crude chemical: Fe (C ₅ H ₅ ) ₂ ). Arsenic can also be used, but its toxicity makes it a solid propellant whose use is less envisaged.

Advantageously, diiodine (I ₂ ) will be used as solid propellant.

avec :
P, la pression en Torr ;This propellant indeed has several advantages. One represented on the figure 13 , a curve providing, in the case of the diode (I ₂ ), the evolution of the pressure P of the iodine gas as a function of the temperature T. This curve can be approximated by the following formula: $$\mathrm{Log}\left(\mathrm{P}\right)=-3512.8*\left(1/\mathrm{T}\right)-2013*\log \left(\mathrm{T}\right)+13.374$$

with:
P, the pressure in Torr;

T, the temperature in Kelvins.

This formula can be obtained in "The Vapor Pressure Iodine", GP Baxter, CH Hickey, WC Holmes, J. Am. Chem. Soc., 1907, 29 (2) pp. 12-136 . This formula is also cited in "The normal Vapor Pressure of Crysta / line Iodine", LJ Gillespie, & al., J. Am. Chem Soc., 1936, vol. 58 (11), pp 2260-2263 . This formula has been the subject of experimental verifications by various authors.

When the thruster goes from an off mode to a nominal operating mode, the temperature can be considered to increase by about 50K. In the temperature range between 300K and 400K, we note on this figure 13 that the pressure of the iodine gas increases practically by a factor of 100, for a temperature increase of 50K.

Also, when the thruster is in stop mode, the leakage of diode gas through the or each orifice 22 is very small, and of the order of 100 times less than the quantity of diode gas passing through the orifice (s) 22. towards the chamber 10, when the thruster 100 is in nominal operation.

A greater difference between the nominal operating temperature of the propellant according to the invention and its stopping temperature will only reduce the relative losses by leakage of propellant in the gas state.

Consequently, a propellant 100 according to the invention using the diode (I ₂ ) as propellant does not need to implement a valve for the or each orifice, unlike document D2. This further simplifies the design of the thruster and ensures good reliability. The flow control of propellant in the gas state is carried out by controlling the temperature of the tank 20, by means of the power supplied to the coil 40 by the radiofrequency alternating voltage source 30 and possibly, as specified. previously, by the presence of a heat exchanger connected to the tank 20. The check is therefore different from that which is carried out in document D3.

The thruster 100 also comprises a means 50 for extracting and accelerating the charged particles of the plasma, positive ions and electrons, out of the chamber 20 to form a beam 70 of charged particles at the outlet of the chamber 20. On the figure 1 , this means 50 comprises a grid 51 located at one end E (outlet) of the chamber 10 and an electrode 52 housed inside the chamber 10, this electrode 52 having by construction a larger area than that of the grid 51 In certain cases, the electrode 52 can be formed by the wall itself, conductive, of the reservoir 20.

The electrode 52 is isolated from the wall of the chamber by an electrical insulator 58.

The grid 51 may have orifices of different shapes, for example circular, square, rectangular or in the form of slots, in particular parallel slots. In particular, in the case of circular orifices, the diameter of an orifice may be between 0.2mm and 10mm, for example between 0.5mm and 2mm.

To ensure this extraction and acceleration, the means 50 is connected to the radiofrequency alternating voltage source 30. The radiofrequency alternating voltage source 30 therefore ensures, in addition, the control of the means 50 for extracting and accelerating the charged particles outside. of the chamber 10. This is particularly advantageous because it makes it possible to further increase the compactness of the thruster 100. In addition, this control of the means 50 of extraction and acceleration by the source 30 of radiofrequency alternating voltage makes it possible to better control the beam 70 of charged particles, unlike the techniques proposed in article D1 in particular. Finally, this control also makes it possible to obtain a beam with very good electroneutrality at the outlet of the chamber 10, without using any external device for this purpose. In other words, the assembly formed by the means 50 for extracting and accelerating the charged particles from the plasma and the radiofrequency alternating voltage source 30 therefore also makes it possible to obtain a neutralization of the beam 70 at the outlet of the chamber 10. compactness of the thruster 10 is thus increased, which is particularly advantageous for the use of this thruster 100 for a small satellite (<500kg), in particular a micro-satellite (10kg-100kg) or a nano-satellite (1kg-10kg), for example of the “CubeSat” type.

