Plasma thruster and method for generating a plasma propulsion thrust

Abstract

The invention, which relates to a miniaturizable plasma thruster, consists of: igniting the plasma by microhollow cathode discharge close to the outlet and inside the means for injecting the propellant gas, said injection means being magnetic and comprising a tip at the downstream end thereof; bringing the electrons of the magnetized plasma into gyromagnetic rotation, at the outlet end of said injection means; sustaining the plasma by means of Electron Cyclotron Resonance (ECR), said injection means being metal and being used as an antenna for electromagnetic (EM) emission, the volume of ECR plasma at the outlet of said injection means being used as a resonant cavity of the EM wave; accelerating the plasma in a magnetic nozzle by diamagnetic force, the ejected plasma being electrically neutral.

Claims

1. A plasma thruster comprising: a discharge chamber comprising an internal cavity and an outlet opening; at least one injection means comprising an injection nozzle capable of injecting into the discharge chamber a propellant gas along a predefined axis, said injection nozzle having an outlet end; a magnetic field generator capable of setting electrons of the propellant gas present in the discharge chamber in gyromagnetic rotation; and an electromagnetic wave generator capable of irradiating the propellant gas present in the discharge chamber by generating at least one electromagnetic wave the electric field of which has a right-hand circular polarization and a frequency equal to the frequency, f.sub.ECR, of gyromagnetic resonance of the electrons of the propellant gas magnetized by said magnetic field generator, wherein said magnetic field generator is capable: on the one hand, of generating a magnetic field having: a first local maximum of intensity inside the injection nozzle and at the outlet end of the injection nozzle; field lines which determine an iso-field surface, known as the ECR surface, with an intensity equal to that allowing a cyclotron resonance of the electrons under the effect of said electromagnetic wave, said ECR surface enveloping the outlet end of said injection nozzle, the volume delimited by this ECR surface being the resonant cavity of the electromagnetic wave; a second local maximum of the intensity of the magnetic field inside the injection nozzle, separated from the first local maximum by a local minimum of the intensity of the magnetic field inside said injection nozzle; on the other hand, of giving said field lines the shape of a nozzle, so as to generate a diamagnetic propulsion force; said injection means: is produced from an electrically conductive material and is electrically connected to the electromagnetic wave generator so as to also operate as an electromagnetic antenna emitting said electromagnetic wave into the propellant gas at the outlet of said injection nozzle; is produced from a magnetically conductive material, making it possible to achieve, inside the latter, said second local maximum of the intensity of the magnetic field; and comprises, at the downstream end of said injection nozzle, an injection channel with an external diameter of less than a few millimeters.

2. The plasma thruster according to claim 1, in which the magnetic field generator comprises as magnetic field source at least one permanent magnet with a toric shape arranged coaxially to the predefined axis and having a first magnetic pole and a second magnetic pole, a first magnetic element integral with the first magnetic pole and a second magnetic element integral with the second magnetic pole, said first and second magnetic poles being arranged at a first distance and, respectively, a second distance from the predefined axis; the second distance being longer than the first distance, the first magnetic pole and the second magnetic pole being arranged upstream and, respectively, downstream of the injection nozzle with respect to the direction of flow of the propellant gas, the field lines intersecting with the injection nozzle and forming an angle comprised between 10 and 70 with said predefined axis.

3. The plasma thruster according to claim 1, in which the length, defined along the predefined axis, of the internal cavity of the discharge chamber is 5 to 10 times smaller than the half-wavelength of said electromagnetic wave in a vacuum, the discharge chamber having an internal cross-sectional area comprised between 0.7 square centimeters and 30 square centimeters; in which the central injection channel has an internal cross-sectional area comprised between 0.7 square millimeters and 3 square millimeters.

4. The plasma thruster according to claim 1, in which the magnetic field intensities of said first local maximum, local minimum and second local maximum are, respectively, 0.18 tesla, 0.01 tesla and 0.05 tesla.

5. The plasma thruster according to claim 1, in which said electromagnetic wave is capable of propagating along an axis parallel to the predefined axis and in which, at the predefined axis, the magnetic field gradient is parallel to the predefined axis; said magnetic field gradient being negative from upstream to downstream in a direction defined by the direction in which the propellant gas is ejected.

