Electronegative plasma thruster with optimized injection

Abstract

A plasma thruster includes extraction of a stream of positive ions. The plasma thruster includes a single ionization stage; injecting ionizable gas for said ionization stage comprising injecting a first gas and an electronegative second gas; creating an RF electric field to cause the gases to ionize in the ionization stage creating a first zone called the hot zone, in the ionization stage; the first gas being distributed in the hot first zone, the second gas being distributed in a second zone less hot than said first zone; extracting a stream of negative ions and a stream of positive ions, these being both connected to the ionization stage; and the extraction of a stream of positive ions and the extraction of a stream of negative ions, ensuring that the thruster is electrically neutral.

Claims

1. A plasma thruster for extraction of a stream of positive ions, the plasma thruster comprising: a single ionization stage; means configured for injecting ionizable gas for said ionization stage, said means configured for injecting comprising at least a first means for injecting a first gas and a second means configured for injecting an electronegative second gas; means for creating a radio frequency (RF) electric field to cause the first gas and the second electronegative gas to ionize in the single ionization stage, said means for creating configured to create a first zone, said first zone being a hot zone, in the single ionization stage, the first gas being distributed in the hot zone, the electronegative second gas being distributed in a second zone of said ionization stage, said second zone being less hot than said first zone; and at least one means for extracting a stream of negative ions and for extracting a stream of positive ions, the at least one means for extracting the stream of negative ions and the stream of positive ions being connected to the single ionization stage, and ensuring that the plasma thruster is electrically neutral.

2. The plasma thruster according to claim 1, wherein the first gas and the electronegative second gas are identical.

3. The plasma thruster according to claim 1, further comprising: two compartments, the two compartments being constituents of the first zone and second zone.

4. The plasma thruster according to claim 1, wherein the first means for injecting the first gas are located on a first face of the single ionization stage, the second means for injecting the electronegative second gas being distributed along a second face transverse to said first face so as to deliver a series of electronegative second gas streams into the single ionization stage.

5. The plasma thruster claimed in according to claim 4, wherein the second means for injecting the electronegative second gas deliver streams of different flow rates into the single ionization stage.

6. The plasma thruster according to claim 1, wherein the at least one means for extracting the stream of negative ions and the stream of positive ions comprises at least one grid, biased alternately to extract negative ions alternately with positive ions.

7. The plasma thruster according to claim 1, wherein the means for creating the RF electric field comprise two conductive elements placed at ends of the ionization stage in order to put said stage under voltage.

8. The plasma thruster according to claim 1, wherein the means for creating the RF electric field comprise a coil supplied by a radiofrequency current.

9. The plasma thruster according to claim 1, wherein the means for creating the RF electric field comprise a helicon antenna supplied by a radiofrequency (RF) current.

10. The plasma thruster according to claim 1, wherein the electronegative second gas is a dihalogen.

11. The plasma thruster according to claim 10, wherein the electronegative second gas is of the diiodide type.

12. The plasma thruster according to claim 1, wherein the electronegative second gas is SF.sub.6.

13. The plasma thruster according to claim 1, wherein the electronegative second gas is oxygen.

14. The plasma thruster according to claim 5, further comprising: means for generating a static magnetic field within the ionization stage so as to filter electrons.

15. The plasma thruster according to claim 14: wherein the means for generating a static magnetic field within the ionization stage comprises permanent magnets placed on the periphery of the ionization stage in order to create the magnetic field within said ionization stage.

16. The plasma thruster according to claim 14, wherein said at least one means for extracting the stream of negative ions and the stream of positive ions are configured to extract the stream of negative ions and the stream of positive ions in a first direction perpendicular to a first direction of the static magnetic field applied in the ionization stage.

17. The plasma thruster according to claim 1, wherein said at least one means for extracting a stream of negative ions and a stream of positive ions comprises a first means for extracting negative ions and a second means for extracting positive ions, the first means for extracting a stream of negative ions and the second means for extracting the positive ions being connected to the ionization stage.