For this purpose, the gate 51 is connected to the radiofrequency voltage source 30 by means of a means 60 for managing the signal supplied by said radiofrequency voltage source 30 and the electrode 52 is connected to the radiofrequency voltage source. 30, in series, by means of a capacitor 53 and of the means 60 for managing the signal supplied by said radiofrequency voltage source 30. The gate 51 is also placed at a reference potential 55, for example ground. Likewise, the output of the radiofrequency alternating voltage source 30, not connected to the means 60, is also set to the same reference potential 55, the ground according to the example.

In practice, for applications in the space field, the reference potential may be that of the space probe or of the satellite on which the thruster 100 is mounted.

The means 60 for managing the signal supplied by said radiofrequency voltage source 30 therefore forms a means 60 which makes it possible to transmit the signal supplied by the source 30 of radiofrequency alternating voltage in the direction of, on the one hand, or of each coil 40 and of 'on the other hand, the means 50 for extracting and accelerating the ions and electrons from the chamber 10.

est la pulsation plasma des électrons et ${\mathrm{\omega }}_{\mathrm{pi}}=\sqrt{\frac{e_0^2{n}_p}{{\epsilon }_0{m}_i}}$

la pulsation plasma des ions positifs ; avec :

• e₀, la charge de l'électron,
• ε₀, la permittivité du vide,
• n _p , la densité du plasma,
• m_i , la masse des ions et
• m_e , la masse des électrons.

The source 30 (RF - radio frequencies) is set to define a pulse ω _RF such that ω _pi , ≤ ω _RF ≤ ω _pe , where: ${\mathrm{\omega }}_{\mathrm{pe}}=\sqrt{\frac{e_0^2{\mathrm{not}}_p}{{\epsilon }_0{m}_e}}$

is the plasma pulsation of electrons and ${\mathrm{\omega }}_{\mathrm{pi}}=\sqrt{\frac{e_0^2{\mathrm{not}}_p}{{\epsilon }_0{m}_i}}$

the plasma pulsation of positive ions; with:

• e ₀ , the charge of the electron,
• ε ₀ , the permittivity of vacuum,
• n _p , the density of the plasma,
• m _i , the mass of the ions and
• m _e , the mass of electrons.

Note that ω _pi << ω _pe due to the fact that m _i >> m _e .

In general, the frequency of the signal supplied by the source 30 can be between a few MHz and a few hundred MHz, depending on the propellant used for the formation of the plasma in the chamber 10 and this, to be between the plasma frequency of ions and the plasma frequency of electrons. A frequency of 13.56MHz is generally well suited, but the following frequencies can also be considered: 1MHz, 2MHz or even 4 MHz.

The electroneutrality of the beam 70 is ensured by the capacitive nature of the extraction and acceleration system 50 because, due to the presence of the capacitor 53, there are on average as many positive ions as there are electrons which are extracted at the over time.

In this context, the shape of the signal produced by the radiofrequency alternating voltage source 30 may be arbitrary. However, provision may be made for the signal supplied by the source 30 of the radiofrequency alternating voltage to the electrode 52 to be rectangular or sinusoidal.

The principle of operation for the extraction and acceleration of charged particles from the plasma (ions and electrons) with the first embodiment is as follows.

By construction, the electrode 52 has a surface which is greater, and generally significantly greater, than that of the grid 51 located at the outlet of the chamber 10.

In general, the application of an RF voltage to an electrode 52 having a larger surface area than the gate 51 has the effect of generating at the level of the interface between the electrode 52 and the plasma on the one hand, and at the level of the interface between the gate 51 and the plasma on the other hand, an additional potential difference, adding to the RF potential difference. This total potential difference is distributed over a sheath. The cladding is a space which is formed between the grid 51 or the electrode 52 on the one hand and the plasma on the other hand where the density of positive ions is higher than the density of electrons. This sheath has a variable thickness due to the variable RF signal applied to the electrode 52.

In practice, most of the effect of applying an RF signal to electrode 52, however, is located in the sheath of gate 51 (the electrode-gate system can be seen as a capacitor with two asymmetric walls , in this case the potential difference is applied to the part of lower capacitance and therefore of smaller surface).

In the presence of the capacitor 53 in series with the RF source, the application of the RF signal has the effect of converting the RF voltage to voltage. constant DC due to the charge of the capacitor 53, mainly at the level of the sheath of the gate 51.