6. The plasma thruster according to claim 1, wherein the plasma thruster is configured to modulate a power of the electromagnetic wave and control a flow rate of the propellant gas, said power of the electromagnetic wave being between 0.5 watts and 300 watts, and between 0.5 watts and 30 watts in a first operating mode.

7. The plasma thruster according to claim 1, which further comprises, a circulator, arranged at an outlet of said electromagnetic wave generator and, an electrically conductive cylindrical sleeve, arranged downstream of a plane defined by the outlet opening known as an outlet plane of the plasma thruster, wherein a diameter of the electrically conductive cylindrical sleeve is equal to one quarter of the wavelength of the electromagnetic wave and the length of which is equal to three quarters of the wavelength of the electromagnetic wave.

8. The plasma thruster according to claim 1, further comprising two injection means coaxial to the axis, one supplying gas to be ionized to the ECR surface and the other increasing the thrust via a gas flow rate and an arcjet operation.

9. A method for generating a propulsion thrust by means of a plasma thruster comprising the following steps: injection, into a discharge chamber comprising an internal cavity and an outlet opening, using at least one injection means comprising an injection nozzle, of a propellant gas along a predefined axis; generation, using a magnetic field generator, of a magnetic field of setting electrons of the propellant gas present in the discharge chamber in gyromagnetic rotation; emission into the propellant gas present in the discharge chamber, using an electromagnetic wave generator, of at least one electromagnetic wave an electric field of which has a right-hand circular polarization and a frequency equal to the gyromagnetic resonance frequency, f.sub.ECR, of the electrons of the propellant gas magnetized by said magnetic field generator; ignition of a plasma by ionization of the propellant gas; and sustaining of the plasma by cyclotron resonance of the electrons; wherein: the ignition of the plasma is realized by microhollow cathode discharge using the injection means which is made of magnetic material and comprises, at the downstream end of its injection nozzle, an injection channel with an external diameter of less than a few millimeters; the injection of the propellant gas and the emission of the electromagnetic wave are carried out by the injection means and at a location in the discharge chamber, said injection means being produced from an electrically conductive material and electrically connected to the electromagnetic wave generator in order to emit the electromagnetic wave into the propellant gas at the outlet of the gas from said injection nozzle, so as to maximize the level of ionization of the propellant gas on exiting; said magnetic field generation is such that: the magnetic field has: a first local maximum of the intensity of the magnetic field situated inside the injection nozzle and at the outlet end of the injection nozzle; field lines which determine an iso-field surface, known as an ECR surface, with an intensity equal to that allowing a cyclotron resonance of the electrons under the effect of said electromagnetic wave, said ECR surface enveloping the outlet end of said injection nozzle; a second local maximum of the intensity of the magnetic field inside the injection nozzle, separated from the first local maximum by a local minimum of the intensity of the magnetic field inside said injection nozzle; the magnetic field gives said field lines the shape of a nozzle, so as to generate a diamagnetic force; and sustaining of the plasma by cyclotron resonance of the electrons being realized by resonance of the electromagnetic wave in the volume delimited by the ECR surface.

10. The method according to claim 9, in which the plasma thruster moreover comprises a device for modulating the power of the electromagnetic wave, a device for controlling the gas flow rate, a peripheral injection channel capable of injecting the propellant gas into the discharge chamber; and in which the method further comprises the following steps: injection of propellant gas into the discharge chamber via the peripheral injection channel; regulation of the flow rate of propellant gas injected into the discharge chamber via the peripheral injection channel; and modulation of the power of the electromagnetic wave.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be better understood on reading the following description, given by way of example only, and with reference to the drawings, in which:

(2) FIG. 1 is an axial cross-sectional view of a plasma thruster according to the invention;

(3) FIG. 2 is an enlarged view of part of FIG. 1 showing the field lines of the magnetic field generated by a generator of the plasma thruster according to the invention;

(4) FIG. 3 is a diagram of the steps of the method according to the invention;

(5) FIG. 4 is an axial cross-sectional view of a thruster according to a variant embodiment of the invention; and

(6) FIG. 5 is a graph showing the magnetic field along the axis A-A of the thruster.