18. The plasma thruster according to claim 17, wherein the first means for extracting a stream of negative ions is a grid and the second means for extracting a stream of positive ions is another grid.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be better understood and further details will become apparent on reading the following description given by way of nonlimiting example and in conjunction with the appended figures in which:

(2) FIG. 1 shows schematically a conventional plasma thruster according to the prior art, comprising an electropositive gas for generating a stream of positive ions, which is neutralized with an electron beam downstream of the accelerating zone;

(3) FIG. 2 shows schematically a plasma thruster according to the prior art comprising an electronegative gas for simultaneously generating a stream of positive ions and a stream of negative ions;

(4) FIG. 3 illustrates an example of a thruster according to the invention, comprising the injection of two different gases at separate and optimized locations;

(5) FIG. 4 illustrates the variation in the electron temperature as a function of the distance from means for creating an electric field perpendicular to an applied magnetic field creating an electron heating zone;

(6) FIG. 5 illustrates the variation in the ratio of negative ions per electron, generated by attachment collision, as a function of the distance from means for creating an electric field perpendicular to an applied magnetic field creating an electron heating zone;

(7) FIG. 6 illustrates the level of negative ion generation by collision with electrons (attachment) as a function of the temperature and the level of ionization creating positive ions by collision with electrons as a function of the temperature;

(8) FIG. 7 shows schematically a second embodiment of the invention comprising a series of means for injecting the second gas into the ionization stage; and

(9) FIGS. 8a, 8b and 8c illustrate an example of a thruster according to the invention.

DETAILED DESCRIPTION

(10) In general, the thruster of the invention comprises a single ionization stage coupled to means for ionizing one or more gases intended for the thrust, said stage comprising at least first means 32 for injecting a first gas and second means 32 for injecting a second gas. The second gas injected is an electronegative gas and is diffused into the ionization stage in a cooler region compared with a hot zone located close to the means for creating an electric field necessary for ionizing the gases.

(11) These means for coupling the electrical energy into the plasma may be of the type comprising two DC, low-frequency or radiofrequency biased plates, a coil fed with radiofrequency power for inductive coupling, or else a microwave source. According to one embodiment of the invention, the means for creating an electric field comprise two conductive elements 304, 306 placed at the ends of the ionization stage in order to put said stage under voltage.

(12) FIG. 3 shows schematically a first example of an ionization stage comprising a feed with gas G.sub.1 and a feed with electronegative gas G.sub.2, the electrical energy coupling means being represented by a supply power Pe and generating electrons represented by e.sup..

(13) The hot region of the ionization stage is referenced Z.sub.1 close to the RF source, while the cooler region away from the RF source is referenced Z.sub.2. According to the invention, the electronegative gas is injected into the less-hot region.

(14) More precisely, the first gas may be an electropositive or electronegative gas, injected into the hot region Z.sub.1 within the core of the plasma in which the RF power is coupled with the electrons.

(15) The efficient generation of positive ions (G.sub.1.sup.+) and negative ions (G.sub.1.sup.) (using an electronegative gas) starting from the gas G.sub.1 is carried out in this region Z.sub.1.

(16) The second gas is injected into a region Z.sub.2 close to the extraction means, in which region the electrons have a lower temperature. The second gas is chosen to be electronegative, ensuring efficient generation of negative ions (G.sub.2.sup.).

(17) Extraction means Me, for example, at least one biased grid, are provided for extracting the positive ions and the negative ions. According to one embodiment of the invention, the thruster includes a system for the temporal modulation 302 of the ion extraction means.

(18) FIG. 4 illustrates in this case the variation in the electron temperature (T.sub.e) as a function of a distance X within the ionization stage, the distance being measured from the zone located near the point of electric field creation (reference 0) plotted on the horizontal axis in said FIG. 4.

(19) FIG. 5 illustrates the variation of the ratio () of negative ions per electron as a function of the same distance X. It is apparent that the generation of negative ions is very pronounced beyond a distance of about 40 mm in the case considered. Curve 5a relates to an O.sub.2 gas while curve 5b relates to an SF.sub.6 gas.

(20) Moreover, the rate of creation of negative ions is a decreasing function of the electron temperature, whereas the rate of ionization, creating positive ions, by collision with electrons, is an exponential function of the electron temperature.

(21) FIG. 6 illustrates this behavior for an electronegative gas, curve 6a relating to the first phenomenon K.sub.att (attachment reaction) and curve 6b relating to the second phenomenon K.sub.iz (ionization reaction) respectively.

(22) These two processes interfere for electron temperatures between 2 and 4 eV, depending on the gases. The negative ions are created in the low-temperature region and become dominant when the electron temperature is typically less than 1-2 eV, whereas the positive ions are created in a region of high electron temperature and become dominant for energies above about 4-5 eV (the threshold values vary greatly depending on the type of gas).