This constant DC voltage in the sheath of the gate 51 implies that the positive ions are constantly extracted and accelerated (continuously). Indeed, this DC potential difference has the effect of making the plasma potential positive. Consequently, the positive ions of the plasma are constantly accelerated in the direction of the gate 51 (to a reference potential) and therefore extracted from the chamber 10 by this gate 51. The energy of the positive ions corresponds to this difference in DC potential ( average energy).

The variation of the RF voltage makes it possible to vary the difference in RF + DC potential between the plasma and the gate 51. At the level of the sheath of the gate 51, this results in a change in the thickness of this sheath. When this thickness becomes less than a critical value, which happens for a period of time at regular intervals given by the frequency of the RF signal, the potential difference between the gate 51 and the plasma approaches zero (therefore the plasma potential approaches the reference potential), which makes it possible to extract electrons.

In practice, the plasma potential below which the electrons can be accelerated and extracted (= critical potential) is given by Child's law, which relates this critical potential to the critical thickness of the cladding below which this cladding disappears (“sheath collapse” according to Anglo-Saxon terminology).

As long as the plasma potential is lower than the critical potential, then there is an acceleration and simultaneous extraction of electrons and ions.

Good electroneutrality of the beam 70 of positive ions and of electrons at the outlet of the plasma chamber 10 can thus be obtained.

On the figure 2 , there is shown an alternative embodiment to the first embodiment shown in figure 1 .

The same references designate the same components.

The difference between the propellant shown in the figure 2 compared to the thruster shown on figure 1 lies in the fact that the electrode 52 housed inside the chamber 10 is omitted and that a grid 52 'is added at the level of the end E (outlet) of the chamber 10.

In other words, the means 50 for extracting and accelerating the charged particles from the plasma comprises a set of at least two grids 51, 52 ′ located at one end E (outlet) of the chamber 10, one 51 at least of the set of at least two gates 51, 52 'being connected to the radiofrequency voltage source 30 via the means 60 for managing the signal supplied by said radiofrequency voltage source 30 and the other 52 'at least of the set of at least two gates 51, 52' being connected to the radiofrequency voltage source 30, in series, by means of a capacitor 53 and of the means 60 for managing the signal supplied by said radio frequency voltage source 30.

The connection of the gate 52 'to the source 30 of radiofrequency voltage is, on the figure 2 , identical to the connection of the electrode 52 to this source 30, on the figure 1 .

Each grid 51, 52 'may have orifices of different shapes, for example circular, square, rectangular or in the form of slots, in particular parallel slots. In particular, in the case of circular orifices, the diameter of an orifice may be between 0.2mm and 10mm, for example between 0.5mm and 2mm.

Furthermore, the distance between the two grids 52 ′, 51 may be between 0.2mm and 10mm, for example between 0.5mm and 2mm (the exact choice depends on the DC voltage and the density of the plasma).

In this variant, the operation of extraction and acceleration of positive ions and electrons is as follows.

When an RF voltage is applied through the source 30, the capacitor 53 charges. The charge of the capacitor 53 then produces a DC voltage at the terminals of the capacitor 53. An RF + DC voltage is then obtained at the terminals of the assembly formed by the source 30 and the capacitor 53. The constant part of the RF + DC voltage then makes it possible to define an electric field between the two gates 52 ', 51, the average value of the RF signal alone being zero. This DC value therefore makes it possible to extract and accelerate the positive ions through the two gates 51, 52 ', continuously.

Furthermore, when this RF voltage is applied, the plasma follows the potential printed at the gate 52 ', which is in contact with the plasma, namely RF + DC. As for the other gate 51 (reference potential 55, for example the mass), it is also in contact with the plasma, but only during the brief time intervals during which the electrons are extracted with the positive ions, namely when the RF + DC voltage is less than a critical value below which the sheath disappears. This critical value is defined by Child's law.

The electroneutrality of the beam 70 at the outlet of the chamber 10 is thus ensured.

It should also be noted that, for this realization of the figure 2 , the electroneutrality of the beam 70 of ions and electrons can be obtained at least in part by adjusting the duration of application of the positive and / or negative potentials from the radiofrequency alternating voltage source 30. This electroneutrality of the beam 70 ions and electrons can also be obtained at least in part by adjusting the amplitude of the positive and / or negative potentials coming from the radiofrequency alternating voltage source 30.