DETAILED DESCRIPTION OF THE INVENTION

(7) With reference to FIG. 1, the plasma thruster 2 according to the invention comprises a support body 4 supporting a discharge chamber 6 leading to an outlet opening 48.

(8) The support body 4 is a non-magnetic hollow body open at each of its ends 9, 11. It comprises a cylindrical internal cavity 14 with an axis of rotation A-A, hereinafter called the predefined axis A-A.

(9) This cavity 14 comprises a central injection channel 10 coaxial to the predefined axis A-A. This central injection channel 10 is constituted for example by a magnetic metal pipe. It has an external diameter smaller than the diameter of the cavity 14, such that with the support body 4 it forms a peripheral injection channel 12 arranged between the internal wall of the support body 4 and the external wall of the central injection channel 10.

(10) In particular, the central injection channel 10 has an internal diameter comprised between 0.5 and 2 mm, and preferably comprised between 1 mm and 1.5 mm. The peripheral injection channel 12 has an external diameter comprised between 3 and 20 mm, and preferably comprised between 6 mm and 12 mm, the internal diameter of the peripheral injection channel 12 being the external diameter of the central injection channel 10.

(11) In other words, the central injection channel 10 has an internal cross-sectional area comprised between 0.7 square millimeters and 3 square millimeters. As a variant, the central injection channel 10 and the peripheral injection channel 12 have a square cross-section.

(12) The central injection channel 10 is fixed to the support body 4 via an insulation block 16 and a clamping ring 20. In particular, a portion of the central injection channel 10 is fitted into a through hole of the insulation block 16. The insulation block 16 is arranged and fixed in the cavity 14 between a shoulder 18 of the support body 4 and a bearing surface 21 of the clamping ring 20. The clamping ring 20 is screwed onto the outer edge of the end 9 of the support body 4.

(13) A first O-ring 22 is interposed between the insulation block 16 and the shoulder 18. A second O-ring 24 is interposed between the insulation block 16 and the bearing surface 21 of the clamping ring 20.

(14) The central injection channel 10 and the peripheral injection channel 12 form two means of injecting propellant gas into the chamber 6, in the context of the invention.

(15) For this purpose, one end of the central injection channel 10 is connected, by a pipe 28, to a propellant gas source 30. An opening 31 is arranged in the support body 4. This opening 31 leads to the peripheral injection channel 12. This opening 31 is connected by a pipe 44 to the propellant gas source 30 in order to supply the peripheral injection channel 12 with propellant gas, during operation of the plasma thruster in a second operating mode known as the arcjet operating mode, as described hereinafter.

(16) This source 30 is equipped with a device 32 for controlling the flow rate of the gas.

(17) In a first operating mode known as the conventional operating mode, the flow rate of the propellant gas is comprised between 0.1 grams per hour and 40 grams per hour.

(18) In a second operating mode known as the arcjet operating mode, the flow rate of the propellant gas is comprised between 1 gram per hour and 400 grams per hour, and preferably comprised between 10 grams per hour and 400 grams per hour.

(19) The other end of the central injection channel 10 comprises a pointed end 36, for example, formed by a beveling of the annular rim of the channel.

(20) The pointed end 36 extends outwards from the support body 4, into the discharge chamber 6. It contributes to the ionization of the propellant gas by an effect called point discharge. Point discharge makes it possible to concentrate the magnetic field into a volume of the discharge chamber, called the ignition volume. It is not a discharge by corona ionization, which concentrates the lines of the electric field, but a microhollow cathode discharge between the two above-mentioned intensity maxima of the magnetic field in the immediate proximity of an outlet of an injection nozzle.

(21) It is to be noted that the presence of a local maximum of the intensity of the magnetic field in the ignition volume, and therefore inside the injection tube, is possible for two reasons. Firstly because the present diamagnetic force thruster constitutes an open cavity for the magnetic field, or more precisely a coaxial system open at one end. Secondly because the complex magnetic circuit of the thruster comprises parts the role of which is precisely to channel a large portion of the magnetic field into this volume via in particular the injection channel 10 made of magnetic material and above all via its pointed end 36.