(23) The electronegative gas used may advantageously be a dihalogen of the I.sub.2 type. Such a gas has a number of advantagesit is inexpensive compared with other electronegative gases and has the great advantage of being solid at room temperature, thereby greatly simplifying all the packaging and storage processes.

(24) It is also highly electronegative and its ionization threshold is relatively lowit may thus generate not only negative ions but also positive ions very efficiently. It may also be used in a thruster according to the invention just as effectively both as first gas G.sub.1 and as second gas G.sub.2.

(25) According to one embodiment of the invention, the thruster may use as first gas a gas of the xenon type, for generating positive ions, and as second gas a dihalogen capable of generating negative ions.

(26) In the above embodiments of the invention, the thruster comprises two zones, called hot and cold zones respectively, into which a first gas and an electronegative second gas are respectively injected via two injection means.

(27) According to another, more elaborate, embodiment of the invention, it is proposed to use a series of means for injecting the second gas, with injection flow rates that may be optimized according to the variation in the temperature in the ionization stage and therefore as a function of the electron temperature. These injections are thus carried out in a series of regions Z.sub.1, . . . , Z.sub.i, . . . , Z.sub.N with variable flow rates. This embodiment shown schematically in FIG. 7 relates to an example in which the single electronegative gas I.sub.2 is injected so as to generate both positive ions and negative ions.

(28) In these various embodiments, the thrust is therefore provided by the two types of ion (positive ions and negative ions). Neutralization downstream is no longer necessary since the ion beams are neutralized downstream (by recombination) to form a beam of rapidly moving neutral molecules.

(29) As is known, the ionization stage described above may be coupled to a filtration stage, such as that illustrated in FIG. 2.

(30) The filtration stage may be produced in at least two ways: (i) by modulating the creation of the plasma (pulsed plasmas: ON-OFF alternation of the electric power) and using the OFF period for the extraction, during which period the electrons disappear by attachment on the molecules. According to this configuration, the ionization stage and the filtration stage are common; (ii) using a static magnetic field to trap the electrons, the much heavier ions not being trapped.

(31) The thruster of the invention also includes an extraction stage that may be formed from accelerating grids, the dimensions of which are not necessarily similar to those in thrusters having a conventional grid, since the properties of the space charge sheaths are different in the absence of electrons.

(32) Example of a Thruster According to the Invention:

(33) In this example of a thruster according to the invention, the plasma is created by an RF (radiofrequency) antenna, the active surface of which is optimized and designed according to the intended applications. FIGS. 8a and 8b illustrate different views of the RF antenna and two zones, called the hot zone Z.sub.1 and the cold zone Z.sub.2 into which the gases G.sub.1 and G.sub.2 are injected respectively.

(34) A plate 80 seals the enclosure into which the gas G.sub.1 is injected.

(35) The temperature in the volume Z.sub.1 is high enough for creating positive ions by ionization and thus obtaining a high density of positive ions in this region.

(36) An electronegative second gas G.sub.2 is injected into the volume Z.sub.2 in order to produce the negative ions.

(37) The extraction volume is divided into two regions by permanent magnets, two accelerating grids being also installed at the outlet of the volume Z.sub.2.

(38) Permanent magnets 70 are placed on one face and in the middle of the volume Z.sub.2 in order to filter the electrons so as to preserve in the medium only positive ions and negative ions at the outlet of the volume Z.sub.2. In this region, the electron temperature decreases and the negative ions are produced by attachment collision with electrons. The applied magnetic field has two functions: (i) to increase the ionization efficiency by better confinement of the electrons; and (ii) to create the magnetic filter for the electrons, i.e. to magnetize the electrons, in order to prevent them from diffusing towards the extraction means.

(39) Extraction means 40 and 50 shown in FIG. 8c are used to accelerate the ions and expel them from the thruster, the ionic entities A.sup. and A.sup.+ thus being extracted from the thruster.

(40) These means may typically be of the grid type, one grid being able to be used to accelerate the negative ions and another grid being able to be used to accelerate the positive ions.

(41) It is also possible to introduce only a single grid, biased alternately so as to extract negative ions alternately with positive ions. It is also conceivable to use an array of grids.

(42) Finally, the two ion beams extracted, of opposite signs, become neutralized downstream (in space). The neutralization is therefore automatic and does not require an additional electron beam. The two beams may also recombine to form a beam of rapidly moving neutral molecules.