The advantage of this variant is, compared to the embodiment illustrated in figure 1 and implementing a gate 51 at the E end of chamber 10 and an electrode 52 housed in the chamber having a larger surface area than gate 51 to provide better control of the positive ion trajectory. This is linked to the fact that a DC (continuous) potential difference is generated between the two gates 52 ', 51, under the action of the source 30 of radiofrequency alternating voltage and of the capacitor 53 in series and not at the level of the cladding between the plasma and the grid 51 (cf. previously) in the case of the first embodiment of the figure 1 .

Therefore, with the variant embodiment shown in figure 2 , it is ensured that many more positive ions pass through the orifices of the grid 52 ', without touching the wall of this grid 52', with reference to what happens in the case of the first embodiment illustrated on the figure 1 .

In addition, the positive ions passing through the orifices of the grid 52 'do not come into contact with the wall of the grid 51 which is visible, from the point of view of these ions, only through the orifices of the grid 52. '. Accordingly, the service life of the grids 52 ', 51 according to this variant embodiment is improved compared to that of the grid 51 of the first embodiment of the figure 1 .

The life of the resulting propellant 100 is therefore improved.

Finally, the efficiency is improved because the positive ions can be focused by the set of at least two gates 51, 52 ', the flow of neutral species being reduced because the transparency to these neutral species increases. .

The figure 3 shows another variant of the first embodiment of the figure 1 , for which the gate 51 is connected, by its two ends to the source 30 of radiofrequency alternating voltage.

Everything else is the same and works the same.

The figure 4 shows an alternative embodiment to the variant shown in figure 2 , for which the gate 51 is connected, by its two ends, to the radiofrequency alternating voltage source.

Everything else is the same and works the same.

The variants illustrated on figures 3 and 4 therefore do not involve the implementation of a reference potential for the gate 51. In the space field, such a connection ensures an absence of parasitic currents flowing between, on the one hand, the external conductive parts of the space probe or of the satellite on which the thruster 100 is mounted and, on the other hand, the means 50 for extracting and accelerating the charged particles proper.

The figure 5 represents a second embodiment of an ion propellant according to the invention.

This is an alternative to the first embodiment shown in figure 1 and for which there is provided a first source 30 of radiofrequency alternating voltage to manage the extraction and acceleration of the charged particles of the plasma out of the chamber 10 and a second source 30 'of alternating voltage, separate from the first source 30 of radio frequency alternating voltage.

The rest are the same and work the same.

In this case, the means 60 for managing the signal supplied by a single source of radiofrequency alternating voltage 30 as proposed in support of the figures 1 to 4 no longer of interest.

This alternative allows for more flexibility.

Indeed, if the source 30 used for the extraction and acceleration of the charged particles out of the plasma remains a source of radiofrequency alternating voltage whose frequency is between the plasma frequency of the ions and the plasma frequency of the electrons, the source 30 'may generate a different signal.

For example, the source 30 'can generate an alternating radiofrequency voltage signal, associated with one or more coil (s) 40 to heat the casing 21 of the conductive tank 20 (made of a metallic material for example), to evaporate the solid propellant then generate a plasma in the chamber 10, the frequency of which is different from that of the operating frequency of the source 30. The operating frequency of the source 30 ′ may in particular be greater than that of the operating frequency of the source 30. .

According to another example, the source 30 'can generate an alternating voltage signal in frequencies corresponding to microwaves, associated with one or more microwave antenna (s) 40.

The figure 6 shows a variant of the second embodiment shown in figure 5 .

The difference between the propellant 100 shown in the figure 5 and the one shown on the figure 1 lies in the fact that the electrode 52 housed inside the chamber 10 is omitted and that a grid 52 'is added at the level of the end E (outlet) of the chamber 10.

The rest are the same and work the same.

In other words, the difference between the variant shown in the figure 6 and the second embodiment of the figure 5 is the same as that which was presented previously between the variant shown on the figure 2 and the first embodiment of the figure 1 .

The figure 7 shows another variant of the second embodiment of the figure 5 , for which the gate 51 is connected to the source 30 of radiofrequency alternating voltage.

Everything else is the same and works the same.