(22) In the present example, the ignition volume is comprised between 0.5 mm.sup.3 and 5 mm.sup.3. It is arranged 12 mm to 15 mm downstream of the pointed end 36 of the central injection channel 10.

(23) The central injection channel 10 is, moreover, suitable for emitting electromagnetic waves, in particular microwaves. For this, the central injection channel 10 is produced from an electrically conductive material and is electrically connected to an electromagnetic wave generator 38 via a connector 40 fixed, for example by screwing, to the support body 4. The connector 40 is, for example, an SMA (registered trade mark) type connector.

(24) The electromagnetic wave generator 38 is capable of irradiating the propellant gas present in the discharge chamber 6 with at least one electromagnetic wave, the electric field of which rotates in the same direction and at the same frequency as the magnetized electrons of the propellant gas, so as to achieve a total absorption of the electromagnetic energy by the ECR electrons. More precisely, the electric field has a right-hand circular polarization and a frequency equal to the gyromagnetic resonance frequency of the electrons of the propellant gas magnetized by the magnetic field generator.

(25) The electromagnetic field generator 38 is equipped with a device 42 for modulating electromagnetic power. It is suitable for generating electromagnetic waves with a power comprised between 0.5 and 300 watts, and preferably comprised between 0.5 and 30 watts in a first operating mode known as the conventional operating mode, and electromagnetic waves with a power comprised between 50 and 500 watts, and preferably comprised between 200 and 500 watts in the second operating mode known as the arcjet operating mode.

(26) The power of the electromagnetic waves is great enough to achieve ECR and to eject the electrons before they have time to radiate, but not too high, so as to prevent any radiation of these electrons before ejection, which makes it possible to prevent any heating by radiation and to preserve an optimum energy yield. The electromagnetic power that the thruster can absorb without degrading the energy yield is linked to the size of the Larmor radius Rb of the electrons in the plasma. This must remain substantially smaller than the radius of the cavity in order that the electrons do not at any time strike the internal wall of the thruster (plasma known as magnetic levitation plasma). However, for an electron with an electric charge qe and a mass me, in a magnetic field B0 in the order of 0.1 tesla (1000 gauss), a radius of gyration Rb of 1 millimeter would correspond to a speed of the electrons ve=Rb.qe.B0/me=1.7610.sup.7 m/s in a direction perpendicular to the magnetic field. Expressed in electron volts, the kinetic energy corresponding to the spin of the electrons would then be in the order of 0.9210.sup.5 eV. Compared with the ionization energy of the gas in the order of 10 to 20 eV for example, such a limit would seem to be difficult to achieve with the electromagnetic powers of a few tens to a few hundreds of watts which are involved here.

(27) It will also be noticed that in an adiabatic process the acceleration of the electrons in the nozzle preserves the magnetic moment=qe.sup.2. Rb.sup.2 B0/2 me. A decrease in B0 by a factor of 10 for example therefore would only cause an increase by a factor of about 3 in the electron gyration radius Rb.

(28) Finally, if a much greater electromagnetic power had to be used, it is possible, without increasing its dimensions, to increase the upper operating limit of the engine, by correlatively increasing the magnetic field B0 and the frequency of the EM exciter wave. Magnets about ten times more powerful than those used in our experiments are already commercially available.

(29) The discharge chamber 6 comprises a magnetic field generator 46 fixed, for example by screwing, to the end 11 of the support body 4. This generator 46 comprises a magnetic field source 50 having two poles, a washer 52 integral with an end surface constituting a pole of said source 50, a retaining nut 54 in contact with the washer 52, and a washer 58 integral with an end surface constituting the other pole of said source 50.

(30) The discharge chamber 6 moreover comprises an outlet opening 48 for the plasma.

(31) The magnetic field source 50 is constituted, for example, by a permanent magnet with a toric shape coaxial to the predefined axis A-A. To simplify the description, it is hereinafter called magnet 50.