The figure 8 shows an alternative embodiment to the variant shown in figure 6 , for which the gate 51 is connected to the source 30 of radiofrequency alternating voltage.

Everything else is the same and works the same.

The variants illustrated on figures 7 and 8 therefore do not involve the use of a reference potential 55 for the gate 51. As explained previously, in the space field, such a connection ensures an absence of parasitic currents flowing between, on the one hand, the external conductive parts of the space probe or of the satellite on which the thruster 100 is mounted and, on the other hand, the means 50 for extracting and accelerating the charged particles proper.

The figure 9 represents an alternative embodiment of the thruster 100 illustrated in figure 8 .

This variant embodiment differs from that shown in the figure 8 in that the reservoir 20 comprises two stages E1, E2 for injecting propellant in the gas state to the plasma chamber 10.

Indeed, on the figure 8 , and moreover on all figures 1 to 7 , the reservoir 20 comprises a casing 21, one wall of which is provided with one or more orifice (s) 22, thereby defining a reservoir with a single stage.

On the contrary, in the variant shown in figure 9 , the reservoir further comprises a membrane 22 ′ comprising at least one orifice 22 ″ and separating the reservoir into two stages E1, E2. More precisely, the reservoir 20 comprises a membrane 22 ′ situated between the solid propellant PS and the envelope 21 provided with at least one orifice 22, said membrane 22 'comprising at least one orifice 22 ", the area of the or each orifice 22" of the membrane 22' being greater than the area of the or each orifice 22 of the envelope 21 of the tank 20.

This variant is of interest when, taking into account the sizing of the or each orifice 22 on the casing 21 of the reservoir 20 in order to obtain in particular the desired operating pressure P2 in the plasma chamber 10, the result is to define orifices that are too small. These orifices may then not be technically feasible. These holes can also, although technically feasible, too small to ensure that solid propellant dust and more generally impurities, will not block orifices 22 in use.

In this case, the or each orifice 22 "of the membrane 22 'is dimensioned so that it is larger than the or each orifice 22 made on the casing 21 of the reservoir 20, the or each orifice 22 remaining dimensioned to obtain the desired operating pressure P2 in the plasma chamber 10.

Of course, a double-stage tank 20 can be envisaged for all of the embodiments described in support of the figures 1 to 7 .

The figure 10 represents a third embodiment of an ion propellant according to the invention.

This figure is presented as an alternative to the realization of the figure 8 (grids 52 'and 51' both connected to the voltage source). However, it also applies as a variant to the figure 6 (grid 52 'connected to the source and grid 51 connected to ground), to the figure 7 (electrode 52 and gate 51 both connected to the voltage source), to the figure 5 (electrode 52 connected to the source and gate 51 connected to ground) and to the figure 9 .

The propellant 100 presented here makes it possible to form a beam 70 ′ of positive ions at the outlet of the plasma chamber 10. For this, the radiofrequency alternating voltage source 30 is replaced by a direct voltage (DC) source 30 ". In order to ensure the electroneutrality of the beam 70 ', electrons are injected into the beam 70' by an external device 80 , 81 to chamber 10. This device comprises a power source 80 supplying an electron generator 81. The electron beam 70 ″ leaving the electron generator 81 is directed towards the beam 70 ′ of positive ions to ensure electroneutrality.

The figures 11 and 12 represent a possible design for a plasma chamber 10 and its environment for a thruster 100 in accordance with the achievements of the figure 1 , of the figure 3 , of the figure 5 or the figure 7 .

These figures show the plasma chamber 10, the reservoir 20 with its casing 21 and the orifices 22. The reservoir 20 also serves as an electrode 52. In the present case, three orifices have been shown. 22, evenly distributed around the axis of symmetry AX of the reservoir 20. The casing 21 is made of a conductive material, for example metallic (aluminum, zinc or a metallic material coated with gold, for example) or of a metal alloy (stainless steel or brass, for example). As a result, eddy currents and therefore a Joule effect can be produced in the casing 21 of the reservoir 20 under the action of the alternating voltage source 30, 30 'and of the coil 40 or, as the case may be. , the microwave antenna 40. The transmission of heat between the casing 21 of the tank 20 and the solid propellant PS can be effected by thermal conduction and / or thermal radiation.

The chamber 10 is clamped between two rings 201, 202, mounted together by means of rods 202, 204, 205 extending along the chamber 10 (longitudinal axis AX). The chamber 10 is made of a dielectric material, for example ceramic. The fixing of the rings and the rods can be made by bolts / nuts (not shown). The rings can be made of a metallic material, for example aluminum. As for the rods, they are for example made of ceramic or a metallic material.

The assembly thus formed by the rings 201, 203 and the rods 202, 204, 205 allows the fixing of the chamber 10 and its environment, by means of additional parts 207, 207 ', which sandwich one 203 rings, on a system (not shown on the figures 11 and 12 ) intended to accommodate the thruster, for example a satellite or a space probe.

Sizing example.

An ion propellant 100 conforming to that shown in figure 1 has been tested.

The plasma chamber and its environment conform to what has been described in support of the figures 11 and 12 . The materials were chosen for a maximum acceptable temperature of 300 ° C.

The solid PS propellant used is diodine (I ₂ , dry mass of about 50 g).

Several orifices 22 have been provided on the conductive casing 21 of the reservoir 20 to pass the iodine gas from the reservoir 20 to the plasma chamber 10 (single-stage reservoir 20).

A reference temperature T1 for tank 20 has been set at 60 ° C. This can be obtained with a power of 10W at the level of the radiofrequency alternating voltage source 30. The frequency of the signal supplied by the source 30 is chosen to be between the plasma frequency of the ions and the plasma frequency of the electrons, in this case. 13.56MHz.

The pressure P1 of the iodine gas in the reservoir 20 is then known by the figure 13 (case of I ₂ ; cf. the corresponding formula F1), this one providing the link between P1 and T1. In the present case, P1 is 10 Torr (approximately 1330 Pa).

To obtain optimum efficiency, the pressure P2 in chamber 10 must then be between 7Pa and 15Pa with a mass flow m 'of iodine gas less than 15sccm (≅1.8.10 ^-6 kg.s ^-1 ) between the tank 20 and room 10.

We can then estimate that the diameter of the equivalent (circular) orifice is about 50 microns. When the orifice is single, it will therefore have a diameter of 50 microns. When several orifices are provided, which is the case in the test carried out, it is then necessary to determine the area of this orifice and to distribute this area over several orifices in order to obtain the diameter of each of the orifices, which will advantageously be the same. .

However, in order to give some additional sizing elements corresponding to the numerical values provided above, the following points may be noted, in the case of an orifice 22 with surface A.

où :

• P₁ est la pression dans le réservoir 20;
• P₂ est la pression dans la chambre 10 ; et
• v est la vitesse moyenne des molécules de gaz de diiode, déterminée par la relation : $$v=\sqrt{\frac{8{\mathit{kT}}_1}{\mathit{\pi m}}}$$
  
  où:
  • T₁ est la température dans le réservoir 20;
  • k est la constante de Boltzmann (k ≈ 1.38·10^-23 J·K^-1); et
  • m est la masse d'une molécule du gaz diode (m(I₂) ≈ 4.25·10^-25 kg).

The volume flow through the orifice 22 can be estimated by the relation: $$Q=\raisebox{1ex}{$v$}\!\left/ \!\raisebox{-1ex}{$4$}\right.\mathrm{AT}\left(P_1-P_2\right)$$

or :

• P ₁ is the pressure in the reservoir 20;
• P ₂ is the pressure in chamber 10; and
• v is the average speed of the molecules of iodine gas, determined by the relation: $$v=\sqrt{\frac{8{\mathit{kT}}_1}{\mathit{\pi m}}}$$
  
  or:
  • T ₁ is the temperature in the tank 20;
  • k is Boltzmann's constant ( k ≈ 1.38 · 10 ^-23 J · K ^-1 ); and
  • m is the mass of a molecule of the diode gas ( m (I ₂ ) ≈ 4.25 · 10 ^-25 kg).

où:

• M est la masse molaire du diode (for I₂, M ≈ 254 u); et
• R est la constante molaire des gaz (R ≈ 8.31 J/mol·K).

The mass flow m 'of iodine gas through the orifice 22 is then obtained by the relation: $$m^l\left[\mathit{kg}/s\right]=\frac{\mathit{MQ}}{{\mathit{RT}}_1}$$

or:

• M is the molar mass of the diode (for I ₂ , M ≈ 254 u); and
• R is the molar constant of gases ( R ≈ 8.31 J / mol · K).