(32) The magnetic field emitted by the magnet 50 has an intensity comprised between 0.05 tesla and 1 tesla, and preferably comprised between 0.085 tesla and 0.2 tesla.

(33) The washer 52 and the retaining nut 54 form a first magnetic element and the washer 58 forms a second magnetic element in the context of the invention.

(34) The washers 52, 58 are each integral with an annular face of the magnet 50. The washer 52 moreover is fixed, for example by screwing, on the outer periphery of the end 11 of the support body.

(35) The retaining nut 54 comprises a substantially truncated protuberance 62 the axis of rotation of which is the predefined axis A-A. The protuberance 62 extends towards the central injection channel 10.

(36) The washer 52, the retaining nut 54 and the washer 58 are constituted by paramagnetic steel, and preferably by ferromagnetic steel.

(37) With reference to FIG. 2, as the washer 52 and the retaining nut 54 are suitable for guiding the magnetic field emitted by the permanent magnet 50, the end surface of the protuberance 62 closest to the central injection channel 10 forms a first magnetic pole 64 arranged upstream of the injection nozzle 65 with respect to the direction F1 of flow of the propellant gas, and at a first distance D1 from the predefined axis A-A.

(38) As the washer 58 is also suitable for conducting the magnetic field, the end surface of the washer 58 closest to the central injection channel 10 forms a second magnetic pole 66 arranged downstream of the injection nozzle 65 of the central injection channel, with respect to the direction F1, and at a second distance D2 from the predefined axis A-A; said second distance D2 being longer than the first distance D1.

(39) The field lines 68 of the field emitted by the magnetic field generator 46 are in the shape of a nozzle. They intersect with the injection nozzle 65 of the central injection channel 10 and form an angle comprised between 10 and 70 with the predefined axis A-A. In other words, the magnetic field emitted by the magnetic field generator 46 is divergent. At the level of the predefined axis A-A, the magnetic field gradient is parallel to the predefined axis A-A. Moreover, this magnetic field gradient is negative from upstream to downstream with respect to the direction in which the propellant gas is ejected.

(40) The magnetic field moreover has a first local maximum of intensity of the magnetic field at the injection nozzle 65 of the central injection channel. This intensity is sufficient to completely ionize, by ECR, the propellant gas exiting said injection nozzle 65. This intensity is for example comprised between 0.087 tesla (ECR for a microwave frequency of 2.45 GHz), and approximately 0.5 tesla (upper limit that can be achieved with permanent magnets). The particular shape of the field lines 68 results in the ECR surface being very close to said first local maximum of intensity and in this ECR surface enveloping the outlet end 165 of the injection nozzle 65. For an EM wave frequency of 2.45 GHz, the ECR surface is situated at a distance in the order of millimeters downstream of the outlet end 165.

(41) In this patent application, a region of space where the gyration frequency of the free electrons in the local magnetic field is substantially equal to the frequency of the electromagnetic exciter wave is called the ECR surface.

(42) The magnetic field generator 46 moreover is capable of accelerating, by a diamagnetic force, the plasma ignited at the injection nozzle 65 towards the outlet opening 48, said plasma ejected from said thruster being electrically neutral. It is to be noted that one of the main benefits of ECR plasma sources resides in the possibility of acting only on the free electrons of the plasma and not on the ions, which requires only relatively reduced magnetic fields, of approximately 0.1 tesla (1000 gauss) in our example. The electrical neutrality of the plasma is very effectively ensured by the ambipolar electric field, or space charge field, which immediately appears within the plasma and counters any imbalance between the populations of positive ions and electrons. It is therefore not necessary to use a neutralizer. In the absence of an electric field applied by an optional accelerator grid, the ambipolar electric field is not disrupted and the electrons subjected only to the diamagnetic force will then entrain in their movement the non-magnetized positive ions (hence the diamagnetic nature of the plasma). Reciprocally, on exiting the thruster, the electrons connected to the ions by the space charge will be able to escape from the residual magnetic field because of the inertia of these previously accelerated ions inside the thruster. Contrary to the other thrusters of the state of the art, the acceleration of the plasma in the magnetic nozzle therefore does not require the consumption of additional electric power in the case where, as in this example, the magnetic nozzle is generated by simple permanent magnets. This saving on electric power is a significant advantage for a space application.