By combining the relations (R1) and (R3), we deduce the area A of the orifice 22 by the relation: $$\mathrm{AT}=\frac{4m^l{\mathit{RT}}_1}{\mathit{vM}\left(P_1-P_2\right)}$$

The orifice 22 is then dimensioned.

As can be seen in relation (R4), the temperature T ₂ in the plasma chamber 10 does not intervene. A more precise modeling could be obtained by taking this temperature T ₂ into account. For more general data on this dimensioning, one can refer to: A User Guide To Vacuum Technology, third ed., Johan F. O'Hanlon (John Wiley & Sons Inc., 2003 ).

où:

• T₀ est la température du propulseur 100 à l'arrêt;
• P₀ est la pression du gaz dans le réservoir 20 lorsque le propulseur est à l'arrêt, cette pression étant fournie par la formule F1 (cf. figure 13) à la température T₀; et
• v₀ est obtenue en utilisant la relation (R2) en substituant T₁ par T₀.

Once the area A of the orifice 22 is sized, the mass flow rate m ' _leak (kg / s) of diode gas leakage when the propellant 100 is stopped can be determined by the relation: $${m^l}_{\mathit{leak}}\left[\mathit{kg}/s\right]\propto \frac{{\mathit{AP}}_0{\mathit{Mv}}_0}{4{\mathit{RT}}_0}$$

or:

• T ₀ is the temperature of the propellant 100 when stopped;
• P ₀ is the gas pressure in the tank 20 when the thruster is stopped, this pressure being provided by formula F1 (cf. figure 13 ) at temperature T ₀ ; and
• v ₀ is obtained by using the relation (R2) by substituting T ₁ by T ₀ .

End of the example.

It should be noted that the positioning of the or each orifice, shown in the appended figures on a face of the shell of the reservoir 20 facing the plasma chamber 10 could be different. In particular, it is quite possible to arrange the or each orifice on the opposite face of the reservoir 20.

Finally, the thruster 100 according to the invention can in particular be used for a satellite S or a space probe SP.

So the figure 14 represents, schematically, a satellite S comprising a thruster 100 according to the invention and an energy source SE, for example a battery or a solar panel, connected to the or each source of direct voltage 30 "or alternating voltage 30, 30 '(radio frequency or microwave, as the case may be) of the thruster 100.

About the figure 15 , it schematically represents a space probe SS comprising a thruster 100 according to the invention and an energy source SE, for example a battery or a solar panel, connected to the or each source of direct voltage 30 "or alternating voltage 30 , 30 '(radiofrequency or microwave, as the case may be) from the thruster 100.

Claims (14)

1. Ion thruster (100) comprising:

   - a chamber (10),

   - a tank (20) comprising a solid propellant (PS), said tank (20) comprising a conductive jacket (21) provided with at least one orifice (22);

   - a set of means (30, 30', 40) for forming an ion-electron plasma in the chamber (10), said set being able to sublime the solid propellant in the tank (20) in order to form a propellant in the gaseous state, then to generate said plasma in the chamber (10) from the propellant in the gaseous state coming from the tank (20) through said at least one orifice (22);

   - a means (50) for extracting and accelerating at least the ions of the plasma out of the chamber (10), said means (50) for extracting and accelerating comprising:

   • either an electrode (52) housed in the chamber (10) to which is associated a grid (51) located at one end (E) of the chamber (10), said electrode (52) having a surface that is greater than the surface of the grid (51),

   • or a set of at least two grids (52', 51) located at one end (E) of the chamber (10);

   - a DC voltage source (30") or a radiofrequency AC voltage source (30) arranged in series with a capacitor (53) and adapted for generating a signal of which the radiofrequency is between the plasma frequency of the ions and the plasma frequency of the electrons, said DC (30") or radiofrequency AC voltage source being connected, by one of its outputs, to the means (50) for extracting and accelerating at least the ions of the plasma out of the chamber (10), and more precisely:

   • either to the electrode (52),

   • or to one (52') of the grids of said set of at least two grids (51, 52'),

   with the grid (51) associated with the electrode (52) or, according to the case, the other grid (51) of said set of at least two grids (52', 51) being either set to a reference potential (55), or connected to the other of the outputs of said radiofrequency AC voltage source (30);
   said means (50) for extracting and accelerating and said DC or radiofrequency AC voltage source (30, 30") making it possible to form, at the output of the chamber (10), a beam (70, 70') comprising at least ions;
   characterised in that said tank (20) is housed in the chamber (10).
2. Thruster (100) according to claim 1, wherein:

   • the voltage source connected to the means (50) for extracting and accelerating is a radiofrequency AC voltage source (30),

   • the set of means (30, 40) for forming the ion-electron plasma comprises at least one coil (40) powered by this same radiofrequency AC voltage source (30) by the intermediary of a means (60) for managing the signal supplied by said radiofrequency voltage source (30) in the direction on the one hand, of said at least one coil (40) and on the other hand, of the means (50) for extracting and accelerating

   in order to form a beam (70) of ions and of electrons at the output of the chamber (10).
3. Thruster (100) according to claim 1, wherein the set of means (30, 40, 30') for forming the ion-electron plasma comprises:

   • at least one coil (40) powered by a radiofrequency AC voltage source (30') different from the DC (30") or radiofrequency AC (30) voltage source connected to the means (50) for extracting and accelerating; or

   • at least one microwave antenna (40) powered by a microwave AC voltage source (30').
4. Thruster (100) as claimed in the preceding claim, wherein the voltage source connected to the means (50) for extracting and accelerating is a radiofrequency AC voltage source (30), in order to form, at the output of the chamber (10), a beam (70) of ions and of electrons.
5. Thruster (100) according to one of claims 2 or 4, wherein, when the means (50) for extracting and accelerating is a set of at least two grids (52', 51) located at one end (E) of the chamber (10), the electroneutrality of the beam (70) of ions and electrons is obtained at least partially by adjusting the application duration of the positive and/or negative potentials coming from the radiofrequency AC voltage source (30) connected to the means (50) for extracting and accelerating.
6. Thruster (100) according to one of claims 2 or 4, wherein, when the means (50) for extracting and accelerating is a set of at least two grids (52', 51) located at one end (E) of the chamber (10), the electroneutrality of the beam (70) of ions and electrons is obtained at least partially by adjusting the amplitude of the positive and/or negative potentials coming from the radiofrequency AC voltage source (30) connected to the means (50) for extracting and accelerating.
7. Thruster (100) according to claim 3, wherein the voltage source connected to the means (50) for extracting and accelerating is a DC voltage source (30"), in order to form, at the output of the chamber (10), a beam (70') of ions, with the thruster (100) further comprising means (80, 81) for injecting electrons into said beam (70') of ions in order to provide electroneutrality.
8. Thruster (100) according to one of the preceding claims, wherein the tank (20) comprises a membrane (22') located between the solid propellant (PS) and the jacket (21) provided with at least one orifice (22), said membrane (22') comprising at least one orifice (22"), with the surface of the or of each orifice (22") of the membrane (22') being larger than the surface of the or of each orifice (22) of the jacket (21) of the tank (20).
9. Thruster (100) according to one of the preceding claims, wherein the or each grid (51, 52') has orifices of which the shape is chosen from the following shapes: circular, square, rectangle or in the form of slots, in particular parallel slots.
10. Thruster (100) according to one of the preceding claims, wherein the or each grid (51, 52') has circular orifices, of which the diameter is between 0.2 mm and 10 mm, for example between 0.5 mm and 2 mm.
11. Thruster (100) according to one of the preceding claims, wherein, when the means (50) for extracting and accelerating out of the chamber (10) comprise a set of at least two grids (52', 51) located at the end (E) of the chamber (10), the distance between the two grids (52', 51) is between 0.2 mm and 10 mm, for example between 0.5 mm and 2 mm.
12. Thruster (10) according to one of the preceding claims, wherein the solid propellant (PS) is chosen from: diatomic iodine, diatomic iodine mixed with other chemical components, ferrocene, adamantane or arsenic.
13. Satellite (S) comprising a thruster (100) according to one of the preceding claims and a source of energy (SE), for example a battery or a solar panel, connected to the or to each DC (30") or AC (30, 30') voltage source of the thruster (100).
14. Space probe (SS) comprising a thruster (100) according to one of claims 1 to 12 and a source of energy (SE), for example a battery or a solar panel, connected to the or to each DC (30") or AC (30, 30') voltage source of the thruster (100).

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