(43) The central injection channel 10 leads to the start of the divergent portion of the magnetic field, upstream of the ECR zone.

(44) Advantageously, the central injection channel 10 serves both as microwave emission antenna 39 inside the discharge chamber 6 and as injection nozzle 65 for injecting the gas to be ionized. The injection nozzle 65 comprises an outlet end 165.

(45) The magnet 50, the washer 52, the retaining nut 54 and the washer 58 form the discharge chamber 6. This has a diameter comprised between 6 mm and 60 mm, and preferably comprised between 12 mm and 30 mm. The discharge chamber 6 thus has an internal cross-sectional area comprised between 0.7 square centimeters and 30 square centimeters.

(46) The length, defined along the predefined axis A-A, of the internal cavity 14 of the discharge chamber 6 is 5 to 10 times smaller than the half-wavelength in a vacuum of the electromagnetic wave emitted by the electromagnetic wave generator 38.

(47) Advantageously, the discharge chamber has a very small dimension.

(48) The plasma thruster 2 comprises, moreover, a mounting clamp 70 and a lock nut 72 screwed onto the outer edge of the support body 4. An O-ring 74 is moreover arranged between the mounting clamp 70 and the lock nut 72.

(49) Advantageously, the plasma thruster according to the invention can be used by means of permanent magnets that do not consume energy.

(50) Advantageously, the discharge chamber forms a high-frequency resonant cavity having dimensions in the order of centimeters with a relatively low frequency in the order of 2.3 to 2.8 GHz. This is possible because the optical index of the ECR plasma is very high, which makes it possible to have a relatively short wavelength even with a relatively low frequency. As the ECR frequency is proportional to the magnetic field, a cavity of this size is therefore possible even with a magnetic field in the order of 0.08 to 0.1 T, which can easily be produced by annular permanent magnets with small dimensions.

(51) The method for generating a propulsion thrust according to the invention is realized by means of a plasma thruster described above. In the first operating mode known as conventional, it comprises, with reference to FIG. 3, the following steps: generation 90 of a magnetic field 63; emission 100 of the microwaves by the electromagnetic wave generator 38; injection 104 of the propellant gas into the discharge chamber 6 via the central injection channel 10; ignition 101 of the plasma; sustaining 103 of the plasma by ECR modulation 102 of the power of the electromagnetic wave emitted by the electromagnetic wave generator 38, by the modulation device 42; regulation 106 of the propellant gas flow rate in the central injection channel 10 by the control device 32.

(52) Advantageously, the emission step 100 is implemented before the injection step 104 when the user desires to save on propellant gas, and the injection step 104 is implemented before the emission step 100 when the user desires to save on electricity.

(53) In the second operating mode known as arcjet, it moreover comprises the following steps: injection 108 of additional propellant gas via the peripheral injection channel 12; regulation 110 of the propellant gas flow rate in the peripheral injection channel 12 by the control device 32; and modulation, with the modulation device 42, of the power of the microwaves emitted by the electromagnetic wave generator 38, in order to operate in the second operating mode known as arcjet.

(54) Advantageously, the axial injection of the propellant gas is completed in this operating mode by an injection of gas around the central injection pipe. This is generally used during a temporary operation with a strong thrust of the thruster, called here the second operating mode known as arcjet. In this case, the rise in pressure of the discharge chamber 6 makes it possible to ignite therein a plasma of the electric arc typevery dense and very hot under the effect of the injection of high-power microwaves (greater than a hundred watts). This makes it possible to operate the plasma thruster with much greater thrustsin the order of several hundreds of millinewtons, but for a much greater heat dissipation and a more reduced energy yield.

(55) Advantageously, it is possible to optimize, for example over the whole of the mission, the consumption both of gas and of energy, by taking advantage of a regulating range for the gas flow rate in the central injection channel and a regulating range for the power of the electromagnetic waves, the two causing the specific impulse and the thrust of the thruster to vary differently, and, where appropriate, by taking advantage of a regulating range for the gas flow rate in the peripheral channel and a regulating range for the power of the electromagnetic waves.

(56) Advantageously, it is possible to use each propulsion mode independently or in combination, a combination making it possible, for example, to realize fine adjustments of the total thrust, even for high amplitudes of this thrust.

(57) According to the variant embodiment shown in FIG. 4, the plasma thruster 120 moreover comprises, on the one hand, a circulator 80 connected to the electromagnetic wave generator 38 and to the connector 40 screwed onto the support body 4 and, on the other hand, an electrically conductive cylindrical sleeve 85, arranged downstream of the outlet plane of the plasma thruster 120.

(58) The circulator 80 is a device, generally made of ferrite, which is placed in a microwave circuit in order to protect the electromagnetic generator 38 or an optional amplifier against a return of EM waves, for example reflected by the plasma (which is the charge to be irradiated, for the EM wave generator). The flow of EM waves which passes through the circulator 80 in the direction of the plasma is not absorbed by the circulator. The flow reflected in the direction of the EM wave generator rotates in the circulator 80 and leaves again in the direction of the plasma, such that the electromagnetic generator 38 is protected and there is no loss of flow of EM waves by reflection directed upstream.

(59) The sleeve 85 has a diameter larger than the diameter of the permanent magnet 50 and a rim 86 fixed against the washer 58 of the magnetic field generator 46. In particular, the sleeve 85 is, for example, a circular wave guide segment with a diameter equal to wavelength and with a length equal to or wavelength of the EM wave in a vacuum. The sleeve 85 blocks the propagation of the EM wave which would otherwise radiate into free space by diffraction from the outlet opening of the thruster. Instead of being emitted into free space, the flow of ultra high frequency EM waves is thus reflected towards the plasma inside the thruster and the portion of it not absorbed by the plasma is sent to the circulator 80. The circulator 80 then, in turn, returns this reverse flow to the plasma thruster 120, and so on until the absorption of the flow of EM waves by the plasma is complete.

(60) FIG. 5 represents the variation of the magnetic field generated by the generator 46 relative to the distance from the outlet plane D-D of the plasma thruster along the predefined axis A-A. In this figure the zero of the X-axis defines the outlet plane D-D. As can be seen in FIG. 2, the outlet plane is the plane parallel to the central plane of the mounting clamp 70 situated at the outlet opening 48.

(61) As can be seen in this figure, the magnetic field has a first local maximum, A, and a second local maximum, C, situated inside the injection nozzle 65, as well as a local minimum situated between the first local maximum A and the second local maximum C.

(62) The first local maximum A is situated at the outlet end 165 of the injection nozzle 165. The first local maximum A is sufficient to ionize, by cyclotron resonance of the electrons of the propellant gas under the effect of said electromagnetic wave, the propellant gas exiting said injection nozzle 65.

(63) The first local maximum A has an intensity greater than the threshold value B.sub.ECR needed to achieve cyclotron resonance defined by the following formula:
B.sub.ECR=2**f.sub.ECRme/qe,

(64) in which me is the mass of an electron, qe is the electric charge of an electron, f.sub.ECR is the gyromagnetic resonance frequency.

(65) The magnetic field generator 50 is capable of accelerating, towards the outlet opening 48 by the diamagnetic force, the free electrons of the plasma ignited at the injection nozzle (65), the positive, non-magnetized, ions following these free electrons because of the ambipolar electric field, or space charge field, which appears almost immediately within the plasma and counters any imbalance between the populations of positive ions and electrons, this electric field, which is not disrupted by any electric field applied, very effectively ensuring the electrical neutrality of the plasma ejected from said thruster.

(66) By concentrating the magnetic field lines thereon, the pointed end 36 of the injection means 10 makes it possible, starting from the magnetic field generator 50, to achieve, on the one hand, the first local maximum of intensity A and, on the other hand, a microhollow cathode discharge, between the first local maximum A and the local minimum B of the intensity of the magnetic field. This microdischarge is sufficient to ionize at least a portion of the propellant gas present in said injection nozzle 65, whatever its flow rate. The magnetic field generator 50 comprises for example permanent magnets.